=The Shell.=--It is absolutely necessary_Tunneling: A Practical Treatise.

the shell should be smooth, and for this reason the exterior rivet heads

must be countersunk. It is generally admitted, also, that the shell

should be perfectly cylindrical, and not conical. The conical form has

some advantage in reducing the frictional resistance to the advance of

the shield; but this is generally considered to be more than

counterbalanced by the danger of subsidence of the earth, caused by the

excessive void which it leaves behind the iron tunnel lining. For the

same reason the shell plate, which overlaps the forward ring of the

lining, should be as thin as practicable, but its thickness should not

be reduced so that it will deflect under the earth pressure from above.

Generally the shell is made of at least two thicknesses of plating, the

plates being arranged so as to break joints, and, thus, to avoid the use

of cover joints, to interrupt the smooth surface which is so essential,

particularly on the exterior. The thickness of the shell required will

vary with the diameter of the shield, and the character and strength of

the diametrical bracing. Mr. Raynald Légouez suggests as a rule for

determining the thickness of the shell, that, to a minimum thickness of

2 mm., should be added 1 mm. for every meter of diameter over 4 meters.

Referring to the illustrations, Figs. 128 to 132 inclusive, it will be

noted that the St. Clair tunnel shield, 21¹⁄₂ ft. in diameter, had a

shell of 1-in. steel plates with cover-plate joints and interior angle

stiffeners; the shell of the East River tunnel shield, 11 ft. in

diameter, was made up of one ¹⁄₂-in. and one ³⁄₈-in. plate; the

Blackwall tunnel shield, 27 ft. 9 ins. in diameter, had a shell

consisting of four thicknesses of ⁵⁄₈-in. plates; and the Clichy tunnel

shield, with a diameter of 2.06 meters, had a shell 2 millimeters

thick.

=Front-End Construction.=--By the front end is meant that portion of the

shield between the cutting-edge and the vertical diaphragm. The length

of this portion of the shield was formerly made quite small, and where

the material penetrated is very soft, a short front-end construction yet

has many advocates; but the general tendency now is to extend the

cutting-edge far enough ahead of the diaphragm to form a fair-sized

working chamber. Excavation is far more easy and rapid when the face can

be attacked directly from in front of the diaphragm than where the work

has to be done from behind through the apertures in the diaphragm. So

long as the roof of the excavation is supported from falling, experience

has shown that it is easily possible to extend the excavation safely

some distance ahead of the diaphragm. In reasonably stable material,

like compact-clay, the front face will usually stand alone for the short

time necessary to excavate the section and advance the shield one stage.

In softer material the face can usually be sustained for the same short

period by means of compressed air; or the face of the excavation,

instead of being made vertical, can be allowed to assume its natural

slope. In the latter case a visor-shaped front-end construction, such as

was used on some portions of the Clichy tunnel, is particularly

advantageous. The following figures show the lengths of the front ends

of a number of representative tunnel shields.

City and South London 1 ft.

St. Clair River 11.25 „

Hudson River 5²⁄₃ „

Mersey River 3 „

East River 3²⁄₃ „

Blackwall 6.5 „

Two general types of construction are employed for the cutting-edge. The

first type consists of a cast-iron or cast-steel ring, beveled to form a

chisel-like cutting-edge and bolted to the ends of the forward shell

plates. This construction was first employed in the shield for the

London Tower tunnel, and has since been used on the City and South

London, Waterloo and City, and the Clichy tunnels. The second

construction consists in bracing the forward shell plates by means of

right triangular brackets, whose perpendicular sides are riveted

respectively to the shell plates and the diaphragm, and whose inclined

sides slant backward and downward from the front edge, and carry a

conical ring of plating. The shields for the St. Clair River, East

River, and Blackwall tunnels show forms of this type of cutting-edge

construction. A modification of the second type of construction, which

consists in omitting the conical plating, was employed on some of the

shields for the Clichy tunnel. This modification is generally considered

to be allowable only in materials which have little stability, and which

crumble down before the advance of the cutting-edge. Where the material

is of a sticky or compact nature, into which the shield in advancing

must actually cut, the beveled plating is necessary to insure a clean

cutting action without wedging or jamming of the material.

=Cellular Division.=--It is necessary in shields of large diameter to

brace the shell horizontally and vertically against distortion. This

bracing also serves to form stagings for the workmen, and to divide the

shield into cells. The following table shows the arrangement of the

vertical and transverse bracing in several representative tunnel

shields.

+------------------+----------+-------+-------+-------+

+------------------+----+-----+-------+-------+-------+

| |Ft. | In. | No. | Ft. | No. |

|Hudson River |19 |11 | 2 | 6.54 | 2 |

|Clichy |19.4| 0 | 2 | 6.54 | None |

|St. Clair River |21 | 6 | 2 | 6.98 | 3 |

|Waterloo (Station)|24 |10¹⁄₂| 2 | 7.12 | None |

|Blackwall |27 | 8 | 2 | 6.0 | 3 |

|East River |11 | ³⁄₄| None | ... | 1 |

+------------------+----+-----+-------+-------+-------+

Referring first to the horizontal divisions, it may be noted that they

serve different purposes in different instances. In the Clichy tunnel

shield the horizontal divisions formed simply working platforms; in the

Waterloo tunnel shield they were designed to abut closely against the

working face by means of special jacks, and so to divide it into three

separate divisions; in the St. Clair tunnel they served as working

platforms, and also had cutting-edges for penetrating the material

ahead; and in the Blackwall tunnel shield they served as working

platforms, and had cutting-edges as in the St. Clair tunnel shield, and

in addition the middle division was so devised that the two lower

chambers of the shield could be kept under a higher pressure of air than

the two upper chambers. Passing now to the vertical divisions, they

serve to brace the shell of the shield against vertical pressures, and

also to divide the horizontal chambers into cells; but unlike the

horizontal plates they are not provided with cutting-edges. The St.

Clair, Hudson River, and Blackwall tunnel shields illustrate the use of

the vertical bracing for the double purpose of vertical bracing and of

dividing the horizontal chambers into cells. The Waterloo tunnel shield

is an example, of vertical bracing employed solely as bracing. The

vertical division of the East River tunnel shield was employed in order

to allow the shield to be dissembled in quadrants.

=The Diaphragm.=--The purpose of the shield diaphragm is to close the

rear end of the shield and the tunnel behind from an inrush of water and

earth from the face of the excavation. It also serves the secondary

purpose of stiffening the shell diametrically. Structurally the

diaphragm separates the front-end construction previously described from

the rear-end construction, which will be described farther on; and it is

usually composed of iron or steel plating reinforced by beams or

girders, and pierced with one or several openings by which access is had

to the working face. In stable material, where caving or an inrush of

water and earth is not likely, the diaphragm is omitted. The shield of

the Waterloo tunnel is an example of this construction. In more

treacherous materials, however, not only is a diaphragm necessary, but

it is also necessary to diminish the size of the openings through it,

and to provide means for closing them entirely. Sometimes only one or

two openings are left near the bottom of the diaphragm, as in the St.

Clair and Mersey tunnel shields; and sometimes a number of smaller

openings are provided, as in the East River and Hudson River tunnel

shields.

In highly treacherous materials subject to sudden and violent irruptions

of earth from the excavation face, it sometimes is the case that

openings, however small, closed in the ordinary manner, are

impracticable, and special construction has to be adopted to deal with

the difficulty. The shields for the Mersey and for the Blackwall tunnels

are examples of such special devices. In the Mersey tunnel a second

diaphragm was built behind the first, extending from the bottom of the

shield upward to about half its total height. The aperture in the first

diaphragm being near the bottom, the space between the second and first

diaphragms formed a trap to hold the inflowing material. The Blackwall

tunnel shield, as previously indicated, had its front end divided into

cells. Ordinarily the face of the excavation in front of each cell was

left open, but where material was encountered which irrupted into these

cells a special means of closing the face was necessary. This consisted

of three poling-boards or shutters of iron held one above the other

against the face of the excavation. These shutters were supported by

means of strong threaded rods passing through nuts fastened to the

vertical frames, which permitted each shutter to be advanced against or

withdrawn from the face of the excavation independently of the others.

Various other constructions have been devised to retain the face of the

excavation in highly treacherous soils, but few of them have been

subjected to conclusive tests, and they do not therefore justify

consideration.

=Rear-end Construction.=--By the rear end of the shield is meant that

portion at the rear of the diaphragm. It may be divided into two parts,

called respectively the body and the tail of the shield. The chief

purpose of the body of the shield is to furnish a place for the location

of the jacks, pumps, motors, etc., employed in manipulating the shield.

It also serves a purpose in distributing the weight of the shield over a

large area. To facilitate the passage of the shield around curves, or

in changing from one grade to another, it is desirable to make the body

of the shield as short as possible. In the Mersey, Clichy, and Waterloo

tunnel shields, and, in fact, in most others which have been employed,

the shell plates of the body have been reinforced by a heavy cast-iron

ring, within and to which are attached the jacks and other apparatus.

The latest opinion, however, seems to point to the use of brackets and

beams for strengthening the shell for the purpose named, rather than to

this heavy cast-iron construction. In the Hudson River, St. Clair River,

and East River tunnel shields, with their long and strongly braced

front-end construction to carry the jacks, the body of the shield, so to

speak, is omitted and the rear-end construction consists simply of the

tail plating. In the Blackwall shield, the body of the shield shell

provides the space necessary for the double diaphragms and the cells

which they inclose. In a general way, it may be said that the present

tendency of engineers is to favor as short and as light a body

construction as can be secured.

The tail of the shield serves to support the earth while the lining is

being erected; and for this reason it overlaps the forward ring of the

lining, as shown clearly by most of the shields illustrated. To fulfill

this purpose, the tail-plates should be perfectly smooth inside and

outside, so as to slide easily between the outside of the lining plates

and the earth, and should also be as thin as practicable, in order not

to leave a large void behind the lining to be filled in. In soils which

are fairly stable, the tail construction is often visor-shaped; that is,

the tail-plates overlap the lining only for, say, the roof from the

springing lines up, as in one of the shields for the Clichy tunnel. In

unstable materials the tail-plating extends entirely around the shield

and excavation. The length of the tail-plating is usually sufficient to

overlap two rings of the lining, but in one of the Clichy tunnel shields

it will be noticed that it extended over three rings of lining. This

seemingly considerable space for thin steel plates is made possible by

the fact that the extreme rear end of the tail always rests upon the

last completed ring of lining.

In closing these remarks concerning the rear-end construction, the

accompanying table, prepared by Mr. Raynald Légouez, will be of

interest, as a general summary of principal dimensions of most of the

important tunnel shields which have been built. The figures in this

table have been converted from metric to English measure, and some

slight variation from the exact dimensions necessarily exists. The

different columns of the table show the diameter, total length, and the

length of each of the three principal parts into which tunnel shields

are ordinarily divided in construction as previously described:

+---------------------+-----------------------------------+

| | LENGTH IN FEET. |

| NAME OF SHIELD. +---------+-----+-----+------+------+

+---------------------+---------+-----+-----+------+------+

|Concorde Siphon | 6.75 | 2.51| 2.55| 1.16| 6.67|

|Clichy Siphon | 8.39 | 2.51| 2.55| 1.16| 6.16|

|Mersey | 9.97 | 5.61| 2.98| 2.98| 11.58|

|East River | 10.99 | 3.51| 0.32| 3.67| 7.51|

|City and South London| 10.99 | 2.65| 2.82| 1.01| 6.49|

|Glasgow District | 12.07 | 2.65| 2.82| 1.01| 6.49|

|Waterloo and City | 12.99 | 2.75| 2.98| 1.24| 6.98|

|Glasgow Harbor | 17.25 | 2.75| 2.98| 1.08| 8.49|

|Hudson River | 19.91 | 4.82| 2.98| 5.67| 10.49|

|St. Clair River | 21.52 | 4.00| 2.98| 11.25| 15.25|

|Clichy Tunnel |23.7-19.8| 4.00| 2.98| 6.88| 17.22|

|Clichy Tunnel |23.8-19.4| 7.44|11.90| 4.46| 23.65|

|Blackwall | 27.00 | 6.98| 5.90| 6.59| 19.48|

|Waterloo Station | 24.86 | 3.34| 5.51| 1.14| 10.00|

+---------------------+---------+-----+-----+------+------+

A shield of 60 or 100 tons weight can hardly be directed along the line

of the proposed tunnel and also through curves and grades, especially

when driven through loose or muddy soils. The tunnels of the New York

and Hudson River Railroad under the Hudson, and the tunnel of the New

York Rapid Transit Railway under the East River, show marked evidence of

how troublesome this work is. To avoid these and other inconveniences

encountered in every shield, the Author has designed a new shield which

was briefly described at page 251.

[Illustration: FIG. 136.--Elevation and Section of Hydraulic Jack, East

River Gas Tunnel.]

=Jacks.=--The motive power usually employed in driving modern tunnel

shields is hydraulic jacks. In some of the earlier shields screw-jacks

were used, but these soon gave way to the more powerful hydraulic

device. The manner of attaching the hydraulic jacks to the shield is

always to fasten the cylinder castings at regular intervals around the

inside of the shell, with the piston rods extending backward to a

bearing against the forward edge of the lining. In the older forms of

shield, having an interior cast-iron reinforcing ring construction, the

jack cylinder castings were always attached to this cast-iron ring; but

in many of the later shields constructed without this cast-iron

reinforcing ring, the cylinder castings are attached to the shell by

means of bracket and gusset connections. The number and size of the

jacks employed, and the distance apart at which they are spaced, depend

upon the size of the shield and the character of the material in which

it is designed to work. In stiff and comparatively stable clays, the

skin friction of the shield is comparatively small, and an aggregate

jack-power of from 4 to 5 tons per square yard of the exterior friction

surface of the shield has usually been found ample. The cylinders are

spaced about 5³⁄₄ ft. apart, and have a working diameter of from 5 to 6

ins., with a water pressure of about 1000 lbs. per sq. in. In soft,

sticky material, giving a high skin friction, the aggregate jack-power

required per square yard of exterior shell surface rises to from 18 to

24 tons; the jacks are spaced about 3 ft. apart; and the working

cylinder diameter and water pressure are, respectively, about 6 or 7

ins., and from 4000 lbs. to 6000 lbs. per sq. in. With these high

pressures, power pumps are necessary to give the required water

pressure; but where the pressure required does not exceed 1000 lbs. per

sq. in., hand pumps may be, and usually are, employed. Fig. 136 shows

the hydraulic jacks used in the East River Gas Tunnel at New York. The

number of jacks required depends upon the diameter of the shield, and,

of course, upon the distance apart which they are placed. In the City

and South London tunnel shield six jacks were used, and in the Blackwall

shield 24 were used. The mechanical construction of the jacks for tunnel

shields presents no features out of the usual lines of such devices used

elsewhere. The jacks used on the East River tunnel shield are shown by

Fig. 136.

Two general methods are employed for transmitting the thrust of the

piston rods against the tunnel lining. The object sought in each is to

distribute the thrust in such a manner that there is no danger of

bending the thin front flange of the forward lining ring. In English

practice the plan usually adopted is to attach a shoe or bearing casting

to the end of the piston rod, which will distribute the pressure over a

considerable area. An example of this construction is the shield for the

City and South London tunnel. In the East River and St. Clair River

tunnels built in America, the tail of the piston rod is so constructed

that the thrust is carried directly to the shell of the lining.

LINING.

Either iron or masonry may be used for lining shield-driven tunnels but

present practice is almost universally in favor of iron lining. As

usually built, iron lining consists of a series of successive cast-iron

rings, the abutting edges of which are provided with flanges. These

flanges are connected by means of butts, the joints being packed with

thin strips of wood, oakum, cement, or some other material to make them

water-tight. Each lining ring is made up of four or more segments, which

are provided with flanges for bolted connections similar to those

fastening the successive rings. Generally the crown segment is made

considerably shorter than those forming the sides and bottom of the

ring. The erection of the iron segments forming the successive rings of

the lining may be done by hand in tunnels of small diameter where the

weights to be handled are comparatively light, but in tunnels of large

size special cranes attached to the shield or carried by the finished

lining are employed. The construction of the iron lining for the Hudson

River tunnel is illustrated in Chapter XX., and that for the St. Clair

River tunnel is shown by Fig. 137.

[Illustration: ~Part Transverse Section.~

~Longitudinal Section.~

FIG. 137.--Cast-Iron Lining, St. Clair River Tunnel.]

CHAPTER XX.

SUBMARINE TUNNELING (Continued).

THE SHIELD AND COMPRESSED AIR METHOD. THE HUDSON RIVER TUNNEL OF THE

PENNSYLVANIA RAILROAD.

The shield and compressed air method of excavating subaqueous tunnels is

used when the distance is small between the roof of the tunnel and the

bed of the river. These tunnels are usually driven from the shafts sunk

from each shore. It is very seldom they can be driven also by an

intermediate shaft. This, however, was done in the case of the Belmont

tunnel under the East River. Here the tunnels passed under the

man-of-war reef where a working shaft was sunk.

The plant is located at some convenient point near the head shaft. It

consists of a set of boilers to provide the power for the different

machines. They are low and high pressure compressors, the former supply

the air through the tunnel; the latter, the air for working the drills,

in case rock is encountered, and power for hauling and hoisting

purposes. The various pumps force the water for the hydraulic rams that

drive the shield and work the erector. They also remove the water from

the tunnel which always collects in variable quantities at the bottom of

the excavation. Besides the machines for light and ventilation purposes,

the head shaft is provided with an overhead construction where are

housed the hoisting machines, the telephone and other means of

communication with the work at the front. Usually a long trestle is

built in connection with the head shaft, leading to the dumping place

and yard. On this inclined elevated structure are located, also, the

tracks upon which will run the small cars used inside the tunnel for

hauling purposes.

The shafts are excavated on a square, rectangular or circular plan and

are usually lined with masonry. It is only recently that shafts

excavated through loose soils have been lined with the same cast-iron

lining used in the tunnels, the only difference being that the rings

were laid flat on the ground and attached to those already sunk.

After the shaft has been sunk to the required level, the tunnel is

driven toward the river by any one of the methods used for land work. At

some convenient distance from the shaft, the dimensions of the tunnel

are enlarged for a length of 20 or 30 ft. In this larger space, called

the shield chamber, the shield is assembled, mounted, and, when

completed, it is slowly pushed toward the river. The tunnel is excavated

from the shield chamber on, with dimensions equal to the exterior shell

of the shield.

The construction of the shield and the hydraulic jacks used for its

advance are explained in a preceding chapter.

In very loose soils, a solid bulkhead of masonry is built across the

tunnel, after the shield has advanced to a certain distance and some

rings of the cast-iron lining have been erected. The bulkhead is

provided with three air locks--two near the floor of the tunnel, for

working purposes, and one near the roof, called the emergency lock,

which, as the name suggests, is used only in case of danger. The air

locks are steel cylinders from 10 to 15 ft. long and 6 ft. in diameter,

made up of boiler plates. They are provided with doors at each end,

besides the pipes for the admission and exit of compressed air. The

working locks also have narrow-gauge tracks for hauling purposes. In

rock or more consistent soil the bulkhead is constructed after the

shield is far ahead, since there is no immediate necessity, under these

conditions, to use the compressed air. In both the loose and good soils,

when the shield has been advanced over 500 ft. from the bulkhead, a

second bulkhead, with air locks, is erected in the tunnel. The first is

left in place but used only in case of emergency.

To direct the shield along the center line and through curves and

grades, accurate measurements are taken, and the distance between the

shield and the last ring inserted in the iron lining is regulated

accordingly. The alignment inside the tunnel is maintained in a very

simple way. For this purpose, points corresponding to the center line

are marked on the roof at distances of 100 ft. Nearly 100 ft. from the

shield, a transit is set up on a strong scaffold spanning the tunnel,

and it is supported by the flanges of the iron lining. A plumb-line is

hung from one of the points of the roof already determined, as

indicating the center line; and the transit man aligns his instrument

with this plumb-line; after this he “plunges” his telescope. A rodman

next places a horizontal rod of special construction between the flanges

of the last ring of the lining. This rod has in the center an open slot

which carries a glass with a black vertical line. The slot is graduated,

the zero of graduation remains in the center while the vertical line is

moved right and left. The rodman places a lamp behind the slot and the

transit-man tells him how to move the dark line until it coincides with

the axis of the tunnel. If the ring, just erected, be a little out of

alignment, it is readjusted by pushing the shield a little more on the

side that has swerved from the axis of the tunnel. As the shield is

pushed forward, it is kept in place by four men with graduated rods, one

man on each side of the shield, one on top and the other on the floor.

As the shield progresses, they repeat aloud in succession, the distance

indicated on the rods, which is the distance from the shield to the

outer circumferential flange of the last ring of the lining. When an

advance of one foot has been made, readings are taken at every inch; and

when very near the required distance, they are taken at every quarter of

an inch. In this way it is not difficult to bring the shield back into

line, in case it may have shifted a little to the right or left. When

curves are met, the rings are no longer cylindrical segments but tores,

so that the segments at one side are longer than those on the other. In

this case, the shield is advanced more on one side by a quantity equal

to the difference of the two sides of the ring to be erected. At each

advance the shield is moved 2 ft. or 2¹⁄₂ ft. ahead, the distance

corresponding to the length of the cast-iron rings of the lining. Within

the space now open between the shield and the lining another ring is

inserted. The ring is composed of different segments provided with

flanges and holes bored so they can be bolted together. The segments of

the lining are very heavy and difficult to handle but they are easily

set by means of the erector.

When the erector is not mounted on the shield, it is located in the

middle of a girder placed across the iron rings of the lining and just

at the rear end of the shield. The girder, at both extremities, has

flanged wheels resting on rails which are placed on brackets. These

brackets are attached temporarily to the flanges of the iron lining. The

erector is provided with an arm capable to swing in a full circle. Its

movements are regulated by two hydraulic jacks, located horizontally on

the spanning girder. On the extreme end of the revolving arm are

projections with holes for the bolts. Each segmental plate of the lining

has a kind of plug in the center which is cast together with the plate

and is provided with holes for the bolt. In placing the segmental plates

of the lining, the arm of the erector is swung over the plate to be

lifted, then two bolts are passed through the holes in the projection of

the erector, and through those in the plug. The arm of the erector is

then moved upwards until the plate, free from all obstacles, is swung

very near its intended position. There it is adjusted and held until

bolts are inserted to fix it to the plates of the preceding ring.

In connection with the method of excavating submarine tunnels by means

of shield and compressed air, the excavation varies with the quality of

soil encountered. In compact rock the usual heading and bench method, so

common in land tunnels, is also employed in this case. The shield is

left behind in presence of good rock.

The men at the front attack the rock with air drilling machines and

charges of dynamite. The holes are driven at a smaller depth than in

land work; very light charges of dynamite are used and only a few holes

fired at each round. Every precaution is taken in order not to disturb

the shield and the bed of the river any more than is possible, because

at a shallow depth the blast would tend to widen the existing crevices

in the rock and thus permit an inflow of water. When the rock is

fissured or disintegrated and the roof of the excavation at the front

requires timbering, the shield should be kept closer to the front. In

this way the quantity of timber for strutting is greatly reduced, so

lessening the probabilities of fires. It is very difficult, in

compressed air, to extinguish fires and in almost every instance the

only way is to flood the tunnel. This was done at the Manhattan end of

the tunnel under the East River for the extension to Brooklyn of the New

York Subway.

The excavation is made by hand in loose but compact soils such as clay.

The men work on platforms located at the front of the shield and they

are protected from the caving-in of the roof by a hood added for working

through loose soils. The men excavate the material which is shoveled

inside the tunnel and is carried away in small cars. The shield is very

close to the front of the excavation in loose soil. The East Boston

tunnel, under Boston Harbor, connecting with the Boston Subway, was

excavated through blue clay. The minimum distance between the bottom of

the water and the roof of the excavation was 18 ft. The tunnel was

excavated by means of compressed air and the shield which was only used

for the roof. It slid on top of concrete side walls built in two drifts

which were excavated nearly 100 ft. ahead of the shield. The tunnel was

lined with concrete, the arch being reinforced by longitudinal steel

rods which received the thrust of jacks used for advancing the shield.

The material in the drifts under the shield and the bench was removed by

hand and carried away in small cars.

Subaqueous tunnels driven through very loose soils can be excavated by

simply leaving the doors open while the shield is pushed ahead. The

material, dislodged by the cutting edge of the shield, is forced through

the doors and falls on the floor whence it is removed in small cars. In

very loose soils the excavation has been made in a still more economic

way; the shield with closed doors is simply squeezed through the soil.

This method is financially convenient, because all the excavating and

hauling operations are eliminated and the tunnel progresses from 40 to

50 ft. per day, but clearly indicates a lack of stability. In this

manner, the Hudson River tunnel of the New York and New Jersey Railroad

was constructed.

The pressure of the air in the tunnel depends upon the depth and as a

rule it varies between 20 and 40 or even more pounds per square inch

above atmospheric pressure. Working in compressed air causes a peculiar

disease commonly known as “bends” or “caisson disease” often proving

fatal. To prevent and remedy the disease, the engineers should order a

set of rules to be strictly observed. The preventative measures should

be, first, to employ only sober, strong and healthy men, never one who

has not successfully passed the examination of the attending physician;

second, to order the lock tenders never to allow any man in or out of

the tunnel unless he has spent at least ten minutes within the locks.

Both compression and decompression should be thorough and it cannot be

in less than this time. A stop of only a few minutes in the locks is not

sufficient and this incomplete compression or decompression is the real

cause of the bends. The men become careless after they have been in the

compressed air for some time, and they try to reduce this tiresome

operation to a minimum, hence the duty of the engineer to strictly

enforce this rule. The remedial measures should consist of constant

medical attendance near the shafts and the erection of a compressed air

hospital where the men affected by bends for lack of decompression may

be attended and cured.

THE HUDSON RIVER TUNNELS OF THE PENNSYLVANIA RAILROAD.[13]

The tunnels constructed under the Hudson River for the Pennsylvania

Railroad, consist of two parallel tubes driven side by side 14 ft.

apart. The tubes are of circular cross-section, 23 ft. exterior

diameter, and are lined with cast-iron rings. The tunnels were driven

from two shafts, one on the eastern shore of the Hudson River near 32nd

St. and 11th Ave., New York; the other at Weehawken, New Jersey, near

the piers of the Erie Railroad. The horizontal distance between the

shafts was 6550 ft. The permanent one at Weehawken was built on a square

plan, 130 ft. to a side. It was lined with concrete masonry and the

walls were battered in such a way as to become the shape of an inverted

frustum of a pyramid. It was provided with five openings at the bottom,

four of these are used by trains that run in the open, the fifth one

leads to a power house near by. During the construction of the tunnels

one-third of this shaft was used for the land portion of the tunnel

under Bergen Hill, while the remaining two-thirds were devoted to the

construction of the tunnel under the river. The working shaft on

Manhattan Island was a side shaft of rectangular plan 30 ft. by 22 ft.,

the tunnel proper being connected by two drifts 10 ft. by 10 ft. each.

The shield rooms 23 ft. long, were situated on both sides of the river

just in front of the shafts. On the New York side, the shields, one for

each tube, were built inside the iron lining of the shield chamber, and

the hoisting tackle was slung from the iron lining. The erection on the

Weehawken side was done in the bare rock excavation where timber

falsework was used. After the shields were finished and in position, the

first two rings of the lining were erected in the tail of the shield.

These rings were firmly braced to the rock and the chamber lining; then

the shields were shoved ahead by their own jacks, another ring was built

and so on.

[13] Condensed from paper by James Forgie, “Eng. News,” Vol. LVI, and

by H. B. Hewett and W. L. Brown, “Proc. Am. Soc. C. E.”, Vol. XXXVI.

[Illustration: ~Rear Elevation of Shield.~

~Vertical Section.~

~Half Section A-B.~

~Half Section C-D.~

~Horizontal Section.~

FIG. 138.--General Elevations and Sections of Shield.]

=Shield.=--The shields used in these tunnels were designed by Mr. James

Forgie, M. Inst. C. E. and M. Am. Soc. C. E., and were provided with

three innovations: the segmental doors, the sliding platforms and the

removable hood. The shields, Fig. 138, were circular, 23 ft. 6¹⁄₄ ins.

in external diameter, and were 16 ft. long, exclusive of the hood. The

tail of the shield overlapped the lining, the maximum being 6 ft. 4¹⁄₂

ins. during ordinary working; the minimum, 2 ft. during the operation of

taking any ram out for repairing. The shields had only one transverse

bulkhead made up of two continuous horizontal platforms and three

vertical partitions stiffening angular web plates fore and aft the ram

chambers. They were connected by angles and skin plates which formed a

ring-shaped frame 25 ins. thick radially and nearly 5 ft. long. Between

the vertical and horizontal partitions were left openings which either

were partially or entirely closed by segmental doors pivoted on an axis

parallel to the face of the shield bulkhead. There were nine of such

openings on each shield, the clear width being 2 ft. 7 ins., the height

varying from 2 ft. 2 ins. to 3 ft. 4 ins., according to the location.

The hood at the front of the shield was designed so as to be detached

underground and was made of complete segments to permit easy erection or

detachment. The hood was extended as far as the upper platform, thus

protecting only the roof of the excavation. It was attached to the

shield by means of bolts, and, when removed, it was replaced by the

cast-steel cutting-edge, built in 24 sections and placed all around the

shield. The eight sliding platforms, another characteristic of this

shield, could be extended 2 ft. 9 ins. in front of the shield by means

of hydraulic rams, and, when so extended, were able to stand a pressure

of 7900 lbs. per sq. ft. These sliding platforms were used as hoods for

the protection of the men working through loose soils, while in rock

they enabled the drilling and blasting to be carried on at three levels.

A water trap or bird fountain was constructed, at the rear of the

bulkhead of the shield, by means of angle irons to which steel plates

were bolted. The opening to the face was so spacious that in an

emergency the men could readily escape by getting over this trap into

safety. Besides, with the assistance of compressed air, it was

sufficient to perfectly trap the water-bearing ground, in case the face

collapsed. Including rams and erector, the total weight of the shield

was 193 tons.

=Hydraulic Rams.=--The shield was operated by hydraulic pressure. The

machines were designed for a maximum pressure of 5000 lbs., to a

minimum of 2000 lbs., while the average working pressure was 3500 lbs.

per sq. in. The forward movement of the shield was obtained by means of

24 single-acting rams 8¹⁄₂ in. in diameter and with 38 in. stroke. Each

ram exerted a pressure of nearly 100 tons, so that the combined action

of the 24 rams was equal to 2400 tons. Each sliding platform was

operated by two single-acting rams 3¹⁄₂ ins. in diameter and with 2 ft.

9 in. stroke. The rams were attached to the rear face of the shield and

the front ends of the cylinders to the front ends of the sliding

platforms, and since the cylinders were movable and free-sliding so also

were the platforms.

=Erector.=--The erector, a box-shaped frame mounted on a central shaft,

revolved in bearings attached to the shield. Inside this frame there was

a differential hydraulic plunger of 4 in. and 3 in. diameters and 48 in.

stroke. To the plunger head were attached two channels which slide

inside the box frame and to the projecting ends of which the grip was

attached. At the opposite end of the box frame was attached a

counter-weight which balances about 700 lbs. of the tunnel segment at 11

ft. radius. The erector was revolved by two single-acting rams fixed

horizontally to the back of the shield, above the erector pivot, through

double chains and chain wheels which were keyed to the erector shaft.

=Air Locks.=--Two bulkhead walls, forming the rear closure of the

pneumatic sections, were built in each end of each tunnel, one just

ahead of the shield chamber, the other about 1200 ft. ahead of the

first. The walls were built of Portland concrete 10 ft. thick, and they

were grouted with Portland cement, under a pressure of nearly 100 lbs.

per sq. in., to make them thoroughly air-tight. Each wall had in it

three locks; for man, material and emergency. Each was equipped with

hand valves arranged to be operated from either outer end or from

within. The floors of the man and material locks were on a level with

the working platform of the tunnel, about 3 ft. 6 ins. above the invert;

the floor of the emergency lock was about 5 ft. above the horizontal

axis of the tunnel. The locks were made of steel plates and shapes, with

iron fittings riveted and bolted together. The man lock was 11 ft. long

of elliptical cross-section, 6 ft. vertical diameter and 5 ft.

horizontal; the material lock was 25 ft. long, with circular

cross-section, 7 ft. diameter, and the emergency lock was 20 ft. long,

of elliptical cross-section, 4 ft. vertical and 3 ft. horizontal

diameters. Fig. 139 shows the elevation of the air lock used in the

Pennsylvania tunnel.

[Illustration: ~Sectional Elevation~

~Horizontal Section~

FIG. 139.--Plan and Elevation of First Bulkhead Wall in South Tube

Manhattan.]

=Excavation.=--In driving these tunnels almost any kind of material was

encountered, viz., rock, partly rock, and partly loose soil, sand and

gravel, and finally silt.

=Rock.=--Much of the rock excavation was made before the shields were

erected in order to avoid the handling of rock through the narrow

openings of the shield doors. Throughout the cross-section the shield

traveled on a cradle of concrete in which 2 or 3 steel rails were

imbedded. At the points where the excavation had been made for the full

section of the tunnel, it was only necessary to trim off the projecting

corners of rock. Where only the bottom heading had been driven the

excavation was completed just in front of the shield; the drilling below

the axis level being done from the heading itself, and above that from

the front sliding platforms of the shield. The holes were placed near

together and were drilled short; very light charges of powder were used

in order to lessen the chance of knocking the shield about too much.

=Mixed Face.=--When the rock dipped to such an extent that the front of

the tunnel was excavated partly in rock and partly in loose soil, the

compressed air was turned on, starting with a pressure varying from 12

to 18 lbs. When the surface of the rock was penetrated, the soft face

was held up at first by horizontal boards braced from the shield until

the shield was shoved. The braces were then taken out and, after the

shield had been shoved, were replaced by others. As the amount of soft

ground in the surface increased, the system of timbering was gradually

changed to one of 2-in. poling-boards. These rested on top of the shield

and were supported by vertical breast-boards which in turn were held by

6-in. by 6-in. walings, braced through the upper doors to the iron

lining and from the sliding platforms of the shield.

=Sand and Gravel.=--Sand and gravel were only met at Weehawken, where

two different methods were used. The first method was employed when the

roof of the excavation was through sand. It consisted of excavating the

ground 2 ft. 6 ins. ahead of the cutting-edge, the roof being held in

place by longitudinal poling-boards. These boards rested on the outside

of the skin at their back end, and at the forward end on vertical

breast-boards, braced from the sliding platforms and through the shield

doors to cross timbers in the tunnel.

The second method of timbering was used in the presence of gravel at the

upper part of the excavation. In such a case, the excavation was only

carried 1 ft. 3 ins. (half a shove) ahead of the cutting-edge, the roof

being supported by transverse boards held by pipes which rested in holes

left in the shield. After a small section of the ground had been

excavated a board supported by a pipe that was inserted underneath and

wedged to it was placed against the ground. These polings were kept

below the level of the hood, so that when the shield was shoved, they

would come inside of it; in addition they were braced with vertical

posts from the sliding platform. The upper part of the face was held by

longitudinal breast-boards braced from the sliding platform by vertical

pieces. The lower part of the face was supported by vertical sheeted

poling, braced to the tunnel through the lower doors. Straw and clay

were used in front of the boards to prevent the escape of air which was

very large, when the tunnel was excavated through sand and gravel. The

average rate of progress in these materials was 5.1 ft. per day.

=Silt.=--When silt was encountered, the shield was shoved into the

ground without any excavation being done by hand ahead of the diaphragm.

As the shield advanced the silt was forced through the doors into the

tunnel. Forcing the shield through the silt resulted in raising the bed

of the river, the amount that the bed was raised depending on the

quantity of material brought into the shield. When the whole volume of

the excavation was brought in, the surface of the bed was not affected;

when about 50% was taken in, the surface was raised about 3 ft.; if the

shield was driven blind, the bed was raised about 7 ft. When the shield

was driven blind, the tunnel began to rise for about 2 ins., and the

iron lining was distorted, the vertical diameter increasing and the

horizontal one decreasing by about 1¹⁄₄ ins. It was found, however, that

the tunnel was not affected when part of the excavation was taken, but

if all of it was taken in or the shield was shoved with open doors, the

tunnel was lowered. A powerful aid was thus found for the guidance of

the shield; for, if high, the shield could be brought down by increasing

the quantity of muck taken in, if low, by decreasing it.

The junction of the shields under the river was made as follows: When

the two shields of one tunnel, which had been driven from opposite sides

of the river, approached within 10 ft. of each other, they were stopped;

a 10-in. pipe was driven between them, and a final check of lines and

levels was made through the pipe. One shield was then started up with

all doors closed, while the doors of the stationary shield were opened

for the muck driven ahead by the moving shield. This was continued until

the cutting-edges came together. All doors in both shields were then

opened and the shield mucked out. The cutting-edges were taken off and

the shields moved together again, edge of skin to edge of skin. As the

sections of the cutting-edges were taken off, the space between the skin

edges was poled with 3-in. stuff. When everything except the skin had

been removed, iron lining was built up inside the skins; the gap at the

junction was filled with concrete and long bolts were used from ring to

ring on the circumferential joint.

=Lining.=--The tunnels were lined with cast-iron circular rings of the

segmental bolted type. In some special cases, cast steel was used

instead of cast iron. The rings were made 30 ins. long, with an internal

diameter of 21 ft. 2 ins. and an external one of 23 ft. The rings were

composed of nine equal segments of 77¹⁄₂ ins. external circumferential

length each, except the two segments adjoining the key which were equal

to the other segments with the difference, that one end joint was not

radial but formed so as to make an opening 12.25 ins. wide at the

outside and 12.60 ins. at the inside, which was closed by the key

segment. Each segment had six bolts in the circumferential joint, the

key had one, so that there were 67 bolts in one circumferential joint.

Each of the twelve longitudinal or radial joints had five bolts, in all

127 bolts per ring. The circumferential flanges of each plate were

strengthened by two transverse webs or feathers on each flange. Each

segment was provided with a 1¹⁄₂ in. grout hole closed with a screw

plug. In order to pass around curves, whether horizontal or vertical, or

to correct deviation from the line or grade, tapering was used; by this

is meant the placing of rings in the tunnels which were wider than the

standard rings, either at one side (horizontal tapers or liners), or at

the top (depressors), or at the bottom (elevators). Tapers ¹⁄₂, ³⁄₄ or

even 1 in. were used. The taper rings were made by casting a ring with

one circumferential flange much thicker than usual and then machining it

off to the taper.

=Grouting.=--From the exterior of the tunnel already lined with

cast-iron rings, grout was forced through the holes closed by

screw-plugs, at a pressure of 90 lbs. per sq. in. The grout was composed

of 1 Portland cement and 1 sand by volume and was forced in by a

specially constructed machine, so it formed a shell of cement nearly 3

ins. thick around the exterior of the iron lining. The grouting began at

the lower segment; the cement was forced in until it reached the hole

above, then the hole was plugged, and the grouting was carried on from

the consecutive hole and so on until all the tunnel was finally encased

in grout, as it filled every crevice between the outside of the lining

and the ground as excavated. The cast-iron rings of the tunnel were

covered with a concrete lining which was placed in the following order:

First, on the invert; second, on the duct benches; third, on the arch;

fourth, on the ducts; fifth, on the face of the bench. Before any

concrete was placed, the surface of the iron was cleaned by scrapers and

wire brushes and by washing it with water. The invert was built in

sections 30 ft. long and the duct benches were constructed soon after.

These duct benches were built with several steps for the ducts to be

laid later. They were built by means of a traveling stage on wheels

which ran on tracks on the working platform of the tunnel. The arch was

constructed soon after. First the portion from the duct benches to the

haunches, then the arch proper, was built on traveling centers on tracks

laid on the steps of the duct benches. The concrete was received in

³⁄₄-cu.-yd. dumping buckets, from the flat cars on which they were run;

the buckets were hoisted to the level of the lower platform of the arch

by a small Lidgerwood compressed air hoister. At this level the concrete

was dumped on a traveling car or stage and moved in that to the point

on the form where it was to be placed. For the lower part of the arch

the concrete was thrown directly into the form from this traveling part

of the stage. Fig. 140 shows the cross-section of the tunnel with the

iron lining and concrete.

[Illustration: Section in Sand and Gravel or Rock

Section in Hudson River Silt, with foundations

FIG. 140.--Typical Cross-Sections of One Tube of Pennsylvania Railroad

Tunnel Under the Hudson River.]

=Hauling.=--A working platform, made up of 5-ft. sections, was built

inside the tunnel and kept close to the shield. On this platform two

lines of industrial railway tracks with switches and sidings at the

locks, and a heading, were laid for hauling materials and spoils. These

lines converged into a single track in passing through the air locks. At

the shaft elevators, they terminated in a steel plate floor to avoid

switches. Between the locks of the bulkheads was installed an

electrically driven cable system, to haul the loaded muck up grade and

to empty the flat cars. From the first bulkhead to the shaft, the cars

were hauled up grade by a steam hauling engine. At the Manhattan end

there was one 10-H.P. engine for each tunnel, while at Weehawken one

25-H.P. engine served for both tunnels. Each shaft contained two

elevators driven by a double-cable, reversible single-drum

steam-hoisting engine. A grouty frame was built over the shafts, and on

the platforms over this frame were narrow-gauge tracks, extending from

the elevators to the muck-chutes and to points where the lining segments

were loaded on the cars. The elevators were arranged to stop at both the

ground and the grouty platform levels. The rolling-stock at each of the

tunnels consisted of 75 flat cars for moving the tunnel segments, and of

about 50 muck cars, each of 1¹⁄₄ cu. yd. capacity.

=Plant.=--The plants located at each end of the tunnel near the shafts

were almost identical. Each consisted of three 500-H.P. Stirtling

boilers, which supplied steam at 150 lbs. pressure. Feed water was

supplied by three 13¹⁄₂ metropolitan injectors, and two Blake duplex

pumps. Two Worthington surface condensers, each of 2250 sq. ft.

condensing surface, took care of the exhaust from the engines and

compressors. Condensing water was pumped from the river through a 16-in.

pipe. The high-pressure air was supplied by a duplex Ingersoll-Sergeant

compressor, with cross-compound steam end 14 × 26 × 30 ins. and simple

water-jacket air cylinders 13¹⁄₄ × 36 ins. Its capacity at 100 r.p.m.

was 1085 cu. ft. free air per minute. The maximum pressure was 130 lbs.

per sq. in. The air for the pneumatic working was supplied by three 14 ×

26 × 30 in. duplex Ingersoll-Sergeant compressors. The maximum capacity

of the three was 12,000 cu. ft. free air per minute at 125 r.p.m. and a

discharge pressure of 50 lbs. per sq. in. The suction air was taken from

the outside about 10 ft. above the roof of the engine house. Three

aftercoolers, 32¹⁄₂ ins. × 11 ft. 4 ins., each having 809 sq. ft.

cooling surface of tinned brass tubes, cooled the low-pressure discharge

to within 10° F. of the temperature of the cooling-water. From the

aftercoolers, the air passed into three steel receivers each 54 × 12

ft., placed outside the engine room and fitted with weighing safety

valves. The receivers were connected to two 10-in. mains; one serving

the north, the other the south tunnel. A fourth receiver of the same

size was built to receive the discharge of the high-pressure compressor,

through a 4-in. pipe. The high-pressure water required for the shield

was furnished by three Blake direct-acting, duplex pumps with outside

packed plungers. The steam end was 16 × 18 ins., the water end 2¹⁄₁₆ ×

18 ins. At 55 r.p.m. pumping against 5000 lbs. per sq. in., the capacity

of each pump was 57 gals. per minute. Two of them, one on each tunnel,

were sufficient to run the shields and the third was held in reserve.

The high-pressure water was conveyed to the front by means of a 2-in.

double, extra strong pipe which was buried between the engine room and

the shaft, in a trench, to prevent freezing in cold weather. The

electric current for light and power was supplied by two 100-K.W.

250-volt G.E. direct-current generators directly connected to Ball &

Wood high-speed engines running at 250 r.p.m. The switchboard had two

machine panels, two distributing panels and one panel carrying a circuit

breaker for the traction circuit.

=Illumination.=--The tunnel was lighted by electricity, there being two

rows of lamps, one in the crown and one in the south axial fine. The

lamps were 16-c.p., 240-volt, two-wire system, and were spaced 35 ft.

apart in the crown and 12¹⁄₂ ft. apart on the axial line. In addition,

five nests of 5 lamps each were used at the front. Candles were supplied

for miscellaneous and emergency uses. The sockets for electric globes

were fitted to a wooden reflector, coated with white enamel paint on the

inside.

CHAPTER XXI.

SUBMARINE TUNNELING (Continued); TUNNELS AT VERY SHALLOW DEPTH. THE

COFFERDAM METHOD. THE PNEUMATIC CAISSON METHOD. THE JOINING TOGETHER

SECTIONS OF TUNNELS BUILT ON LAND.

The tunnels on the river bed or at such a shallow depth that only a few

feet of material will remain between the bottom of the river and the

roof of the tunnel can be built in three different ways, viz., (1) by a

cofferdam; (2) by pneumatic caissons; (3) by sinking and joining

together whole sections of tunnels that were built on land.

=The Cofferdam Method.=--=The Van Buren Street Tunnel, Chicago

River.=--According to the cofferdam method, the work is attacked at one

of the shores, and the tunnel built in sections of such length as not to

interfere with the flow of water or the navigation of the river. Round

the entire exterior line of the first section a double-walled cofferdam

is built, and strongly braced transversely, so as to withstand the

pressure of the water. When the water is pumped out, a single-walled

cofferdam is built within the first, leaving sufficient distance between

the two to allow of the construction of the masonry. The soil is then

removed within the inner cofferdam, and the tunnel constructed from the

foundation. When the end of the tunnel reaches the channel end of the

cofferdam, a crib-wall is erected over the end of the completed tunnel.

This crib, in turn, forms the end wall of another cofferdam, built in

continuation of the first, so as to allow the second section to be

proceeded with, and at the same time to facilitate the removal of the

cofferdams of the first section. The work goes on continuously in this

way until the distant shore is reached.

VAN BUREN STREET TUNNEL, CHICAGO.

The Van Buren Street tunnel, built to carry a double-track street

railway under the Chicago River, was completed in 1894 by the cofferdam

method. The special features of the tunnel[14] are: (1) the unusually

large dimensions of the cross-section of 30 ft. × 15 ft. 9 ins.; (2) its

construction inside of cofferdams of great length and width; (3) the

construction under some very high buildings calling for great care and

very strong temporary and permanent supports.

[14] “Eng. News,” April 12, 1892.

The special feature of the work for our present purpose was the

construction of the tunnel across the river. To accomplish this a

cofferdam was built out from the west shore of the river to its middle,

and the tunnel constructed within it like the building of any other

structure within a cofferdam. Transverse and longitudinal sections of

this cofferdam are shown by Fig. 141. As will be seen, it was a simple

double-wall cofferdam, with a clear width between the walls of 58 ft.,

and braced transversely as shown. Inside of this a single-wall cofferdam

of piles was constructed, with a clear width just sufficient to allow

the construction of the masonry within it. When the tunnel end reached

the channel end of the cofferdam, a crib-wall was built over the end of

the completed tunnel, as shown by the drawings. This crib-wall was

intended to form the end wall of another cofferdam, which was built out

from the east shore, and within which the remaining half of the tunnel

was built as the first half had been. The drawings show the character of

the tunnel masonry and of the centering upon which it was built.

[Illustration: ~TRANSVERSE SECTION OF COFFERDAM AND TUNNEL~

~SECTION SHOWING METHOD OF CONSTRUCTING CRIB DAM.~

FIG. 141.--Sections of Cofferdam, Van Buren St. Tunnel, Chicago.]

The Van Buren Street tunnel was the last of the three tunnels under the

Chicago River, constructed according to the cofferdam method. At the

time the tunnels were constructed the bed of the river was 17 ft.

deep. In connection with the harbor and river improvements, the Federal

Government ordered the Chicago River to be lowered so as to give a depth

of 26 ft. of water. This necessitated the lowering of the tunnel roof

and the excavation for a deeper floor which was a very difficult

operation. This work was described in “Eng. News,” Sept., 1906.

THE PNEUMATIC CAISSON METHOD.--THE TUNNEL UNDER THE HARLEM RIVER.

In the early seventies Prof. Winkler proposed to construct a tunnel

under the River Danube to connect the various portions of the Vienna,

Austria, underground railway, and to use caissons in the construction.

Prof. Winkler proposed to build caissons from 30 ft. to 45 ft. long,

with a width depending upon the lateral dimensions adopted for the

tunnel masonry. The caisson was to be made of metal plates and angle

iron with riveted connections on all sides except those running

vertically transverse to the tunnel axis, whose connections were to be

bolted. In the middle of the roof an opening was to be left; this was

for the shaft having the air-locks to allow the passage of men,

materials, and compressed air.

Across the river two parallel rows of piles were to be driven into the

river bed, to fix the place where the caisson was to be sunk. Then the

first caisson near the shore was to be lowered in the ordinary way, and

a second caisson was to be immediately sunk very close to the first one.

When both caissons had reached the plane of the tunnel floor, the sides

which were in contact were to be unbolted and removed, and the small

space between made water-tight. The chambers of the two caissons were to

be opened into a single large one communicating above by means of two

shafts. At the same time that the masonry was being built in the first

two caissons, from the inverted arch up, a third caisson was to be sunk;

and when by excavation it had reached the plane of the projected tunnel

floor, the partitions were to be removed so that the three caissons were

in communication, forming a large single caisson. Then the outer

partition of the first caisson was to be removed, and the masonry of the

submarine tunnel connected with the portion of the tunnel built on land.

In a similar manner all the caissons were to be sunk; and when the last

one was placed, and the masonry lining constructed, and connected with

the portion of the tunnel built on the other shore of the river, the

partition walls were to be battered down, and the submarine tunnel

completely constructed and open to traffic.

=The Harlem River Tunnel.=--The pneumatic caissons method was employed

in the construction of the tunnel under the Harlem River for the New

York Rapid Transit Railway. The tunnel proper consisted of two parallel

tubes riveted to each other and surrounded by a cradle of concrete as

shown in Fig. 121, page 216. The tunnel was built in three

sections:--The first, from the Manhattan shore well towards the middle

of the river; the second, from the shore of the Bronx towards the middle

of the river; and the last, the section uniting the other two and

completing the tunnel.

Each section was built within a specially constructed working-chamber,

consisting of timber side walls forming a wooden caisson, so constructed

that compressed air could be used. This working-chamber of Mr. McBean

presented some novel features, inasmuch as the caisson was not built on

land, but under water.

In building the tunnel, the Harlem River was dredged to a certain depth,

so as to leave only 6 ft. or 8 ft. of excavation to be done before

reaching the line of sub-grade of the proposed structure. Two service

platforms were built on piles 10 ft. apart longitudinally, and cut off

at a point above mean high-water mark, braced in the usual manner, and

covered with heavy planks, to serve as the floor of the platform. On

this platform were placed rails for the trains used in the

transportation of materials. These platforms were also used in

maintaining the perfect alignment of the caissons.

Within the platforms and along the dredged channel four longitudinal

rows of piles were sunk. These piles were accurately brought to line by

beams bolted together, and placed across and above the water-level. A

few beams were also added for the purpose of bracing the piles

transversely, after which they were cut off under water and capped.

[Illustration: FIG. 142.--Showing Working Platforms and Piles Sunk in

the Dredged Channel.]

Fig. 142 shows the manner in which the working platforms were

constructed, and also the rows of piles sunk in the dredged channel.

Between the piles a very strong frame was placed, made up of waling

pieces and two transverse beams 14 ins. by 14 ins. each, placed one

below the other at a distance of 5 ft. 8 ins., and strongly braced

together. Guiding-beams were fixed on each side of the frame for the

sheeting piles. The frames were built in sections of different lengths,

and placed directly above the cap-pieces of the pile-bents sunk in the

dredged channel.

The longitudinal sides of the caisson were constructed by sinking two

rows of sheeting piles, each row being close to a service platform. The

sheeting piles were made up of yellow-pine timbers 12 ins. by 12 ins.;

three piles bolted together formed a section 3 ft. wide. Each section

was grooved and tongued, so as to be firmly connected with the adjacent

sections to be sunk. The lower ends of the piles were cut wedge-shaped,

with a sharp edge to offer a small resistance while penetrating the

soil. The sheeting-piles were then cut off under water, which operation

was successfully carried out by means of a circular saw operated by a

pile-driving machine. The sheeting was also extended between two

platforms to make a bulkhead, and in this way the four sides of the

caisson were built up. Particular attention was always given to the

alignment of the sheeting piles, which was obtained by guiding the piles

with the timbers placed longitudinally, one below the water-line in

connection with the frames located between the pile-bents, and the

second along the inner edge of the service platform, as shown in Fig.

143.

[Illustration: FIG. 143.--Showing Sheeting-Piles for the Sides of the

Caisson and Trussed Beam for the Roof.]

The caisson was completed by placing a roof covering the sides. This

roof was 40 ins. thick, made up of three layers of 12-in. beams placed

transversely to the axis of the caisson, while between the beams planks

2 ins. thick were placed lengthwise and bolted together, so as to make a

firm, solid structure. The roof was built ashore, in sections each

varying from 39 ft. to 130 ft. long. The edges of the roof fitted the

sides of the caisson perfectly; and when each section was towed to the

proper spot, it was sunk and made secure. Under the roof were placed six

longitudinal beams, 12 ins. by 14 ins., called “rangers,” resting on the

cap-pieces of the pile-bents that were laid across the space of the

proposed tunnel; while the extreme rangers were used for the purpose of

fitting above the sheeting-piles of the caisson, in order to make the

latter water-tight. The two extreme rangers were provided with

=T=-irons, the flat side being laid on the sheeting-piles, while the web

penetrated the ranger by reason of the weight of the load resting on the

roof, for the purpose of sinking it to the required point. Earth was

next heaped on the roof, and in this way a large working-chamber was

prepared, as shown in Fig. 144.

[Illustration: FIG. 144.--Showing the Caisson with the Working-Chamber.]

The working-chamber built on the Manhattan side of the Harlem River was

216 ft. long, provided with two rectangular shafts 7 ft. by 17 ft.,

rendered water-tight, and rising above the high-water mark of the river.

Within these shafts the air-locks of the tunnel tubes were placed, so

that the work could be carried on by means of compressed air. The

pressure of the air was used to expel the water, being sufficiently

intense to equilibrate a column of water equal to the depth of the

lowest point of the roof of the caisson.

When the working-chamber was constructed, the tunnel proper was begun by

excavating the soil down to the required level; the concrete was then

laid on. It was just at this point, when a large part of the roof was

constructed and supported only by the sheeting-piles of the sides of the

caisson, that the writer of this article feared that this novel method

of tunneling would prove a failure. The tendency of the timber to float,

aided as it was by the air pressure within the caisson, was counteracted

only by the weight of the earth heaped on the roof, and by the friction

of the soil against the feet of the sheeting-piles. This friction was

only a small quantity, as the soil was loose, so that it was considered

rather risky and dangerous to place reliance on such a feeble quantity.

This fear was, unfortunately, justified on two occasions, when on

cutting off a portion of the pile-bents some of the sheeting-piles got

loose and water flooded the whole chamber, but, happily, without loss of

life. As the chamber was one of large dimensions, the workmen had time

enough to effect their escape. It may be remarked that during these

troubles only a few of the sheeting-piles were displaced, while the

caisson itself offered a stout and successful resistance, due to its

being strongly braced transversely. The accidents were, therefore,

limited to a few piles, instead of affecting the entire caisson. On the

occasion of the first, the repairs were effected by sinking the piles to

a greater depth, continuing down until rock was encountered. After that,

the water was pumped out and the work resumed. In repairing the second

accident, the sheeting-piles were driven down to bed-rock, and the

surrounding soil strengthened by cement forced through the loose soil

around the piles. This remedy proved effective, and no further trouble

occurred to delay the work on the Manhattan side of the Harlem River.

[Illustration: FIG. 145.--Showing the Tunnel Constructed within the

Caisson.]

On the concrete bed of the tunnel the segments of the metal lining were

placed and surrounded by concrete, as required by the plans and

specifications (Fig. 145). The contractors had planned to unbolt the

roof from its holdings, to remove by means of dredgers the earth which

had been heaped on it, and thus set the roof afloat, after which it was

to be towed within the two working platforms already erected on the

Bronx shore. But Mr. McBean devised a simpler and more economic, but at

the same time more dangerous, way of constructing this second section of

the tunnel. He thought that the upper half of the tunnel proper could be

used instead of the timber roof, thereby reducing the capacity of the

working chamber, and limiting the use of compressed air. In this way he

dispensed with the removal of timber, and also of the earth heaped on

the roof.

In building this Bronx section, a channel was dredged along the line of

the tunnel to a depth of 5 ft. from the foundation-bed of the proposed

tunnel. The working platforms were constructed on both sides of this

channel, quite similar to those erected on the other half of the tunnel;

and between them pile-bents were sunk, capped with 12-in. by 12-in.

beams. Over the cap-pieces rangers were placed longitudinally, which

also rested on the sides of the wooden working caisson, Fig. 146. The

sheeting-piles were cut off at level, but much lower down than in the

first half of the tunnel.

The roof was built on floats made of 12-in. by 12-in. timber laid

transversely 4 ft. apart and supporting a floor of 3-in. by 12-in.

planks rendered water-tight. The sides of the floats were made by

verticals, 4 ins. by 6 ins., and planks, 3 ins. by 12 ins., carefully

caulked. A temporary floor was built on the base of the float,

consisting of transverse beams, 16 ins. by 16 ins., placed 8 ft. apart.

A center piece, 10 ins. by 16 ins., was laid so as to correspond with

the axis of the tunnel; and on each side of it, other parallel beams, 16

ins. by 16 ins., corresponding to each center of the circular metal

lining of the tunnel; the beams, longitudinal and transversal, were

strongly bolted together. The temporary floor was completed by boarding

the spaces left between the various beams.

[Illustration: FIG. 146.--Showing Sides of the Caisson and Supports for

the Roof.]

On this float, the upper half of the tunnel was constructed by erecting

the segments of the metal lining, which were strongly supported, so as

to prevent any settling or distortion; the concrete was then built up in

a large flange with vertical suspension rods, four to each bar. The

rings of the tunnel were 6 ft. each, the weight of each lining being

41,000 lbs., the concrete covering 618 cubic feet. The second part of

the tunnel was 300 ft. long, with roof constructed in three

sections--two of 90 ft. in length each and the third of 84 ft. Each of

these sections alternated with a smaller section, 12 ft. long, provided

with air-locks. The shortest of the three sections was the first one set

up, and was constructed close to the Bronx side of the Harlem River. For

this purpose the two extreme ends of the section were closed by means of

steel plates forming diaphragms, built 6 ft. inward, thus leaving one

ring projecting out at each end. Openings were left on the top of these

projecting rings for access by divers. The exterior of the upper half

section of the permanent tunnel was filled with water until it was

lowered into position. It was directed by means of tackles attached to

vertical eye-bars, which were strongly fixed to the flanges of the

springing line of the arch, and bolted to the beams of the temporary

floor. In this way the roof was towed into place, and lowered by means

of stone ballast, until it rested on the cap-pieces and frames of the

pile abutments, the sides of the roof remaining just on top of the

sheeting-piles that formed the sides of the caisson, as shown in Fig.

147. Perfect alignment was obtained by wires strung at each end and

along the side of the roof, corresponding to points fixed on the working

platforms and sighted with transits. Such accuracy was obtained that the

circumferential flanges of the outer 6-ft. ring were brought into

contact with those of the 12-ft. section already constructed. A diver

then entered by the opening left in the projecting ring, and bolted this

section of the roof to the preceding one. By removing the iron

diaphragm, the consecutive sections were united into one. When the diver

completed his work, the opening was closed up, and compressed air used

to keep the water out of the box included between the roof and the

temporary flooring.

[Illustration: FIG. 147.--Showing the Roof of the Caisson Formed by the

Upper Half of the Tunnel.]

The remaining sections of the tunnel roof were built in the same way,

until the last abutted against the part of the work constructed within

the caisson under the high wooden roof on the Manhattan side of the

river. The following method was adopted for the purpose of connecting

the few parts of the tunnel which had been differently constructed. The

diaphragm at the end of the last section of the tunnel roof was

constructed so as to abut against the last circumferential flanges of

the iron lining without leaving a projecting ring. It was continued

above the metal and concrete lining of the roof in a rectangular form,

and of the same height and width as the wooden bulkhead of the

working-chamber on the Manhattan side of the river. The diaphragm was

made of riveted plates and angles, with an opening 20 ins. by 30 ins.,

bolted so as to be removable at will. The diaphragm was of the same

height as the roof and was connected with a roof-plate to the rangers

supporting the thick wooden roof. Other steel plates, placed vertically,

were riveted to the diaphragm and bolted to the caisson. All this work

was carried on by divers. The wooden bulkhead was cut to the

springing-line of the arch; and between the two parts of the tunnel,

built by different methods, a bulkhead was placed, made of steel plates

14 ins. long, which prevented the entrance of water into the

working-chamber.

[Illustration: FIG. 148.--Showing the Tunnel Completed by Building the

Lower Half within the Caisson.]

When the different sections were joined together, and all the openings

closed and made water-tight, cement-grout was poured on the roof, and

earth was heaped up to a height of 5 ft. The 300 ft. of the roof,

resting on sheeting-piles and provided with diaphragms at the extreme

ends, formed a water-tight working-chamber, or caisson, communicating

with the exterior by means of the shafts and air-locks. The lower

portion of the tunnel was built under air-pressure. The pile-bents were

first cut off at the plane of the tunnel sub-grade, after which the

foundation-bed of concrete was laid. The lower segments of the iron

lining were then placed in position, and the structure made continuous

by building up the lateral walls, consisting of concrete (Fig. 148). No

accidents occurred while building the second part of the tunnel.

The Harlem River tunnel was completed in contract time, although the

opening of the subway was delayed by difficulties encountered in

tunneling through rock in the borough of the Bronx. The writer

endeavored to obtain information regarding the expense per linear foot,

but all his efforts were rewarded with a general assurance that it

proved to be the cheapest method.

SINKING AND JOINING TOGETHER SECTIONS OF TUNNELS BUILT ON LAND. THE

SEINE. THE DETROIT RIVER TUNNELS.

In the year 1896, Mr. Erastus Wyman secured a patent for building

subaqueous tunnels close to the river, by sinking and joining together

small sections of tunnels previously built on land. Each section would

have been provided with a long vertical tube for the air-lock when

compressed air was to be admitted to expel the water and permit the

construction of the lining within the sunken shell. Thus each section of

the tunnel would have acted as a pneumatic caisson; being, however, an

improvement on Professor Winkler’s suggestion inasmuch as the caisson

was a portion of the tunnel itself, instead of a simple inclosure for

facilitating the construction of the shield. Mr. Wyman proposed to use

this method in the construction of a tunnel between South Brooklyn and

Stapleton, Staten Island; a charter was granted him but the tunnel was

never built.

=The Tunnel under the Seine River.=--The caisson method of building

tunnels under water was used at Paris, France, in the construction of

the Metropolitan Railroad under the Seine River.

The caissons designed by Mr. L. Chagnaud were for a double track line.

They were sunk, ends to ends, and formed a portion of the tunnel lining

which was enveloped by a framework of metal embedded in concrete.

Built-up frames carried a shell of steel plating on the sides, from toes

to springing lines, and on the sides and roof of the working-chamber. A

temporary plate diaphragm closed the open ends. This construction formed

a vessel capable of floating with a very light draft.

The method of sinking the caissons was as follows: The caisson was

erected on the river bank and when completed it was launched and towed

into position between pile stagings which served the double purpose of

guiding the descent at the beginning of the sinking and of forming a

working platform. The caisson when launched and, consequently, before

the cast-iron lining had been put in place within it, weighed 280 metric

tons; but, beyond some difficulty in taking it under the bridges in the

way, the towing was accomplished without serious trouble.

[Illustration: FIG. 149.--Transversal Section of the Caissons for the

Tunnel under the Seine River.]

Previous to placing the caisson in position between the stagings, the

portion of the river bed it was to rest upon had been leveled by

dredging. Once in position, the first work was the erecting of the

cast-iron lining segments within the framework. Work was then begun by

filling the annular space between the lining and the shell with

concrete; this additional weight gradually sunk the caisson to the river

bottom. The working shafts, made up of steel cylinders, were placed as

the sinking progressed to this point.

[Illustration: ~Section A-B.~

~Section C-D.~

~Plan at Joint.~

FIG. 150.--Showing the Joining of the Caissons at the Pont Mirabeau

Tunnel under the Seine River.]

After the caissons had been sunk to the required place and in

continuation of one another, a space of nearly 5 ft. was left between

them. The construction of the tunnel within the bank of earth

separating the two caissons was as follows: A cofferdam was built around

this space. It was formed by two diaphragms closing the ends of the

tunnel, and by two longitudinal walls sunk as temporary caissons, one on

each side of the tunnel and inclosing their ends. This cofferdam was

covered with a metal working-chamber whose lower edges rested on top of

the four walls of the cofferdam. The joints were made tight by means of

rubber or packed clay. The water in the cofferdam was then pumped out,

the earth excavated, and the masonry built in continuation of the two

ends of the tunnel sections. The submerged sections of the tunnel which

were allowed to remain full of water to render them more stable and to

save effort in pumping them, were now made dry; the diaphragms were

removed from the ends of the caisson tunnels and the work made

continuous. Fig. 149 shows the cross-section of the caissons.

At the Pont Mirabeau crossing of the Seine, a slightly different method

was used, described in “Eng. News,” May 18, 1911. The caissons were sunk

to the required line and grade with an intervening longitudinal space of

15³⁄₄ ins. between two adjoining caissons. At each end of this space,

which was filled with the river marl, was sunk against the edges of the

caissons a hollow cylinder 20 ins. outside diameter. The interior of

these cylinders was excavated and filled with concrete, thus forming a

continuous wall on both sides of the two adjoining caissons. The earth

from the intervening space was then removed and concrete deposited from

bottom opening tremies up to the level of the top of the caisson. After

nearly one month the tunnel was entered from the shaft and an opening

the shape and size of the tunnel section cut through the diaphragms of

the 15³⁄₄-in. wall and the concrete tunnel lining made continuous

between the two sections. Fig. 150 shows the method of joining the

caissons.

=The Detroit River Tunnel.=[15]--With some modifications which permitted

dispensing with compressed air, the tunnel under the Detroit River was

built for the Michigan Central Railroad, connecting Detroit with

Windsor, Canada. The tunnel is 6625 ft. long; of this, however, only

2625 ft. are under the river, while the approach on the American side is

2000 ft. long and that on the Canadian side, 4000 ft. The tunnel

consists of two parallel circular tubes 23 ft. in diameter, built up of

³⁄₈-in. steel plate. They are placed 26 ft. apart, center to center, and

are connected by diaphragms at 12-foot intervals.

[15] Condensed from a paper by B. H. Ryder.

Each section of the subaqueous tunnel is approximately 262 ft. long.

There are ten of these sections and an eleventh a little over 60 ft.

long. These tubes were built at the shipyards of the Great Lakes

Engineering Works at St. Clair, about 30 miles from Detroit. After the

assembling was completed, the ends of each tube were closed by temporary

wooden bulkheads to make them float, and the outside sheathed

horizontally with heavy timbers bolted to the diaphragms. This sheathing

running lengthwise of the tube made a form or pocket, into which the

inclosing jacket of concrete was placed. The sections were then launched

and towed down to the tunnel site and sunk separately in a trench on the

river bottom that had been previously dredged to receive them. This

trench was dug to a width of 50 ft. and depth varying from 25 to 50 ft.

by clamshell buckets, swung from a scow, working to a depth below the

water level of 60 to 90 ft.

As a foundation for the sections, a grillage was constructed on the

surface and sunk in place in the trench by derricks swung from a scow.

The grillage was placed underneath each joint between the sections and

built up of I-beams imbedded in concrete. This grillage is the width of

the trench and about 30 ft. long, with posts projecting downward from

the four corners, and these were seated into the river bottom, by means

of pile drivers, to the desired grade.

Then the eleven sections of the tunnel were lowered and connected, one

at a time. By the aid of air tanks placed on each section the movement

was controlled until the final sinking upon the grillage in the trench.

This operation called into play the greatest engineering skill and

ingenuity. When it is considered that the current velocity at the river

bed is about 2 ft. per second and much higher along the surface, some

idea can be gained of the problems to be overcome. The movement of the

enormous sections must be absolutely under control. Thirty-five-ton

blocks of concrete were sunk in the river bottom up and down stream to

act as anchors, and through them cables were rigged and connected back

to the hoisting engines on the derrick scows. These were prevented from

moving by spuds at each corner, securely driven into the river bottom at

depths sometimes as great as 90 ft. Controlling cables were also run

from the sections to the tremie scow to pull one structure close to the

adjoining section previously sunk, and the divers made the necessary

connection. Fig. 151 shows cross-sections and plans of the tunnel as

given in “Eng. Record,” March 2, 1907.

[Illustration: ~HALF CROSS SECTION Y-Y~

~HALF CROSS SECTION Z-Z~

~HALF HORIZONTAL SECTION X-X~

~HALF TOP VIEW~

FIG. 151.--Cross-Sections and Plans of the Detroit River Tunnel.]

Steel masts had been previously attached to each end of the sections to

enable the engineers on shore to determine the alignment and locate the

exact position during the sinking.

Concrete was then deposited in the pockets, completely surrounding the

tubes, forming a solid monolithic structure from end to end.

This was done by means of the tremie process.

A 32-ft. by 160-ft. scow was equipped with a concrete mixing plant and

the tremie pipes, three in number, through which the concrete was

deposited. Each pipe is 12 ins. in diameter, of spiral riveted steel, 80

ft. long. These pipes could be raised or lowered, reaching from the

receiving hoppers on the scow to the bottom of the trench. When the

pipes were filled with concrete and lowered into position, a continuous

flow was maintained. As fast as the concrete escaped at the bottom end

of the pipe it was replenished at the top; this process continuing until

the entire space surrounding the section was filled to the desired

level, and under the pressure produced not only by the depth of water

under which it was submerged, but also by the weight of the long column

of concrete contained in the tubes. It is interesting to note that this

is the first time a large amount of concrete has been deposited at a

depth of 70 ft. by this method, and upon the accomplishment of this task

in a measure depended the successful building of the tunnel.

Inside the tubes was placed a lining of reinforced concrete 20 ins.

thick. Side walls were built up from this ring to provide ducts, which

carry the electrical cables for the distribution of power, lighting,

signal and telegraph wires. They also serve to provide a footwalk along

the side of the tunnel.

There are cross passages in the tunnel every 200 ft., and also various

niches for the different equipment needed in connection with the

signaling, telephone and fire alarm system. The tunnel is lighted with

800 16-candle-power incandescent lights.

The track construction is new. There is no ballast used, the ties being

laid in concrete. A ditch in the center of each track carries the

rainfall that will flow down from the summits to sumps which are drained

by centrifugal pumps.

One remarkable feature of its construction is that compressed air was

not used in the building of the subaqueous tunnel, but it was necessary

in building the approach tunnels. This is contrary to the usual program

where compressed air is required in subaqueous work, and not ordinarily

used in approach or land tunnel construction.

The trains are operated by very heavy electric locomotives, operated by

the third-rail system.

The tunnel was constructed under the supervision of W. S. Kinnear, Chief

Engineer of the Detroit River Tunnel Co.; Butler Bros. of New York were

the general contractors.

CHAPTER XXII.

ACCIDENTS AND REPAIRS IN TUNNELS DURING AND AFTER CONSTRUCTION.

In the excavation of tunnels it often happens that the disturbance of

the equilibrium of the surrounding material by the excavation develops

forces of such intensity that the timbering or lining is crushed and the

tunnel destroyed. To provide against accidents of this kind in a

theoretically perfect manner would require the engineer to have an

accurate knowledge of the character, direction and intensity of the

forces developed, and this is practically impossible, since all of these

factors differ with the nature and structure of the material penetrated.

The best that can be done, therefore, is to determine the general

character and structure of the material penetrated, as fully as

practicable, by means of borings and geological surveys, and then to

employ timbering and masonry of such dimensions and character as have

withstood successfully the pressures developed in previous tunnels

excavated through similar material. If, despite these precautions,

accidents occur, the engineer is compelled to devise methods of checking

and repairing them, and it is the purpose of this chapter to point out

briefly the most common kinds of accidents, their causes, and the usual

methods of repairing them.

=Accidents During Construction.=--Accidents may happen both during or

after construction, but it is during construction, when the equilibrium

of the surrounding material is first disturbed, and when the only

support of the pressures developed is the timber strutting that they

most commonly occur.

=Causes of Collapse.=--Collapse in tunnels may be caused: (1) by the

weight of the earth overhead, which is left unsupported by the

excavation; (2) by defective or insufficient strutting; and (3) by

defective or weak masonry.

(1) The danger of collapse of the roof of the excavation is influenced

by several conditions. One of these is the method of excavation adopted.

It is obvious that the larger the volume of the supporting earth is,

which is removed, the greater will be the tendency of the roof to fall,

and the more intense will be the pressures which the strutting will be

called upon to support. Thus the English and Austrian methods of

tunneling, where the full section is excavated before any of the lining

is placed, and where, as the consequence, the strutting has to sustain

all of the pressures, present more likelihood of the roof caving in than

any of the other common methods.

The character and structure of the material penetrated also influence

the danger of a collapse. A loose soil with little cohesion is of course

more likely to cave than one which is more stable. Rock where strata are

horizontal, or which is seamy and fissured, is more likely to break down

under the roof pressures than one with vertical strata and of

homogeneous structure. Soft sod containing boulders whose weight

develops local stresses in the roof timbering is likely to be more

dangerous than one which is more homogeneous. A factor which greatly

increases the danger of collapse, especially in soft soils, is the

presence of water. This element often changes a soil which is

comparatively stable, when dry, into one which is highly unstable and

treacherous. The liability of the material to disintegration by

atmospheric influences and various other conditions, which will occur to

the reader, may influence its stability to a dangerous extent, and

result in collapse.

(2) Collapse is often the result of using defective or insufficient

strutting. Of course, in one sense, any strutting which fails under the

pressures developed, however enormous they may be, can be said to be

insufficient, but as used here the term means a strutting with an

insufficient factor of safety to meet probable increases or variations

in pressure. Insufficient strutting may be due to the use of too light

timbers, to the spacing of the roof timbers too far apart, to the

yielding of the foundations, to insufficient bearing surface at the

joints, etc. Collapse is often caused by the premature removal of the

strutting during the construction of the masonry. The masons, to secure

more free space in which to work, are very likely, unless watched, to

remove too many of the timbers and seriously weaken the strutting.

(3) The third cause of collapse is badly built masonry. Poor masonry may

be due to the use of defective stone or brick, to the thinness of the

lining, to poor mortar, to weak centers which allow the arch to become

distorted during construction, to poor bonding of the stone or bricks,

to the premature removal of the centers, to driving some of the roof

timbers inside it, etc.

=Prevention of Collapse.=--Tunnels very seldom collapse without giving

some previous warning of the possible failure, and also of the manner in

which the failure is likely to occur. From these indications the

engineer is often able to foresee the nature of the danger and take

steps to check it. The danger may occur either during excavation or

after the lining is built. During excavation the danger of collapse is

indicated beforehand by the partial crushing or deflection of the

strutting timbers. If the timbers are too light or the bearing surfaces

are too small, crushing takes place where the pressures are the

greatest, and the timbers bend, burst, or crack in places, and the

joints open in other places. The remedy in such cases is to insert

additional timbers to strengthen the weak points, or it may be necessary

to construct a double strutting throughout. When the distance spanned by

the roof timbers is too great, failure is generally indicated by the

excessive deflection of these timbers, and this may often be remedied by

inserting intermediate struts or props. In some respects the best

remedy under any of these conditions is to construct the masonry as

soon as possible.

When collapse is likely to occur after the masonry is completed, its

probability is generally indicated by the cracking and distortion of the

lining. A study of the cause is quite likely to show that it is the

percolation of water through the material surrounding the lining which

causes cavities behind the lining in some places, and an increase of the

pressures in other places. When it is certain that this water comes from

the surface streams above, these streams may often be diverted or have

their beds lined with concrete to prevent further percolation. When

percolating water is not the cause of the trouble, a usually efficient

remedy is to sink a shaft over the weak point, and refill it with

material of more stable character. These, and the remedies previously

suggested, are designed to prevent failure without resorting to

reconstruction. When they or similar means prove insufficient,

reconstruction or repairs have to be resorted to.

=Repairing Failures.=--Tunnels may collapse in several ways: (1) The

front and sides of the excavation may cave in; (2) the floor or bottom

may bulge or sink; (3) the roof may fall in; (4) the material above the

entrances may slide and fill them up.

(1) One of the most common accidents is the caving of the front and

sides of the excavation. This may often be prevented by taking care that

the face of the excavation follows the natural slope of the material

instead of being more or less nearly vertical. When, however, caving

does occur it may usually be repaired by removing the fallen material,

strongly shoring the cavity, and filling in behind with stone, timber,

or fascines.

(2) The bulging or rising of the bottom of the tunnel may usually be

considered as a consequence of the squeezing together of the side walls.

It usually occurs in very loose soils, and is chiefly important from the

fact that the reconstruction of the side walls is made necessary. The

sinking of the tunnel bottom is a more serious occurrence. It seldom

happens unless there is a cavity beneath the floor, due either to

natural causes or to the fact that mining operations have gone on in the

hill or mountain penetrated by the tunnel. When the bottom of the tunnel

sinks, three cases may be considered: (_a_) when the sinking is limited

to the middle of the tunnel floor; (_b_) when only a portion of the

foundation masonry is affected; and, (_c_) when the entire lining is

disturbed. In the first case repairs are easily made by filling in the

cavity with new material. In the second case the unimpaired portion of

the masonry is temporarily supported by shoring while the injured

portion is removed and rebuilt on a firm foundation. The remaining

cavity is then filled. In the case of the complete failure of the

lining, the method of repairing employed when the roof falls, and

described below, is usually adopted.

(3) The most dangerous of all failures is the falling of the tunnel

roof. In such casualties two cases may be considered: (_a_) When the

falling mass completely fills the tunnel section, and (_b_) when it

fills only a portion of the section.

[Illustration: FIG. 152.--Tunneling through Caved Material by Heading.]

When the whole section is filled by the fallen material, the problem may

be considered as the excavation of a new tunnel of short length inside

the old tunnel, and under rather more difficult conditions. The first

task, particularly if men have been imprisoned behind the fallen

material, is to open communication through it between the two uninjured

portions of the tunnel. It is advisable to do this even when there is no

danger to life because of imprisoned workmen, since it enables the work

of repairing to be conducted from both directions. The excavation of a

passageway through the fallen material is rendered difficult, both

because the fallen material is of an unstable character, and also

because it is usually filled with the lining masonry, timbering, etc.

When, therefore, the accident has happened before the full section of

the original material has been removed, the first heading or drift is

driven through this original material rather than through the fallen

débris. Any of the regular soft-ground methods of tunneling may be

employed, but it is usually better to select one which allows the

masonry to be built with as little excavation as possible at first. For

this reason the German method of tunneling is particularly suited to

repair work of this nature. The Belgian method may also be used to

advantage, particularly when the caving extends to the surface of the

ground above, and the upper portion of the débris is, therefore,

practically the same material as that through which the original tunnel

was driven. The greatest defect of the Belgian method for making repairs

is that the roof arch is supported by a rather unstable mass of mingled

earth, stone, and timber, which constitutes the bottom layer of the

fallen material. The method of strutting the work when the German or

Belgian method is used is shown by Fig. 152. It sometimes happens that

the fallen débris is so unstable that it will not carry safely the arch

masonry in the Belgian method or the strutting in the German method, and

in these cases one of the full-section methods of excavation is usually

adopted. The nature of the strutting employed is shown by Fig. 153. When

the section has been opened and the new masonry built, great care should

be taken to fill the cavity behind the masonry with timber or stone; and

should the disturbance reach to the ground surface it is often a good

plan to sink a shaft through the disturbed material, and fill it with

more stable material.

[Illustration: FIG. 153.--Tunneling through Caved Material by Drifts.]

When the fallen débris fills only a part of the section, the first thing

to provide against is the occurrence of any further caving; and this is

usually done by building a protecting roof above the line of the future

roof masonry. Figs. 154 and 155 show two methods of constructing this

temporary roof, which it will be noticed is filled above with cordwood

packing. As soon as the temporary roof is completed, the lining masonry

is constructed.

[Illustration: FIGS. 154 and 155.--Filling in Roof Cavity Formed by

Falling Material.]

[Illustration: FIG. 156.--Timbering to Prevent Landslides at Portal.]

(4) Landslides which close the tunnel entrance are repaired in a variety

of ways. Fig. 156 shows a common method of preventing the extension of a

landslide which has been started by the excavation for the entrance

masonry. Fig. 157 shows a method often adopted when the slope is quite

flat and the amount of sliding material is small. It consists

essentially of removing the fallen material and building a new portal

farther back; that is, the open cut is extended and the tunnel is

shortened. When the amount of the sliding material is very large, the

contrary practice of lengthening the tunnel and shortening the open cut,

as shown by Fig. 158, may be adopted.

[Illustration: FIG. 157.--Shortening Tunnel Crushed by Landslide at

Portal.]

=Accidents After Construction.=--Accidents after the completion of the

tunnel may be divided into two classes: first, those which entirely

obstruct the passage of trains, of which the collapse of the roof is the

most common; and second, those which allow traffic to be continued while

the repairs are being made, such as the bulging inward of a portion of

the lining without total collapse. In the first case the first duty of

the engineer is to open communication through the fallen débris, so that

passengers at least may be transferred from one part of the tunnel to

the other and proceed on their way. This is done by driving a heading,

and strongly timbering it to serve as a passageway. If the tunnel is

single tracked this heading is afterwards enlarged until the whole

section is opened. In double-track tunnels the method generally adopted

is to open first one side of the section and timber it strongly, so as

to clear one track for traffic. While the trains are running through

this temporary passageway the other half of the section is opened and

repaired; the traffic is then shifted to the new permanent track, and

the temporary structure first employed is replaced with a permanent

lining. When the accident is such that the repairs can be made without

obstructing traffic entirely, various modes of procedure are followed.

In all cases great care has to be exercised to prevent accident to the

trains and to the tunnel workmen. The work should be done in small

sections so as to disturb as little as possible the already troubled

equilibrium of the soil; the strutting should be placed so as to give

ample clearing space to passing trains, and the trains themselves should

be run at slow speeds past the site of the repairs. To illustrate the

two kinds of accidents and the methods of repairing them, which have

been mentioned, the accidents at the Giovi tunnel in Italy and at the

Chattanooga tunnel in America have been selected.

[Illustration: FIG. 158.--Extending Tunnel through Landslide at Portal.]

=Giovi Tunnel Accident.=--In September, 1869, at a point about 220 ft.

from the south portal of the Giovi tunnel, a disturbance of the masonry

lining for a length of about 52 ft. was observed. Accurate measurements

showed that the lining was not symmetrical with respect to the vertical

axis of the sectional profile. It was concluded that owing to some

disturbance of the surrounding soil unsymmetrical vertical and lateral

pressures were acting on the masonry. Close watch was kept of the

distorted masonry, which for some time remained unchanged in position.

In 1872, however, new crevices were observed to have developed, and

shortly afterwards, in January, 1873, the injured portion of the masonry

caved in, obstructing the whole tunnel section. The fallen material

consisted chiefly of clay in a nearly plastic state. The surface of the

ground above was observed to have settled. Investigation showed also

that the cause of the caving was the percolation of water from a nearby

creek. The water had soaked the ground, and decreased its stability to

such an extent that the masonry lining was unable to withstand the

increased vertical and lateral pressures.

=The Shell.=--It is absolutely necessary that the exterior surface of