CHAPTER V._Tunneling: A Practical Treatise.

METHODS OF TIMBERING OR STRUTTING TUNNELS.

The purpose of timbering or strutting in tunnel work is to prevent the

caving-in of the roof and side walls of the excavation previous to the

construction of the lining. As the strutting has to resist all the

pressures developed in the roof and side walls, which may be exceedingly

troublesome and of great intensity in loose soils, its design and

erection call for particular care. The method of strutting adopted

depends upon the method of excavation employed; but in every case the

problem is not only to build it strong enough to withstand the pressures

developed, but to do this as economically as possible, and with as

little hindrance as may be to the work which is going on simultaneously

and which will come later. Only the latter general problems of strutting

peculiar to all methods of tunnel work will be considered here. For this

consideration strutting may be classified according to the material of

which it is built, under the heads of timber structures and iron

structures.

[Illustration: FIG. 17.--Joining Tunnel Struts by Halving.]

[Illustration: FIG. 18.--Round Timber Post and Cap Bearing.]

TIMBER STRUTTING.

Timber is nearly always employed for strutting in tunnel work. So long

as it has the requisite strength, any kind of timber is suitable for

strutting, since, it being only temporarily employed, its durability is

a matter of slight importance. Timber with good elastic properties, like

pine or spruce, is preferably chosen, since it yields gradually under

stress, thus warning the engineer of the approach of danger; while oak

and other strong timbers resist until the last moment, and then yield

suddenly under the breaking load. Soft woods, moreover, are usually

lighter in weight than hard woods, which is a considerable advantage

where so much handling is required in a restricted space. Round timbers

are generally employed, since they are less expensive, and quite as

satisfactory in other respects as sawed timbers. In the English and

Austrian methods of strutting, which are described further on, a few of

the principal struts are of sawed timbers.

The various timbers of the strutting are seldom attached by framed

joints, but wedges are used to give them the necessary bearing against

each other. Where framed joints are employed they are made of the

simplest form usually by halving the joining timbers, as shown by Fig.

17. Fig. 18 shows a form of joint used where round posts carry beams of

similar shape. The reason why it is possible to do away with jointed

connections to such a great extent, is that the strains which the

timbers have to resist are either compressive or bending strains, and

because the timbers are so short that they do not require to be spliced.

=Strutting of Headings.=--The method of strutting the heading that is

employed depends upon the material through which the heading is driven.

In solid rock strutting may not be required at all, or only for the

purpose of preventing the fall of loose blocks from the roof, then

vertical props are erected where required, or horizontal beams are

inserted into the side walls, as shown by Fig. 19. These horizontal

beams may be used singly at dangerous places, or they may be placed from

2 ft. to 3 ft. apart all along the heading. In the latter case they

usually carry a lagging of planks, which may be placed at intervals or

close together, and filled above with stone in case the roof of the

excavation is very unstable. Planks used in this manner are usually

called poling-boards. Where the side walls as well as the roof require

support, vertical side posts are employed to carry the roof beams, as

shown by Fig. 20; and, when necessary, poling-boards are inserted

between these posts and the walls of the excavation.

[Illustration: FIG. 19.--Ceiling Strutting for Tunnel Roofs.]

[Illustration: FIG. 20.--Ceiling Strutting with Side Post Supports.]

[Illustration: FIG. 21.--Sill, Side Post and Cap Cross Frame Strutting.]

[Illustration: FIG. 22.--Reinforced Cross Frame Strutting for

Treacherous Materials.]

_Frame Strutting._--In very loose soils not only the roof and side

walls, but also the floor of the heading require strutting. In these

cases frame strutting is employed, as shown by Fig. 21. It consists

simply of a rectangular frame; at the top there is a crown bar supported

by two vertical side posts setting on a sill laid across the bottom of

the heading. These frames are spaced at close intervals, and carry

longitudinal planks or poling-boards. The sill of the frame is sometimes

omitted when the soil is stable enough to permit it, and in its place

wooden footing blocks are substituted to carry the side posts. In soils

where the pressures are great enough to bend the crown bar, a secondary

frame is employed, as shown by Fig. 22, the two inclined roof members,

or rafters, of which support the crown bar at the center.

[Illustration: FIG. 23.--Longitudinal Poling-Board System of Roof

Strutting.]

[Illustration: FIG. 24.--Transverse Poling-Board System of Roof

Strutting.]

It is the more common practice in driving headings through soft soils to

use inclined poling-boards to support the roof. Fig. 23 shows one method

of doing this. The method of operation is as follows: Assuming the

poling-boards _a_ and _b_ to be in place, and supported by the frames

_A_, _B_, _C_, as shown, the first step in continuation of the work is

to insert the poling-board _c_ over the crown bar of frame _C_, and

under the block _m_. Excavation is then begun at the top, and as fast as

the soil is removed ahead of it the poling-board _c_ is driven ahead

until its rear end only slightly overhangs the crown bar of frame _C_.

The remainder of the face of the heading is then excavated nearly to the

front end of the poling-board _c_, and another frame is set up. By a

succession of these operations the heading is advanced. The

poling-boards at the sides of the heading are placed in a similar manner

to the roof poling-boards. A second method of using inclined

poling-boards is shown by Fig. 24. Here the poling-boards run

transversely, and are supported by the arrangement of timbering shown.

The chief advantage of using these inclined poling-boards, particularly

in the manner shown by Fig. 23, is that the excavators work under cover

at all times, and are thus safe from falling fragments or sudden

cavings.

_Box Strutting._--In very treacherous soils, such as quicksand, peat,

and laminated clay, box strutting is commonly employed. The method of

building this strutting is to set up at the face of the work a

rectangular frame, and use it as a guide in driving a lagging or boxing

of horizontal planks into the soft soil ahead. These planks have sharp

edges, and are driven to a distance of 2 ft. or 3 ft. into the face of

the heading, so as to inclose a rectangular body of earth. This earth is

excavated nearly to the ends of the planks, and then another frame is

inserted close up against the new face of the excavation, which supports

the planks so that the remainder of the earth included by them may be

removed. These two frames, with their plank lagging, constitute a “box;”

and a series of these boxes, one succeeding another, form the strutting

of the heading.

=Strutting the Face.=--In some cases it is found necessary to strut the

face of the heading in order to prevent it from caving in. This is

generally done by setting plank vertically, and bracing them up by means

of inclined props whose feet abut against the sill of the nearest cross

frame. This strutting is erected while the workmen are placing the side

and roof strutting, and is removed to permit excavation.

=Full Section Timber Strutting.=--For strutting the full section two

forms of timbering are employed, known as the polygonal system and the

longitudinal system.

Longitudinal strutting consists of a timber structure so arranged as to

have all the principal members supporting the poling-boards parallel to

the axis of the tunnel. This system of strutting is peculiar to the

English method of tunneling. The longitudinal timbers rest on this

finished masonry at one end, and are carried on a cross frame or by

props at the other end. At intermediate points the longitudinals are

braced apart by struts in planes transverse to the tunnel axis. This

construction makes a very strong strutting framework, since the

transverse struts act as arch ribs to stiffen the longitudinals; but the

use of transverse poling-boards requires the excavation of a larger

cross-section than is necessary when longitudinal poling-boards are

employed, and this increases the cost both for the amount of earth

excavated and the greater quantity of filling required.

In polygonal strutting the main members are in a plane normal to the

axis of the tunnel. They form a polygon whose sides follow closely the

sectional profile of the excavation. These polygonal frames are placed

at more or less short intervals apart, and are braced together by short

longitudinal struts lying close to the sides of the excavation, and

running from one frame to the next, and also by longer longitudinal

members which extend over several frames. The polygonal system of

strutting is peculiar to the Austrian method of tunneling, and is fully

described in a succeeding chapter. One of its distinctive

characteristics is that the poling-boards are inserted parallel to the

tunnel axis. Polygonal strutting is generally held to be stronger than

longitudinal strutting under uniform loads, but it is more liable to

distortion when the loads are unsymmetrical.

[Illustration: FIG. 25.--Shaft with Single Transverse Strutting.]

[Illustration: FIG. 26.--Rectangular Frame Strutting for Shafts.]

[Illustration: FIG. 27.--Reinforced Rectangular Frame Strutting for

Shafts in Treacherous Materials.]

=Strutting of Shafts.=--Tunnel shafts are strutted both to prevent the

caving-in of the sides and to divide them into compartments. When the

material penetrated is very compact, and caving is not likely, a single

series of transverse struts, one above the other, running from the top

to the bottom of the shaft, as shown by Fig. 25, is used to divide it

into two compartments. In softer material, where the sides of the shaft

require support, Fig. 26 shows a form of strutting commonly employed. It

consists of vertical corner posts braced apart at intervals by four

horizontal struts placed close to the walls of the shaft. The longer

side struts are also braced apart at the center by a middle strut which

divides the shaft into two compartments. A lagging of vertical plank is

placed between the walls of the shaft and the horizontal side struts. In

very loose soils the form of strutting shown by Fig. 27 is employed.

This is practically the same construction as is shown by Fig. 26, with

the addition of an interior polygonal horizontal bracing in each half of

the shaft. Referring to Fig. 27, the timbers _a_, _a_, etc., are

vertical and continuous from the top to the bottom of the shaft; and the

horizontal timbers, _b_, _b_, etc., are spaced at more or less close

intervals vertically. The lagging planks may be laid with spaces between

them, or close together, or, in case of very loose material, with their

edges overlapping. The manner of constructing the strutting is also

governed by the stability of the soil. In firm soils it is possible to

sink the shaft quite a depth without timbering, and the timbering can

be erected in sections of considerable length, which is always an

advantage, but in loose soils the timbering has to follow closely the

excavation.

The solid wall shaft struttings which have been described are

discontinued at the point where the shaft intersects the tunnel

excavation; and from this point to the floor of the tunnel an open

timbering is employed, whose only duty is to support the weight of the

solid strutting above. This timbering is made in various forms, but the

most common is a timber truss or arch construction which spans the

tunnel section.

=Quantity of Timber.=--The quantity of timber employed in strutting a

tunnel varies with the character of the material through which the

tunnel is excavated: it is small for solid-rock tunnels, and large for

soft-ground tunnels. In the Belgian method of excavation a smaller

quantity of timber is used than in any of the other ordinary methods.

For single-track tunnels excavated by this method there will be needed

on an average about 3 to 3¹⁄₃ cu. yds. of timber per lineal foot of

tunnel. Practical experience shows that about four-fifths of the timber

once used can be employed for the second time. In any of the methods in

which the whole tunnel section is excavated at once, the average amount

of timber required per lineal foot is about 8.7 cu. yds. Of this amount

about two-thirds can be used a second time. In the Italian method, in

which the upper half and the lower half are excavated separately, about

5 cu. yds. of timber are required per lineal foot of tunnel, about

one-half of which can be employed a second time. For quicksand tunnels

the amount of timbering required per lineal foot varies from 3 to 5

cubic yds. Shaft strutting requires from 1 to 1¹⁄₂ cu. yds. of timber

per lineal foot.

=Dimensions of Timber.=--The dimensions of the principal members

composing the strutting of headings, full section, and shafts, are given

in Table I. The planks used for lagging or the poling-boards are usually

from 4 ins. to 6 ins. wide, with a length depending upon the method of

strutting employed.

TABLE I.

Showing Sizes of Various Timbers Used in Strutting Tunnels Driven

Through Different Materials.

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

| | ROCK. | SOFT SOILS. |

| +-----+-----+--------+------+------+

| +-----+-----+--------+------+------+

| | ins.| ins.| ins. | ins. | ins. |

| Cap-pieces and vertical struts | 6 | 8 | 10 | 12 | 14 |

| Sills | | | 8 | 10 | 12 |

| Struts | 5 | 5 | 6 | 7 | 8 |

| feet | 6 | 4.5| 3 | 2.6 | 2.6 |

| | | | | | |

| Crown bars | 12 | 14 | 14 | | |

| supporting the crown bars | 10 | 12 | 14 | | |

| Sills | 8 | 8 | 10 | | |

| Cap-pieces or saddles | 10 | 12 | 14 | | |

| Struts to stiffen the structure | 6 | 8 | 10 | | |

| feet) | 4.5 | 4 | 3 | | |

| | | | | | |

| Cap-pieces and contour pieces | 8 | 10 | 12 | 14 | 16 |

| Vertical struts on top | 10 | 12 | 14 | 16 | 18 |

| Vertical struts below | 12 | 14 | 16 | 20 | 24 |

| Intermediate sills | 12 | 14 | 16 | 20 | 24 |

| Lower sills | | | 12 | 16 | 18 |

| Raking props | 10 | 10 | 10 | 12 | 12 |

| feet) | 6 | 4.5 | 4 | 3 | 3 |

| | | | | | |

| frame | 8 | 8 | 10 | 12 | 14 |

| Transverse beams | 8 | 8 | 8 | 10 | 12 |

| frames | 8 | 8 | 10 | 12 | 12 |

| Struts to reënforce the frame | | 6 | 8 | 8 | 8 |

| (in feet) | 6 | 4.5 | 4 | 3 | 2.6 |

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

IRON STRUTTING.

In 1862 Mr. Rziha employed old iron railway rails for strutting the

Naensen tunnel, and his example was successfully followed in several

tunnels built later where timber was scarce and expensive. The

advantages which iron strutting is claimed to possess over the more

common wooden structure are: its greater strength; the smaller amount of

space which it takes up; and the fact that it does not wear out, and

may, therefore, be used over and over again.

[Illustration: FIG. 28.--Strutting of Timber Posts and Railway Rail

Caps.]

[Illustration: FIG. 29.--Strutting made entirely of Railway Rails.]

=Iron Strutting in Headings.=--In strutting the headings the cross

frames have a crown bar consisting of a section of old railway rail

carried either by wood or iron side posts. When wooden side posts are

used their upper ends have a dovetail mortise, and are bound with an

iron band, as shown by Fig. 28. The base of the rail crown bar is set

into the dovetail mortise and fastened by wedges. When iron side posts

are employed they usually consist of sections of railway rails, and the

crown bar is attached to them by fish-plate connections, as shown by

Fig. 29. The iron cross frames are set up as the heading advances, and

carry the plank lagging or poling-boards, exactly in the same manner as

the timber cross frames previously described.

[Illustration: FIG. 30.--Rziha’s Combined Strutting and Centering of

Cast Iron.]

[Illustration: FIG. 31.--Cast-Iron Segment of Rziha’s Strutting and

Centering.]

=Full Section Iron Strutting.=--The iron strutting devised by Mr. Rziha

for full section work is shown by Fig. 30. Briefly described, it

consists of voussoir-shaped cast-iron segments, which are built up in

arch form. Fig. 31 shows the construction of one of the segments, all of

which are alike, with the exception of the crown segment, which has a

mortise and tenon joint which is kept open by filling the mortise with

sand. The segments are bolted together by means of suitable bolt-holes

in the vertical flanges, and when fully connected form an arch rib of

cast iron. This arch rib, A, Fig. 30, carries a series of angle or

T-iron frames bent into approximately voussoir shape, as shown at B,

Fig. 30. Above these frames are inserted the poling-boards, running

longitudinally, and spanning the distance between consecutive arch ribs.

By removing the bent iron frames the cast-iron rib forms a center upon

which to construct the masonry. Finally, to remove the cast-iron rib

itself, the sand is drawn out of the mortise and tenon joint in the

crown segment, which allows the joint to close, and loosen the segments

so that they are easily unbutted.

The illustration, Fig. 30, shows longitudinal poling-boards; more often

longitudinal crown bars of railway rails span the space between

connective arch ribs, and support transverse poling-boards. In building

the masonry, work is begun at the bottom on each side, the bent iron

frames being removed one after another to give room for the masonry. As

each frame is removed, it is replaced with a sort of screw-jack to

support the poling-boards until the masonry is sufficiently completed to

allow their removal. The interior bracing of the arch rib shown at _a a_

and _b b_ consists of railway rails carried by brackets cast on to the

segments. A similar bracing of rails connects the successive arch ribs.

These lines of bracing serve to carry the scaffolding upon which the

masons work in building the lining.

[Illustration: FIG. 32.--Cast-Iron Segmental Strutting for Shafts.]

=Iron Shaft Strutting.=--In soft-ground shaft work, the use of an iron

strutting, consisting of consecutive cast-iron rings, has sometimes

been employed to advantage. Fig. 32 shows the construction of one of

these rings, which, it will be seen, is composed of four segments

connected to each other by means of bolted flanges. The holes shown in

the circumferential web of the ring are to allow for the seepage from

the earth side walls. The method of placing this cylindrical strutting

is to start with a ring having a cutting-edge. By means of excavation

inside the ring, and by ramming, the ring is sunk into the ground a

distance equal to its height. Another ring is then fastened by special

hooks on top of the first one, and the sinking continued until the

second ring is down flush with the surface. A third ring is then added,

and so on until the entire shaft is excavated and strutted. As in timber

shaft strutting, the solid iron ring strutting is carried down only to

the top of the tunnel section, and below this point there is an open

timber or iron supporting framework.