Produced by Louise Hope, Juliet Sutherland and the Online
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[Transcriber's Note:
Two other papers from ASCE _Transactions_ LXVIII (September 1910) are
referenced in this paper:
No. 1150, "The New York Tunnel Extension..." by Charles W. Raymond,
available from Project Gutenberg as e-text #18229.
No. 1151, "The North River Division" by Charles M. Jacobs, e-text
#18548, generally cited as "the paper by Mr. Jacobs".
The word "Figure" is used in two ways. It refers either to individual
numbered Figures (1-21), or to any of the four pictures that make up
each Plate, identified in the form "Fig. 2, Plate XXI". Figures 1-4
are always discussed as a group.
Single letters in boldface are shown as =A=. Typographical errors are
listed at the end of the text.]
* * * * *
* * * *
* * * * *
American Society of Civil Engineers
Instituted 1852
TRANSACTIONS
Paper No. 1154
THE NEW YORK TUNNEL EXTENSION OF THE PENNSYLVANIA RAILROAD.
THE BERGEN HILL TUNNELS.[1]
By F. LAVIS, M. Am. Soc. C. E.
[Footnote 1: Presented at the meeting of April 6th, 1910.]
_Location._--That section of the Pennsylvania Railroad's New York
Tunnels lying west of the Hudson River is designated Section "K," and
the tunnels are generally spoken of as the Bergen Hill Tunnels. Bergen
Hill is a trap dike (diabase) forming the lower extension of the Hudson
River Palisades.
There are two parallel single-track tunnels, cross-sections of which are
shown on Plate VIII of the paper by Charles M. Jacobs, M. Am. Soc. C. E.
The center line is a tangent, and nearly on the line of 32d Street, New
York City, produced, its course being N. 50° 30' W. The elevation of the
top of the rail at the Weehawken Shaft (a view of which is shown by
Fig. 2, Plate XXII), on the west bank of the Hudson River, is about 64
ft. below mean high water; and at the Western Portal, or Hackensack end,
the rail is about 17 ft. above; the grade throughout is 1.3%, ascending
from east to west. The length of each tunnel between the portals is
5,920 ft.
A general plan and profile of these tunnels is shown on Plate I of the
paper by Charles W. Raymond, M. Am. Soc. C. E. At Central Avenue a shaft
212 ft. deep was sunk. It is 3,620 ft. from the Weehawken Shaft.
[Illustration: Plate XXI.
Fig. 1: K 94. P.R.R. Tunnels, N. R. D. Section K. (Bergen Hill
Tunnels.) from Hackensack Poral, North Cut and Cover Section, and
Portal looking East from Sta. 323. Dec. 8, 05.
Fig. 2: K 71. P.R.R. Tunnels, N. R. Div. Sect. K. (Bergen Hill
Tunnels) Method of using Cross-Section Rod in getting Sections of
Tunnel. Aug. 30, 06.
Fig. 3: K 115. P.R.R. Tunnels, N. R. Div. Sect. K. (Bergen Hill
Tunnels) Weehawken Shaft, North Tunnel Conveyor used by King Rice
and Garney for handling and placing concrete. June 3, 07.
Fig. 4: K 116. P.R.R. Tunnels, N. R. Div. Sect. K. (Bergen Hill
Tunnels) Weehawken Shaft, North Tunnel. View of conveyor for placing
concrete, with bucket suspended over hopper above belt. Steel forms
in fore ground. June 4, 07.]
_History._--The contract for this work was let on March 6th, 1905, to
the John Shields Construction Company; it was abandoned by the Receiver
for that company on January 20th, 1906, and on March 20th, of that year,
was re-let to William Bradley, who completed the work by December 31st,
1908.
The progress of excavation and lining in the North Tunnel is shown
graphically on the progress diagram, Fig. 9, that of the South Tunnel
being practically the same.
_Geology._--Starting west from the Weehawken Shaft, the tunnels pass
through a wide fault for a distance of nearly 400 ft., this fault being
a continuation of that which forms the valley between the detached mass
of trap and sandstone known as King's Bluff, which lies north of the
tunnels, and the main trap ridge of Bergen Hill.
The broken ground of the fault, which consists of decomposed sandstone,
shale, feldspar, calcite, etc., interspersed with masses of harder
sandstone and baked shale, gradually merges into a compact granular
sandstone, which, at a distance of 460 ft. from the shaft, was
self-supporting, and did not require timbering, which, of course, had
been necessary up to this point.
A full face of sandstone continued to Station 274 + 60, 940 ft. from the
shaft, where the main overlying body of trap appeared in the heading.
The full face of the tunnel was wholly in trap at about Station 275 +
30, and continued in this through to the Western Portal, where the top
of the trap was slightly below the roof of the tunnel, with hardpan
above. The contact between the sandstone and the overlying trap was very
clearly defined, the angle of dip being approximately 17° 40' toward the
northwest.
The sandstone and trap are of the Triassic Period, and the trap of this
vicinity is more particularly classified as diabase.
The character of the trap rock varied considerably. At the contact,
at Station 275, and for a distance of approximately 200 ft. west,
corresponding to a thickness of about 60 ft. measured at right angles to
the line of the contact, a very hard, fine-grained trap, almost black in
color, was found, having a specific gravity of 2.98, and weighing 186
lb. per cu. ft. The hardness of this rock is attested by the fact that
the average time required to drill a 10-ft. hole in the heading, with a
No. 34 slugger drill, with air at 90 lb. pressure, was almost 10 hours.
The specific gravity of this rock is not as high as that of some other
specimens of trap tested, which were much more easily drilled. This rock
was very blocky, causing the drills to bind and stick badly, and, when
being shoveled back from the heading, as it fell it sounded very much as
though it were broken glass.
The remainder of the trap varied from this, through several changes of
texture and color, due to different amounts of quartz and feldspar, to a
very coarse-grained rock, closely resembling granite of a light color,
though quite hard. The speed of drilling the normal trap in the heading
was approximately 20 to 25 min. per ft., as compared with the 60 min.
per ft. noted above, the larger amounts of quartz and feldspar
accounting for the greater brittleness and consequently the easier
drilling qualities of the rock. The normal trap in these tunnels has a
specific gravity varying from 2.85 to 3.04, and weighs from 179 to 190
lb. per cu. ft.
The temperature of the tunnels, at points 1,000 ft. from the portals at
both ends, remained nearly stationary, and approximately between 50° in
winter and 60° in summer, up to the time the headings were holed
through, being practically unaffected by daily changes in the
temperature outside. At the western end, after the connection with the
Central Shaft headings was made, there was almost always a current of
air from the portal to the shaft, and ascending through the latter. This
tended to make the temperature in this part of the tunnel correspond
more nearly with the outside temperature; in fact, the variation was
seldom more than 5° Fahr.
_Timbering._--These tunnels have been excavated entirely by the center
top heading method, almost invariably used in the United States.
Timbering, where required, was of the usual segmental form with outside
lagging, as shown in several of the photographs. In a few places it was
necessary to hold the ground as the work progressed, and, in such cases,
crown bars were used in the headings.
There was some little trouble at the Western Portal, where the top of
the rock was very near the roof of the tunnel, as shown by Fig. 1, Plate
XXI. A side heading was driven at the level of the springing line until
a point was reached where the roof was self-supporting, and the
timbering was brought out to the face of the portal from that point.
[Illustration: Plate XXII.
Fig. 1: K 26. P.R.R. Tunnels, N. R. D. Sect. K. (Bergen Hill
Tunnels,) Weehawken Shaft. Scaffold car in South Tunnel at Sta.
267+60. Jan. 11, 06.
Fig. 2: K 31. P.R.R. Tunnels, N. R. Div. Sect. K. (Bergen Hill
Tunnels) Weehawken Shaft. Headhouse at ? elevator frame work,
looking West. Oct. 17, 06.
Fig. 3.--Round Holes in Concrete Forms.
Fig. 4.--Round Holes in Concrete Forms Completed.]
_Drilling._--Where no timbering was required, several different methods
were used in drilling and excavating the solid rock, though in all cases
a center top heading was driven. The four diagrams, Figs. 1, 2, 3,
and 4, give typical examples of these methods and show, in the order of
their numbers, the general tendency of the development from a small
heading kept some distance ahead of the bench, to a large heading with
the bench kept close to it. The notes on each diagram give the general
details of the quantity of drilling and powder used, methods of
blasting, etc., and on the progress profile, Fig. 6, is indicated those
portions of the tunnels in which each method was used.
All the drills used throughout the work by Mr. Bradley were Rand No. 34
sluggers, with 3-5/8-in. cylinders, and the steel was that known as the
"Black Diamond Brand," 1-3/8-in., octagon. It was used in 2, 4, 6, 8,
10, and 12-ft. lengths; toward the end of the work it was proposed to
use 14-ft. lengths, but owing to some delay in delivery this length was
never obtained. The starters, 18 to 24 in. long, were sharpened to 2¾ to
3-in. gauge, which was generally held up to depths of 6 ft.; then the
gauge gradually decreased until it was 1¾ to 2¼ in. at the bottom of a
12-ft. hole. Frequently, as many as three or four starters were used in
starting a hole, and generally two sharpenings were required for each
2 ft. drilled, after the first 6 ft. It is estimated that about ¼ in. of
steel was used for each sharpening, and that there was an average of one
sharpening for every foot drilled.
The total quantity of steel used up, lost, or scrapped on the whole work
was almost exactly 1 ft. for each 10 cu. yd. excavated, equal to 1¼ in.
of steel per yard, distributed approximately as follows:
Sharpening ¾ to 7/8 in.
Other losses ½ to 3/8 "
---------------
Total 1¼ in. per cu. yd.
An "Ajax" drill sharpener was used, and proved very satisfactory. Rubber
and cotton hose, covered with woven marlin, was used for the bench
(3 in. inside diameter, in 50-ft. lengths), for drills (1 in. in
diameter, in 25-ft. lengths), and for steam shovels (2½ in. in diameter,
in 50-ft. lengths). Hose coverings of wound marlin, and of woven marlin
with spiral steel wire covering were tried, but were not satisfactory,
owing to the unwinding of the marlin and the bending of the steel
covering.
[Illustration: Fig. 1. {Drilling Method No. 1}
CROSS-SECTION, LONGITUDINAL SECTION, PLAN]
Drilling Method No. 1: Small heading, 60 to 80 ft. long. Two columns
used in heading, with two drills on each. Drills on sub-bench and
main bench mounted on tripods.
+--------------------------------------------------+
| Per Round |
+---------+---------+---------+----------+---------+
| | Total | No. of | Pounds | |
| | Depth | Cubic | of | Advance |
| | Drilled | Yards | Dynamite | |
+---------+---------+---------+----------+---------+
| Heading | 140-155 | 18-21.6 | 93-131 | 5-6 |
+---------+---------+---------+----------+---------+
| Bench | 110-120 | 53-60 | 76-97 | 3½-4 |
+---------+---------+---------+----------+---------+
+---------+--------------------+-----------------------------+
| | Per Cubic Yard | Per linear Foot of Tunnel |
| +---------+----------+-------+---------+-----------+
| | Linear | Pounds | Cubic | No. of | Pounds |
| | Feet | of | Yards | Feet | of |
| | Drilled | Dynamite | | Drilled | Dynamite |
+---------+---------+----------+-------+---------+-----------+
| Heading | 8-9 | 5-6 | 3.6 | 29.-32 | 18-22 |
+---------+---------+----------+-------+---------+-----------+
| Bench | 2 | 1.4-1.6 | 15.4 | 30.-31 | 21.5-24.6 |
+---------+---------+----------+-------+---------+-----------+
| Total | 19 | 59.63 | 39.5-46.6 |
+------------------------------+-------+---------+-----------+
| Per cubic yard, whole tunnel section | 3 to 33 | 2.1-2.5 |
+--------------------------------------+---------+-----------+
+---------------------------------------------------+------------+
| | Number |
| Blasting Notes: | of Sticks |
| +------------+
| Heading: First Round: 6 sticks, 60% in each cut | |
| hole, cut generally blasted twice | 36 to 72 |
| Second Round: 3 side holes each side, | |
| 5 sticks, 40% ea. | 30 |
| Third Round: Rest of side holes and dry | |
| holes, 5 sticks, 40% each | 40 |
| Stub holes, say | 5 to 15 |
| +------------+
| Total Sticks | 111 to 157 |
| +------------+
| Total Pounds | 93 to 131 |
| +------------+
| Sub-bench: 4 widening holes; 2 to 3 sticks, | |
| each, 40% | 10 to 12 |
| 6 down holes; 5 to 7 sticks, each, 40% | 30 to 42 |
| Bench: 6 holes; 6 to 8 sticks each, 40% | 36 to 48 |
| Taking up bottom, average, say | 15 |
| +------------+
| Total Sticks | 91 to 117 |
| +------------+
| Total Pounds | 76 to 97 |
+---------------------------------------------------+------------+
[Illustration: Fig. 2.
CROSS-SECTION, LONGITUDINAL SECTION, PLAN]
Drilling Method, No. 2: Five drills in heading, mounted on three
columns; the holes marked with a cross (X) were drilled with the
drills on the center column.
+--------------------------------------------------+
| Per Round |
+---------+---------+---------+----------+---------+
| | Total | No. of | Pounds | |
| | Depth | Cubic | of | Advance |
| | Drilled | Yards | Dynamite | |
+---------+---------+---------+----------+---------+
| Heading | 190-220 | 35-42 | 134-196 | 6½-8 |
+---------+---------+---------+----------+---------+
| Bench | 110-130 | 55 | 79-106 | 4 |
+---------+---------+---------+----------+---------+
+---------+--------------------+-----------------------------+
| | Per Cubic Yard | Per linear Foot of Tunnel |
| +---------+----------+-------+---------+-----------+
| | Linear | Pounds | Cubic | No. of | Pounds |
| | Feet | of | Yards | Feet | of |
| | Drilled | Dynamite | | Drilled | Dynamite |
+---------+---------+----------+-------+---------+-----------+
| Heading | 5.4-6.0 | 3.9-5.0 | 5.3 |28 to 32.| 20.7-26.5 |
+---------+---------+----------+-------+---------+-----------+
| Bench | 2.-2.4 | 1.4-2.0 | 13.7 | 27.-33. | 19.2-27.4 |
+---------+---------+----------+-------+---------+-----------+
| Total | 19 | 55.-65. | 39.9-53.9 |
+------------------------------+-------+---------+-----------+
| Per cubic yard, whole tunnel section | 2.9-3.4 | 2.1-2.8 |
+--------------------------------------+---------+-----------+
+---------------------------------------------------+------------+
| | Number |
| Blasting Notes: | of Sticks |
| +------------+
| Heading: First Round; 2 to 3 relieving holes | |
| sprung with 4 to 5 sticks each | 8 to 15 |
| 8 cut holes, 7 sticks each | |
| (sometimes shot twice) | 56 to 112 |
| First side round, 6 holes, 6 sticks | |
| each | 36 |
| Widening and dry holes, 10 to 12, | |
| 6 sticks each | 60 to 72 |
| +------------+
| Total Sticks | 160 to 235 |
| +------------+
| Total Pounds | 134 to 196 |
| ----------------+------------+
| Sub-bench: 8 holes, 4 to 6 sticks, each | 32 to 48 |
| | |
| Bench: 8 holes, 6 to 8 sticks, each | 46 to 64 |
| Taking up bottom, average | 15 |
| +------------+
| Total Sticks | 95 to 127 |
| +------------+
| Total Pounds | 79 to 109 |
+---------------------------------------------------+------------+
[Illustration: Fig. 3.
CROSS-SECTION, LONGITUDINAL SECTION, PLAN]
Drilling Method No. 3: Heading same as second method, but larger lift
taken off bench, and lift holes drilled in bottom bench in order to
get down to grade in floor. Bench kept closer to heading.
+---------------------------------------------------------+
| Per Round |
+---------+------------+-----------+------------+---------+
| | Total | No. of | Pounds | |
| | Depth | Cubic | of | Advance |
| | Drilled | Yards | Dynamite | |
+---------+------------+-----------+------------+---------+
| Heading | 190 to 220 | 35 to 42 | 134 to 196 | 6½ to 8 |
+---------+------------+-----------+------------+---------+
| | | | | |
| Bench | 145 " 190 | 90 to 110 | 118 " 167 | 6½ " 8 |
+---------+------------+-----------+------------+---------+
+-----+-------------------------+----------------------------------+
| | Per Cubic Yard | Per linear Foot of Tunnel |
| +------------+------------+-------+-----------+--------------+
| | Linear | Pounds | Cubic | No. of | Pounds |
| | Feet | of | Yards | Feet | of |
| | Drilled | Dynamite | | Drilled | Dynamite |
+-----+------------+------------+-------+-----------+--------------+
| Hd. | 5.4 to 6.0 | 3.9 to 5.0 | 5.3 | 28 to 32 | 20.7 to 26.5 |
+-----+------------+------------+-------+-----------+--------------+
| B. | 1.6 " 1.9 | 1.3 " 1.8 | 13.7 | 22 " 36 | 17.8 " 24.7 |
+-----+------------+------------+-------+-----------+--------------+
| Total | 19 | 50 " 58 | 38.5 " 51.2 |
+-------------------------------+-------+-----------+--------------+
| Per cubic yard, whole tunnel section | 2.6 " 3.1 | 2.0 " 2.6 |
+---------------------------------------+-----------+--------------+
+---------------------------------------------------+------------+
| | Number |
| Blasting Notes: | of Sticks |
| +------------+
| Heading: First Round: 2 to 3 relieving holes | |
| sprung, with 4 to 5 sticks each | 8 to 15 |
| 8 cut holes, 7 sticks each | |
| (sometimes shot twice) | 56 to 112 |
| First side round, 6 holes, 6 sticks | |
| each | 36 |
| Widening and dry holes, 10 to 12 holes, | |
| 6 sticks each | 60 to 72 |
+---------------------------------------------------+------------+
| Total Sticks | 160 to 235 |
+---------------------------------------------------+------------+
| Total Pounds | 134 to 196 |
+---------------------------------------------------+------------+
| | |
| Sub-bench: 4 widening holes, 4 to 5 sticks each, | |
| 2 rounds | 32 to 40 |
| 6 down holes, 5 to 7 sticks each, | |
| 2 rounds | 60 to 84 |
| | |
| Bench: 4 down holes, 5 to 7 sticks each | 20 to 28 |
| 6 to 8 lift holes, 5 to 6 sticks each | 30 to 48 |
+---------------------------------------------------+------------+
| Total Sticks | 142 to 200 |
+---------------------------------------------------+------------+
| Total Pounds | 118 to 167 |
+---------------------------------------------------+------------+
[Illustration: Fig. 4.
CROSS-SECTION, LONGITUDINAL SECTION, PLAN]
Drilling Method No. 4: 8 drills on 4 columns used in heading. Bench
taken off in one lift. Bottom taken up with lift holes.
+--------------------------------------------------+
| Per Round |
+---------+---------+---------+----------+---------+
| | Total | No. of | Pounds | |
| | Depth | Cubic | of | Advance |
| | Drilled | Yards | Dynamite | |
+---------+---------+---------+----------+---------+
| Heading | 310-320 | 63-71 | 215-257 | 8-9 |
+---------+---------+---------+----------+---------+
| Bench | 190-210 | 89-100 | 107-155 | 8-9 |
+---------+---------+---------+----------+---------+
+---------+--------------------+------------------------------+
| | Per Cubic Yard | Per linear Foot of Tunnel |
| +---------+----------+-------+----------+-----------+
| | Linear | Pounds | Cubic | No. of | Pounds |
| | Feet | of | Yards | Feet | of |
| | Drilled | Dynamite | | Drilled | Dynamite |
+---------+---------+----------+-------+----------+-----------+
| Heading | 4.5-5.1 | 3.4-5.7 | 7.9 | 35.6-45. | 26.9-45.0 |
+---------+---------+----------+-------+----------+-----------+
| Bench | 1.9-2.2 | 1.2-1.7 | 11.1 | 21.1-24. | 13.3-18.9 |
+---------+---------+----------+-------+----------+-----------+
| Total | 19 | 56.7-69. | 40.2-63.9 |
+------------------------------+-------+----------+-----------+
| Per cubic yard, whole tunnel section | 3.-3.6 | 2.1-3.4 |
+--------------------------------------+----------+-----------+
+---------------------------------------------------+------------+
| | Number |
| Blasting Notes: | of Sticks |
| | |
| All holes of whole round are cleaned and loaded | |
| before blasting is started | |
| | |
| First Round: 5-6 lift holes, 7 to 9 sticks each | 35 to 54 |
| First row, sub-bench, 6 holes, 6 to 8 | |
| sticks each | 36 to 48 |
| | |
| Second Round: Second row, sub-bench and widening | |
| holes, 8 to 10 holes, 6 to 8 sticks each | 48 to 64 |
| Stub holes | 10 to 20 |
| | |
| Bench: Total Sticks | 129 to 186 |
| Total Pounds | 107 to 155 |
| | |
| Third Round: 8 cut holes, 7 sticks each, often | |
| requires 3 to 4 charges | 112 to 224 |
| Fourth Round: 8 holes, First side round, 5 to 7 | |
| sticks each | 40 to 56 |
| Fifth Round: 8 holes, Second side round, 5 to 7 | |
| sticks each | 40 to 56 |
| 2 dry holes 5 to 7 sticks each | 10 to 14 |
| Sixth Round: 4 to 6 widening holes and dry holes, | |
| 6 sticks each | 36 to 48 |
| Stub holes | 20 to 30 |
| | |
| Heading: Total Sticks | 258 to 428 |
| Total Pounds | 215 to 357 |
+---------------------------------------------------+------------+
The average quantity of powder used on the whole work was about 2.9 lb.
per cu. yd. The tables on the diagrams, Figs. 1, 2, 3, and 4, show that
the quantity actually used in making the advance at the main working
faces was about 2.5 lb. The difference is accounted for by the larger
percentage of powder used for trimming the sides, breaking out the
cross-passages between the tunnels, and the excavation of the ditches,
the latter operation not being done until the concrete lining was about
to be put in.
There was some time, too, during the earlier stages of the work, when it
is believed that an excessive quantity of powder was used; for one or
two months it ran up to 4 lb. per cu. yd.
[Illustration: Fig. 5.
MUCK CAR USED AT WEEHAWKEN SHAFT]
The dynamite used was "Forcite." At first, both 40% and 60% were used,
the 60% generally only for blasting the cut in the headings; during the
latter part of the work, however, the 60% was used exclusively.
The rock as a rule broke very well, and only a comparatively small
quantity could not be handled by the shovels without being broken up
further by block-holing. In the sandstone the quantity of powder per
cubic yard was much more than for any of the trap.
In drilling the Central Shaft, a 6-hole cut was made approximately on
the center line, east and west, the enlargement requiring about 18 more
holes, which were generally about 6 ft. deep, the average advance being
about 4 ft. per day of 24 hours.
[Illustration: Fig. 6.
PROGRESS PROFILES OF NORTH AND SOUTH TUNNELS SHOWING MONTHLY
EXCAVATION]
The drills were run by steam until a depth of about 150 ft. had been
reached, air from the plant at Hackensack being available after that
time. Four drills were used most of the time, and six later when air was
available. This work was done entirely by the John Shields Construction
Company, and a depth of 205 ft. was sunk in 6 months (from July 15th,
1905, to January 15th, 1906). A derrick was used for hoisting and
lowering men and tools during the sinking, elevators being put in later.
[Illustration:
PLATE XXIII.]
_Drilling Data._--During the progress of the work, both general and
detailed observations were made of the drilling, the results of which
are shown in the tables. Table 1 has been compiled from the records as
platted daily on the chart from the inspectors' reports, as shown by
Plate XXIII, and described on page 113. Table 2 contains some data
relating to the drilling in the headings.
The general results of these observations show that the average time the
drills were "actually working" was 5.2 hours per shift, and that they
were actually "hitting the rock" about half of this time, or about 2.5
hours per shift. The average depth drilled per hour, during the time the
drills were "actually working," was 2.66 ft.
The "actual working time," as noted above, covers the period from the
time the drills were first set up in the heading after blasting until
they were taken down for the next blast; it does not include the time
occupied in setting up or taking down, which would probably average 30
min. more per shift. It is believed that this figure will also apply
very closely to drills working on the bench, though no actual
observations were taken to determine this, on account of the
irregularity with which they were worked.
The actual working time of the drills in the 736 shifts (7,360 hours)
covered by Table 1, was 3,826 hours, or 5.2 hours per shift. The average
depth drilled per yard, as shown in the last column of Table 1, agrees
fairly well with the figures on the diagrams, Figs. 1, 2, 3, and 4.
Table 2 has been compiled from detailed timed observations of individual
drilling of down holes in the bench, for periods of 7 or 8 hours each,
in January, 1907. The work at that time was in fairly normal condition
at all points.
The figures in the third column of Table 2 include the time required for
moving from one hole to another, when this occurred during the
observation, the time required for changing bits, oiling drills, etc.,
and all delays of all kinds. A close record of the delays was kept, and
it was considered that, of the 93 hours, 48 min., in Table 2, the
unnecessary delays amounted to 5 hours, 7 min., or about 5½ per cent.