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between the construction of a rock-and-mud tunnel, on which he was
engaged 32 years ago, and this up-to-date subway. This is exemplified
by the difference between a small tallow candle and a large electric
searchlight.

The success of the blasting operations on this work might also be
contrasted with a case in N"ew York Harbor some 15 years ago, when
a coffer-dam was built as far out as it was considered economical, and
the rock excavated inside the coffer-dam in the dry. From the finished
excavated space (inside the coffer-dam), some fifty holes were drilled
into the rock to be removed, outside the coffer-dam lines.

To ensure success the contractor secured a man from the explosives
manufacturer to load the holes and connect the exploders in such a
manner as to make a complete circuit. As the current would have to
travel through this circuit, it seemed impossible to have a hitch; but
only three or four exploders (at different parts of the circuit of fifty)
were set off at the first attempt. The wires were then connected again,
and a number, but not all, of the remaining charges went off. This
naturally put the rest of the work in a very uncertain condition.

In order to ascertain the cause of the failure, the speaker took
twenty-five exploders from the same stock, connected them up in the
same manner as before, but without the dynamite, threw them into the
water, turned on the current, and only four (at intervals) exploded.
The remaining twenty-one were again connected up, and all went off
together, showing that some of them were just a trifle quicker than
the others.

* New York City.



DISCUSSION ON SUBWAY TUNNEL AVORK Q^d

The author speaks of the considerable quantity of extra rock exca- Mr.
vation which the contractor was obliged to do without compensation, ^^*''"***^-
This is not equitable, but it seems to be difficult to draw up a contract
which will allow for these changes ; and, on public work, very few engi-
neers dare to be just, against the terms of the contract and specifica-
tions, as they would be accused of graft, etc.

This refusal of the authorities to give the engineers a chance to use
their own judgment, however, is costing the country millions of dollars,
as the contractors have to bid higher than they would otherwise. As
an example, an engineer who, in his time, had a reputation of the very
highest, and did an enormous amount of work, got the reputation of
refusing to allow any change in his plans and insisting on refinements
which did not improve the job. The result was that his clients paid
far more for what they got than they would have done otherwise. For
instance, the manager of a large manufacturing concern once told the
speaker that he always figured work for that engineer just as he would
for anybody else and added 25% to the price to cover such arbitrariness.

John H. Myers,* Assoc. M. Am. Soc. C. E. — The speaker thinks Mr.
that engineering literature contains many references to tunnels which ^y®*"®'
approximate the size of the two-track sections described in this paper,
and that he is correct when he says that there are few, if any, such
references to tunnels similar to the four-track sections. He refers
especially to the tunnel of varying cross-section by which is accom-
plished the transition from double-deck (or two tracks over two) to
four tracks abreast between 98th and 102d Streets. He thinks, there-
fore, that Mr. Werbin has done a valuable service in compiling this
accurate and complete record of this work.

There is one theme that runs all through the paper which should
be emphasized, and that is the character of the design which per-
mitted what may be termed sectional construction. By this is meant
the design which allowed the tunnel lining to follow quickly after
the excavation and enabled the constructor to excavate and line part
of the tunnel before excavating and lining the remainder. The methods
of construction which such a design made possible are illustrated in.
Figs. 1, 15, 16, 17, IS, 25, and 29, and Plate VII. It is the speaker's
opinion that this design aided materially in the successful completion
of this very unusual work through a portion of the city which pre-,
sented many natural difficulties.

The fact that a structure of the magnitude of that described has
been built through the heart of the city without material interruption
of its mode of living, or of its varied industries, is in itself worthy
of note, and the record of the methods used in bringing this about,
which the paper provides, is, the speaker believes, of permanent
technical and historical value.

• New York City.



Moulton



400 DISCUSSION ON SUBWAY TUNNEL WORK

Mr. n. G. Moulton,* Esq. — The tiinnels on Lexington Avenue, especially

the double-deck sections, approximately 40 ft. high and 40 ft. wide, are
quite unusual in size, from a civil engineering standpoint; yet they are
small in comparison with vmderground excavations required in mining
operations. The work on these sections presents problems more nearly
comparable to metal mining than those encountered anywhere else on
subway construction in New York City.

In mining large bodies of ore, it is often necessary to carry a stope,
for a width in excess of 100 ft., to a height of 100 ft. (the usual distance
between levels), and for a length of several hundred feet on the strike
of the vein, and to carry the walls, roof, and face of the excavation on
timbers. As the work on Lexington Avenue is so similar in type, though
on a much smaller scale, it is interesting to consider the possibility of
adapting to work of this kind the methods used in extracting irregular
ore bodies, the dimensions of which are too large to permit the walls
and roof to stand for any length of time without support.

There are two marked differences between the excavation methods
described in this paper and those which would have been used under
metal-mining practice. The first difference is in the method of attack —
the use of the top heading instead of a bottom heading, as in mining
operations. The second is in the method of timbering the opening —
I-beams, segmental sets, etc.

In the very early days of western mining development, it was com-
mon practice to remove ore bodies in very much the same manner as
that described in this paper. The method is known as underhand stop-
ing, and consists of carrying the upper level of the excavation in ad-
vance of the lower, and following in successive levels with the exca-
vation of the bench. As mining methods developed, it was realized that
this was the least economical way of attack, and it was abandoned in
favor of overhand stoping or, in other words, bottom heading work, in
which the lowest part of the excavation is carried well in advance and
the back is taken down in successive slices, stepping back consecutively
toward the roof. This method has the advantage of less shoveling and
also of cheaper breaking, as the rock is broken from an under-cut hori-
zontal face at less cost than from any other position, and also because
the drilling can be done with the most efficient types of drills — the so-
called stope hammers or buzzers.

In order to hold the walls and roof, and keep the men close to the
working face, timbering is placed in sections as the work progresses.
The usual dimensions of the timber sets common in metal-mining prac-
tice are 7 ft. high, 5 ft. wide between posts, and 5 ft. longitudinally
between bents or adjacent rows of posts. The posts, caps, and girts are
all framed to exact lengths and provided with horns in daps, so that
connection is secured for each member of the set in every direction.

• New York City.



DISCUSSION ON SUBWAY TUNNEL WORK 401

The rock is broken out in small sections, say, 5 ft. square and 7 ft. high, Mr.
and another set of timbering is placed. On all slices above the bottom
heading, the new post rests directly on top of one of the posts set in the
bottom heading, and the cap and girt join with the set at the side and
to the rear. In this way the entire mass of rock is removed in a series
of inverted steps in such a manner that not more than 5 ft. in either
direction is without timber support at any time. The probability of an
extensive rock fall is eliminated, and it is possible to work exactly to
the pay line of the excavation.

To apply square-set stoping to such an operation as that described
in this paper, the first step would be to sink a shaft at one side of the
excavation and drive a bottom heading in each direction in the lower
corner of the section nearest to the shaft. This bottom heading would
be continued through to the next shaft and timbered continuously with
the foundation or sill floor timbers of the square-set system. At
different places along the heading, the stopes would be started, widened
to the opposite side of the section, carried up to the roof, and then con-
tinued in both directions until several hundred feet had been opened
and timbered. After this, a start would be made on steel erection, the
timber sets being pulled out row after row, after alternate bays of steel
had been erected and concreted between them. Where no heavy pres-
sures had been developed, the sets would be removed quite readily after
knocking out the side and top blocking and wedging. Where heavy
pressures had developed from loose ground, it would be necessary to
do considerable work with axe and saw on the bottom posts, and ruin
part of the timber. Yet, even under the worst circumstances, it is pos-
sible that 75% of the timbers could be recovered and used over and
over again as the work progressed.

As to the cost of this method, ore is mined on an extensive scale in
various districts at costs ranging from $1.50 to $2.50 per ton by square-
set stoping at reasonable depths, exclusive of any items corresponding
to cost of plant or contractors' profits. This would be equivalent to
about $3 to $5 per yd. The timber required would be approximately,
30 ft. b. m. per cu. yd. of material taken out, and, with timber at $60
per ft. b. m. in place, the cost would amount to $1.80 per cu. yd. Con-
sidering that most of the timber would be used over and over again,
and also that round timbers would be the most advantageous to use,
and much less expensive than sawed stuff, it is probable that $1 per cu.
yd. would represent the maximum cost for timbering. Against this
additional cost there would be certain credits, as compared with carry-
ing the excavation open and with no timbering at all. One such credit
would be in cheaper breaking and mucking costs, due to shooting the
rock down from an under-cut face and catching it on heaving blocking
of poles resting on the caps of the timber sets. It could then be shoveled



403 DISCUSSION ox SUBWAY TUXXEL WORK

Jir. into chutes and drawn into cars in the main bottom heading. Another
saving in cost would be realized by working exactly to the pay line, and
saving the excess cost of hauling out irregular masses of material,
broken away outside of the section, and afterward concreting and back-
filling these spaces. In blocky ground, such as was often encountered
on Lexington Avenue, it is probable that the credits from the items
mentioned would more than offset the additional cost of $1 per cu. yd.
for timbering, so that this method might well be cheaper than the ones
used.

As to the question of safety, there can be little doubt that full
square-set timbering would have the advantage. The great principle
involved in keeping rock excavation safe is to have the smallest pos-
sible quantity of ground standing without support. On Lexington
Avenue, this was done by building the finished structure in sections;
but the same thing could have been accomplished, and even greater
protection secured, by putting in the timber in small sections and hav-
ing only the extent of one set open at any one time. Then, the steel
structure could be built in its entirety between the timbers at any dis-
tance desired behind the face.

As to speed, all the advantages are with the bottom-heading method
of attack, as it is possible to open up as many faces as desired dependent
only on the hoisting capacity of the shafts.

It is well recognized that even the worst ground may be held with
comparative ease before any movement starts, but if once it starts, it
may demolish any timbers that can be put in. Close timbering, to pre-
vent the initial slip, may mean all the difference between absolute safety
and serious collapse. The wedging and arching action of blocks of rock
in unsound ground, together with the frequent resistance on seams, and
probably to a certain extent atmospheric pressure, renders valuable as-
sistance to the timbers in holding the ground, and the timber system
should provide for this by preventing the initial movement which may
destroy all these valuable allies.

As to the timbering methods used in unsound ground on Lexington
Avenue, and described in this paper, the speaker is of the opinion that
although they served their purpose in giving protection while the work
was being carried on, nevertheless, they were unnecessarily expensive
and inefficient, as compared with the usual mine-timbering methods.
The segmental set, for example, which is used so widely in large tunnel
headings, represents the most inefficient and uneconomical manner in
which timber can be used. It usually comes down at the joints long
before the timber can develop its strength as an arch, and wherever
heavy ground is encountered in the roof, the segmental sets must be
reinforced with longitudinal horseheads under the top cap and radial
posts at the joints. About all that segmental sets will accomplish is the



DISCUSSION ON SUBWAY TUXNEL WORK 403

prevention of spalling and light rock falls from the roof. Where really Mr.
heavy ground is encountered, as in extracting ore by the caving system, ^^



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