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in advance of the remainder of the section, which is the
bench. The drills working in the heading are mounted
upon columns, two drills on each column. The drills work-
ing on the bench are mounted upon tripods. The air pipe
is carried to within about 50 feet of the bench, where a bench
hose of equal diameter is attached to the air pipe, lead-
ing directly to the bench. At the end of the hose is a metal
nozzle called a manifold, containing hose connections for
each of the drills.

A section of heading showing arrangement of drill holes
in the face is given in Fig. 435. The two middle rows of
holes A B and CD converge at an angle of about 20,
nearly meeting on the center line E F of the tunnel, and are
called the center cut holes. The mass of rock included
by these holes is wedge-shaped and shown in plan at A in
Fig. 43G. The removing of this wedge by blasting is called
breaking the cut. Fig. 437 shows a longitudinal section
through the center cut holes. The rows of holes G, H, K,



and L, Fig. 435, on each side of the center cut holes, are
called side rounds. If but one row on each side, they are


FIG. 438.

called single side rounds ; if two rows, double side
rounds. A longitudinal section through the side holes is

FIG. 439.

FIG. 440.

FIG. 441.

FIG. 442.

given in Fig. 438. The cut and side rounds are loaded at
the same time. The cut is fired first (see Fig. 439), followed



by the side rounds, which are fired either single, i. e., one
row on each side of the cut (see Figs. 440 and 441), or
double fired, i. e., both rows fired simultaneously, as shown
in Fig. 442.

1512. Enlarging the Heading. In that portion of
the heading shown in the preceding figures, the holes are
drilled directly into the face of the heading. After the
holes are fired and the material removed, side holes are

FIG. 444.

FIG. 445.

drilled at an angle of about 60 with the center line denoted
by the letters C. L., as shown in section in Fig. 443 and
plan in Fig. 444.

1513- Removing the Bench. The bench is taken
out in two sections, B and B' , as shown in section in Fig.
445. The full tunnel section is shown by dotted lines.

The holes in the bench are inclined backward from a ver-
tical line. A longitudinal section through the center line,
showing the usual mode of drilling headings and benches, is
given in Fig. 446. The center cut holes in the heading H
and all the bench holes at B and B' are usually fired



together, followed by double side rounds in the heading.
The center cut offers the greatest resistance to blasting.
The holes are consequently loaded with more powerful
explosives than are used for either side rounds or bench.

In driving the New York aqueduct tunnel, the cut was
loaded with dynamite containing from 60 to 80 per cent, of
nitro-glycerine, while the average bench powder contained
but 40 per cent, of nitro-glycerine. On some sections,
where rock of special hardness was encountered, the cut
was loaded with pure nitro-glycerine. This operation is

always attended with great danger. After several prema-
ture explosions, resulting in considerable loss of life, the use
of pure nitro-glycerine was abandoned. The effect of firing
the cut is generally to pulverize the rock, and all tunnel
blasting is intended to so break the rock as to render the use
of the sledge-hammer unnecessary in reducing masses of
rock to sizes convenient for loading.

The execution of the powder depends largely upon the
judgment used in locating the holes and the angle at which
they are bored. The position of the machine while drilling
holes at foot of the bench is shown at C, Fig. 446.


1514. Drainage. The grade of a tunnel must be
established with reference to securing complete drainage.
When the grade at both portals is the same, the grade of the
tunnel is made to ascend from both ends, the grades being
united by a flat vertical curve. The grade of the St. Goth-
ard tunnel is 0.1 per cent. ; that of Mt. Cenis 0.05 per cent.,
but these grades are very light. A grade of 0.25 per cent,
will insure complete drainage and need not be exceeded. If
the tunnel is short, a continuous grade may be employed.
In such a case, if the tunnel is driven from both ends, it will
be necessary to remove the water from the descending por-
tion by pumping. In tunnels of considerable length, the
grade is usually made to ascend from both ends. This pro-
vides complete drainage during construction and also i educes
the cost of removing the excavated material, as the loaded
cars will either run of themselves or with small draft.
When shafts are sunk, the water is removed from the tunnel
by pumping.

1515. Shafting. When the tunnel is of considerable
length, and dispatch in driving is of great importance, ad-
ditional working faces are obtained by sinking one or more
shafts, each shaft affording two additional working faces.
Shafting adds very considerably to the cost of tunnel dri-
ving; for, besides the cost of sinking the shaft, there is the
constant expense of hoisting the excavated material to the
surface, to which must be added the expense of pumping.
On the New York aqueduct tunnel, which has a total length
of about 33 miles, a shaft was sunk at each interval of one
mile, the shafts varying in depth from 80 to 380 feet. Where
shafts are employed, the greatest care and skill are neces-
sary in transferring the alinement from the surface of the
ground to the tunnel at the foot of the shaft.

1516. Shaft Lining. When the shaft is sunk through
solid rock, the walls are self sustaining, and no timber lining is
required except a curb at the top of the shaft. If, however,
the material is earth, loose rock, or shale, the shaft must be
timbered. The timbers are put in place as the shaft is sunk.



Frames (sets, they are commonly called) of timber, shown
at A in Fig. 447, are placed about 4 feet apart, and behind
these frames, lagging B, of either sawed timber or half round
poles split from young trees, is placed on end and in close
contact. As each frame is placed in position it is supported
by struts footing on the bottom of the shaft, or if the walls
are sufficiently firm, the frames are held in place by wedges,

until another set is required, when timber struts C, mortised
into the frames, form the permanent support. These struts
are placed one above the other, .and, together with the
frames into which they are mortised, form continuous "tim-
ber columns extending from the bottom to the top of the
shaft. With each set of timbers a horizontal timber Z>,
called a bunion, is placed with ends abutting against the


vertical timber E. A beveled seat with a square shoulder
is cut on the vertical timber for each bunton. The buntons
are held in place by wedges shown at F and F '. These
wedges are forced between the bunton and the shoulder of
the beveled seat. As the wedges are tightened, the bunton
is forced downwards until it is perfectly rigid. Vertical tim-
bers G, G ' are spiked to the buntons and to the ends of the
frames, to serve as guides for the carriage. A detail of the
splice of the carriage guide is shown at H and of the wedges
at K. As all shafts are moist, and many decidedly wet,
iron should only be used in timbering when no substitute
can be found. The pressure of earth or loose rock against
the timbers is usually sufficient to hold them in place, and
most of the joints do not require keying. Treenails
(wooden pins) should be used in place of iron. The lagging
must be put in place as fast as the work progresses, and all
spaces behind it filled with well-rammed earth. In very
wet ground or in quicksand, special devices are employed
to meet the needs of the situation. Shaft sinking in bad
ground is exceedingly dangerous work, and every known
precaution is essential if loss of life would be avoided.

1517. Removing Excavated Material. The ma-
terial excavated in tunnel driving is called muck. The
muck is loaded into dump cars at the foot of the bench,
which, when there is a sufficient descending grade, run by
gravity either to the foot of the shaft, where they are hoisted
to the surface, or to the dump outside the tunnel. When
there is not sufficient grade to run the cars of themselves,
mules are employed to haul them.

A single track A, Fig. 448 (ordinarily of 3 feet gauge), is
laid on the center line of the tunnel, with passing branches
at suitable intervals. At a distance of about 100 feet from
the bench a simple switch is built, and two tracks C and D
laid to the bench, which permit the loading of two cars at
the same time, and provide for shifting cars. On a level
with the top of the bench, and directly over the cars, a scaf-
fold E is erected, and upon it a runway of planks /MS laid,



extending from the scaffold to the heading. The heading
muck is loaded into barrows and wheeled on this runway to
the .scarf old, and emptied directly into the cars.

A simple and very effective bench scaffold is made of
wrought iron pipe supports and shown in Fig. 448. Each
support consists of two pieces of pipe, one telescoping within

FIG. 448.

the other, and provided with clamps by means of which
they are adjusted to any 'desired length. The plank is laid
directly upon these pipe supports. The air for working the
drills is carried from the air pipe G to the bench by means
of the bench hose //. The manifold K attached to the e*nd
of the bench hose contains hose connections for all the drills.
A ditch L on the opposite side of tunnel from the air pipe
drains the tunnel.



As the work advances from the tunnel entrance, or from
the foot of the shaft, more time is required to haul away
the loaded cars and bring back the
empty ones. When one mule is no
longer able to perform the work a
second one is added, and passing
branches are built in the main track
at suitable intervals, where returning
empty cars are switched while the
loaded cars are passing outward. A
passing branch contains two switches,
which are made self-acting by the
following simple device, shown in
Fig. 440. The points of the switch
rails a and b are connected by a
clamp rod, attached to a spring c d,
which is constantly acting, and holds
the point a close against the main
rail ef, and the switch is constantly
set for the passing branch k I. The
switch points m and n of the second
switch are kept in place by the spring
o /, the switch being always set for
the main track q r. An outgoing
car running in the direction r q finds
the switch Y set for the main track.
Upon reaching the switch X, the
flange of the right-hand wheel, pass-
ing between the rail e f and the
switch point a, forces the switch
point b against the rail s f, and the
car passes the switch in safety. A
returning empty car finds the switch
X set for the passing branch k /, and in passing from the
branch to the main track, the flange of the head wheel, in
passing between the rail u t and the switch point m, forces
the switch point n against the main rail e f, and the car
passes safely on to the main track. The springs c </and op


are elastic young saplings, kept in place by strong staples
driven into the switch ties.

1518. Care of Track. A common fault of contract-
ors and their employes is neglect of track. Usually poor
material is furnished, worn cut and crooked rails, poor
fastenings, poor ties, and often no ties, requiring every
sort of makeshift. With such material a good track is im-
possible, and requires constant tinkering. Derailments are
continually occurring, involving costly delays. Tunnel
tracks should be built of good material and in a thorough
manner. Short rails of varying lengths are required in
keeping the track well up to the bench. With proper care
of tracks, the cars may be kept within easy shoveling dis-
tance of the bench. The foreman in charge of the muckers
should keep a small stock of ties and rails constantly on
hand, together with the necessary track tools, and do his
own track work.

1519. Keeping Down to Grade. The invariable
tendency in tunnel work is to keep above grade. The prin-
cipal cause is the unconscious effort to avoid the water which
is constantly accumulating. This tendency can only be
avoided by establishing grade stakes at every 25 feet and
keeping the excavation on true lines. The holes drilled at
the foot of the bench should penetrate a foot below grade,
which will insure the removal of the entire section. Much
of the muck in the bottom of the tunnel will require severe
use of the pick to remove it. Wedges and a heavy sledge
are often of great service in this work.

15 2O. Timbering. When the material is rotten rock
or earth, the tunnel must be timbered. The timbered sec-
tion should be enough larger than the standard section to
admit of a masonry lining. When the material is such that
the side walls will stand of themselves for a time, hitches
A and B are excavated near the springing line and sets of
timbers placed as shown in Fig. 450. Iron clamps shown at
C and D hold the timbers together while the lagging is



being placed. It is laid over the timbers lengthwise of the
tunnel and as close together as possible. The spaces G, H

[FIG. 450.

between the lagging and the roof are filled with dry rubble
or cordwood.

Roof timbers should be either 12 in. X 12 in. or 12 in.
X 14 in. in cross-section, and placed with 2 to 4 feet clear
space between each set. Where the ground is very soft
with a tendency to expand, larger timbers may be necessary.
Hemlock, yellow pine, or spruce is commonly used. In
special cases, where great pressure is to be resisted, oak is
used. When the side walls will not support the roof tim-
bers, they are carried on supports arranged as shown in
Fig. 451. Four posts A, C, D and B, resting on sills O, P, Q
and R, are mortised into the cap E F.

The roof timbers G, H, K are clamped together as in Fig.


450, and mortised into the cap at spring line. The pressure
against the roof timbers is relieved by the struts L, M, N, U,
and F, which transfer the stress to the posts C and D and
the cap E F. The dimensions of timber given in the draw-
ing are such as are used where the pressure is great ; they
will meet the requirements of most situations. The lagging
may be either sawed timber or split poles, obtained by split-

FlG. 451.

ting in half straight grained chestnut or oak saplings. The
backing may be either dry rubble or cordwood. The latter
is preferable, as it is light, portable, and uniform in shape.
The side walls are all of well-scabbled rubble of good-sized
stones, even beds, and laid in courses with cement mortar.
The impost courses 5 and T should be of well-cut stone,
twelve inches in thickness and of full width of wall. The
arch is either brick or rubble. The caps and roof struts



interfere somewhat with arching. Holes are left in the
masonry where these timbers interfere until a section of the
arch is complete, when they are removed and the gaps filled
with masonry, the joints being thoroughly grouted. All
other timbers are left in place. The spaces A", Z, etc., Fig.
450, between the arch and roof timbers, are usually filled
with concrete.

When the material through which the tunnel passes is
very soft, with slight coherence, all the energy and skill
of engineer and workmen are required to make headway.
It is considered the better practice to drive the heading at
the bottom of the tunnel instead of the top, as by the time

FIG. 452.

the heading is driven the ground composing the remainder
of the section will have become thoroughly drained, and may
be taken out with much greater safety and less expense
than with a top heading. The mode of driving a heading
through such material is illustrated in Fig. 452, in which
A represents a cross-section and B a longitudinal section of
the heading, with complete system of timbering.

A full section of timbers is called a set, of which the up-
right timber C is called the leg; the horizontal timber D the
sill, and E the cap or collar. The short boards F, F, which
extend from collar to collar, and are in direct contact with
the sustained material, are called poling boards. They are
sharpened to a cutting edge, and are driven into the face of
the heading with sledges, a wedge-shaped block G being


placed above them to keep them at a proper angle. The
planks H which protect the sides of the heading are termed
lagging. The flooring K serves to exclude the liquid mud,
which would otherwise be forced from underneath by the
external pressure. The horizontal cross timber L, as well
as the longitudinal timbers M and A 7 , are called struts. The
floor O serves as a footing for the workmen while driving
the poling boards.

If the material penetrated is wet enough to run, it is
necessary to constantly maintain a bulkhead of planks
P, called face boards, which is held in place by struts Q. As
the poling boards are driven forward, the top face board is
removed, allowing the released material to flow into the
gangway. This forms a cavity in the face of the heading,
and immediately another bulkhead is started by placing a
face board R in advance of those at /-*, with a strut S to
keep it in place. When the heading is advanced half the
length of the poling boards, a new set of timbers is put in
place, the collar of which takes the strain from the poling
boards, which would otherwise be soon broken by the great
pressure above them.

As the section is enlarged, other timbers are substituted,
until the complete section is excavated. The masonry
lining should follow immediately. The less important tim-
bers may be removed as the masonry advances, and their
stresses transferred to it ; but the main supports should re-
main in place, and the masonry be built around them, and
not disturbed until the arch is keyed. They can then be
removed with safety, and the vacancies in the masonry
carefully filled and grouted. All open spaces between the
masonry and the timber should be filled with well-rammed

1 521 . Centering. Tunnel centers are built on much
the same plan as those used in arched culverts, a fu41
description of which was given in Arts. 1475 and 1476.

1522. Portals. Tunnel portals correspond to the
face and wing walls of an arched culvert. Usually some


regard is paid to architectural effect, the walls being of
dressed stone laid in courses.

1523. Alinement and Levels. During construc-
tion, the alinement and levels must be frequently tested.
At least once a week the heading should be carefully
centered, and a grade stake set at the foot of the bench.

In running the center line, plummet lamps are used in-
stead of the transit poles used on surface lines. A plummet
lamp consists of an oil reservoir of brass of the shape of an
ordinary plumb-bob, the stem of which contains the wick.
The lamp is suspended by a bail, at the crown of which is
an eye for the cord which suspends the lamp. When sus-
pended, the cord, the flame of the lamp, and the point of the
plumb-bob are in the same vertical line. A man holds the
cord against the roof of the heading, moving it right or
left until the intersection of the cross wires coincides with
the flame of the lamp. A point is then marked on the roof,
and a hole accurately drilled by hand to a depth of about
three or four inches. A plug of dry pine is then driven
into the hole, projecting two inches below the roof. The
plug is carefully centered and a screw-eye securely fastened
in its center, from which a plummet lamp may be suspended.
A piece of copper wire equal in length to the full height of
the tunnel is attached at one end to the screw-eye and the
other end fastened to the wall of the tunnel.

When the full section of the tunnel is excavated, a plumb-
bob is suspended from the wire, just touching the floor of
the tunnel. A hole is drilled at the point, plugged, and
centered, as on a surface line. For bench marks, holes are
drilled in the tunnel wall about two feet above the floor,
and plugs of either wood or iron are firmly driven and
allowed to project far enough from the wall to allow the
rod to be held upon them in a vertical position. In testing
the grade of the roof, the rod is held in an. inverted position,
the foot of the rod being placed against the roof. In this
case the elevation of the roof is obtained by adding the rod-
reading to the height of instrument. For example, suppose



the tunnel section is 24 feet in height, the floor grade at
say Station 160, is 240.5 feet and the height of the instru-
ment, which is standing on the bench, is 259.6 feet, what
should the rod read to give roof grade for Station 160 ? The
floor grade at Station 160 being 240.5 feet, the roof grade
will be 240.5 feet + 24 feet (the height of the tunnel section)
which is 264.5 feet. As the height of instrument is 259.6
feet, the rod-reading for roof grade will be 264.5 259.6 =
4.9 feet. A common bulls-eye lantern is used to illuminate
the cross hairs, and a small headlight reflector affords the
best light for reading the level rod and tape, and for taking

1524. Measuring Excavation. Various methods
are adapted for checking the dimensions of the tunnel sec-

PlG. 453.

tion and measuring up the work. The best device is the
following, shown in Fig. 453. A semi-circular protractor
A B of a diameter from 8 to 10 feet, and made of light pine,


is set up at right angles to the center line of the tunnel.
The diameter A B of the protractor is brought into a hori-
zontal position by means of the spirit level C and placed at
any desired height above the floor of the tunnel. A sliding
rod D E, one end of which is fastened to the center D of
the protractor, measures the distances to the tunnel walls on
radial lines. The angles which these lines make with the
horizontal are read directly from the protractor. The tun-
nel section and the actual working measurements are then
platted on cross-section paper, from which the amount of
excavation is readily calculated.

1525. Plumbing Shafts. When a shaft is sunk to
increase the number of working faces, the process by which
the center line is transferred from the surface of the ground
to the bottom of the shaft is called plumbing the shaft.
Fig. 454 illustrates the process. Two pieces of plank C and
D are spiked to the shaft timbers where the center line
crosses, the edges of the plank projecting over the shaft

Slots E and F are cut in the plank on the center line.
An iron plate with a carefully drilled hole in its center is
placed over each slot with the center hole exactly on the
center line of the tunnel. Holes are drilled in the corners
of the plates for screws, by means of which the plates are
fastened to the plank.

Plumb-bobs weighing from 20 to 30 pounds are suspended
by fine steel wire which passes through the eye-hole in the
plate. On the shaft bottom a pail of oil is placed directly
under each plumb-bob, which is entirely immersed in the
oil to check the vibrations. When the plumb-bobs come to
rest, the lines which suspend them are exactly in the center
line as laid down on the surface of the ground. A transit is
set up at / (the tunnel having been driven some distance
from the foot of the shaft) and moved until both wires are
exactly in the line of sight. A plug is then set on the line
at K, after which the instrument is moved to K and a plug
set at /, thus establishing the line.



1526. Ventilation. In clear weather, the gases
formed by the combustion of the powder used in blasting
pass rapidly from the tunnel to the outer air. In heavy
weather, and especially when the heading is at a consider-
able distance from the portal or shaft, several hours are
required to clear the tunnel of powder smoke. Under such


FIG. 454.

conditions the question of ventilation becomes an important
one. In order that a man may do effective tunnel work, he
should have a supply of 100 cubic feet of pure air per min-
ute. And as there is an average force of 25 men in each


heading, the aggregate supply of pure air per heading should

Online LibraryInternational Correspondence SchoolsThe elements of railroad engineering (Volume 2) → online text (page 24 of 35)