June 14, 1872. THE BUILDING NEWS. 485
NOTES ON EARTHWORK.—VIII.
LOCOMOTIVE engine on rails cannot
draw its due load up steep inclines, and
in many parts of this country a long line of
railway cannot be made without cutting
deeply through hills. When the depth much
exceeds 100ft., a tunnel can generally be made
at a less cost than an open cutting, especially
when the ground is such that it willnot stand
with steep slopes.
Tt is calculated that in the United Kingdom
about 80 miles of railway tunnelling have
been already made, the greatest length of
any one tunnel being alittle more than three
miles. It isnot the length, however, that is
the greatest difficulty to be overcome. Very
long tunnels are generally through hard rock,
to get through which requires certainly
more time, but less ingenuity, and calls for
less risk of life or limb than is necessary in
some other kinds of ground. Considering
the size of the excavation required to be made
in bad ground—nearly 30ft. in height, and
about the same in width—to make such an
excavation through clay or loose ground,
great exertions must be made, and many
precautions taken, to ensure success, and
especially where much water is met with.
Much water, however, is perhaps more often
met with in the rock tunnels.
Foul air, chiefly carbonic acid gas, is met
with in passing through coal-beds and the
contiguous strata, and in old coal hollows,
from which the coal has been removed.
Otherwise, the foul airthat causes the greatest
inconvenience in tunnelling is that produced
by exploding powder in excavating the
tunnel. ‘The shafts are generally sufficiently
numerous to give good ventilation after the
heading has been driven through, and until
then a circulation of fresh air is kept up by
blowing it down the shafts with fans, through
sheet iron pipes or wooden trunks.
As the number of tipping places that can
be arranged at the head of an embankment
governs the time required for the completion
of an open cutting, so does the number of
shafts that can be sunk govern the time in
which a tunnel can be driven. Shafts are
usually from 100 to 200 yards apart. In ordi-
nary ground they should not exceed 100 yards
apart if the work is to be completed in
twelve months, but more often the distance
is 200 yards apart, and sometimes more,
owing to the great cost of sinking deep shafts,
the use of which, moreover, for the most
part, ceases when the tunnel has been opened.
For the sake of opening the line for traffic
at the earliest possible moment, however, a
few extra thousands spent in shaft-sinking
may be justifiable enough. It is only a
question whether the traflic will repay the
outlay, and as the presumption is that the
traffic will repay the main outlay, it would
seem to be true policy to ensure that the
tunnels shall be completed by the time the
other works are completed, at whatever cost
almost, although it has often happened that
the opening of a line otherwise ready has
been delayed because a tunnel was not so.
The line of a tunnel is sometimes curved, but
is usually straight. The line of a straight
tunnel is set out with the transit instrument,
the telescope of which is mounted on high
standards, so that it can be turned completely
round on its axis. For accuracy, it is neces-
sary to have the instrument firmly fixed ona
solid bed of masonry, and as there are usually
obstructions to the line of sight, caused by
erections at the mouth of the shaft, the transit
instrument is elevated on a pier of masonry,
sometimes 30ft. or more, which is surrounded
by an outer casing or weather shell of poles
and boarding, entirely separated from the
masonry tower, so that the vibration caused
by wind cannot be communicated to the in-
strument. ‘The use of these erections ceasing
before the completion of the tunnel the ma-
terials are used up in the works. When the
tunnel is curved, however, a portable theodo-
tangential angles. The curve is not difficult to set out on the surface, but below it is so. There are difficulties also with the centreing of the arches; but, perhaps, the most im- portant objection to a curved railway tunnel is that the engine drivers cannot see the ends of the tunnel in passing through it. Some of the shafts of a tunnel are usually left open for ventilation, but the number re- quired for this purpose is seldom so many as are required for the due expedition of the work, and working shafts, in addition to the permanent shafts, are therefore sunk for the latter purpose and filled in again. The per- manent shafts require to be lined with masonry—steined, but the sides of the work- ing shafts, except in rock, are sufficiently supported by timbering. About 9ft. square is a usual dimension, but the proper size depends on the size of the skips intended to
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CRISS_SECTION be used, and this, again, upon the power to be employed in winding; for the shaft should be large enough to allow one skip to be drawn up at the same time that another is being lowered, and to pass each other with a good clearance. 9ft. allows clearance for skips 2ft. Gin. square at the top. Where the ground in which the tunnel is driven throughout is so hard that but small progress can be made in a given time the skips need not be so large, for there will be a comparatively small quantity of material to be brought up daily, and the size of the shafts can be diminished accordingly ; but, on the other hand, the rate of progress may be such as to oblige a large quantity of stuff to be sent up the shaft at each turn, and then 9ft. might not be large enough. But 8ft. square is not an unusual dimension. Fig. 9 represents the vertical section of a timbered shaft, and Fig. 10 lite is necessary with which to set out the } the plan.
After the shaft has been sunk to a depth
of about 7ft., horizontal bars are laid along
the sides, the ends of two of them being
notched to receive the ends of the other two,
in such a manner as to bring the pressure of
the earth behind one pair to bear longitudi-
nally against the other pair. The dimensions
must be greater or less according to the
nature of the ground. Whole timbers 12in.
or 13in. square are in some cases necessary,
while in others a dimension of 6in. or 8in. is
sufficient ; but round timber does very well;
larch of 8in. or 9in, diameter being often used.
Behind the horizontal bars a sheeting of
battens or deals is placed against the sides of
the shaft. The battens or deals are cut to a
length of about 6ft., and their tops are secured
by inserting another set of horizontal bars
similar to the first. To support these, props
are set up at the angles, resting upon the first
set of bars. To secure the head and foot
of each prop, 6in. spikes, made with a head
turned in one direction only, are driven into
the struts. They are thus capable of being
drawn out by inserting the end of a small
crow-bar under the head. To hold up this
first set of timbering while another is being
put in below, raking struts are inserted under
the lower set of bars, by cutting a trench
across the bottom, sloping down to the centre
from each side, the depth of which at the
centre should be sufficient to allow the
excavation to be carried down far enough to
get in another set of horizontal bars, say 6ft.
down. When the raking struts are got in
the remainder of the earth can be excavated
down to the level of the centre hole, and
another set of bars put in, behind which and
the bars above another set of sheeting can be
inserted, and props set up and secured as
before. In rock that will stand without
timbering the shaft is sunk circular, and both
sorts of sinking sometimes occur in the same
shaft.
The head frame which supports the pulleys
over which the winding ropes are carried
should be of considerable height above the
ground in order to allow a bank to be formed
around the mouth of the shaft, for, consider-
ing the large quantity of earth that must be dis-
posed of, itisadvantageous to have it delivered
at such a height above the surface that it can
be easily taken away. For sinking the shafts
and driving the headings, however, a common
windlass is often used, the horse-gins being
set up only on the commencement of the
tunnel itself, except when large quantities of
water have to be drawn out. Water barrels
are hung from the bale rods a little below the
centre, and secured to the bale rods by a
catch at the top, so that by releasing the
catch they are easily turned over and emptied.
The centre line being set out on the sur-
face a mark is made on each side of the shaft
in that line, and the corresponding points at
the bottom found by plumb lines and marked
on the roof of the heading. From these
marks candles are suspended from time to
time, and the workmen in each heading guide
themselves in the true centre line of the
tunnel by bringing another candle in line with
the two suspended at the bottom of the shaft.
After the work has proceeded some distance
other permanent marks are correctly aligned
from the shaft forwards, which are used in a
similar manner.
Before the tunnel is excavated to the full
dimensions a small heading 5ft. or 6ft. high
and 4ft. or 5ft. wide is pushed forward each
way from the shaft to get the line, and from
which to commence the widening out of the
excavation to its full dimensions. This
widening out is done in short lengths—9ft. or
10ft. in bad ground and 15ft. or 20ft. in good
ground, The tunnel being thus constructed
in “lengths” different parts of it take appro-
priate names. ‘The length immediately under
the shaft is the shaftlength. The first length
on each side of the shaft is the side length, and
each succeeding length the leading length for
the time being, until another can be got in,
