Page 476 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.76 THE WORK

The sheeting is then broken out from the spiles down to the lower line of holes, and the ground
allowed to run into the tunnel until it assumes its natural angle (C). The resulting slope will nor-
mally not extend back to the points of the forepoles, but will end at some intermediate position.
Next a horsehead, or false set, is placed under the poles about 2 feet beyond the sheeting. This
consists of a cross piece under the spiles, and a center post set on a small supporting block in the
dirt (D). The spiles are then driven to their full penetration, substituting the support of the horse-
head for that of the cap rear timber.
Earth is then raked in until the points of the spiles are almost uncovered. A board the width of
the cut and about 18 inches high is set vertically immediately under them. This serves as a breast
board to keep more dirt from flowing in, and supports the spiles (E).
A cap timber is then set to line and grade, and is temporarily supported by a single center post. A
“bridge” of 2 6 planks is fastened to the top of the cap but separated from it by 4-inch blocks (F).
The remainder of the side spiles are now driven. Some of these are tapered and are used wide
end forward, so as to reverse the upward slope of the roof spiles and the upper few wall spiles.
The forward cap is now supported by a pair of beams resting on short temporary posts and wedged
down from a cross timber. The remainder of the sheeting below the first cap is now broken out from
the top down, and the dirt pulled into the tunnel and hauled away. Additional breast boards are set
under each other as space becomes available, and held in place by cleats nailed to the side spiles.
When the floor is cut to grade, side posts (legs) are set on below-grade blocks and wedged up
until they take the weight of the cap. Wedges are driven between the posts and the side spiles to
tighten them. Sometimes a trench jack must be used to force the side spiles out while setting the legs.
The next set of roof spiles is entered through the bridge slot on top of the second cap, and is
driven at the same upward angle. Space for side spiles outside the legs is obtained by knocking
out the wedges as the spiles are placed for driving.
All spiles should be driven skintight (touching throughout their length) except at the corners,
where 1-inch boards (lacing) are tacked on. When necessary, cracks are stuffed with excelsior,
salt hay, or other packing to prevent inward leakage of soil.
Each timbering set must be braced securely to that behind it, as any shifting will severely
weaken the structure. Spiles are usually driven with a sledgehammer or air hammer. Sometimes
they are jacked in—a very tedious job—to avoid jarring the soil.
If the tunnel floor tends to get muddy, it should be floored, for convenience of workers and to
avoid possible shifting or settling of the foot blocks. Sometimes floor spiles are driven if the bottom
tends to boil up, but compressed air is a better way to combat this and other difficulties with
excessively soft ground.
This method is relatively easy to follow in many soils, but it takes an experienced crew to get
through boulders, flowing mud, and other difficult conditions.
Forepoles may also be used with steel ribs instead of timber sets.
The standard soft ground tunneling hand tool is a short-handled, round-pointed shovel, aided
when necessary by a grub hoe (mattock), pickaxe, or crowbar, and often by paving breakers. Special
grub hoes have one hammer face for use in driving wedges. In soft clay a curved two-handled
draw knife can be used to advantage. It is pulled by two workers or a power winch, and slices the
clay off in strips.
Liner Plates. Corrugated steel liner plates, curved to match the tunnel rim and supplied with
drilled bolt holes in flanges or overlaps for fastening to each other, are increasingly used for soft
ground tunneling. They are made in various sizes, with 16 36 inches in common use. A plate
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of this size made of ⁄8-inch metal weighs about 27 pounds, and if ⁄4-inch stock, 53 pounds. Short
plates are available for fitting into the tunnel circumference.
Stiffening ribs are used when the tunnel is over 10 feet in diameter, and for heavy loads in any
size opening. They are generally not used when the same strength can be supplied by a heavier-
gauge plate.
Liner plates are usually a temporary support to hold the tunnel until a concrete lining has been
installed, usually a matter of hours or a few days after the digging. They are sometimes “robbed”
for reuse immediately before the concrete is placed. The safety of this practice depends on the
character of the soil, which is a matter for engineers to pass on in each case.