Page 150 - Civil Engineering Project Management, Fourth Edition

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Site surveys, investigations and layout
Percussion drilling
A percussion drill may be used to penetrate rock or boulders if no cores are
required. A percussion chisel, usually of cruciform shape with a string of tools
to give it weight, all suspended on a wire rope, is raised and let fall repeatedly
on the rock base of the hole. The chisel has to be let fall with a clean blow on
the base, and it is caused to rotate a little with each blow by the suspension
wire having a left hand lay, and a friction grip attachment which lets the wire
re-set from time to time. The drill chisel must be sharpened regularly. The rock
chips are removed by water flush in small holes; in larger holes a bailer, very
similar to a shell, has to be lowered at intervals to collect the chippings.
Sometimes it is only necessary to find the depth at which hard material,
such as rock exists, or to drill sufficiently far into rock to ensure it is not a boul-
der overlying soft material below. In such situations a down-the-hole hammer
drill can be used at a relatively small diameter. The blows to the cruciform bit
are applied by a compressed-air operated hammer adjacent to the bit. The rock
fragments are either blown out to the surface, or washed out by drilling water
supplied through waterways inside the drill rods and can be examined. Small
percussion drills of this type, 50–75mm size, are fast and can penetrate some-
thing of the order of 6m of rock or concrete per hour.

12.9 Other means of ground investigation

The hydraulic digger (or backhoe) is useful as a means of revealing the nature
and extent of shallow overburden material on site. It can excavate a trench up
to 3–4m deep in soft or moderately hard material, at a fast rate and cheaply.
The substantial cross-section of material then revealed for inspection can be
more informative than samples from a few borings. However, trenches of this
depth must be securely supported before access for inspection is allowable.
A sampling tube can sometimes be pushed into the base of the trench using
the digger bucket, and then dug out by the same machine.
The auger may be used for boring holes in soft materials. A lining may be
required to keep a hole open during and after boring. Large augers, machine
driven, are used for sinking shafts for the formation of in situ concreted
piles. For site investigations, the hand-auger is a simple little tool, usually of
75–150mm diameter, for penetrating shallow depths of soft material. About
300mm of material is penetrated at a time before the tool is withdrawn and
the material taken out of it and examined. Two men are usually required to
twist the auger, the hole being watered from time to time if necessary in order
to reduce friction. Penetration is usually of the order of 1.5–2.5m; to get a hole
deeper than 3m the ground has to be very soft. Gravel or cobbles cannot be
penetrated. The tool is useful for locating the extent and depth of shallow,
very soft overburden material.