Civil Engineering Reference
In-Depth Information
140 or
194 mm
18
30
mm
clack
cutting shoe
Clay cutter
Shell, with clack fitted
Chisel
Fig. 6.4
Cable percussion cutting tools and samplers.
●
the cutting tool drives into the soil and the borehole soil is forced inside the tool;
●
the winch raises the tool from the ground, the soil inside the tool is cleaned out (and sampled if appro-
priate) and the now clear tool is raised once again to the required free fall height and the process
repeated over and over;
●
as the borehole is advanced, the sides of the borehole are supported by sequentially installed tube
sections of steel known as the borehole
casing
. These sections are advanced as the borehole itself is
advanced;
●
undisturbed samples of soils are retrieved in steel or plastic sampling tubes (see Section
6.4)
;
●
the driller operating the equipment records a log of the progress of the borehole and makes any obser-
vations such as soil types, obstructions, groundwater level etc.
This is an extremely versatile and relatively inexpensive means of forming a borehole. As seen, it can be
used in different soil types and this is essential for any borehole forming equipment. If boulders or cobbles
are encountered, these can be broken down using a heavy chisel in place of the cutting tool until the
obstruction is clear, then progress can continue.
In clay soils, the soil is simply wedged inside the clay-cutter and is removed by hand from inside using
steel bars pushed through the side slots. In granular soils the material is retrieved by means of the shell.
This cutting tool is fitted with a clack (a hinged lid) that closes as the shell is withdrawn and retains the
loose particles inside. The soil is removed and sampled by opening the clack once the shell is at ground
level.
Rotary drilling
Rotary drilling involves using a high-powered, truck mounted motor to rotate drilling rods connected to
a drill bit into the ground as shown in Fig
6.5.
The technique was traditionally used mainly for boring and
sampling rock, although the technique is becoming increasingly used in soils work too. The heavy-duty
drill bit (interchangeable types exist for whether boring or sampling is taking place) is attached at the end
of the drilling rods and rotates at high speeds to cut into the ground and move downward.
The drilling rods are hollow so that a water-based coolant mixture can be pumped down them and out
through the holes in the drill bit into the surrounding space within the borehole. This fluid has several
functions: it acts as both a coolant and as a lubricant to aid the cutting process, it provides pressure
balance during drilling to resist inflow of groundwater to the borehole and it provides the means by which
the cuttings of soil and rock are pumped up around the drilling rods to the surface for removal.
