Civil Engineering Project Management (4th Edition)

134 Civil Engineering Project Management
to the rock. The recovery percentage must be checked and recorded and may
indicate the need for a change in equipment or technique. The cores must be
labelled ‘top’ and ‘bottom’, the depth must be marked on them, and they must
be placed for safekeeping and later inspection, in sequence, in purpose-made
core boxes. A label should be attached to the box stating the borehole reference,
date of start of drilling, etc.
When drilling, the need to get complete and reliable information on the
groundwater is important. The water level at the beginning and end of each
day’s work should be measured, and preferably before and after each break or
stoppage for testing. The sinking of the hole disturbs the natural groundwater
conditions, but the changes in level recorded give valuable information on the
probable natural conditions and the rate of inflows and outflows at various
levels. On completion of a hole it is valuable to install a piezometer by which
the longer term natural fluctuation of the groundwater levels can be recorded.
Particular attention should always be paid to any hole which the driller
reports as difficult to sink – the drill bit gets jammed or the drill goes off line,
or the hole has to be abandoned. Any of these can be the sign of a geological
fault, unconformity of strata, a change of inclination of strata and so on. It is
surprising how often one finds the drilling records for the cutoff of an old dam
show a borehole missing in the very area where trouble is later experienced.
So if a boring has to be abandoned it can be important to sink another one
very close by, perhaps using a different technique for core recovery.
Light cable percussion drilling
Light cable percussion driven lined holes in soft ground are usually of larger
diameter than rotary drilled holes, often 150 mm diameter to allow U100
samples to be taken. A deeper hole may need starting off at a larger diameter.
The hole is excavated by bumping a ‘shell’ or clay cutter on the base of the
hole. The shell is used on non-cohesive soils (e.g. sands and gravels), and is a
heavy cylindrical tube with a lower cutting edge and some form of non-return
flap valve inside. Material entering the shell is retained and withdrawn with
the shell, which is removed every 0.5 m or so of boring and emptied for examination.
The clay cutter is similar to a shell, but has a retaining ring at the base
to hold the clay in, and has open slots either side for removal of the clay. The
material inside the shell or clay cutter is partly disturbed but its nature can be
inspected and logged. To take an undisturbed sample a 100 mm diameter
sampling tube attached to rods is pushed or driven into the base of the boring,
given a slight twist to break off the sample and withdrawn. Alternatively a
down-the-hole hammer can be used to drive the tube. The sampling tube has
a detachable cutting shoe with a small internal lip to retain the sample.
If the ground is very weak it may be necessary to push temporary lining
down as the hole is deepened. After this it may be necessary to use a shell or
cutter of slightly smaller diameter to continue drilling.