Exploratory holes can generally be classified into three kinds:
• rotary core drilling by diamond drill to obtain samples of soil and rock;
• cable percussion driven lined holes in soft ground, sunk by clay cutter or shell;
• uncored holes drilled by rotary percussion drill in hard ground.
It is, of course, necessary to have an idea what sort of ground must be penetrated before the right type of investigation can be chosen; also it is necessary to know the kind of information required. It is not always possible to know the nature of the ground beforehand; soft ground can contain large boulders, and hard ground bands of soft or loose material. Mixtures of this type will cause delay.
Rotary core drilling
Arotary core drill uses a circular, diamond-embedded drill bit which cuts out a core of rock. The standard sizes in use are given in BS 5930. The most usual starting size adopted is ‘H’ (nominal hole diameter 99 mm) to give good sized cores of 76 mm diameter which are less liable to fracture during the cutting process and which permit size reduction to deepen a borehole. It is important that cores are inspected immediately upon withdrawal in order to note whether fractures are fresh and caused by drilling, or whether they are natural 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. Adeeper 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.
A percussion drill may be used to penetrate rock or boulders if no cores are required. Apercussion 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 boulder 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, 50–75 mm size, are fast and can penetrate something of the order of 6 m of rock or concrete per hour.