24.12 Wash boring and other methods
24.12.1 Wash boring
COMMENTARY ON 24.12.1
The drill rig consists of a simple winch and tripod. The ground at the bottom of the borehole is broken up by the percussive action of a chisel bit, and washed up to the surface by water that is pumped down the hollow drill rods. Borehole sizes are typically between 65 mm and 150 mm. In collapsing ground, casing is driven down to support the sides of the borehole, or drilling mud can be used. The fragments of soil brought to the surface by the wash return are not representative of the character and consistency of the strata that are being penetrated. These properties are determined by carrying out standard penetration tests at the top of each stratum and at frequent intervals of depth (see 25.4.5 and Clause 39). Open-tube samples or piston samples can be taken in fine soils.
Wash boring is best suited to sands, silts and clays and should be carried out using appropriate size tools and casings. Its use in gravels is impractical and should be avoided.
NOTE Wash boring is widely used in North America, but not often in the UK. The technique tends to be used for rock head profiling in near-shore or other aquatic environments where there exists a ready source of water for flush and the nature of superficial deposits is of no concern.
24.12.2 Other methods of drilling
COMMENTARY ON 24.12.2
There are many methods of drilling, which have been developed mostly to obtain maximum penetration speed, e.g. rotary percussive drilling for blast holes and grouting. When such boreholes are sunk for ground investigations, limited information about ground conditions can be obtained, provided that the boreholes are drilled under controlled conditions.
When rotary percussive drilling is used for ground investigation, the rate of penetration should be measured and drilling characteristics should be observed; in addition samples can be taken of the drilling flushings (see Horner and Sherrell, 1977 ). The use of suitable instrumentation (DPR or MWD), in order to record the progress of the drilling rig can considerably enhance the results obtained (see 24.11.2).
NOTE Rotary percussive drilling is commonly used for locating disused mineshafts or other cavities (e.g. mines, workings or dissolution cavities).
24.13 Backfilling excavations and boreholes
Backfilling should be carried out in such a way as to ensure that there is no, or very limited, settlement of the ground and that the backfilled hole does not form a pathway for groundwater or ground gas.
NOTE 1 Guidance on backfilling in aquifers is provided in the Environment Agency document Decommissioning Redundant Boreholes and Wells .
NOTE 2 Water flow through inadequately backfilled boreholes or excavations can cause very serious inconvenience if these investigatory holes are on the site of future deep excavations, tunnels or water-retaining structures. It could also lead to the migration of contamination and in turn the future pollution of an aquifer.
NOTE 3 Backfilling with arisings is often unsuccessful in preventing the flow of water.
For boreholes, the backfill material should be of lower permeability than the surrounding ground (see BS EN ISO 22475-1:2006, 5.5). The borehole should be refilled with a cement-based grout introduced at the lowest point by means of a tremie pipe. A cement and bentonite grout, mixed with no more water than is necessary to permit the grout to flow or to be pumped, should be used. The addition of an expanding agent might be necessary. Cement alone, other than Shallow Oil Well cement, should not be used to seal a borehole as it shrinks slightly.
NOTE 4 The need and specification for the grout is project dependent.
Pits and trenches are often backfilled with arisings and in these circumstances, the backfill of these excavations should be compacted by means of the excavator bucket or other mechanical means. In special cases, weak concrete or a specified coarse-grained fill might be required. On sites which are categorized as BDA yellow or red, trial pits should be backfilled with care, taking account of any contamination present (see BS 10175 and BS ISO 18400-102 [in preparation]). Appropriate arrangements should be made for the removal and suitable reuse or disposal of any excess arisings.