Trial pits are often a useful adjunct to borehole exploration for a piling project. Shallow
trial pits are excavated in filled ground to locate obstructions to piling such as buried timber
or blocks of concrete (note that by statute all excavations deeper than 1.2 m must be
supported). Deep trial pits, properly shored, may be required for the direct inspection of a
rock formation by a geologist, or to conduct plate-bearing tests to determine the modulus of
deformation of the ground at the intended pile base level (see Sections 4.7 and 5.5). It may
be more convenient and economical to make these tests at the preliminary test piling stage.
It is not the intention to describe ground investigation techniques in detail in the following
section of this chapter. Details of drilling and sampling methods, geophysical surveying, and
the various forms of in-situ tests are given in BS5930. Detailed information on soil testing pro-
cedure is given in BS1377 (methods of test for soils for civil engineering purposes). Eurocode
EC7-Part 2: 2007, Ground investigation and testing, gives ‘guidance for planning and inter-
pretation of geotechnical laboratory and field tests’. However, it is not as comprehensive as
code of practice BS5930 and does not provide the detail in testing methods given in BS1377;
reference has to be made to this standard and various other procedural documents such as
ISRM ‘suggested methods’. The UK National Annex to EC7-2 has not yet been published.
11.1.2 Boring in soil
Cable percussion borings give the most reliable information for piling work. Operation of
the boring tools from the winch rope gives a good indication of the state of compaction
of the soil strata. If the casing is allowed to follow down with the boring and drilling, and
water to aid drilling is used sparingly, reliable information can be obtained on groundwater
conditions, but where groundwater fluctuates seasonally and tidally, standpipe readings over
a period are essential. Such information cannot be obtained from wash borings or by drilling
in uncased holes supported by bentonite slurry. Borings by continuous flight auger are
satisfactory provided that there is a hollow drill stem down which sample tubes can be
driven below the bottom of the boring and measurements of the groundwater level obtained.
Information on the size of boulders is essential for a proper assessment of the difficulties
of driving piles past these obstructions or of drilling through boulder deposits for bored
piles. Light cable percussion tools or flight auger drilling cannot penetrate large and hard
boulders, and it is the usual practice to bring a rotary drill over the hole to core through the
boulder, so obtaining information on its size and hardness. To avoid delays and standing
time of the two types of drilling equipment it is more economical to continue rotary drilling
past the boulder down through the remaining soil overburden to bedrock while the percussion
rig is moved to an adjacent hole. It is usually possible to obtain information on the soil
conditions below the level of the boulder from these adjacent boreholes.
Investigation of glacial tills for piled foundations presents particular problems in addition to
the potential for random boulders, for example, identification of mixed sequence of strata, lam-
inations of silty clay, perched water tables, infilled buried channels and samples for testing. Care
is needed to ensure that compressible clays are identified below the anticipated pile tip so they
are not overloaded, also soft clays which may be subjected to lateral loads from the pile shaft.
In UK practice ‘undisturbed’samples of fine-grained soils are obtained from cable
percussion boreholes by means of 100 mm open-drive thick-wall sample tubes. Recent
developments in ground investigation techniques include the use of thin-wall sample tubes
pushed into the soil and core drilling using triple core barrels to obtain continuous samples
in over-consolidated clay and some coarser soils.(11.1)The samples from these techniques are
of high quality with very little disturbance of the fabric of the material.
Ground investigations, contracts and testing 501