Ground investigations, contracts and testing 507Thefalling-head testconsists of filling the borehole with water and measuring the time
required for the level to drop over a prescribed distance. In the constant-head testwater is
poured or pumped into the borehole and the quantity required to maintain the head at a con-
stant level above standing groundwater level is recorded. The procedure for obtaining the
coefficient of permeability is described in BS5930. Pumping-in tests made through packers
in a borehole, or pumping-out tests with observations of the surrounding drawdown are too
elaborate for most piling investigations. Sufficient information to evaluate groundwater
problems can often be obtained by baling the borehole dry and observing the rate at which
the water rises to its standing level. Such simple tests provide useful background information
for contractors tendering for bored piling work and it is false economy to neglect them.
It was noted in Section 11.1.2 that high-quality undisturbed samples of soil can be obtained
by means of pushed-in samples in thin-walled tubes. Equipment has also been developed for
measuring very small strains in samples undergoing triaxial compression tests. Thus reliable
Young’s modulus values can be obtained from samples of soil or weak rock without the need
for employing special in-situ testing equipment or making plate-bearing tests in deep boreholes.
Small-strain Young’s modulus values are useful for calculating consolidation settlements in the
mass of ground beneath pile groups rather than the base settlement of individual piles. They are
also applicable to the determination of the deflection of laterally loaded piles. The adoption of
improved sampling and triaxial compression testing techniques will result in undisturbed shear
strength values higher than those obtained in earlier practice, particularly in very stiff to hard
clays. These higher values may require modification of correlations established between the
shear strength of clays and shaft friction and end-bearing resistance of piles.
Laboratory tests on rock cores should include the determination of the unconfined
compression strength of the material, either directly in the laboratory or indirectly in the
field or laboratory by means of point load strength tests. Young’s modulus values of rock
cores can be obtained by triaxial compression testing using the transducer equipment for
small strain measurements.
The point load test(11.14)is a quick and cheap method of obtaining an indirect measurement
of the compression strength of a rock core specimen. It is particularly useful in closely
jointed rocks where the core is not long enough to perform uniaxial compression tests in the
laboratory. The equipment is easily portable and suitable for use in the field. The tests are
made in the axial and diametrical directions on cores or block samples. The failure load to
break the specimen is designated as the point load strength (Is) which is then corrected
to the value of point load strength which would have been measured by a diametral test on
a 50 mm diameter core using a standard correction (Table 11.1) to obtain Is(50).
Table 11.1Relationship between uniaxial compression strength (qcomp) and point load
strength (Is(50)) of some weak rocks
Rock description Average qcompMN/m^2 qcomp/Is (50)
Jurassic limestone 58 22
Magnesium limestone 37 25
Upper Chalk (Humberside) 3 – 818
Carbonate siltstone/sandstone (UAE) 2 – 512
Mudstone/siltstone (Coal Measures) 11 23
Tuffaceous rhyolite (Korea) 15 – 90 8
Tuffaceous andesite (Korea) 40 – 160 10