Pile Design and Construction Practice, Fifth edition

506 Ground investigations, contracts and pile testing
bridge strain gauged cantilever springs with a maximum capacity of 20 MN/m 2 . Two hundred
and forty pressuremeter tests using the HPD to determine in-situ deformation properties
in Trias and Carboniferous strata were carried out to depths up to 50 m for the piling to
the viaduct piers on the Second Severn Crossing (11.7) . The Marchetti dilatometer (11.8) is a
spade-shaped device which is pushed or hammered into the soil. A load-cell is mounted on
the vertical face of the spade and pushed against the soil or rock. The pressuremeter should
be distinguished from a borehole jack which applies forces to the sides of boreholes by
forcing apart circular plates, imposing different boundary conditions on the test.
The cone pressuremeter, developed by Fugro and Cambridge Insitu (Whittle (11.9) ) initially
for offshore site investigations, is a self-boring device which incorporates a 1500 mm 2 cone
below the friction sleeve of the pressuremeter module and is pushed into the soil using
standard cone rods. A piezo-cone may replace the cone to assist in identifying soil for
pressuremeter testing.
As noted in Section 6.3.7, the pressuremeter has useful applications to determinations of
the ultimate resistance to lateral loads on piles and the calculation of deflections for a given
load. Because the pressuremeter only shears a soil or rock (there is no compression of the
elastic soil or rock) the slope of the pressure/volume change curve in Figure 6.34 gives
the shear modulus G. This can be converted to the Young’s modulus from equation 6.49. In
recently developed instrumentation, the data points on the load/unload loop are now very
frequent so that the change in strain can be accurately measured for each successive point from
a selected zero – with the smallest increment being around 0.01% radial strain. G calculated
in this way more accurately reflects actual strain produced in the ground by structures, and is
greater than G obtained from slopes of lines through the loops. When using the pressuremeter
to obtain E values for pile group settlements using the methods described in Chapter 5, it is
necessary to take into account the drainage conditions in the period of loading.
Plate-bearing tests can be used to obtain both the ultimate resistance and deformation
characteristics of soils and rocks. When used for piling investigations these tests are
generally made at an appreciable depth below the ground surface, and rather than adopting
costly methods of excavating and timbering pits down to the required level it is usually more
economical to drill holes 1 to 1.5 m in diameter by power auger or grabbing rig. The holes
are lined with casing and the soil at the base is carefully trimmed by hand and the plate
accurately levelled on a bed of cement or plaster of Paris (11.10) . The deformation of the soil
or rock below the test plate can be measured at various depths by lowering a probe down a
tube inserted in a drill hole beneath the centre of the plate. This device (11.11) is helpful in
obtaining the modulus of deformation of layered soils and rocks. The load is transmitted to
the plate through a tubular or box-section strut and is applied by a hydraulic jack bearing
against a reaction girder as described for pile loading tests (see Section 11.4.1). Loading
tests on 500 mm diameter plates were carried out in 600 mm holes 20 m deep drilled
offshore from a jack-up platform to determine deformation properties in Trias rocks for the
main span foundations for the Second Severn River crossing (11.7) .
Small-diameter plate loading tests can be made using a 143 mm plate in a 150 mm
borehole, but it is, of course, impossible to trim the bottom of the hole or to ensure even
bedding of the plate. However, these tests can be useful means of obtaining the ultimate
resistance of stiff to hard stony soils (11.12) or weak rocks (11.13) . They do not give reliable
values of the deformation modulus.
Simple forms of in-situ permeability test can give useful information for assessing
problems of placing concrete in bored and cast-in-place piles in water-bearing ground.