Ground investigations, contracts and testing 503for shearing strength on conventional ‘undisturbed’soil samples indicate shearing values
that are somewhat lower than the true in-situ strengths. The lateral resistance of piles is
particularly sensitive to the shearing strength of clays at shallow depths, and if the calcula-
tion methods can be refined to a greater degree of certainty than that exists at present then
the vane test may have a useful application.
The most useful all-round test for piling investigations is the standard penetration test
(SPT), which in clays, silts, and sands is performed with an open-ended tube and in gravels
and weak rocks is made by plugging the standard tube with a cone end, when the test is
sometimes known as the dynamic cone penetration test(CPT). The blow counts (blows per
300 mm of penetration desegnated the ‘N-value’) for the SPT and CPT have been correlated
with the angle of shearing resistance of coarse-grained soils (Figure 4.10) by Peck et al.(4.18).
Terzaghi and Peck(11.2)have given the following approximate correlation with the consistency
of fine-grained soils:
N-value (blows/300 mm) Consistency Approx. unconfined compressive
strength kN/m^2
Below 2 Very soft Below 25
2 – 4 Soft 25 – 50
4 – 8 Medium 50 – 100
8 – 15 Stiff 100 – 200
15 – 30 Very stiff 200 – 400
Over 30 Hard Over 400Stroud(5.7) has established a relationship between the standard penetration test and
the undrained shear strength of stiff over-consolidated clays as shown in Figure 5.22. The
cone-ended standard penetration test can also be made in weak rocks and hard clays. Useful
correlations have been established between the blow-count values of stiff to hard clays and
the modulus of volume compressibility (see Figure 5.22). The test should also be made if
percussion borings are carried down below rockhead.
The standard penetration test is liable to give erroneous results if the drilling operations
cause loosening of the soil below the base of the borehole. This can occur if the borehole is
not kept filled with water up to ground level, or above ground level, to overcome the head
of groundwater causing ‘blowing’of a granular soil. Careful manipulation of the ‘shell’or
baler is also necessary to avoid loosening the soil by sucking or surging it through the clack
valve on the baler. It is particularly necessary to avoid misinterpretation of SPT data on piling
investigations since denser conditions than indicated by the test may make it impossible to
drive piles to the required penetration level.
The standard penetration test cannot be performed satisfactorily at deep-sea locations,
say, for example, in ground investigations for piled foundations for offshore oil production
platforms. This is because the hammer is operated at the surface and the inertia of 100 m or
more of drill rods from the hammer to the SPT sampler would make it impossible to achieve
anything like the standard blow as performed at normal drilling depths on land investiga-
tions. Underwater hammers operating in air in sealed containers are available for deep-water
ground investigations.
The application of the static cone penetration testto the design of individual piles is
described in Section 4.3.6 and to the design of pile groups in Section 5.3. Because of the