formation of a very soft sludge at the bottom of the drillhole which, apart from weakening
the base resistance, makes it difficult to identify the true character of the rock at the design
founding level. The sludge should be baled out, accompanied if necessary by flushing with
air and water. Standard or cone penetration tests can be used to assess the rock quality at
base level but the procedure is time consuming. It is better to judge the founding level by
examination and testing of cores taken from boreholes at the site investigation stage
(Section 11.1.3), with later correlation by examining drill cuttings from the pile boreholes.
Assessment of pile base levels from rock cores is particularly necessary in thinly bedded
strata where weak rocks are interbedded with stronger layers. In such cases the allowable
bearing pressure should be governed by the strength of the weak layers, irrespective of the
strength of the material on which the pile is terminated.
The introduction of powerful mechanical augers of the type described in Section 3.3 has
eliminated most of the rock identification problems associated with percussion drilling.
Where pile boreholes are socketted into strong rock the allowable base pressure is gov-
erned by the permissible compression strength of the concrete forming the pile shaft
(Section 2.4.2). In weak rocks which are decomposed to a soil-like consistency (complete
weathering), the ultimate base resistance of the material can be determined, in the case of
sandstones, by making standard or cone penetration tests in the weathered layers at the site
investigation stage, with calculations from the test results as described for bored piles in
coarse-grained soils in Section 4.3.6.
It should be possible to obtain undisturbed samples of completely weathered mudstones,
siltstones and shales, from which shear strength tests can be made and base resistance
calculated as described in Section 4.2.3. In the case of moderately weathered mudstones,
siltstones and shales, uniaxial compression tests are made on rock cores, or in the case of
poor core recovery, point load tests (Section 11.1.4) are made to obtain the compression
strength. The base resistance is then calculated using the relationship with qucand RQD as
shown in Table 4.14. Alternatively, the parameters cand can be obtained from this table
and used in conjunction with equation 4.40.
In the absence of compression strength data, published relationships between the weath-
ering grade, undrained shear strength and elastic properties of the above rocks can be used
to determine the base resistance from equation 4.40. CIRIA Report 181(4.57)gives these
properties as shown in Table 4.18.
High values of base resistance resulting from the calculations described above should
be adopted with caution because of the risk of excessive base settlement. This can be of the
order of 20% of the pile width at the toe which is required to mobilize the ultimate base
Resistance of piles to compressive loads 209Table 4.17 and values of weak mudstones
related to weathering grades
Grade
IV–III 0.45 —
IV–II 0.45 0.5
II 0.3 1.71
IV–III 0.3 0.86
III 0.31–0.44 0.86–1.06
IV–II 0.45 —
IV–II 0.375 —