AKC Smith, Webber Associates, formerly of Fugro.
During the last 35 years, there have been considerable developments in the understanding of foundations in chalk, as described for example by CIRIA (1994). However, there is still considerable uncertainty concerning design parameters, particularly if only routine site investigation methods are employed (and in particular if values of design parameters are based on the standard penetration test (SPT)).
The static cone penetration test (CPT) has a number of advantages over the standard penetration test. It is not nearly so operator-dependent and is much faster than shell and auger boring. The CPT gives a continuous profile of end resistance and sleeve friction, rather than discrete values of blow count as with SPTs, and the effects of flints on the test can more easily be identified and discounted (although sometimes flints prevent further penetration). Many authors (eg Montague, 1990, Mortimore et al, 1990 and CIRIA, 1994) discuss various shortcomings of SPTs in chalk.
There is very little published information on interpreting the cone resistance for design purposes. Hodges and Pink (1971) correlated cone resistance with end bearing capacity and shaft friction for driven steel pipe piles at Portsmouth. Searle (1979) presented expressions relating chalk relative density and degree of cementation to the cone resistance and friction ratio. Power (1982) carried out CPTs at Mundford and correlated the results with the well-known Mundford classification (Ward et al, 1968 and Wakeling, 1969).
Bracegirdle et al (1990) reported the use of CPTs to identify soft zones in the area of a cofferdam and concluded that it provided “a quick and effective means of profiling the chalk and assessing the extent of cavities or zones of disturbed material”. Finally, Illingworth and Chantler (1999) used cone tests to assess depths of weathering on a site at Reading, and converted cone resistance to an equivalent SPT N value to carry out pile design.
It seems likely that the identification of solution features and other variations is the main purpose for which CPTs are used in chalk. Since the cone test produces continuous numerical output, it is considered that it should also be valuable for shallow foundation and pile capacity design. However, before it is possible to carry out such design with confidence, it is necessary to have a better idea of how chalk can be classified from CPTs.
This paper presents results from a number of sites across the chalk outcrop, and shows how the relationship between end resistance and friction ratio can vary. It also presents some examples showing the difficulties of interpreting cone test results in chalk and some examples of the lateral variation of cone resistance resulting from weathering and solution features.
There is very little published information to enable cone test results to be used directly for design, apart from a few correlations with SPTs. The paper discusses how such information might be obtained.