Numerical simulation of piezocone dissipation test in clays

The piezocone is one of the most widely used in-situ tools with which to characterise the soil profile, distinguishing different types of sediment, and to provide quantitative data on the soil strength and consolidation characteristics. For fine-grained soils where penetration occurs under nominally undrained conditions, consolidation characteristics are obtained from dissipation tests conducted with the piezocone brought to rest. Dissipation testing is interpreted to yield a coefficient of consolidation, by comparison of the excess pore pressure decay with theoretical solutions. It is generally acknowledged that the coefficient of consolidation derived from in-situ dissipation tests will be greater, by a factor of perhaps 2 to 10, than values derived from oedometer tests on samples recovered from the corresponding depth. Apart from potential differences in horizontal and vertical permeability, differences arise from the influence of the rigidity index on the initial excess pore pressure field and the more complex stress paths undergone by soil elements surrounding the piezocone by comparison with an oedometer test. In order to advance the science underlying interpretation of dissipation testing, a careful parametric study has been undertaken using finite-element analysis, with the soil modelled as modified Cam Clay. The aim has been to derive an ‘operative' coefficient of consolidation, linked to the consolidation and swelling parameters of the soil model, thus allowing quantitative treatment of the difference in coefficients of consolidation derived from laboratory oedometer or in-situ dissipation testing. The effect of partial consolidation during the penetration phase is also considered.