Numerical and physical models of rate effects in soil penetration

Marcelo Furtado Silva, Cambridge University
Geotechnical Engineering Group


Standard piezocone tests are widely used to deduce soil properties through empirical or analytical correlations between soil properties and both the measured penetration resistance and pore water pressure. Cavity expansion methods are commonly used to simulate cone penetration, allowing reliable correlations to be derived. For practical applications, the cavity expansion analyses are reduced to either fully drained or undrained problems, from which constitutive models can be derived. In reality, soils are the most heterogeneous of materials. Depending on the physical and mechanical characteristics of the soil, such as permeability and compressibility, partially drained conditions during the standard piezocone test may occur. However, only when drainage conditions during a piezocone test are fully understood can the piezocone measurements be correlated to the soil properties. Otherwise, the piezocone data are, in principle, only able to identify the layering and characteristic grain size of soils.

During penetration, the degree of drainage can best be inferred by varying the rate of penetration in a particular soil. In this dissertation, numerical and experimental methods are performed to investigate the effect of penetration rate on soils. Firstly, a set of cavity-expansion finite-element analyses simulating piezocone tests in clays is presented, using the assumption that the soil deformation is entirely in the horizontal plane, taking place between the cone tip and cone shoulder. A coupled formulation is used, allowing any generated excess pore pressure to dissipate during simulation. The influence of penetration rate on the stress, pore pressure and voids ratio distributions are examined, demonstrating that partially drained penetration is permitted by volume change in the near field, in addition to radial movement in the far field. The radial distribution of excess pore pressure after slow penetration differs from the undrained scenario, with a relatively low pressure gradient existing near the cone. As a result, the dissipation curves after slow penetration lag behind those following fast penetration. Normalised curves of velocity against excess pore pressure and net cone resistance, capturing the transition from undrained to drained penetration, are then derived.

Secondly, centrifuge piezocone tests in silica flour are described. These tests were performed at 50g with a 12-mm diameter piezocone at different penetration rates. The trend for dilatancy in the silica flour is observed from the measurements of negative excess pore pressures in the soil, as well as on the piezocone. The results are then compared with cavity expansion analyses, where the soil obeys a state-dependant dilatancy model. These analyses are performed with a set of geotechnical parameters derived from conventional triaxial tests on silica flour under similar stress conditions to that in the centrifuge tests. Comparison of the results suggests that the excess pore pressures can be adequately modelled by cylindrical cavity expansion methods.

Finally, it is suggested that the degree of drainage during penetration should be evaluated by the penetration speed, which is rendered dimensionless by normalisation with the coefficient of horizontal consolidation and the cone diameter. The implications of a partially drained piezocone test are discussed, and the use of multi-rate piezocone tests to deduce the consolidation coefficient of clays is presented, in light of these analyses.