Executive Summary : | Constant rate penetration tests are widely used for ins-situ soil strength characterization or pile capacity determination due to their fast experimentation time and cost-effectiveness. The strength predicted from these rapid tests can differ remarkably from their static counterpart owing to the higher strain rate. Hence, special attention is required while inferring the quasi-static strength of sand from such rapid in-situ tests. The model experiments and field prototype tests on instrumented piles clearly exhibit an increased tip resistance and initial stiffness during the fast penetration tests along with accumulation of excessive negative pore water pressure at the pile toe region. On the other hand, the fast laboratory experiments (triaxial) on sand indicate some typical trends like higher strength and dilation rate, increased initial stiffness, early peaks followed by enhanced softening and faster localization onset. The governing factors leading to rate-dependency in sand has already been explained in the available literatures in light of these rapid laboratory tests. However, effort is still required to connect these element test observations to the same from rapid field tests. This step is essential for assessing the predicted strength from these field tests and a numerical approach can be employed to bridge this gap.
The project envisions to employ a FEM based numerical framework for investigating the strain-rate issues associated with the rapid field penetration tests in sand. The numerical framework requires implementation of a rate-dependent constitutive model which will aptly represent the rate-sensitive behaviour of sand over the strain-rate regime under consideration. The visco-plastic model proposed by Mukherjee (2016a) shows the potential to capture the typical behaviour of sand at higher strain rate when tested against biaxial loading and can be augmented with such FEM based numerical framework. A 2D axisymmetric coupled Eulerian-Lagrangian FEM framework will be developed and validated against the available model penetration tests. The developed numerical framework will be further used to simulate the field rapid penetration tests and the rate effect phenomena will be explored in detail in terms of strength enhancement, lateral deformation and excess pore pressure development. The simulation results from both static and rapid penetration tests will be compared and guidelines will be proposed for the quasi-static strength prediction using the rapid field penetration tests. |