Correlating CPT data with stiffness parameters of sand in FEM

Abstract

Making foundation designs is one of the major tasks of a geotechnical engineer. Toachieve such a design, knowledge of the soil stiffness is required. Information about the soil stiffness can be obtained by either laboratory tests or in situ tests. Laboratory testing is time consuming, costly and sample disturbance is inevitable. In situ tests are done directly on the soil encountered on the site and therefore are a good representation of the in situ soil state. CPT's are the most performed in situ tests in geotechnical engineering. CPT's are used to evaluate the subsurface based on the mechanical response translated to cone resistance and sleeve friction. Therefore it is desirable to find a correlation between soil stiffness and CPT results. A lot of research has been done to correlate CPT results, such as the cone resistance, with the stiffness of sand. It has been found that these correlations are highly variable and site specific. State parameters as consolidation state have a high influence on the soil stiffness but are difficult to evaluate from CPT results only. In this thesis the correlation between cone resistance and stiffness parameters for sand is investigated based on a series of Zone Load Tests done at a site in Kuwait. In a Zone Load Test a footing is loaded in steps and the settlement of the footing is measured. First an analytical settlement analysis is done with existing methodologies which use correlation between cone resistance and soil stiffness. The predicted settlements according to these methods are compared with settlements measured in the field. Afterwards, a new site specific correlation between cone resistance and soil stiffness is proposed using regression analysis. A verification of the new proposed correlation is done with the finite element program PLAXIS 2D. The numerical calculations were done with the Hardening Soil model using an axisymmetric approach. Multiple Zone Load Tests are simulated with PLAXIS 2D where the input parameters of the Hardening soil model are obtained from the proposed correlation. The numerical calculated settlement of the footing are in agreement with the measurements in the field and therefore it can be concluded that the correlation found for this site is valid. The general application of the proposed correlation is not confirmed. The research done in this thesis is related to direct settlement. Time dependent behaviour is excluded but could be of significant influence. Carbonate sands, as encountered at the site, are sensitive for particle crushing. Particle crushing can lead to creep effects and therefore it is advised to perform Plate Load Tests to get better insight in the creep behaviour in the sand fill.