Bosco Poon

BE
Postgraduate Research Student
Centre for Geotechnical Research

Research project - Behaviour of Circular Footings on Silica Sand Subjected to Inclined Load

Supervisor: Challis Professor John Carter
Associate Supervisor: Associate Professor David Airey

The behaviour of shallow circular foundations on silica sand under inclined loads is investigated by performing experimental model-scale tests and numerical and semi-analytical analyses.

A series of inclined loading tests on circular footing on dense, medium and loose Sydney sand, under three surcharge pressures (0kPa, 25kPa, and 50kPa), is conducted using a displacement controlled apparatus. The design of the loading system enables the footing to rotate and move freely in vertical and horizontal directions, and avoids any kinematic restraints from the loading rod, which is being used to apply load on the footing. The testing system designed is intended to apply axially inclined loads on the footing. However, as the footing rotates about the load rod, load eccentricity occurs and the results in moments being applied to the footing, in addition to the applied inclined load

All the classical bearing capacity theories assume the soil to obey the Mohr-Coulomb failure criterion, using a single value of friction angle. However, because of the variation of stress level, the soil beneath the footing possesses a significant variation of mobilised frictional strength. This leads to uncertainty as to what value is to be used in the conventional bearing capacity solutions. In this research, a modified conventional method, which accounts indirectly for the non-linear strength behaviour of sand through the use of Bolton's (1986) strength-dilatancy equation, is used to predict the vertical test results for maximum load. Overall, this method is shown to give good predictions for the vertical test results on dense sand. However, this method is shown to have some uncertainties when applied to medium and loose sands due to the compressible nature of these sands.

Numerical analyses corresponding to the model tests are performed using a semi-analytical finite element approach incorporating the Mohr-Coulomb stress-strain model. It is demonstrated that this model, together with a non-associated flow rule for the sand, can provide a reasonable match between the numerical predictions and the experimental observations in most cases. In order to achieve good agreement, a non-associated flow rule is essential, but the high non-associativity required to match experimental data is often accompanied by instability in the numerical solution. The model predictions indicate significant dependence of the maximum bearing resistance on the dilation angle of the soil, and this becomes much more significant as the applied load becomes inclined.

Comparisons are also made between the experimental results and the predictions obtained by the strain hardening system-level plasticity model, known as "Model C", developed by Houlsby and Cassidy (2002). In general, Model C gives realistic predictions for the experimental data of the footing tests. The experimental validation of Model C from this research confirms the suitability of the model for predicting the behaviour of circular footings on sands.

Selected publications

• Poon, M.S.B., Cassidy, M.J., Airey, D.W., and Carter, J.P. (2004). "The behaviour of circular footings on silica sand subjected to inclined load." Proceedings, 8th Australia New Zealand Conference on Geomechanics, Auckland, Vol.2, 569-575.
• Carter, J.P., Poon, M.S.B., and Airey, D.W. (2005). "Numerical and semi-analytical techniques for footings subjected to combined loading." Proceedings of the 11th International Conference on Computer Methods and Advances in Geomechanics, Torino/Italy, Vol.4, 163-176.

Test rig