Estimating normal effective stress degradation in sand under undrained simple shear condition

The degradation of the effective normal stress in soil surrounding the pile caused by undrained cyclic loading affects the shaft capacity of the pile and can lead to structural instability. In order to investigate this phenomenon, a series of constant volume monotonic and cyclic simple shear tests on Fontainebleau sand has been conducted. Based on the experimental results, the shear stress at phase transformation state is first determined for different initial void ratios and initial normal effective stresses. Then, the number of cycles to liquefaction is estimated as a function of both the cyclic and the average shear stresses normalised by the shear stress at phase transformation. An empirical equation to evaluate the normal stress degradation is formulated and the procedure of parameter identification is presented. The performance of the suggested formulation has been analysed, based on simple shear test results on Fraser River sand and Quiou sand, and triaxial tests on Karlsruhe sand, and it has been validated by a series of additional tests on Fontainebleau sand. All the results indicate that the proposed formulation is able to estimate with good accuracy the effective normal stress degradation in sand subjected to undrained cyclic shearing.