卸载对粗砂抗剪强度影响的直剪试验研究

采用粗砂试样进行一系列卸载后的室内直剪试验，分析卸载作用、超固结比OCR、先期固结压力和剪胀性对粗砂抗剪强度的影响。试验结果表明：对于同样的卸载后竖向应力，OCR越大，相同剪切位移对应的剪应力越大；当OCR较小时，峰值强度基本上随超固结比的增加而增加，当OCR较大时，其对峰值强度的影响较小；当卸载后竖向应力较小时，峰值强度随竖向应力几乎线性增加，当竖向应力增加到一定数值以上时，竖向应力变化对峰值强度几乎没有影响；当卸载后竖向应力较小时，与正常固结砂样相比，超固结砂土试样均有一定的剪胀性，其中在低竖向应力下这种现象尤为明显，摩擦强度和剪胀强度均对剪胀性砂土的峰值强度有所贡献；根据试验结果，得到粗砂的峰值内摩擦角与最大剪胀角之间的线性拟合关系。

Direct shear test study on the effect of unloading on shear strength of coarse sand

 A series of laboratory direct shear tests were carried out on unloaded coarse sand samples in order to analyze the effects of unloading action, overconsolidation ratio, pre-consolidation pressure and dilatancy on shear strength of coarse sand. The test results show that shear stress under certain shear displacement increases with increasing OCR for the same vertical stress after unloading; peak strength increases with increasing OCR for low overconsolidation ratios, while the effect of OCR on peak strength can be almost neglected for large overconsolidation ratios; peak strength increases linearly with increasing vertical stress when the vertical stress is quite small after unloading, while the vertical stress has little effect on peak strength when it reaches a certain level; compared with normally-consolidated sand specimens, overconsolidated sand specimens exhibit dilatancy to some extent under relatively small vertical stresses after unloading, which is more evident especially for those low vertical stresses, and both frictional and dilatancy strengths contribute to peak strength of the sand with dilatancy. Based on the experimental results, the linearly fitted relationship between peak internal friction angle and maximum dilatancy angle for coarse sand was accordingly obtained.