反压对土体强度特性的影响试验研究及其影响机理分析

Effect of back pressure on shear strength of sand:experimental study and mechanism analysis

为提高试样饱和度,室内试验普遍采用反压饱和法,但现行规范中对其反压大小未做具体规定。该文利用竖向-扭转双向耦合剪切仪及全自动真三轴仪,针对福建标准砂(相对密实度30%、58%、70%)及细砂,进行了不同反压下单调排水与不排水剪切试验,分析不同反压下砂土的应力-应变关系、超静孔隙水压力发展规律。试验结果表明:反压对剪胀性砂土的固结不排水剪切强度具有显著影响,且剪胀性越大,其影响更显著;反压对剪缩性砂土的不排水抗剪强度影响不显著;反压对砂土固结排水抗剪强度的影响相对较小;反压对砂土有效抗剪强度指标基本没有影响。本文认为反压对剪胀性砂土的固结不排水抗剪强度产生影响的原因是:反压不同时,土体中的气液混合体的体积模量并不相同,因此相同的体胀趋势或相同的体积回弹下所引起的超静孔隙水压力不同,进而直接影响有效围压及其砂土抗剪强度;而土体发生剪缩时,气液混合体越接近刚体,其体积模量对反压的依赖性越小,因此,相同的剪缩趋势或相同的体积压缩量下,所产生的超静孔隙水压力与反压的大小没有显著的依赖关系,因此在剪缩状态下,反压对砂土的应力-应变关系及抗剪强度没有显著的影响。在固结排水试验中无论反压设定多大,不存在超静孔隙水压力,有效围压与反压大小无关,因此反压对砂土排水抗剪强度没有显著影响。

Back pressure saturation method was commonly adopted to improve the saturation of samples,but specific provisions haven＇t been given for the magnitude of back pressure in current Chinese standards. By employing the vertical-torsional coupling shear apparatus and the automatic true triaxial apparatus, the monotonic drained and undrained shear tests were carried out under different back pressures for the Fujian standard sands（ with relative density as 30%,58% and 70%,respectively） and fine sand,so that the stress-strain relationships for sandy soil and the development law for excess pore water pressure under different back pressures can be analyzed. The test results show that the back pressure has an obvious effect on the shear strength of dilatant sand in consolidated-undrained condition,and with the increase of dilatancy,the effect tends to be more evident; back pressure has no significant influence on both the undrained shear strength of shear shrinkage sand and the shear strength of sand in consolidated-drained condition,while it almost has no influence on the effective shear strength indexes of sand. According to this study,the obvious effect of back pressure on the shear strength of dilatant sand in consolidated-undrained condition may be attributed to the fact that with different back pressures,the bulk modulus of gas-liquid mixture in soil shall be different so that in the same volumetric expansion or in the same volumetric rebound,the induced excess pore water pressures shall be different,which may further directly affect the confining pressure and shear strength of sand. During the shear shrinkage process of soil,the closer the gas-liquid mixture approaches to a rigid body,the less the bulk modulus depends on the backpressure,so that in the shear shrinkage condition, the back pressure has no obvious effect on the stress-strain relationship and shear strength of sand. In the consolidated-drained tests,no matter how large the back pressure is,there is no excess pore water pressure,and the effective confining pressure is irrelevant to the back pressure so that the back pressure has no significant effect on the shear strength of sand in drained condition.