基于孔穴扩张理论的黏土不排水抗剪强度计算方法对比研究

Comparative study of calculation methods for undrained shear strength of clay based on cavity expansion theory

为了利用旁压试验准确获取不排水黏土抗剪强度,对比研究了3种基于孔穴扩张理论的不排水黏土抗剪强度评价方法。其中,Gibson和Anderson提出的直接传统法忽略了土体超固结比和旁压探头实际长度的影响;Cao提出的基于修正剑桥模型的解析法考虑了土体超固结比的影响;基于修正剑桥模型的有限元法同时考虑了土体超固结比和旁压探头实际长度的影响。在两个试验场地的轻度超固结黏土和超固结黏土中开展了一系列旁压试验,采用上述3种方法对比研究了土体超固结比和旁压探头长度的有无限性对不排水抗剪强度的影响,结果表明:单独考虑超固结比的影响时,由直接传统法估算的黏土不排水抗剪强度较解析法小,且二者的差值随着超固结比的增大而增大;单独考虑旁压探头长度有无限性的影响时,由解析法得到的黏土不排水抗剪强度较有限元法高,且旁压探头的无限长度效应随着超固结比的增大急剧减弱。当以特定长度的旁压探头在黏土中进行旁压试验时,存在一个超固结比临界值。低于这个临界值时,旁压探头的无限长度效应对不排水强度的影响大于超固结比;高于这个临界值时,旁压探头的无限长度效应与超固结比的影响相互抵消。对于例1中的粉质黏土,超固结比临界值约为23;对于例2中的粉质黏土,超固结比临界值为34。在实际工程中,有限元法能够同时考虑旁压探头实际长度和超固结比对黏土抗剪强度的影响,为最佳方法。当黏土处于重度超固结状态(超固结比大于临界固结比)时,直接传统法和有限元法计算结果相近,可利用直接传统法计算黏土抗剪强度。

To accurately obtain the shear strength of undrained clay by pressuremeter test(PMT), three types of evaluation method for shear strength for undrained clay were contrastively studied based on cavity expansion theory in this study. Among them, Gibson and Anderson’s direct traditional method neglected the influence of over-consolidation ratio of soil and the actual length of side pressure probe;Cao’s analytical method based on modified Cambridge model considered the influence of over-consolidation ratio of soil;and the finite element method based on modified Cambridge model considered the influence of both over-consolidation ratio(OCR) of soil and the actual length of the pressuremter probe. A series of pressuremeter tests was carried out on slightly overconsolidated and overconsolidated clays in two test sites. The above three methods were applied to study the influence of OCR of clay and the infinity of the pressuremeter probe on the undrained shear strength. The results show that the undrained shear strength of clay estimated from the direct traditional method is lower than that estimated from the analytical method when the effect of OCR is considered alone, and difference between them is increased with the increase of OCR. The undrained shear strength obtained from the analytical method is higher than that obtained from the FEM when the effect of the infinity of the pressuremeter probe is considered alone, and the effect of infinite length of pressuremeter probe decreases rapidly with the increase of OCR. When PMT is conducted on clay with a given pressuremeter length, there is a critical value of OCR. Under this critical value, the effect of infinite length of pressuremeter probe is greater than the OCR;above this critical value, the influences of the infinite length effect of the pressuremeter probe and the over-consolidation ratio cancel each other out. The critical OCR value is predicted to be about 23 for the silty clay in case 1;the critical OCR value is predicted to be 34 for the silty clay in case 2. In practical engineering, only finite element method can consider the effect of the actual length of the pressuremter probe and overconsolidation ratio on shear strength at the same time,which is the best method. When the clay is in a state of severe overconsolidated(the overconsolidation ratio is greater than the critical consolidation ratio), the results of direct method and the finite element method are similar. The direct traditional method can be used to calculate the shear strength of clay.