Abstract: 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.

Key words: pressuremeter test, expansion of a cylindrical cavity, undrained shear strength, direct traditional method, modified Cam-c1ay model, finite element method