中文摘要：

采用空心圆柱仪对上海原状软黏土进行了不排水剪切试验，研究了主应力轴旋转条件下中主应力系数对饱和软黏土变形与强度特性的影响。采用等压固结模式对软黏土空心薄壁试样进行固结，并在3种不同主应力轴旋转角度下，对试样进行不同中主应力系数的不排水剪切试验。试验前提为剪切过程中平均应力、中主应力系数与主应力轴旋转角度均保持不变，而偏应力逐渐增大，直至试样破坏。试验结果表明：在不同中主应力系数下，天然软黏土的变形与强度特征存在明显的差异，在3种主应力轴旋转角度下，随着中主应力系数的增加，临界应力比均呈降低趋势，相应的峰值剪切强度减小；在主应力轴旋转角度为0。时，中主应力系数为0．25和0．50的试样均出现了轻微的应变局部化现象，剪应力在达到峰值后呈逐渐降低的趋势；在主应力轴旋转角度为90。时，中主应力系数为0．50和0．75的试样所对应的状态为内外压不等的非轴对称拉伸状态，二者的峰值剪切强度比较接近，而中主应力系数为1．00的试样对应的为内外压相等的轴对称拉伸状态，其峰值剪切强度相比前二者降低了25％；在内外压相等的加载条件下，主应力轴旋转角度由O°增加为90°的同时，中主应力系数由0增加为1．00，试样破坏时对应的临界应力比与不排水剪切强度均逐渐降低。

英文摘要：

A series of undrained shear tests were carried out on Shanghai undisturbed soft clay by using hollow cylinder apparatus. Under the rotation of principal stress axes, the influence of intermediate principal stress coefficient on the deformation and strength character of saturated soft clay was researched. The hollow thin-walled samples of soft clay were consolidated under isotropic consolidation modes. Under 3 kinds of rotation angles of principal stress axes, a series of undrained shear tests were carried out with different intermediate principal stress coefficients. During the shear tests, the deviator stress increased until the sample was destroyed, while average stress, intermediate principal stress coefficient and the rotation angle of principal stress axes remained unchanged. Test result indicates that the deformation behavior and strength character of natural soft clay were obviously different under different intermediate principal stress coefficients. The critical stress ratio and undrained shear strength decrease with the increase of intermediate principal stress coefficient under the three kinds of rotation angles of principal stress axes. When the rotation angle of principal stress axes is 0°, the shear stresses of samples with the intermediate principal stress coefficients of 0. 25 and 0. 50 decrease after arriving the peak values because of slight strain localization. When the rotation angle of principal stress axes is 90°, and intermediate principal stress coefficients are 0. 50 and 0. 75, the peak values of shear strength are similar, and the sample states are non-axisymmetrical tensile with unequal inner and outer pressure. Moreover, when the intermediate principal stress coefficient is 1.00, comparing with the coefficients of O. 50 and O. 75, the peak shear strength decreases by 25%, and the sample states are axisymmetrical tensile with equal inner and outer pressure. If the equal internal and external pressures are applied, when the rotation angle of principal stress axes increase from 0° to 9