软土卸荷时效性及其孔隙水压力变化试验研究

EXPERIMENTAL STUDY ON TIME-DEPENDENT PROPERTIES AND PORE WATER PRESSURE OF SOFT SOIL UNDER UNLOADING

采用英国GDS公司生产的STDTTS+UNSAT(7kN/1700kPa)型号三轴测试系统,对上海淤泥质软土进行一系列室内试验研究,系统地探讨基坑不同区域的卸荷时效性特性及其孔压变化规律。试验结果表明,软土卸荷后蠕变可出现3个阶段:衰减蠕变、等速蠕变、加速蠕变。当应力水平较低时,蠕变曲线只出现蠕变的第1阶段;当卸荷应力水平增大到一定值时,蠕变曲线出现第1,2阶段;当应力水平较高时,变形急剧增加,土样很快就出现破坏。但不会出现从蠕变的第2阶段(等速蠕变阶段)直接过渡到蠕变第3阶段(加速蠕变阶段)的情况。孔压系数随时间而变化,并不是常数。卸荷时基坑不同区域孔压均减小,然后在不排水蠕变阶段逐渐增加到最大值,此时基坑安全系数达到最小。

Using standard stress path triaxial system, STDTTS＋UNSAT（7 kN/1 700 kPa） production of GDS, UK, a series of laboratory tests are conducted with undisturbed muddy clay samples obtained from Shanghai. Time-dependent properties and pore water pressure in passive zone and active zone in excavation are researched. It is shown that three stages appear under unloading： weakening stage, constant speed stage and accelerating stage. Creep curve is in weakening stage when unloading is small and then changes from weakening stage to constant speed stage with the increase of unloading. Soil sample is damaged quickly at high unloading stress level with deformation sharply increased. However, it is not found that creep curve of soil changes with time from constant speed stage to accelerating stage directly. Parameter of pore water pressure is not constant and changes with time. Pore water pressure decreases under unloading whether in passive zone or in active zone of excavation, then increases to the maximum value in undrained creep stage and safety ratio is the smallest at this time.