干湿循环作用下膨胀土变形特性

Deformation characteristic of expansive soil under wet-dry cycles

为探究自然界干湿循环作用下膨胀土的膨胀变形特性及其影响机理,以南宁膨胀土为研究对象,分别测定0~4次干湿循环后试样膨胀量与时间的关系曲线,分析了膨胀曲线不同阶段特征及其影响机理,研究了不同干湿循环次数下的土水特征曲线,分析了基质吸力在膨胀变形中的作用。研究发现不论是否经历干湿循环作用,膨胀量与时间的关系曲线(膨胀曲线)都是非线性的;未经历循环的试样的膨胀速率最低,约在230 min后完才成总膨胀量的90%;经历了1~4次循环后,试样均在约100 min就完成膨胀量的90%,之后缓慢达到稳定值;不同循环次数下,膨胀速率随时间增加而逐渐降低,膨胀曲线大致分为快速膨胀、减缓膨胀和缓慢稳定3个阶段,膨胀曲线可用3段直线进行近似拟合;经历1次循环后,试样总膨胀量增加约20%,之后随着循环次数的增加而逐渐减小;在低含水率下(ω<3%),不同循环试样的基质吸力较为接近,随着含水率增大,基质吸力的差异也增大。非饱和土吸水过程中的水封闭、双开敞、气封闭等3个阶段的三相特征差异,是造成膨胀速率变化的主要原因。经历不同干湿循环次数后,试样破碎化程度和基质吸力的差异,是导致膨胀曲线及总膨胀量差别的重要因素。

To explore the swelling characteristics of expansive soil when experiencing natural wet-dry cycle and its influencing mechanism,expansive soil of Nanning was taken as the object and the swelling relation between the expansion and time(swelling curve)was measured after 0~4 cycles.Stage characteristic of each swelling curve and their influencing mechanism were analyzed;soil-water characteristic curves under different wet-dry cycles were studied,and the effect of matrix suction on expansion deformation was analyzed.It is found that the swelling curves are all non-linear whether wet-dry cycle is experienced;a lowest expansion rate is observed on the specimens undergo no cycles,in which 90% of the total expansion deformation is achieved after about 230 minutes.After 1~4 cycles,however,the 90% deformation is completed in about 100 minutes.For all swelling curves,the expansion rate decreases with an increasing time;these curves are roughly divided into three stages:rapid expansion,slow expansion and slow stabilization,and 3 straight lines are used to fit the swelling curves.After cycle one,the total expansion deformation increases by about 20%,and it then decreases gradually with cycle number increased.With lower moisture content(less than 3%),the matrix suctions of specimens experienced various wet-dry cycles are relatively close,and the gap of matrix suctions increases as the moisture content increased.Differences in the three stages,unsaturated soil-water sealing,double opening and gas sealing,is the main reason for the difference of expansion rate.The distinction between fragmentation and matrix suction is an important factor leading to the difference of swelling curve and expansion deformation.