【作者】 商翔宇；

【导师】 余海岁； 周国庆；

【作者基本信息】 中国矿业大学 ， 岩土工程， 2009， 博士

【摘要】 本文以室内试验、理论分析以及数值分析为研究手段,对饱和重塑深部粘土力学特性随应力水平不同的变化特性、其内在机理和高压粘土力学本构模型进行了较为深入、系统的研究。本文研究选用粘土属于典型的华东、黄淮地区深厚表土层深部粘土,对其进行的矿物组成成分定量分析及基本物理指标测试结果表明该粘土是一种蒙脱石、高岭石及伊犁石矿物之和占粘土矿物总量68.6%的高液限粘土。利用DRS-1超高压直剪试验系统,进行了固结压力从0.2 MPa到15 MPa范围内共16个压力等级的排水剪切试验研究。试验结果表明,高压力下饱和粘土压缩指数普遍小于中常压下压缩指数,高压力下饱和粘土的压缩曲线可以用双折线表示,双线交汇点在1.6 MPa左右;由试验结果推算出的粘土渗透系数随孔隙比呈指数关系变化,其变化曲线形态表明1.6 MPa左右是渗透系数变化率的转折点;试验饱和粘土抗剪强度随竖向压力增大而增大,但中常压下获得的内摩擦角大于高压下相应值,具体相差近9°,抗剪强度包络线可以用双折线表示,双折线交汇点在1.6 MPa左右。直剪试验结果还表明饱和粘土剪切变形特性随竖向压力增大,总体呈现从塑性向脆性转变的特点;饱和粘土剪切过程中体积变形均呈现出压缩特点,且压缩量总体随竖向压力增大而减小。利用TSZ30-2.0中压三轴试验系统和TATW-500高压三轴试验系统,进行了饱和粘土围压从0.2 MPa至9 MPa共11个压力等级下的三轴固结不排水剪切试验研究。研究结果表明临界剪切应力比随围压增大而减小,且可以用双线性方程来近似描述这种变化趋势,分界点在1.8 MPa左右;高压下饱和粘土抗剪强度中包括内聚力项,如果不考虑其影响将会造成临界剪切应力比偏大;随围压增大,土样最终破坏形式由塑性外鼓破坏转变为脆性破裂破坏。对直接剪切试验和三轴不排水剪切试验结果的综合分析表明,由直接剪切试验剪应力峰值反算出的临界剪切应力比与三轴试验相应实测值具有较好的对应关系,前者小于后者,两者相差倍数约为1.5;饱和重塑粘土中常围压下临界剪切应力比是高围压下相应值的2倍左右;饱和重塑粘土中常压下饱和粘土的压缩指数是高压下相应值的1.11倍;1.6~1.8MPa是饱和粘土强度、压缩变形性质的分界点,其根源很可能与孔隙水性质的转变有关;不同于抗剪强度和压缩变形随压力水平变化的规律,剪切变形特点随压力水平增大由塑性转变为脆性的转折点在0.8 MPa左右,分析其原因在于剪切变形受控于另外不同的机制--颗粒间胶结作用。考虑到本文三轴试验结果-偏应力峰值点对应的轴向应变随压力水平变化并不显著,再结合工程实际应用中强度特征普遍是更为重要的指标,因此,本文建议以1.6~1.8 MPa作为深部饱和粘土高低应力水平的分界点。结合本文进行的压汞试验结果,对现有利用双电层理论预测计算高压粘土压缩曲线的计算方法进行改进,结果表明改进算法预测计算结果能够较好地与与实测结果相吻合,说明粘土压缩变形等宏观性质实际上由其内在物理化学机制所决定;用弹性粘着摩擦理论计算表明,在压力为0.2 MPa作用下土的摩擦系数是1.6 MPa压力条件下相应值的2倍,这与实测分析结果相一致。根据上述关于高压粘土饱和粘土力学特性的分析,在新近提出的临界土力学模型CASM的基础上,提出了适应高压饱和粘土具有内聚力这一性质的CASM-hc模型;并将上述两模型嵌入到ABAQUS中;利用该程序对三轴不排水剪切试验进行了数值模拟,结果表明上述两个模型在模拟中常压和高压饱和粘土临界强度特性方面性能较好;但在反映常压下土样塑性变形特点以及孔隙水压变化特点等方面存在不足之处,有待进一步研究。更多还原

【Abstract】 The mechanical characteristics of saturated remolded clay under different stress level and its constitutional relationship and intrinsic mechanism were studied deeply and systemically by means of three methods which include laboratory test, theoretical and numerical analysis. The clay used in the experiment is the typical deep clay distributed in East China. The quantitative analysis of mineral composition and the measurement of basic physical index for the clay show that it is a kind of high liquid limit clay including 68.6 percents of Montmorillonite /illite / kaolinite.The drained direct shear test of saturated remolded clay was carried out using the DRS-1 high normal stress direct and residual shear apparatus under 16 different consolidation pressures which vary from 0.2 MPa to 15 MPa.The experiment result shows that the compression index of saturated clay in high pressure was lower than the one in medium-low pressure.The compression curve of saturated clay in high pressure can be fitted by double broken line and the meeting point was at approximately 1.6 MPa.The relationship computed by experiments between clay’s permeability coefficient and void ratio had an exponential relation, from which it can be seen that the permeability coefficient change rate underwent a turning point at 1.6 MPa.The shearing strength of the saturated clay used in experiment went up with increase of vertical pressure,however the internal friction angle in medium-low pressure is 9°higher than the one obtained in high pressure.The shearing strength envelope curve can also be fitted by double broken line and the meeting point was at about 1.6 MPa.At the same time,the direct shear test also proved that the shear deformation turned from plasticity to brittleness gradually with vertical pressure increasing. The volume deformation during shearing showed compression without exception and the compression volume decreased with increase of vertical pressure.Then TSZ30-2.0 medium pressure triaxial experiment apparatus and TATW-500 high pressure triaxial experiment apparatus were introduced to carry out triaxial consolidated undrained shearing test for saturated clay under 11 different consolidation pressures which vary from 0.2MPa to 9 MPa.The experiment results indicated the critical shear stress ratio decreased with rise of confining pressure and the trend can be approximately described by bilinear equation with a dividing point of some 1.8 MPa.The shear strength of saturated clay in high pressure includes cohesion.The critical shear stress ratio would be slightly larger without consideration of cohesion. The final failure form turned from plastic outward squeeze to brittle failure with significant fracture plane with confining pressure increasing.The comprehensive analysis of direct shearing experiment and triaxial undrained shearing experiment showed that critical shear stress ratio that was back-calculated from peak of shear stress in direct shearing experiment corresponds quite well with the one measured in triaxial experiment,in which the former one is 1.5 times lower than the latter. The critical shearing stress ratio of saturated remodeling clays in general and medium pressure was 2 times of the one in high pressure.The compression index of saturated remodeling clays in medium-low pressure was 1.11 times of the one in high pressure. The dividing point of strength and compression characteristics in high pressure and low pressure was about 1.6~1.8 MPa.It seemed that the origin of mechanics property difference for saturated clay in higher or lower pressure lied in the change of pore water. The dividing point of shear deformation characteristics in high pressure and low pressure was about 0.8 MPa, which is different from strength and compression with the reason that shear deformation’s intrinsic mechanism is related to the cementation between clay particles.Based on the result of mercury injection experiment of this paper and electric double layer theory, the present algorithm which can be used to predict compression curve in high pressure was modified. It showed that the clay’s compression property was determined by its inherent physiochemical mechanism. With the elastic adhesive friction theory,it showed that the friction coefficient in 0.2 MPa was 2 times of the one in 1.6 MPa,which is consistent to the measured result.According to the analysis aiming at mechanics property of saturated clay in high pressure,the paper put forward the CASM-hc model applicable to the saturated clay with cohesion which was based on the critical soil mechanics model CASM. The author embedded these models into the ABAQUS. The numerical simulation of the triaxial undrained shearing test based on the program showed that the two models above can simulate the critical strength property of saturated clay in medium and high pressure quite well, however it presented some deficiency on the analysis of the plastic deformation and pore water pressure changing, which required further research.更多还原