高围压下卸载速率对饱和黏土力学性质的影响试验及其微观机理

发布时间：2018-11-11 00:37

【摘要】：本文采用高压三轴试验和微观分析方法,对重塑饱和黏土和原状饱和黏土进行了固结不排水卸载试验,研究了在高围压下,卸载速率、围压以及土样性质对饱和黏土的应力应变、孔隙水压力等力学特性的影响。主要工作与获得的认识如下:(1)试验获得了不同卸载速率对土样应力应变特性的影响特点。当卸载速率为5k Pa/min和50k Pa/min时,重塑土样应力应变曲线呈应变软化型,且卸载速率越小,峰值点所对应的轴向应变越大。当卸载速率为10k Pa/min~50k Pa/min时,最大偏应力值随卸载速率的增大而增大,且卸载速率越大,应力应变曲线的起始坡度越陡;当卸载速率减小到5k Pa/min时,应力应变曲线的变化趋势有所改变,最大偏应力不是减小反而增大,曲线起始坡度也变陡,即在5k Pa/min~10k Pa/min之间存在一个“转换卸载速率”;原状土样与重塑土样的应力应变曲线变化规律相似,但“转换卸载速率”值有所不同,原状土样“转换卸载速率”在20k Pa/min~40k Pa/min范围内。(2)试验获得了不同的卸载速率对土样孔压特性的影响特点。整体上原状土样的孔隙水压力峰值比重塑土样的峰值大,但是原状土样和重塑土样在相同的卸载速率下孔隙水压力随偏应力变化曲线的发展规律是不相同的。重塑土样孔压随轴向应变的发展存在一个“拐点”,即“拐点”前后孔压随轴向应变的增长速率有所改变,在“拐点”前孔压随轴向应变增长快,在“拐点”后增长速率变慢,且卸载速率越大,“拐点”所对应的轴向应变值越大。孔压-轴向应变变化曲线表明,卸载速率在20k Pa/min~50k Pa/min范围内存在一个“转换卸载速率”,使得在这一卸载速率上下土样的孔压特性有所改变。(3)试验获得了不同固结围压对土样应力应变特性的影响特点。同一卸载速率20k Pa/min,当固结围压为2MPa、3MPa时,原状土样应力应变曲线为应变硬化型,固结围压越大,偏应力峰值越大。当固结围压为6MPa时,应力应变曲线为应变软化型,偏应力峰值变小;在同一固结围压下,当偏应力大小相等时,围压越小,土样变形越大。(4)通过环境扫描电子显微镜对卸载试验后土样微观分析。重塑土样和原状土样经过三轴卸载试验后,土样结构变得松散,孔隙变多,发生剪切变形破坏的土样在剪切面有明显的定向性擦痕;剪切面上有数条拉张裂缝,且张裂缝大多处于较大的颗粒周围,主裂缝旁边衍生出许多细小的次级裂缝。(5)利用液氮吸附试验研究卸载试验前后土样孔隙变化。发现随着卸载速率的增加,平均孔直径减小,对于原状土样,直径由5.77nm减小到4.76nm,对于重塑土样,直径由8.38nm减小到6.73nm;且在相同围压下卸载速率增大,吸、脱附曲线类型有所变化,由L2类曲线变为L3类曲线,即孔隙的连通性由差变好。
[Abstract]:In this paper, high pressure triaxial test and microscopic analysis are used to study the undrained unloading test of remolded saturated clay and undisturbed saturated clay, and the unloading rate under high confining pressure is studied. The influence of confining pressure and soil sample properties on the mechanical properties of saturated clay such as stress strain pore water pressure and so on. The main results are as follows: (1) the influence of different unloading rates on the stress-strain characteristics of soil samples has been obtained. When the unloading rate is 5k Pa/min and 50k Pa/min, the stress-strain curve of the remolded soil shows strain softening type, and the smaller the unloading rate, the greater the axial strain corresponding to the peak point. When the unloading rate is 10 k Pa/min~50k Pa/min, the maximum deflection stress increases with the increase of unloading rate, and the higher the unloading rate, the steeper the initial slope of the stress-strain curve. When the unloading rate decreases to 5 k Pa/min, the variation trend of the stress-strain curve changes, the maximum deflection stress increases instead of decreasing, and the initial slope of the curve becomes steeper. That is, there is a "conversion unload rate" between 5k Pa/min~10k Pa/min; The stress-strain curves of undisturbed soil samples are similar to those of remolded soil samples, but the values of "transfer and unloading rate" are different. The "conversion and unloading rate" of undisturbed soil samples is in the range of 20k Pa/min~40k Pa/min. (2) the effects of different unloading rates on pore pressure characteristics of soil samples are obtained. On the whole, the peak value of pore water pressure of undisturbed soil sample is larger than that of remolded soil sample, but the development law of pore water pressure with deviation stress curve is different at the same unloading rate between undisturbed soil sample and remolded soil sample. There is a "inflection point" between pore pressure and axial strain of remolded soil sample, that is, pore pressure changes with the growth rate of axial strain before and after "inflection point", and increases rapidly with axial strain at "inflection point" and slows down after "inflection point". The larger the unloading rate, the greater the axial strain value of the inflection point. The change curve of pore pressure and axial strain indicates that there is a "conversion unloading rate" in the range of 20k Pa/min~50k Pa/min. The pore pressure characteristics of soil samples at this unloading rate have been changed. (3) the effects of different consolidation confining pressures on the stress-strain characteristics of soil samples have been obtained. At the same unloading rate of 20k Pa/min, when the consolidation confining pressure is 2 MPA, the stress-strain curve of the undisturbed soil is of strain hardening type. The larger the confining pressure of consolidation is, the greater the peak value of deflection stress is. When the consolidation confining pressure is 6MPa, the stress-strain curve is strain-softening type, and the peak value of deflection stress becomes smaller. Under the same consolidation confining pressure, the smaller the confining pressure is, the larger the deformation of soil sample is. (4) the microcosmic analysis of soil sample after unloading test is carried out by environmental scanning electron microscope. After triaxial unloading test, the structure of the remolded soil sample and the undisturbed soil sample become loose, the pores become more and more, and the shear deformation failure of the soil sample has obvious directional scratches on the shear surface. There are several tensile cracks on the shear surface, and most of the tensile fractures are around the larger particles, and many small secondary cracks are derived from the main cracks. (5) the pore changes of soil samples before and after unloading test are studied by liquid nitrogen adsorption test. It is found that with the increase of unloading rate, the average pore diameter decreases from 5.77nm to 4.76 nm for undisturbed soil samples and from 8.38nm to 6.73 nm for remolded soil samples. Under the same confining pressure, the unloading rate increases, the type of absorption and desorption curves changes, from L2 type curve to L3 type curve, that is, the connectivity of pore becomes better from difference.
【学位授予单位】：中国矿业大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TU411

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