Stress-strain relationship of unsaturated cohesive soil

A moisture-content based constitutive model was proposed based on the hyperbolic model as an attempt to move towards the implementation of unsaturated soil mechanics into routine geotechnical engineering practice. The stress-strain behavior of in-situ soil at a depth of 5 m was investigated by conducting undrained triaxial compression tests using the remolded soil samples. The test results show that the stress-strain relationship of unsaturated cohesive soil is still hyperbolic. The values of parameters a and b given in the model decrease with increasing the confining pressure for soil samples with the same moisture content and increase with increasing the moisture content for soil samples under the same confining pressure. The relationships between parameters a, b and moisture content were studied for confining pressures of 100, 150, 200 and 250 kPa. The comparison between the measured and predicted stress-strain curves for an additional group of soil samples, having a moisture content of 25.4%, shows that the proposed moisture content-dependent hyperbolic model provides a good prediction of stress-strain behavior of unsaturated cohesive soil.

反复吸湿循环对原状残积土剪切特性的影响

为了研究反复吸湿循环下天然土体因吸力路径变化引起道路、桥梁等工程土体剪切特性变化的规律,开展了不同围压、吸力和反复吸湿条件下的非饱和土三轴试验研究,从有效应力原理、矿物成分和土水特征曲线(SWCC)滞回特性出发,对不同吸水条件引起的非饱和土剪切特性差异进行多角度分析和解释;同时,研究不同吸水次数后非饱和土体积变化趋势和SWCC滞回特性引起饱和度变化对土体强度的影响,深入分析土体抗剪强度吸水软化的原因。结果表明:脱湿路径下原状残积土表现出的延性和剪缩特性与正常固结土的剪切性状相似,而吸湿路径下土样具有的脆性和剪胀特性与超固结土相符;同一吸力条件下,第1次脱、吸湿路径的抗剪强度差值并不随有效应力的增加而增加;而相同围压下,第1次脱、吸湿路径的抗剪强度差值表现出随吸力增加而减小的趋势;多次吸水后,残积土总体积增加具有不可逆和滞后的特点,土体膨胀性和剪破特性与有效应力和吸力水平有关,应力水平越大,土体应变软化效应越明显;土体软化与水力滞回特性引起土体饱和度的变化和产生的不可逆变形(或累积不可逆变形)有关,多次脱吸湿循环产生的累积不可逆变形是导致土体强度下降的主要原因。研究结果对残积土边坡稳定性分析评价及其治理设计具有重要意义。