粉土液化后剪切强度特性研究

Characteristics of the Shear Strength of Post liquefied Silt

摘要:

设计了分析粉土液化后单调荷载下剪切强度的三轴试验。对粉土施加动荷载使其发生液化后，在不排水条件下施加单调静荷载，直至土体达到强度稳定停止试验。试验结果表明，不同初始有效固结压力、初始孔隙比对液化后土体不排水剪切强度影响较大；液化后粉土表现出明显的剪胀特性，颗粒结构重组，孔压在不排水条件下逐渐消散，土体强度则逐渐增加并最终趋于某一稳定值；剪切强度与初始有效固结压力呈线性关系；孔隙比越小，其液化后剪切强度越大。

Abstract:

Movement of the ground is the key factor in predicting structure deformation.Particularly，for prediction of liquefied ground，large deformation may destroy its superstructure even for horizontal ground.This paper presents the behavior of liquefied silt soil on the basis of lab tests.The post-liquefaction stress and strain of silt was evaluated by test results.The possibility of triggering silt liquefaction and post-liquefaction has been documented by previous studies and historical cases.The behavior of silt differed from sand in some aspects such as pore water pressure dissipation.A series of monotonic tests of liquefied silts was performed using special triaxial apparatus in which the shear strength and large deformation were evaluated for silt ground.The isotropic consolidation samples applied various effective confining pressures for different initial void ratio samples.Cyclic loading was then applied to the samples in undrained conditions. The samples were examined from initial liquefaction to the specified level of liquefaction.The undrained monotonic triaxial compression tests，known as post-liquefaction tests， were then performed on the liquefied silts. Undrained shear strength，pore water pressure，and axial strain were automatically recorded at each stage to determine the shear strength behavior post-liquefaction.In addition，large deformation was evaluated from the relationship of the stress and strain of post liquefaction.The liquefied silt behaved as dilative soil during the post-liquefaction stage.The soil resembled fluid with nearly zero effective stress.The axial strain increased up to 3% at nearly zero effective stress，after which time the pore water pressure decreased and the effective stress began to increase.The increasing axial strain was associated with nearly zero effective stress at the beginning of post-liquefaction. It is noteworthy that soil experienced perfect plastic deformation during the zero effective stress stage. Soil particles lost contact when initial liquefaction occurred during cyclic loading.The condition at the end of cyclic loading was the initial condition for the post liquefaction tests.Therefore，the soil particles with no contact showed nearly zero effective stress.The soil particles regained contact with each other with the dissipation of pore water pressure，and the soil strength also began to increase. It was demonstrated that the initial effective confining pressure and initial void ratio had a great effect on the undrained shear strength of the liquefied silts.Steady shear strength was reached as pore water pressure dissipated.A linear relationship was noted between post-liquefaction steady shear strength and initial confining pressure.The shear strength of the liquefied silt was larger than that with a smaller initial void ratio.