冻融循环作用下粉质黏土原位动剪切模量及其衰减特征

根据深季节冻土区高速铁路周边土体实际工程状态,开展现场地震波扁铲侧胀试验和室内共振柱试验,结合现场土体刚度和室内刚度衰减规律,综合评价了冻融环境下粉质黏土原位刚度性状及衰减特征。结果表明:受冻融循环作用的影响,小应变条件下粉质黏土最大动剪切模量G_max由未经冻融时97.9 MPa降至53.6 MPa,最大衰减出现在冻融循环初期;经历循环后,土体归一化G/G_max-γ曲线出现上移,同等应变条件下,冻融循环后土体的G/G_max值较高;粉质黏土原位最大动剪切模量G_0显著高于室内最大动剪切模量G_max。说明取样、运输及制样等过程存在对土样的扰动作用,仅借助室内试验难于还原土体原位刚度特性;原位剪切模量随着冻融循环次数的增加而减小,冻融作用的影响随着应变的增大逐渐减弱,当应变超过工作应变γ_DMT时,可忽略冻融循环对土体动剪切模量的影响,根据工程实际应变范围和冻融作用合理选用土体刚度值是深季节冻土区工程设计的关键。

In-situ dynamic shear modulus and decay characteristics of silty clay under cyclic freezing-thawing

According to the actual soil state of geotechnical structures in deep seasonally frozen region,systematic in-situ seismic dilatometer tests and laboratory resonant column tests were conducted.The in-situ G-γdecay characteristics of silty clay were investigated based on the in-situ maximum shear modulus and laboratory decay characteristics.The results show that freezing and thawing cycles exert great impacts on the G-γdecay characteristics of silty clay.The maximum dynamic shear modulus G max drops from 97.9 MPa to 53.6 MPa over successive freezing-thawing cycles.The maximum attenuation occurs in the prophase of cyclic freezing-thawing process.After several freeze-thaw cycles,the normalized G max-γcurves shift upwards.The G/G max-value of the specimens subjected to freezing and thawing cycles is relatively higher,which indicates that cyclic freezing and thawing can relieve the shear modulus decay.On the other hand,the in-situ dynamic shear modulus G 0 by in-situ SDMT was greater than the G max from laboratory RCT.This is due to the soil disturbance during sampling and transportation,and suggests the necessity of in-situ tests.G 0 decreases with freezing and thawing cycles.Moreover,the influence of freezing and thawing decreases with shear strain and is negligible when the shear strain exceeds the working strainγDMT.Rational selection of the value of soil modulus with the consideration of actual strain level and freezing-thawing is the key for the engineering design in deep seasonal frozen area.