常压至高压下中砂剪切特性及应力-剪胀关系

Shear characteristics and stress-dilation relation of medium sand under normal to high pressures

为研究高围压范围内砂土相对密实度和围压对土体强度和变形特性的影响,对3种不同相对密实度砂土试样在常至高围压下进行常规三轴固结排水剪切试验,获得偏应力-轴向应变-体应变关系曲线,同时进行颗粒破碎分析。结果表明:在常至中压范围(0.8 MPa≤σ3≤2 MPa),应力-应变曲线均表现出不同程度的应变软化,其剪胀性随相对密实度增加和围压的降低而增强;当进入高围压范围时(σ3>2 MPa),应力-应变曲线逐渐向应变硬化型转变,试样体积逐渐趋于剪缩。颗粒破碎程度随着围压和密实度的增大而增大,在高围压时由于中密和密砂剪切后期出现了明显的颗粒破碎,导致剪切过程中出现了二次相变。不同密实度土体的破坏内摩擦角和对数围压表现良好的线性关系,拟合确定了破坏内摩擦角随对数围压增加的衰减率,同时基于Bolton应力-剪胀关系拟合确定了试验砂土的临界状态内摩擦角,建立了剪胀指标与初始相对密实度及平均有效应力的关系式,为高压情况下砂土地基稳定性分析等提供强度参数。

The triaxial shear tests on the sands with various relative densities under normal to high pressures are carried out to investigate the effects of density and confining pressure on sand strength and deformation characteristics.Their mechanical properties are analyzed.Among the pressures ranging from normal to medium(0.8 MPa≤σ3≤2 MPa),the strain-stress curves show strain softening to different extents,and the shear dilatancy increases with the increase of relative density and the decrease of confining pressure.When entering high confining pressure range(σ3>2 MPa),the strain-stress curves gradually turn into strain hardening type and the sample volume gradually turns into shear contraction.Prominent particle breakages will occur in the later shearing period of medium and dense sands at high confining pressure,which may lead to the secondary phase transition in the shearing process.The failure friction angles decrease with the logarithmic confining pressure linearly,and the attenuation rates of failure friction angles for different density sands are determined by linear fitting.Based on the Bolton’s stress-dilation relation,the critical state friction angle of the sand is determined,and the relationship among dilatancy index,initial relative density and average effective stress is established to provide strength parameters for stability analysis of sandy soil foundation under normal to high pressures.