冻结粉质黏土滞回曲线形态变化特征及其影响因素

Morphological variation characteristics and its influencing factors of hysteretic curve of frozen silty clay

冻土动力学特性与冻土区基础设施工程的设计、运营、维护有着密切的联系。冻土的动力学特性是从冻土的滞回曲线中分析得到的,因此全面分析冻土滞回曲线形态特征及其影响因素对明确冻土动力学特性有着重要的意义。冻土滞回曲线的形态特征可用滞回曲线的轴向动应力幅值、不闭合程度、宽窄程度及面积4种参数来表征。冻土的轴向动应力幅值和动应变幅值所围成的骨干曲线反映了冻土等效变形的大小、非线性效应以及冻土的动弹性特性,冻土滞回曲线的不闭合程度反映了冻土的塑性变形性能,冻土滞回曲线的宽窄程度反映了冻土的黏滞特性,冻土滞回曲线的面积反映了冻土的耗能能力。为此,以我国西北季节冻土区粉质黏土为研究对象开展了负温动三轴试验,试验包括4种土温(-0.3℃、-1℃、-3℃、-5℃),3种土体含水率(14%、16%、18%),3种围压(0.1 MPa、0.2 MPa、0.3 MPa),以及3种加载频率(1 Hz、2 Hz、4 Hz)交叉组成的10种工况,探究了冻结粉质黏土滞回曲线形态特征及其影响因素。根据试验结果,主要分析了4种描述滞回曲线形态特征参数随着振次、围压、土温、含水率、加载频率以及动应变幅值等因素的变化规律。结果表明,振次对冻结粉质黏土滞回曲线形态特征的影响很小。随着动应变幅值增大,冻结粉质黏土的骨干曲线增长模式逐渐由近似线性模式转为明显的非线性模式;滞回曲线的不闭合程度和面积随动应变幅值的增大呈凹形曲线形态增长;宽窄程度呈凸形曲线形态增长。随着围压、含水率、加载频率的增大以及土温的降低,冻结粉质黏土的骨干曲线变得越陡峭,非线性效应表现得越弱;滞回曲线的不闭合程度、宽窄程度和面积都有不同程度的降低趋势。当围压在0.2 MPa以下、含水率在16%以下、土温在-3℃以下时,冻结粉质黏土滞回曲线形态特征随这3种因素的变化更明显。冻结粉质黏土滞回曲线的不闭合程度和宽窄程度受加载频率的影响较为均衡,并没有出现最优影响范围。与冻结粉质黏土滞回曲线的不闭合程度和宽窄程度随各影响因素变化程度相比,滞回曲线的面积受各因素的影响程度相对较小。在4种影响因素中,加载频率和土温对滞回曲线的面积影响较大,含水率的影响次之,围压对滞回曲线面积影响很小。当围压为0.1~0.3 MPa时,冻结粉质黏土的滞回曲线面积-动应变幅值关系曲线随围压变化非常小。

The dynamic properties of frozen soil are closely related to the designing,operating,and maintaining of many infrastructure projects in permafrost areas.The dynamic parameters of permafrost are obtained from the hysteretic curve of frozen soil.Therefore,a comprehensive investigation of the morphological characteristics of frozen soil hysteretic curves is important to clarify the dynamics of frozen soil and its influencing factors.The morphological characteristics of the hysteretic curve of frozen soil can be described by four parameters:the axial dynamic stress amplitude,the degree of non-closure,the width and the area of the hysteretic curve.The specific dynamic characteristics of frozen soil reflected by these four parameters are as follows:the curve enclosed by axi⁃al dynamic stress amplitude and dynamic strain amplitude of frozen soil is called the backbone curve,which re⁃flects the magnitude of equivalent deformation,nonlinear effect,and dynamic elastic properties of frozen soil;the degree of non-closure of frozen soil hysteretic curve reflects the plastic deformation property of frozen soil;the width of the frozen soil hysteretic curve reflects the viscous hysteretic property of frozen soil;the area of the frozen soil hysteretic curve reflects the energy dissipation capacity of frozen soil.To this end,a negative temper⁃ature dynamic triaxial test was carried out on silty clay in the seasonally frozen soil area of Northwest China.The experiment included ten testing conditions,which composed of four soil temperatures(-0.3℃,-1℃,-3℃,-5℃),three soil water contents(14%,16%,18%),three confining pressures(0.1 MPa,0.2 MPa,0.3 MPa)and three loading frequencies(1 Hz,2 Hz,4 Hz).According to the test results,the variation trends of four morphological characteristic parameters describing hysteretic curves with vibration cycle number,confining pres⁃sure,soil temperature,water content,loading frequency,and dynamic strain amplitude are analyzed.The re⁃sults show that the effect of vibration cycle number on the morphological characteristics of the hysteresis curves of frozen silty clay is small.With increasing the dynamic strain amplitude,the growing mode of the backbone curve of frozen silty clay gradually changes from an approximate linear mode to an obvious nonlinear mode.And the degree of no-closure and area of the hysteretic curve increase in a concave curve shape,while the width increases in a convex curve shape.With the increase of confining pressure,water content,loading frequency and the decrease of temperature,the backbone curve of frozen silty clay become steeper and the nonlinear effect is weaker,while the degree of no-closure,the width and area of the hysteretic curve tend to decrease in varying degrees.When the confining pressure is below 0.2 MPa,the water content is below 16%and the temperature is below-3℃,the morphological characteristics of the hysteretic curve of frozen silty clay change more obviously with these factors.The degree of non-closure and width of the hysteretic curve is more balanced by the loading frequency,and there is no optimal influence range.Among the four influencing factors,loading frequency and soil temperature have a greater influence on the area of the hysteretic curve,and the water content has the latter influence,while the overall influence of the confining pressure on the area of the hysteretic curve is small.The area of hysteretic curve-dynamic strain amplitude relationship curves of frozen silty clay under the three kinds of confining pressure used in this test almost coincide.