土的清华弹塑性模型及其发展

Characteristics and development of Tsinghua Elasto-plastic Model for soil

土的清华弹塑性模型具有独特的建模方式。在Drucker假说的基础上,无需其他的附加假设,通过试验资料直接确定塑性势面和塑性势函数,选择适当的硬化参数使屈服函数与塑性势函数一致。通过真三轴试验及平面应变试验,利用该模型的建模方法,在π平面上确定了一种双圆弧的屈服轨迹,从而建立了该模型的三维形式,提出了相应的流动规则表达式;在小浪底堆石料的三轴湿化试验基础上,发现湿化应变作为一种塑性应变与通过该点的屈服面正交,因而只需分别确定干土与饱和土的清华弹塑性模型的参数,同时进行各向等压条件下的湿化试验,测量其湿化体应变,就可以计算出在任意应力状态下浸水湿化的应力应变全过程;与湿化的清华模型相似,进行了不同含水率的土的三轴试验,并进行在干土试样中预加冰屑然后在指定应力状态下使其融化均匀增湿的三轴试验,发现屈服函数不变,硬化参数可以表示为塑性应变和含水率的函数,从而绕过了基质吸力这一变量,建立了非饱和土的清华模型,试验表明它可以合理地预测从干试样增湿到其他含水率的应力应变全过程;密实的永定河砂的试验表明,在相同应力状态下,应变硬化段与应变软化段的塑性应变增量的方向是一致的,将硬化参数表示为塑性功的函数,则可以描述土的应变软化,模型试验结果表明了建立的反映应变软化的清华模型可以合理地计算浅基础的荷载沉降关系;将等向硬化改为旋转硬化,就可以计算砂土在减载和循环加载下的应力应变关系。清华弹塑性模型是一个具有很大发展空间的模型。

Unlike other elasto-plastic models of soil, Tsinghua Elasto-plastic Model was developed in a different way, in which the plastic potential surface and the function of the model was directly determined from experimental data without any other assumptions. According to the Drucker＇s postulation, the yield function was identical with plastic potential function by selecting proper hardening parameter. The yield locus of the model, formed by 2 circular arcs on π plane, was developed by using true triaxial tests and plane strain tests. A three-dimensional Tsinghua Model was then proposed and the three-dimensional flow rules were presented. Based on the wetting test results of Xiaolangdi rock fill, it was found that as a type of plastic strain, the wetting strain vector was normal to the yield locus of the model. Provided that the model parameters of dry soil and saturated soil and the wetting volumetric strain in isotropic compression test were determined, the whole stress-strain relationship during wetting of the sample could be simulated by the model. An unsaturated soil model was also developed following the similar approach to the wetting Tsinghua Model. In the triaxial test of unsaturated soil, some ice crumbs weremixed into the dry sample before test and the ice crumbs were melted at determined stress states. From the test results it was found that yield surfaces of soil with different moisture contents were the same and hardening parameter was the function of plastic strain and Moisture content. In the Tsinghua Model for unsaturated soil, the strain increment produced by increments of stress and moisture content could be calculated conveniently without appearance of matrix suction. The whole stress-strain relationship in triaxial tests with increasing moisture contents was predicted by the model reasonably, Additionally, based on the behavior observed in triaxial compression tests with dense Yongding River sand, the model was still developed to simulate strain-softening characteristics of the sand, in which the hardening parameter was assumed to be the function of plastic work. The model was verified in computing the displacement vs. load of a shallow footing on dense sand by hydraulic gradient model tests. If the rotational kinematic hardening is applied in the model instead of isotropic hardening, it could be used to model the soil behavior with large stress reversals, by which stress strain relationship during load decreasing or cyclic loading can be simulated. It is thought that Tsinghua Elasto-plastic Model is a promising model for soil.