压实黄土平面应变方向的主应力特性

Characteristics of principal stress of compacted loess in plane strain direction

对不同含水率压实黄土进行等小主动主应力σx的平面应变三轴试验,研究了含水率w及小主动主应力σx对加载过程中平面应变方向主应力σy特性的影响,根据试验结果提出了描述平面应变方向与其它方向主应力双线性关系的表达式,验证了基于典型强度准则的中主应力公式描述压实黄土σy变化的适用性。研究结果表明:在等向固结及初始加载阶段,平面应变方向主应力不是中主应力σ2,而是小主应力σ3;平面应变方向主应力比σy/σx随着主动主应力比R的增大先平缓后快速增大,转折点前后主应力之间分别呈线性和非线性关系;转折点处的主动主应力比Rz大于平面应变方向主应力由小主应力转变为中主应力临界点处的主动主应力比Rc,w及σx对Rz的影响较大,对Rc的影响很小。R较小时,w及σx对加载过程中σy/σx几乎没有影响。主应力参数K（=2σy/（σx+σz））与主动主应力比R之间呈双直线;前段为水平直线,K近似为常数Kc;后段为倾斜向上的直线;Kc及直线斜率m与w及σx的大小无关。提出的双线性函数能较好地预测压实黄土加载过程中σy的变化特性,而仅在土样破坏时,基于Lade-Duncan及SMP准则的中主应力公式的预测值与试验值比较接近。

The plane strain triaxial tests in which the minor active principal stress（also called σx） keeps invariable are performed on the compacted loess with different water contents. The influences of σx and water content（also called w） on characteristics of the principal stress in plane strain direction（also called σy） during loading are studied. Based on the test results, the expressions describing the bilinear relationships between the principal stress in the plane strain direction and that in other directions are proposed. It is verified whether or not σy can be predicted by the expressions for the intermediate principal stress based on different strength criteria for compacted loess. The test results show that σy is not the intermediate principal stress（also called σ2） but the minor principal stress（also called σ3） during the isotropic consolidation and the initial loading stage. The ratio of the principal stress in the plane strain direction to the minor active principal stress（also called σy/σx） fast increases after the gentle development stage with the increase of the ratio of the major active principal stress to the minor one（also called R）, and the relationships between the principal stresses are respectively linear and nonlinear before and after the turning point. The ratio of the major active principal stress to the minor one at the turning point（also called Rz） is larger than that at the critical point where σy transforms σ2 to σ3（also called Rc）. w and σx have obvious influences on Rz but little ones on Rc. The effects of w and σx on σy/σx are little as R is small. The relationships between the principal stress parameter（=2σy/（σx+σz）, also called K） and R can be describedas two-stage lines. The one is horizontal and K is constant Kc in the first stage. The other one is inclined upward in the second stage. The slope m and Kc are irrelevant to w and σx. The change of σy during the loading can be better predicted by the proposed bilinear function. The predictedresults are approximately equal to the test ones only at the failure of soil samples, using the expressions for the intermediateprincipal stress based on the Lade-Duncan and the SMP strength criteria.