加卸载条件下吸力对黄土变形特性影响的试验研究

为了探讨基质吸力和净围压对黄土的变形–强度及卸载–再加载模量的影响,用非饱和土三轴仪对原状Q_3黄土及其重塑土共做了3组27个试验,即9个控制吸力和净围压等于常数的原状Q_3黄土的三轴固结排水剪切试验、9个原状Q_3黄土和9个重塑Q_3黄土的控制吸力和净围压等于常数的三轴固结排水卸载–再加载剪切试验。研究结果表明:基质吸力和净围压均对试样的强度及变形特性有显著影响;原状Q_3黄土的偏应力–应变曲线随净围压的增大从理想弹塑性型演变为硬化型,因其结构性导致偏应力–应变曲线的初始阶段存在交叉现象,试样一直处于剪缩状态;重塑试样在净围压较小时由剪缩逐渐变为较强的剪胀,在净围压较大时则只发生剪缩;表观凝聚力、切线杨氏模量和卸载–再加载模量均随着吸力的增大而增大;而吸力对有效内摩擦角及参数n(原状Q_3黄土的初始杨氏模量随净围压增大而变化的指数)、n_(ur)(原状Q_3黄土的卸载—再加载模量随净围压增大而变化的指数)和n_(ur)'(重塑Q_3黄土的卸载—再加载模量随净围压增大而变化的指数)的影响很小,皆可视为常数。基于试验研究结果,提出了考虑吸力和净围压影响的表观凝聚力、切线杨氏模量和卸载—再加载模量的修正计算公式,完善了非饱和土的增量非线性模型。

Experimental research on influences of suction on deformation characteristics of loess under unloading-reloading conditions

In order to explore the influences of matrix suction and net confining pressure on the deformation-strength and unloading-reloading modulus of loess, three groups of 27 tests are performed using the unsaturated triaxial apparatus for undisturbed Q₃ loess and remolded loess, namely, 9 triaxial consolidated drained shear tests on undisturbed Q₃ loess under controlled suction and net confining pressure, 9 unloading-reloading triaxial tests on undisturbed Q₃ loess and 9 unloading-reloading triaxial tests on remoulded loess under controlled suction and net confining pressure. The research results show that the matric suction and net confining pressure have great influences on the strength and deformation characteristics of the soils. The behavior of deviatoric stress-strain curve of the undisturbed Q₃ loess turns from the ideal elastic-plastic model into hardening type, and the curves cross at the initial stages because of their constitutive property, and the volume of the samples decreases during shearing. And the behavior of the remoulded samples exhibits gradually strong dilatancy when the net confining pressure is small, and shear contraction only occurs when the net confining pressure is larger. Apparent cohesion, tangent Young's modulus and unloading-reloading modulus increase with the increase of the suction. The influences of suction on the friction angle and parameter (the index that the initial Young's modulus of undisturbed Q₃ loess changes with the increase of net confining pressure), (the index that the unloading-reloading modulus of undisturbed Q₃ loess changes with the increase of net confining pressure), and (the index that the unloading-reloading modulus of remoulded Q₃ loess changes with the increase of net confining pressure) are insignificant and can be regarded as constants, respectively.