砾粒土卸载-再加载力学特性大型平面应变试验研究

Large-scale plane strain tests on mechanical properties of gravelly soils under unloading and reloading conditions

针对水利和交通工程中平面应变类工程重复加卸荷问题,利用压力室平面应变改造后的西安理工大学大型土工真三轴仪在不同围压条件下对两种砾粒土进行固结排水平面应变卸载-再加载试验,研究了砾粒土在平面应变状态下的应力-应变关系和卸载回弹应变和卸载-再加载回弹坡度等力学特性。结果表明:砾粒土的颗粒圆度、孔隙比、母岩强度和围压均会影响剪切峰值应力及剪胀特性,颗粒圆度好、孔隙比小的砾粒土峰值应力较低且更易表现出剪胀特性,母岩强度低会促进应变软化并降低峰值应力,围压增大可抑制剪胀并使峰值应力增大;体积应变和广义剪应变卸载回弹量与应力水平和围压有关;回弹坡度随应力水平增大先增后减,密实度较大时回弹坡度较大;卸载-再加载过程中出现主应力轴旋转现象,零应变方向的应力并非一直为中主应力;回弹坡度显著大于初始切线坡度,回弹坡度与初始切线坡度之比随围压增大快速减小。

Aiming at the repetitive loading and unloading issue in the plane strain of water conservancy and transportation projects,the large-scale geotechnical triaxial apparatus of Xi'an University of Technology modified with a pressure chamber of plane strain,was used to conduct consolidation and drainage plane strain unloading-reloading tests on two types of gravelly soils under different confining pressures.The mechanical properties investigated include the stress-strain relationship and the unloading rebound strain and unloading-reloading rebound gradient of gravelly soils under plane strain conditions.The results show the particle roundness,pore ratio,parent rock strength and confining pressure of the gravelly soils can all affect the peak shear stress and shear dilation characteristics.The gravelly soils with good particle roundness and small pore ratio have lower peak stress and are more likely to exhibit shear dilation characteristics.Low parent rock strength promotes strain softening and reduces the peak stress,while the increasing confining pressure can suppress shear dilation and increase the peak stress.The volumetric strain and shear strain unloading rebound amount are related to the stress level and confining pressure.The rebound modulus first increases and then decreases with the increase of the stress level,and when the density is high,the rebound modulus is higher;The unloading rebound volumetric strain and the generalized shear strain are influenced by the stress level during unloading and the consolidation confining pressure,while the unloading-reloading rebound gradient is related to the proportion of elastic strain during the loading process.During the unloading-reloading process,the rotation of the principal stress axis is observed,and the stress in the non-strain direction is not always the intermediate principal stress.The rebound gradient is significantly greater than the initial tangent gradient,and the ratio of the rebound gradient to the initial tangent gradient decreases with the increasing confining pressure.