热湿循环作用下砂岩加卸荷力学特性研究

Experimental study on loading and unloading mechanical properties of sandstone under heat and wet cycles

夏季高温季节频繁的降雨与蒸发过程,使岩石(体)经常性的处于热湿循环的交替作用。鉴于此,以三峡库区砂岩为研究对象,开展不同热湿循环后的三轴加载与卸荷试验,从变形、强度以及破坏模式等方面研究热湿循环对砂岩力学特性的影响规律。研究结果表明:(1)砂岩三轴加载、卸荷的峰值强度随着热湿循环次数的增加而不断减小,这种劣化效应在循环前期表现的较为明显。可采用对数函数对其变化规律进行描述。(2)砂岩热湿循环三轴压缩破坏模式以剪切破坏为主,主要表现为单剪切面的剪切破坏和"X","Y"型共轭剪切破坏,在破坏面上出现明显的滑移现象,在其端部均形成含有圆锥的对角破坏。(3)砂岩热湿循环三轴卸荷破坏以剪切破坏为主,伴有张拉劈裂破坏与拉剪破坏,与三轴压缩破坏相比,砂岩破坏的主裂纹和次生裂纹更多,破坏形式更复杂,破坏程度也更为严重。随着热湿循环次数的增加,砂岩的裂纹扩展情况越复杂。(4)砂岩的力学性质参数随着热湿循环作用呈下降趋势,且变化趋势一致。定义热湿循环劣化系数W,即归一化函数中ln(n+1)的系数绝对值来评价热湿循环作用对力学参数的劣化强弱程度,该参数可为后续建立岩石随热湿循环劣化的损伤模型提供一定的理论基础。

During the high temperature season of summer,frequent rainfall and evaporation result in that rock（rock mass） is under the alternate action of heating and wetting cycles. In this study,the variation of mechanical properties and the effect of heating and wetting cycles were investigated through the triaxial loading and unloading tests to intact sandstone samples under cyclic heating and wetting. The peak strength of triaxial loading and unloading of sandstone was found to decrease with the increasing of heating and wetting cycles and the degradation effect is more obvious in the early cycles. The failure mode of sandstone under triaxial compression after heating and wetting cycles is mainly the shear failures,which are the one with a single shear plane and the conjugated shear failure of ＂X＂and＂Y＂type. The slipping phenomena appears in the failure surface and the damage occurs along the cone at the end of the sample. The triaxial unloading failures of sandstone under heating and wetting cycles are mainly the shear failure accompanied by the tensile splitting failure. Some specimens have the tensile and shear failures at the same time. The sandstone has more primary and secondary cracks,the modes of failure are more complex,and the extent of damage is more severe than the case of triaxial compression failure. The mechanical properties of sandstone decrease with the increase of heating and wetting cycles,and the trend of variation is consistent. The deterioration coefficient W due to heating and wetting cycles,that is,the absolute value of the coefficient of the term ln（n＋1） in the normalized function,was defined to evaluate the deterioration degree of the mechanical parameters in the heating and wetting cycles.