干湿循环效应下花岗岩残积土结构损伤的多尺度研究

A multi-scale study on structure damage of granite residual soil under wettingdrying environments

为了明晰炎热多雨气候对花岗岩残积土结构的损伤特征,对0~8次的干湿循环过程中的原状样开展三轴固结不排水剪切(CU)、计算机层析识别(CT)、核磁共振(NMR)与扫描电子显微镜(SEM)试验,从宏观–细观–微观尺度分析土体的损伤机制。结果表明:干湿循环损伤效应在宏观上表现为强度的衰减,细观上表现为裂隙的发展,微观上表现为颗粒与孔隙结构的演变;有效黏聚力和有效内摩擦角随着循环次数增加呈指数型的衰减规律,但黏聚力的衰减幅度明显更高;细观裂隙的发展过程可以划分为萌发期(0~1次循环)、发展期(2~5次循环)和稳定期(5~8次循环);孔隙体积分布曲线呈双峰分布,根据孔径大小将孔隙分为4个类别,随着循环次数增加,微孔的体积占比降低,中孔和大孔的体积占比增加;黏土颗粒在干湿循环过程中逐渐疏松,粒间孔隙不断扩张、贯通,形成连通裂隙;在干湿交替过程中,浸水时亲水性黏土矿物发生膨胀,干燥时微观拉应力增加,同时胶结物质分解流失,3种因素共同驱动细观裂隙的扩展和连通,引起结构的疲劳损伤,最终导致宏观力学性能的衰减。该研究成果可为花岗岩残积土力学特性与环境损伤效应的认识提供有益参考。

To study the environmental effect with hot and rainy feature to the structural characteristics of granite residual soil under multiple dry-wet cycles,the triaxial consolidation undrained(CU),computer tomography(CT),nuclear magnetic resonance(NMR)and scanning electron microscope(SEM)tests were performed.Then,a multiscale study from microscopic,mesoscopic,and macroscopic perspective about the damage mechanism of the soil.The test results show that the wetting-drying effect shows a change in particle and pore structure at microscopic scale,an expansion of fractures at mesoscopic scale,and a decrease in strength at macroscopic scale.The effective cohesion decreases exponentially with the increase of the wetting-drying cycles and the effective internal friction angle decays in a small range.The development process of meso-fractures can be divided into three phrases,which are the germination stage(0 to 1 cycle),the development stage(2 to 5 cycles),and the stable stage(5 to 8 cycles).The pore volume distribution curves of granite residual soil show a bimodal shape.The pores can be divided into four types according to pore size.With the increasing number of wetting-drying cycles,the volumetric proportion of microscopic pores decreased,while that of medium pores and large pores increased drastically.During the wetting-drying process,the clay particles gradually loose and the intergranular pores expand and connect to form connected cracks.Due to the wetting-drying effect,the hydrophilic clay minerals expand during a humidifying process,the microscopic tensile stress increases during a drying process,and the cemented materials decompose and lose.The three factors jointly drive the expansion and connectivity of fractures and cause fatigue damage to the soil structure,leading to the attenuation of mechanical properties of soil.This study provides a useful reference for the understanding of mechanical properties and structural damage rules of granite residual soil in hot and rainy climate.