干湿交替作用下非均质土体的变形与失稳机制

Granite weathering profiles with heterogeneous soil structure are widely distributed in South China. The significant spatial variation of particle size distribution generates the nonlinear and nonuniform distribution of soil structure and physico-mechanical properties along soil profile. The expansion and shrinkage deformation resulted from wetting and drying cycles influencing the formation of soil cracks induce the irreversible degradation of soil structure and the attenuation in soil shear strength, which accelerate soil failure and the development of soil erosion. Physico-mechanical processes of granitic soil profile including expansion and shrinkage, soil crack distribution, shear strength and slope failure are investigated in this project with the techniques consisting of filed investigation, in situ monitoring, laboratory analysis and simulation. In accordance with the obtained results from laboratory study, the quantification system of soil shrinkage in 3D is established, and the disciplines about the interaction of soil texture and wetting-drying cycles on soil volume deformation and soil shear strength degradation are explored. Response models with regard to physico-mechanical properties and soil pore structure facilitate the explanation of the deformation process and slope failure mechanism in heterogeneous soil profile during the wetting and drying cycles. The achievements from this project will contribute to the investigation of hydraulic-gravity compound erosion in South China, and facilitate the improvements of regional soil erosion evaluation and management.

南方花岗岩风化壳作为典型的非均质土体其土壤颗粒组成具有明显的剖面异质性，导致其土壤结构、物理力学性质的非线性和非均匀变化；在干湿交替环境中，土壤的胀缩形变影响裂隙开闭，引起剖面土壤结构和强度的不可逆劣化和衰减，加速土体失稳，促进土壤侵蚀的发生和发展。本项目以花岗岩发育的红壤为例，拟通过野外调查、室内分析与模拟、原位监测等手段，研究不同质地土壤层状构型的非均质土体在干湿交替作用下的胀缩形变、裂隙发育、强度衰减、土体失稳等过程；建立表征不同颗粒组成土壤裂隙分布的三维指标体系；明确土壤颗粒组成与干湿交替共同影响下，土体的胀缩形变与强度损伤规律；揭示非均质层状土在水分变化下的变形过程与失稳机制，构建物理力学行为对土壤孔隙结构变化的响应模型。项目的开展有助于深化对南方地区水力重力复合侵蚀过程机理的认识，对区域土壤侵蚀评价与水土保持治理具有重要的科学价值和理论意义。

土体应力的非均匀分布是其变形及失稳的主要因素。我国南方花岗岩出露面积大，土壤侵蚀严重。本项目以花岗岩风化土体为研究对象，以干湿交替作用下土体的变形与失稳为主线，通过室内控制试验和原位监测等手段，研究了花岗岩风化土体水力学特征、孔隙结构及抗剪强度等对土壤水分变化及干湿循环的响应，主要研究结果如下：（1）花岗岩风化程度的空间异质性是岩土特性非均匀性变化的根本原因，在全风化区内，黏粒、1:1型黏土矿物及游离氧化铁铝的含量随着风化程度的增加而增加；（2）花岗岩风化土体颗粒组成的非均匀性是决定其结构稳定性的关键因子，黏粒含量显著促进了土壤的脱水收缩变形，而砂粒和砾石则对土体的收缩变形具有抑制作用；（3）在自然干湿交替的作用下，花岗岩风化土体体积含水量变异系数达33%，且随着干湿交替程度的增加>1000μm的孔隙含量及不规则孔隙占比逐渐增加，在室内干湿交替控制条件下，土壤中活性孔隙（>3μm），尤其是通气孔隙（>29μm）的含量随着干湿交替次数的增加而增加，而非活性孔隙（<3μm）在10次干湿交替之后呈现不同程度的降低，CT扫描测定的大孔隙含量略低于水分特征曲线计算结果，但两者随干湿交替的变化规律一致且显著线性相关（R2=0.91）；（4）土壤持水能力随干湿循环频度增加而减小，饱和导水率则反之，且土壤进气值及残余基质吸力受干湿循环频度影响更大，而>30μm大孔隙的变化受干湿循环强度的影响更大，高强度的干湿交替加快了裂隙的发育过程，且随着干湿交替次数的增加，裂隙的连通性和复杂性逐渐增加；（5）花岗岩风化土体抗剪强度由颗粒组成和土壤含水量共同决定，当含水量为14~21%时土体的总体抗剪强度最大，且基于基质吸力与抗剪强度的非线性关系，建立了花岗岩风化土体的应力应变模拟方程；（6）干湿交替促进了花岗岩土壤的崩解程度与崩解速率，降低了土壤的抗蚀能力，而干湿交替过程中土体的稳定性对有效抗剪强度及持水能力的变化率最敏感。