干湿循环作用下红壤土孔隙结构与强度特性试验研究

Experimental study on pore structure characteristics and strength properties of red soil subjected to dry-wet cycles

【目的】周期性库水位升降使三峡库区消落带土体受反复干湿交替过程,为了分析干湿循环作用对土体的孔隙结构和强度特性的影响,【方法】针对三峡库区某滑坡消落带红壤土,开展反复干湿循环下的三轴试验和扫描电镜、核磁共振测试,分析干湿循环作用对红壤土微细观孔隙结构与强度特性的影响,研究土体孔隙结构演化规律及其与强度特性之间的关系。【结果】结果显示:干湿循环对消落带土体强度有明显弱化作用,随着干湿循环次数增加,土体剪切强度逐渐减小并趋于稳定;在干湿循环作用下,孔隙扩张向大孔隙演化,土颗粒间距增加,裂隙发育后团聚体崩解、分散为微小团聚体,孔隙间连通性增强,原有微观孔结构基本丧失,逐渐形成新的稳定孔隙结构状态。【结论】土体结构呈现明显的双重分形特征,土体黏聚力和内摩擦角与中巨孔隙分形维数呈现明显关联关系,干湿循环作用使中巨孔隙分布复杂程度降低,孔隙结构更松散,颗粒间咬合作用降低,黏聚力和内摩擦角减小。

[Objective]Periodical rise and drawdown of reservoir water level cause the soils along riparian zones of the Three Gorges Reservoir to experience repeated dry-wet cycles,leading to irreversible changes in the pore structure and strength properties.[Methods]Taking a red soil from a landslide in Three Gorges Reservoir as an example,triaxial tests,scanning electron microscopy and nuclear magnetic resonance tests under dry-wet cycles were performed to analyze the influence of dry-wet cycles on the microstructure and strength properties of the soil sample.The evolution and correlation of soil pore structure and strength properties are addressed.[Results]The results show that applying dry-wet cycles leads to a significant deterioration of shear strength.With increase of dry-wet cycles,the shear strength decreases and tends to a stable level.Under the influence of dry-wet cycles,pore structure expands and the distance between the soil particle enlarges.Aggregate disintegration with fracture development produces a smaller size aggregate and improves the pore connectivity.The microstructure transmits into a new steady state of pore structure.[Conclusion]A double fractal distribution of pore structure is observed,and the cohesion and internal friction angle are closely related to the fractal dimension of bigger pores.The dry-wet cycles reduce the complexity of big pore size distribution,loosen the pore structure and weaken intergranular occlusion,Resulting in a decrease of cohesion and internal friction angle.