压实黏土剪切带渗透特性试验研究

针对中国在建和拟建的300m级高堆石坝心墙黏土(尤其是接触黏土)在大剪切变形、高水头作用下渗透安全性评价的重大需求,采用新研发的土体环剪渗透试验装置对某高堆石坝心墙黏土进行了系列竖向压缩-环向剪切-径向渗流试验,测定了不同压实密度的黏土剪切带在不同竖向压力下导水系数随剪切位移的变化过程,揭示了压实黏土在大剪切变形过程中渗透特性的演化规律、内在机理及影响因素。在高竖向压力下,剪切带在剪切过程中被压缩得更密实,其导水系数在剪切过程中并不增大,剪切带在经历大剪切变形后仍具有良好的抵抗渗流的能力。在低竖向压力下,剪切带内土体破碎、错动、形成空隙,从而导致其导水系数急剧增大。对于所用的黏土,压实过程导致的前期固结压力可以粗略作为判别产生剪切渗漏带的门槛应力值。超固结的压实黏土易产生剪切渗漏带的试验发现,比目前传统的水力劈裂假设可以更好地解释和判别坝体中集中渗漏发生的条件及部位。

Experimental study on permeability of shear bands in compacted clay

Several 300 m-high rock-fill dams with clay core are being constructed or will be constructed in China, raising an urgent need for evaluating the safety against seepage failure of the clay core after experiencing large shear deformation and subjected to high water heads. A series of vertical-consolidation, circumferential-shear and radial-seepage tests are conducted on a clay using a newly-invented ring shear permeameter. The variations of the hydraulic transmissivities of the shear bands are measured for the specimens with different compaction densities and under different surcharge pressures. When being largely sheared, the original arrangement and bonding among particles in a shear band are damaged. Under high surcharge pressure, the shear band is compressed more tightly and becomes denser, and thus little increase can be observed in its hydraulic transmissivity. Conversely, under low surcharge pressure, the shattered fragments in the shear band glide, rotate, roll and climb across each other, leading to a looser packing, and as a result, the hydraulic transmissivity of the shear band increases sharply. It seems that the preconsolidation pressure originating from compaction may serve as a threshold value to determine whether the permeability will increase for the test clay. The test results highlight an important fact that the heavily over-consolidated clay can generate shear bands of remarkably reduced seepage resistance during shearing, which can be used to explain more rationally the trigger conditions and positions of concentrated leakages happened in earth dams than the conventional hydraulic fracturing hypothesis.