Experimental Study And Failure Mechanism Research On The Permeability Of Shear Bands In Compacted Clay

Several 300m-high rock-fill dams of a 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 subjecting to high water heads.In this paper,through the combination of laboratory test,theoretical analysis and numerical calculation,the seepage properties of high earth-rock dam core wall clay under large shear deformation conditions are studied.The variation rule and internal mechanism of permeability coefficient of compacted clay during shear deformation are revealed.A method to evaluate the safety of dam deformation and seepage control system that under influences of core wall deformation-osmotic coupling is developed.The main work and results are as follows:(1)A series of vertical compression,circumferential shear and radial seepage tests were conducted on a clay using a newly-invented ring shear permeameter.The variations of the hydraulic transmissivities of the shear bands were measured for the specimens of different compaction densities and under different surcharge pressures.Shear seepage failure tests of clay was carried out to studying the seepage stability of samples with different different compacted densities under large shear displacement with consolidation pressure and shear displacement.The seepage calculation model in the process of annular shear deformation is constructed.The results of shear percolation test of soil are explained mathematically.(2)Series of test results and experimental phenomena revealing evolution law,internal mechanism and influencing factors of seepage properties of compacted clay when is being largely sheared.Under high surcharge pressure,the shear band are compressed more tightly and becomes denser,and it ’ s hard to observe hydraulic transmissivity changes of shear band,the shear plane still has good resistance to seepage after undergoing large shear deformation.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,the seepage resistance of the soil shearing plane is greatly reduced,and it is easy to cause osmotic damage.(3)From the test results and the simulation results,it can be found that the over-consolidation ratio affects shear plane’s seepage behavior during annular shear.It seems that the preconsolidation pressure originating from compaction may serve as a threshold value to determine whether the permeability would increase for the clay tested.The test results highlighted an important fact that the heavily over-consolidated clay could 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 conventional hydraulic fracturing hypothesis.