Experimental Study On Permeability Of Contact Clay In The Core Wall Of High Rockfill Dams During Shearing

Clay is extensively used as a construction material of core wall in rockfill dams because of its low permeability and wide availability.However,the deformation moduli of the compacted clay core,the dam shell and the bedrock of the canyon are different,that may cause concentrated large shear deformation in the clay core in the form of shear bands,which parallel to the flow direction of the dam,due to the differential settlement.Concentrated leakage may occur in the large shear bands under the action of high seepage water pressure after impoundment,which gives a high risk of seepage failure.In order to coordinate the deformations of the dam abutment and the canyon,and make the deformations of core wall concentrated,the contact clay is usually filled in this area.So it is necessary to research the permeability evolution of compacted contact clay in the process of large shear deformation.Hence,a series of seepage tests were conducted by using a newly improved ring-shear radial-seepage apparatus.This paper studied the permeability evolution of the compacted contact clay with different compaction densities under the coupling of surcharge pressure,shear displacement and seepage pressure,and then revealed the seepage mechanism and seepage failure pattern of the contact clay during shearing.The main work and conclusions are as follows:(1)For the urgent need of the permeability safety evaluation of the shear bands in the clay core of high rockfill dams,a ring-shear radial-seepage apparatus was improved rely on a patent.Then illustrated the testing principle,showed the specimens preparation,and deduced the calculation formulas of the tests to ensure that apparatus can meet the functional requirement of the research on permeability in compacted clay during large shear deformation.(2)The contact area between the abutment and the canyon has a high risk of concentrated leakage in the core wall.To study the permeability of this area during shearing,the testing soil adopted a typical contact clay in the core wall of a high rockfill dam.For the clayey soil,the basic physical parameters were determined according to the standard.Then the reliability of the apparatus is verified using the contact soil.(3)The seepage tests during shearing and seepage failure tests after shearing were conducted on hollow cylindrical specimens with different compaction densities,then the transmissivity coefficients and the critical hydraulic gradients of the compacted specimens were measured under different surcharge pressures after experiencing large shear displacement.The test results showed that the change of permeability of compacted clay after shearing depends on the relative magnitude of the applied surcharge pressure and the preconsolidation pressure.When the surcharge pressure is less than the preconsolidation pressure,the overconsolidated soil in the shear bands breaks into small fragments,which stagger and climb to form cracks and leakage channels,resulting the increasing of the transmissivity coefficients and the decreasing of the critical hydraulic gradients obviously;on the contrary,when the surcharge pressure is greater than the preconsolidation pressure,the soil in the shear bands will be compacted more tightly,the transmissivity coefficients of the shear bands do not change significantly during shearing,it still has a good resistance to against seepage failure.Compared with the conventional hydraulic fracturing hypothesis,this testing conclusion can explain the position of concentrated leakage in the core wall and determine the triggering hydraulic conditions of seepage failure more rationally,which is more in line with the engineering practice.