Three Axis Experimental Study On Cyclic Deformation Characteristics Of Rockfill

Rockfill material is the main filling material for earth rock dam and other geotechnical structures.The dynamic Poisson ratio,dynamic shear modulus and damping ratio of the rockfill materials are the key calculation parameters for the seismic response analysis and seismic design of these geotextiles.However,due to the end effect of three axis specimens and the limitation of measurement technology,it is difficult to accurately measure the radial deformation of the specimen under dynamic load.In view of the above problems,this paper uses the newly developed large three axis instrument for long cube specimen and combined with local test technology to realize the direct measurement of the local radial strain of the specimen,and solve the difficult problem of measuring the inaccurate measurement of the radial deformation of the specimen by the traditional method.The main research results in this paper are as follows:(1)In view of the problem that the traditional large three axis instrument can not directly measure the radial deformation of the specimen,a large three axis instrument for measuring the axial and radial deformation of the three axis specimen is developed in this paper.The axial and radial deformation of the local position are measured directly by the placement of multiple local displacement LDT at different positions of the specimen,and the dynamic deformation characteristics of the rockfill materials at different heights are studied.(2)According to the test results,the difference between radial deformation and dynamic Poisson's ratio of samples obtained by local measurement method and conventional measurement method is compared.Under the same test conditions,due to the influence of the"reservoir area"in the end of the sample,the average radial deformation obtained by conventional measurement method is larger than that directly measured by the local measurement method.Under the conditions of different confining pressure,the conventional measurement method is concentrated about 0.45 of the dynamic Poisson's ratio,and the local measurement method is obtained.The dynamic Poisson ratio is distributed between 0.20~0.40and the results of the analysis show that the results measured by the local measurement method are more consistent with the deformation characteristics of the soil,and can reflect the true deformation state of the specimen.(3)The influence of dynamic Poisson's ratio on dynamic shear strain and dynamic shear modulus of rockfill is compared.The experimental results show that the shear deformation of the rockfill material will be underestimated when the confining pressure is low,and the shear deformation will be overestimated when the confining pressure is low.Under the same confining pressure,the variation of the Poisson's ratio will lead to the obvious difference between the?_d~?_a and G_d~?_d curves.There is a growing difference between the increase of confining pressure.(4)The dynamic deformation characteristics of specimens at different heights were studied.Three dynamic parameters of the dynamic Poisson ratio,the dynamic shear modulus and the damping ratio are studied.The influence of the factors such as the confining pressure,the dynamic stress ratio,the dynamic shear strain and the mean principal stress on the dynamic parameters of the rockfill are analyzed and summarized,and the fitting formula of the shear modulus and the damping ratio with the dynamic shear strain attenuation is given.Based on the test results,the maximum dynamic shear modulus and maximum damping ratio of rockfill are obtained.