Dynamic Properties Of Clay-sand Interbedded Soft Clay In Ningbo Area And Long-term Deformation

The marine and alluvial plains in the southeastern coastal area of China are widely distributed with sand intercalated strata,including sandy lenticular bodies and interlayers of small thickness.The survey results of Ningbo subway show that there are a large number of loose sandy lenticular layers with thickness only about 10 cm in the strata of the subway tunnel project.In this paper,by investigating and collecting geological survey data in Ningbo metro project,the strength characteristics and deformation characteristics of in-situ sandy lenticular soft clay obtained by drilling and sampling for representative sandy lenticular layers in the metro project are studied under long-term cyclic loading;and the dynamic characteristics and long-term deformation of remodeled soft clay with different sand content lenticular layers are also studied.The research results are as follows:(1)In order to study the real stress state of soft clay soils with natural sand interlayers under static load embankment,isotropic consolidation undrained and consolidation drained shear tests of soft clay soils with in-situ sand interlayers were carried out,and the test results showed that the stress-strain curves of both consolidation undrained and consolidation drained shear tests conformed to the strain hardening curves;the reason for this is that the porosity of the sand interlayers in the soft clay soils was reduced so that the resistance of the specimens to deformation was increased.(2)An experimental study on the dynamic properties of soft clay soils in the in-situ sandwich layer under different loading frequencies was carried out.It is shown that the shear modulus of unidirectional vibration,the dynamic shear modulus of the specimen develops faster at the early stage of vibration,while the dynamic shear modulus changes in a smaller interval under bidirectional vibration,i.e.,the development of the accumulated plastic strain of the specimen will be promoted by unidirectional vibration,and the compressive stress generated by bidirectional vibration will hinder the development of the accumulated plastic deformation.In addition,with the increase of cyclic stress ratio,when r_c=0.225,the phenomenon of stress path gradually approaching the K’_f line appears.In this paper,the critical cyclic stress ratio of soft clay soil in the in situ sandwich layer is around r_c=0.225.(3)The dynamic properties of remodeled soft clay soils with different sand contents were investigated,and it was shown that:the higher the loading frequency,the stronger the resistance of the specimen to deformation under the same circumferential pressure and cyclic dynamic stress;and the higher the frequency,the smaller the corresponding final damping ratio under the same load.In addition,the gradual increase of sand content,there is a critical sand content rate(r_s=20%),because at this time,the sand particles in the soil do not participate in the structure of the soil,but on the contrary,it reduces the content of the clay particles composing the soil skeleton,which severely damages the structure of the soil,resulting in a smaller dynamic stress under the same dynamic strain,a decrease of the dynamic shear modulus,and an increase of the damping ratio instead.(4)The relationship between the cumulative plastic deformation due to long-term cyclic loading with the number of cycles was investigated for soft clay soils with in-situ sand interlayers and soft clay soils with remodeled sand interlayers of different sand content at different frequencies using a double logarithmic coordinate system.It is shown that there is a good linear relationship at N=4000 cycles in a double logarithmic coordinate system.The cumulative strain and cyclic vibration times for remodeled soft clay soil with different sand content still have good linear relationship at N=4000 times in double logarithmic coordinates.When the sand content r_s=20%,the cumulative strain develops rapidly after a large number of cycles.(5)The deformation pattern and prediction model of the reshaped soft clay soil under long-term cyclic loading with different loading frequencies were investigated.It is shown that the frequency has a large effect on the accumulated axial deformation of the specimens.With the increase of load frequency,the axial displacement of the specimen also increases.Meanwhile,by introducing the quadratic function of load frequency,the prediction model of accumulated axial deformation under different frequencies and different sand contents can be obtained,and the test results are in good agreement with the results of the improved prediction model.(6)The deformation patterns and prediction models of remodeled soft clay soils with different sand content under long-term cyclic loading were investigated.It is shown that the sand content also has a great influence on the cumulative deformation of the remodeled specimens.With the increase of the sand content of the specimen,the axial displacement of the specimen reaches the maximum at r_s=20%.Meanwhile,by introducing the cubic function of sand content,the prediction model of cumulative axial deformation under different sand content can be obtained,and the experimental results are in good agreement with the improved prediction model results.