Study On The Static And Dynamic Characteristics Of Sandy Soil Under The Coexistence Of Clay And Silt

Lots of saturated soils in the ocean and on land are often subjected to static and dynamic loads such as typhoons,waves,earthquakes,traffic vibrations,and rainfall,and a large part of these soils are mixed soils which composed of various particles,such as sand,silt,and clay.It is of great theoretical significance and engineering value to study the influence of fines type and fines content(FC)on the static and dynamic characteristics of sandy soil.In this paper,through a series of static and dynamic consolidation undrained tests and mesoscopic observation tests,the influence of clay content(CC)on the consolidated undrained strength of sandy soil,the influence of FC and clay silt ratio(CS)on the static and dynamic strength of sandy soil was found,and the influence mechanism was clarified;Through the one-dimensional consolidation test and meso observation test,the influence of FC and CS on the compression and swelling curve of soil was found,and the influence mechanism was clarified.On the basis of considering the FC,fines particle size,plasticity index of clay,CS and internal friction angle,an improved super-subloading Cam-Clay model was proposed and its validity was verified.The main research results of this paper are as follows:(1)Based on the strain controlled triaxial apparatus,the effect of CC on consolidated undrained shear characteristics of sandy soil was studied under the same sand skeleton void ratio.The results indicated that the static liquefaction occurred for the specimens with CC=0,3%,5%,7%,and 10%under different confining pressures.While no static liquefaction occurred for the specimens with CC=12%and 15%.From the mesoscopic observation tests of dyeing sandy soil with different CC,it could be seen that when CC≤10%,the clay particles were mainly filled in the pores formed by sand particles,distributed on the surface of the sand particles and scattered at the contact points between the sand particles,and this part of clay could lubricate and bond the sand particles.The binding effect of the clay particles on the sand particles was further enhanced when CC=12%and 15%.(2)Based on the strain controlled triaxial apparatus,the effect of FC and CS on the consolidated undrained shear characteristics of sandy soil was studied under the same sand skeleton void ratio and void ratio.The results indicated that the consolidated undrained strength of sandy soils with the same FC and different CS was different.Except for the specimens with FC=5%,the consolidated undrained strength of the specimen decreased and the pore water pressure increased monotonously with the increase of CC.The consolidated undrained strength and the pore water pressure of the specimen with FC=5%-CS=1 was smallest and largest respectively.When the sand skeleton void ratio was constant,for sandy soils with the same CS,the consolidated undrained strength increased and the pore water pressure decreased gradually with the increase of FC,and the internal friction angle of the specimens showed an increasing trend generally.However,when the void ratio was constant,for sandy soils with the same CS,the consolidated undrained strength decreased and the pore water pressure increased gradually with the increase of FC,and the internal friction angle of the specimen showed an decreasing trend generally.(3)Based on the mesoscopic observation tests of sandy soil with dyeing fines,it could be seen that when FC=3%,most of silt particles were filled in the pores formed by sand particles,and the clay particles were scattered on the surface of the sand particles.A small amount of clay aggregates with small diameter were formed and filled in the pores formed by the sand particles when CS=4.When FC=5%,there were more clay-silt aggregates in the specimen with FC=5%-CS=1,and these aggregates were filled in the pores formed by the sand particles,bound the sand particles.When FC=7%and 10%,a large number of silt particles acted as the soil skeleton when CS was small.While there were more clay-silt aggregates and clay aggregates when CS was large,and these aggregates were filled in the pores formed by the sand particles,bound the sand particles and acted as the soil skeletons further.(4)Through the CKC cyclic loader triaxial testing system,the dynamic triaxial test was conducted to study the effect of FC and CS on the dynamic strength of sandy soil under different void ratios.The test results show that the dynamic strength of the same specimen under different failure standards was different.The FC and CS had different effects on the dynamic strength of different specimens under different failure standards.A new dynamic pore water pressure prediction model has been proposed in this paper,which can predict the development trend of dynamic pore water pressure more accurately.Morever,the sudden change point of pore water pressure during its growth can be also captured through this prediction model.(5)The GDG series high-pressure consolidation testing system was used to conduct the consolidation tests on the sandy soil.The effects of FC and CS on the compressibility and resilience characteristics of sandy soil under different void ratios were discussed.The results indicated that the influence of FC and CS on the compressibility,resilience and corresponding indexes of sandy soil was important.(6)Based on the recognition of the characteristics of clay particles,an empirical formula of clay participation coefficient had been proposed,in which the CC,clay size and the plasticity index of clay could be considered.The validity and rationality of the proposed empirical formula of clay participation coefficient were fully demonstrated by the test results in this paper and the existing results.Based on the study of the super-subloading Cam-Clay model,and combined with the mechanical behavior of sandy soil in this paper,the empirical formulas of degradation parameter of overconsolidation state mR and degradation parameter of structure mR*were proposed.In these formulas the FC,CS and the internal friction angle were treated as the independent variables.Finally,the empirical formulas of clay participation coefficient,mR and mR*were used to modify the relevant parameters in the super-subloading Cam-Clay model,and the constitutive model of sandy soil with clay and silt has been established.This model could describe the influence of the clay content,silt content,fines content,clay silt ratio,the plasticity index of clay,the size of clay particle,the size of silt particle,the size of sand particle and the internal friction angle on the characteristics of sandy soil.And the model was verified by simulating the results of dynamic triaxial tests.