Physical And Mechanical Properties Of Shanghai Deep Sand: Laboratory Tests And Constitutive Model

The rapid expansion of infrastructures like tunnels,sub-structures in Shanghai,has increased the importance of research in a deeper layer of soil.Most of the underground research works are carried for the clay up to layer 6representing the depth up to 40m.However,with the rapid urbanization,the importance of underground soil has rapidly increased.In this research,the layers 7,9,and 11 mostly formed of sandy soils representing depth up to 150 m are studied by conducting several experiments to investigate the physical and mechanical properties.The microscopic images and particle size analyzers are used to analyze the shape and size of the particles.One-dimensional oedometer tests are performed to investigate the compressibility behavior of the sandy soils at three different relative densities for each layer.Similarly,drained and undrained consolidated static triaxial tests are conducted to analyze the stress-strain behaviors.The influence of particle sizes,density,and angularity on the stress-strain,compressibility is also discussed.Also,by using cyclic mobility model,the simulation of the triaxial test is conducted for different layers.From the same loose sand and eight initial parameters for different drainage and relative densities,the simulation is performed and e soil structural parameters are determined from the simulated results.Among the three layers of sand,layer 9 is composed with the largest mean grain size particles(D₅₀),layer 11 with the smallest D₅₀,and layer 9 with the intermediate D_50.The compressibility index is inversely proportional with the D₅₀as layer 9 has the lowest compressibility and layer 11 has the highest compressibility index among three layers.Also,D₅₀ affects the strain-strain relationship from the consolidated undrained(CU)and consolidated drained(CD)triaxial test results.For layer 11 with smallest D₅₀,specimens underwent strain-softening behavior for all relative densities at CU triaxial test.However,layer 9 specimen with highest D₅₀exhibits strain softening at only 40%relative density,whereas strain hardening behavior was observed at 60%and 80%relative density and layer 7 with the intermediate grain size shows intermediate behavior of strain hardening at 80%relative density and strain softening behavior at 40%and 60%relative density.Similarly,during the CD triaxial test,layer 7 and 11 shows only contractive behavior for all three relative densities,however layer 9 showed dilative behavior at 60%and 80%relative densities.For higher relative density,the volumetric contraction is small and vice versa.Also,layer 9 has the lowest volumetric contraction and layer 11 has highest volumetric contraction among three layers.Similarly,the brittle behavior and secant modulus of sand increases and axial strain at peak stress decreases with the increase in density and mean grain size for all three layers.The critical state friction angle is independent of the drainage condition and relative density but increases with the particle shape and highest for layer 9.The critical state line(CSL)is presented for three different layer with steepness of layer 7 and 11higher in comparison to layer 9.Also,the maximum dilatancy rate for layer 7and 9 is presented and dilatancy not possible for layer 11is predicted.The numerical simulation by using the unified constitutive model is conducted in three stages:compaction,isotropic consolidation,and shear.The simulated results of stress-pore water and strain curves are compared with the experimental results.The rate of loss of over-consolidation increases and loss of initial structure of soil decreases with increase in particle size as layer 9 has loss of over-consolidation was highest for layer 9 and smallest for layer 11,and loss of initial structure was lowest for layer 11.The experimental and simulated CSL are compared and shows that CD triaxial test is more suitable for the study of critical behavior as small shear strain is required to achieve critical condition.Also,plastic volumetric strain is found higher in CD condition in comparison to CU condition from the simulation result.