Mechanical Behavior And Engineering Application Of Unsaturated Chloride Saline Soil-Structure Interface Based On Freeze-Thaw Action

In the special climate of Northwest China,freezing and thawing,as a weathering process,has repeatedly changed the microstructures and physical properties of the soil,which in turn has strongly influenced the interaction between soil and structure.According to the influence of freezing and thawing on the mechanical properties of saline soil-structure contact surface,the unconsolidated and undrained direct shear tests were carried out to study the effects of freezing and thawing cycles,salt content and matrix suction on the mechanical properties of unsaturated chloride saline soil-steel block and concrete block contact surfaces.Through the use of the finite element software ABAQUS,the experimental parameters and the number of freeze-thaw cycles were introduced to model and the actual project was analyzed.The main research contents include the following points:(1)When the salt was not contained,the mechanical parameters(cohesion and angle of internal friction)of the contact surface increased first and then decreased with the increase of freeze-thaw cycles.When the salt was contained,the cohesive force of the contact surface decreased,but the internal friction angle slightly increased with the increase of the number of freeze-thaw cycles.Before and after freezing and thawing,the mechanical parameters of saline soil interface first decreased and then increased with the increase of salt content,and there were threshold values of salt content.When not experienced freezing and thawing,the threshold value of the mechanical parameters of the steel contact surface was about 8%,and the threshold value of the mechanical parameters of the concrete interface was about 6%,but the threshold changed with the increase of the number of freeze-thaw cycles.The steel contact surface had a threshold of about 10% and the threshold was about 8% for concrete contact surfaces.(2)In the whole direct shear test process,the shear stress-shear displacement curve of the contact surface of the chloride saline soil-steel block was roughly divided into the linear elastic deformation stage and the strengthening stage.When the vertical load was small,it showed weak hardening,and no obvious strain softening phenomenon were observed.For the chloride-salt soil-concrete block contact surface shear,stress-shear displacement was divided into linear elastic deformation stage,strengthening stage,softening stage and flow stage.The suction of the contact surface matrix generally decreased with the increase of the number of freeze-thaw cycles andeventually stabilized.As the salt content increased first,it decreased and then increased.The threshold value of the matrix suction salt content was about 10%.The suction force of the contact surface matrix affected the shear strength of the interface.When the salt content was 0%,the shear strength of the contact surface increased first and then decreased with the decrease of the matrix suction force.When the salt content was greater than or equal to 2%,as the suction of the matrix decreased,the shear strength of the contact surface did not decrease,but was proportional to the area of action of the matrix suction.(3)By appraising different constitutive models,the applicability of contact surface shear-shear displacement was evaluated.It was found that the Gongppaz model can be well matched with the shear stress-shear displacement curve of the steel block contact surface.The statistical damage model and the shear stress-shear displacement curve of the concrete block contact surface were in good agreement.Based on this finding,a large number of experimental data were used to establish the mechanical model of the interface between unsaturated chloride saline soil and steel as well as concrete.The reliability of the mechanical model was also verified,which indicated that the established mechanical model had good applicability.It can provide reference for the interaction between structure and saline soil in seasonal frozen soil area.(4)Using the finite element software ABAQUS,the experimental parameters and the number of freeze-thaw cycles were introduced to model and analyze the actual project.The frictional resistance and vertical shear stress cloud diagram of the contact between the strip-shaped web and the unsaturated chloride saline soil in the model were compared.The frictional resistance and the experimental shear stress calculated by numerical simulation and theoretical method were compared and analyzed.It was found that the difference was within 10%.(5)By extracting the stress,strain and displacement cloud images in the model,for the same basic material,under the same vertical load,the displacement of the base soil gradually increased with the increase of the number of freeze-thaw cycles.For the steel base material,the displacement of the base soil gradually stabilized after 8freeze-thaw cycles.For the concrete base material,the displacement of the base soil gradually stabilized after 6 freeze-thaw cycles.Under the same number of freeze-thaw cycles,as the vertical load increased,the displacement of the soil at the bottom of the foundation would gradually increase.When the vertical load was greater than the load corresponding to the inflection point(the end of the elastoplastic stage),the displacement of the base soil suddenly became larger and the foundation lost stability.For different basic materials,the error between the simulated bearing capacity of the foundation obtained by the finite element software and the bearing capacity calculated by the modified formula was within 10%,which was satisfactory for the simulation results,indicating that the standard formula revised under the freezing and thawing conditions in this paper had high rationality and reliability when calculating the foundation bearing capacity.