Study On Shear Band Thickness Of Silty Clay-concrete Interface In Changchun

With the rapid development of civil engineering and infrastructure construction in China,a lot of engineering problems have emerged caused by the destruction of soilconcrete interface in the context of deep foundation projects,underground integrated corridors,subway tunnels,reservoirs and river berms,road foundations and other projects.These engineering problems are caused by the strain localization area near the interface when the soil and concrete interface,which is called shear band.The development of shear band affects the bearing capacity and deformation characteristics of the soil-concrete interface system.Although the shear band phenomenon is universal,there are essential differences in the thickness and evolution mechanism of shear band in different media.Does shear band exist in silty clay and concrete projects in the seasonal freeze area? What factors affect the thickness of shear band? How to determine the thickness of shear band? What is the nature of soil stresses and deformations in shear band? The investigation of these scientific questions is of great significance for the engineering design and safety protection of structures involving soil and concrete.Based on the above-mentioned scientific problems,this paper investigated the shear band thickness and evolutionary failure mechanism of interface from laboratory direct shear tests,theoretical models of interface,microscopic structural characteristics and numerical simulations for a silty clay-concrete interface in Changchun area.The main contents are as follows:1.The shear stress-shear displacement curves and shear strength values were obtained based on the laboratory direct shear tests on the silty clay-concrete interface with different normal stresses,moisture content of soil samples,concrete surface roughness and number of freeze-thaw cycles.The results show that the shear strength value of the silty clay-concrete interafce is positively correlated with the normal stress and the surface roughness of the concrete,and negatively correlated with the moisture contents of the soil sample and the number of freeze-thaw cycles.The shear strength of the interface decreases slowly after 7 freeze-thaw cycles,and the higher the moisture content,the more obvious the deterioration of the interface strength after freeze-thaw.2.Comparing the results of the direct shear test on the silty clay-concrete interface and the direct shear test on the silty clay itself,the existence of the shear band on the silty clay-concrete interface in the seasonal frozen area was verified.The particle image velocimetry(PIV)technique was introducted to analysis the whole deformation process of the silty clay-concrete interface,and the shear band thickness values under different soil moisture contents,normal stresses,concrete surface roughness and freeze-thaw cycles were obtained.The results show that the shear band exists under the condition that the moisture content is higher than 14.7% or the normal stress is greater than 200 k Pa.Normal stresses,soil moisture contents,roughness and freeze-thaw cycles can affect the shear band thickness of silty clay-concrete interface.Shear band thickness is positively correlated with normal stresses,moisture contents,roughness and the first three freeze-thaw cycles,after which it no longer increased with freeze-thaw cycles.The contact interface area includes the contact interface of silty clay and concrete and a certain thickness of soil from the concrete surface,and the range of the contact interface area is related to the roughness of the concrete surface.3.A shear mechanics model for silty clay-concrete interface considering shear band thickness was established based on adhesion-ploughing friction theory,which can reflect the effects of roughness of concrete surface,normal stress,moisture content of soil sample and number of freeze-thaw cycles,the relationship between shear band thickness and interface shear strength was quantified.The results show that the model reflects the shear force on the interface of silty clay and concrete divided into adhesion force and ploughing force.The shear force is related to the adhesion area and the ploughing area,the adhesion resistance is influenced by the normal stress,moisture content of the soil sample and freeze-thaw cycles,and the ploughing resistance is influenced by the normal stress,surface roughness of the concrete roughness and the number of freeze-thaw cycles.The model can better predict the shear strength and shear band thickness at the silty clay-concrete interface.4.The apparent characteristics of the silty clay-concrete interface after shear damage under the action of normal stress,moisture content of soil sample,roughness of interface and freeze-thaw cycles,and the changes of particle arrangement characteristics and pore characteristics inside and outside the shear band were investigated from the macroscopic and microscopic viewpoint to reveal the shear band damage mechanism.The results show that normal stress,moisture content,concrete surface roughness increased,soil samples remaining on the concrete panel after shear damage increased,and concrete surface wetness increased.The orientation of the particles in the shear band is significantly enhanced under normal stress,and the large pores in the shear band are transformed into small pores.The increase of moisture content can enhance the orientation of particles in the shear band,but the enhancement ability is limited,and the enhancement effect is not obvious near the optimal moisture content.The increase of roughness contributes to the orientation of particles in the shear band,but the effect is limited.The freeze-thaw cycles action causes deterioration damage to the soil in the shear band of the silty clay-concrete interface,and cracks are generated internally.The freeze-thaw cycles reduce the particles orientation in the shear band,but the particles orientation in the shear band is always stronger than that outside the shear band.The freeze-thaw cycles transform the large pores in the shear band into small pores.5.Through the self-prepared continuous-discrete coupling procedure of silty clayconcrete interface,and the numerical model of deformation of silty clay-concrete interface was established.The whole process of damage evolution of shear band between silty clay and concrete interface was simulated.The mechanism of shear band formation was revealed through the cloud diagram of soil deformation,displacement cloud diagram,velocity field and force chain network diagram in the shear process under corresponding conditions.The results show that the shear band is formed by the tumbling motion of internal particles and the change of contact force chain.The shear band is fully formed when the shear displacement reaches 10 mm.The lower concrete is affected by the normal stress and roughness,and the normal stress has a more obvious spreading effect on the internal stress of concrete.