Investigations On Undrained Shear Behaviors Of Loess Subjected To K₀ Consolidation In Triaxial Test

How to determine the coefficient of lateral earth pressure（K₀）is one of the classic problems in the field of geotechnical engineering,and coefficient of lateral earth pressure is a key parameter for calculating the horizontal force distribution of structures.Different from ordinary soils such as sand and clay,the K₀ value of loess is difficult to be accurately evaluated due to its fragile structure,complex mineral composition.In addition,the traditional method for studying the shear characteristics of soil is to conduct triaxial shear tests after consolidation under static water pressure,which is inconsistent with the actual stress situation of soil in foundation engineering.Therefore,this study takes Yan’an loess as the research object and conducts a series of dynamic,static triaxial tests and microscopic tests,focusing on exploring the effects of stress state,pore ratio,structure,stress history,and modifiers on the K₀ value of loess.It also analyzes the undrained shear characteristics of loess under K₀ stress state and uses numerical simulation methods to analyze the impact of changes in K₀ value on underground engineering in loess.The main conclusions are as follows:How to determine the coefficient of lateral earth pressure（K₀）is one of the classic problems in the field of geotechnical engineering,and the coefficient of lateral earth pressure is a key parameter for calculating the horizontal load on structures.Different from ordinary soils such as sand and clay,loess has a fragile structure and complex mineral composition,making it difficult to accurately evaluate its K₀ value.The traditional method for studying the shear characteristics of soil is to conduct triaxial shear tests after consolidation under static water pressure,which is inconsistent with the actual stress situation of soil in foundation engineering.Therefore,this study takes Yan’an loess as the research object and conducts a series of dynamic,static triaxial tests and microscopic tests,focusing on exploring the effects of stress state,pore ratio,structure,stress history,and modifiers on the K₀ value of loess.It also analyzes the undrained shear characteristics of loess under K₀ stress state and uses numerical simulation methods to analyze the impact of changes in K₀ value on underground engineering in loess.The main conclusions are as follows:（1）There is a sudden change in the triaxial K₀ consolidation curve of loess.The K₀ value of the compacted sample of remolded loess（e₀<0.6）undergoes a sudden change during the initial stage of consolidation.For the loose sample of remolded loess（e₀≈0.77）,the location where the sudden change occurs is near the effective confining pressure of 100k Pa,while for the undisturbed sample,it occurs near the effective confining pressure of 350k Pa.The results of nuclear magnetic testing confirm that the pore structure of the sample is an important reason for the above differences;（2）The coefficient of lateral earth pressure of remolded loess samples is more sensitive to changes in initial pore ratio.As the initial pore ratio decreases,the coefficient of lateral earth pressure of remolded samples significantly decreases.The coefficient of lateral earth pressure of undisturbed loess samples is more sensitive to the effective consolidation confining pressure,and decreases with the increase of the effective consolidation confining pressure;When the initial pore ratio is similar and the effective confining pressure is the same,the K₀ value of the undisturbed sample is greater than that of the remolded sample;（3）The existence of OCR over consolidation ratio will affect the K₀ consolidation performance of loess.The K₀ value increases with the increase of OCR.The relationship between K₀ value and OCR in this study is:K_{OC R}=0.42×OCR^0.36;（4）The peak stress(q_peak)of the sample after K₀ consolidation is about 25%higher than that after uniform pressure consolidation.This indicates that,without considering the initial stress state（i.e.,K₀ state）,the shear strength of loess may be underestimated;（5）The q_peak of Nano-Si O₂ improved loess shows a pattern of first increasing and then decreasing with the increase of Nano-Si O₂ content.In this study,the optimal addition content of the modifier Nano-Si O₂ is about 4%;（6）The K₀ value affects the dynamic triaxial undrained shear response.With the decrease of K₀ value,the failure modes of flow failure,cyclic migration,and plastic strain accumulation are presented successively;（7）The numerical simulation results show that under the three working conditions of vertical excavation of foundation pit,tunnel excavation and shield excavation between pile foundation,the decrease of K₀ value has adverse effects on the displacement,deformation and stress of underground engineering soil.