Experiment Study On Dynamic Strength Characteristics Of Unsaturated Loess Under Complex Initial Stress States

Loess and loess-like soil is widely distributed in China and many other countries and regions all over the world. In recent years, the number of construction in the loess region increased, earthquakes and other natural disasters were frequent, conventional triaxial test apparatus and the conventional dynamic torsional shear apparatus which simulated a simple initial stress state has been unable to meet the actual needs of the project, the initial complex cyclic stress state such as traffic load, seismic load and wave load effects propose a new challenge for seismic design in loess area; Therefore, to obtain reasonable dynamic parameters of loess under complex initial stress states to provide more accurate reference for seismic design in the loess region has important practical significance.Currently the research about complex initial stress states is relatively small, and the main materials are sand and soft soil. In this case, the paper based on the improved DTC-199 type electro-hydraulic serve cyclic torsional dynamic triaxial apparatus, carried out experimental study on dynamic strength characteristics of loess under different dry densityρd, moisture content w, initial angles of principal stressα₀, initial coefficient of intermediate principal stress b₀, initial deviator stress ratioη₀ and initial average principal stress pm₀, and the conclusions are as follows:(1) Dynamic strength curve of the loessTaking into account the initial pre-trial shear stress, there is a big residual soil dynamic shear strain, and to compare the advantages and disadvantages of Shen Ruifu's failure criterion, Huang Shaoming's failure criterion of double amplitude strain, Andersen's failure criterion, Wang Hongjin's generalized shear strain failure criterion and Luan Maotian's comprehensive failure criterion, and ultimately the writer decided to use Andersen's failure criterion, that is, within a single cycle, the specimen damage when the sample generated the average shear strain considering the factor of residual shear strain reached 15% or axial deformation reached 5%.Other conditions being the same, the dynamic strength of undisturbed loess is greater than remodeled loess. Other conditions being the same, the bigger dry density, the lower water content, the smaller initial angles of principal stresses, the smaller the initial deviator stress ratios or the bigger the average principal stress, the greater the dynamic strength of loess. Initial coefficient of intermediate principal stresses factor in the dynamic strength characteristics of the loess has range effects, when the initial coefficient of intermediate principal stresses b0 changes in the range of 0～0.5, the larger b0, the greater the dynamic strength of loess; when b0 changes in the range of 0.5～1, the larger b0, the smaller the dynamic strength of loess.(2) Relation curve ofτ_d/σ₃～lgN of loessOther conditions being the same, the smaller the dry density, the greater moisture content the smaller the initial mean principal stress, more obvious the normalization trend ofτ_d/σ₃～lgN curve. Other conditions being the same, initial angles of principal stresses and initial coefficient of intermediate principal stresses has interval effects on the normalization ofτ_d/σ₃～lgN curve. Other conditions being the same, with initial angles of principal stresses increases, the normalization ofτ_d/σ₃～lgN curve experiences a reduction followed by an increase, whenα0=45°, the normalization ofτ_d/σ₃～lgN curve is the least. with initial coefficient of intermediate principal stresses increases, the normalization ofτ_d/σ₃～lgN curve experiences a reduction followed by an increase, when b0=0.5, the normalization ofτ_d/σ₃～lgN curve is the least. The effects of initial deviator stress ratios on the normalization ofτ_d/σ₃～lgN curve is not obvious.(3) Dynamic strength parameters of the loessDifferent initial conditions, with the vibration times increase, the dynamic strength parameters of loess cd andφd decrease, and are stabilized gradually. Other conditions being the same, the same damage vibration times, the bigger dry density, the smaller water content, the smaller initial angles of principal stresses, the smaller initial deviator stress ratios or the bigger initial average principal stresses, the larger the dynamic strength parameters of loess cd andφd. This conclusion is consistent with the regular of dynamic strength of loess under different initial conditions. Initial coefficient of intermediate principal stresses have smaller effect on the dynamic strength parameters of loess, but influence regular is obvious and it is consistent with the regular of dynamic strength of loess under different Initial coefficient of intermediate principal stresses.