Seismic Liquefaction Behaviors Of Saturated Loess And Anti-seismic Treatment Methods

Seismic liquefaction of saturated loess is one of the three serious geo-seismic hazards in loess region,particularly in the Loess Plateau in China.Compared with the unsaturated loess,the structural strength of the saturated loess is obviously weakened result in the partial or total dissolution of salt crystals and other cements in the water.Meanwhile,the pore water pressure generated by load causes a significant reduction of the effective stress of the soil.Thus,the catastrophe risk of the saturated loess under dynamic loads is much more intense.However,the dynamic characteristics of saturated loess are relatively lacking due to the reasons such as insufficient cases related to seismic hazards of the saturated loess in historical earthquakes in loess region.The influence of complex stress conditions on liquefaction characteristics of the saturated loess is still unclear.The physical processes and mechanisms on seismic liquefaction of saturated loess are not immediately clear.Moreover,the application of seismic-improved treatment methods with cost-effectively and environment-friendly has become an urgent issue due to the fragile ecology in loess region.In this paper,based on laboratory tests of the saturated loess,the dynamic properties of the saturated loess,the micro-structural characteristics of the liquefied loess,the characteristics of loess liquefaction disaster and the prevention methods of loess liquefaction are taken as the main clue to illustrate the seismic liquefaction behaviors of saturated loess and provide an economic and environmental anti-seismic treatment method of loess foundation.A variety of methods including field investigation,in-situ test,laboratory test,SEM test and XRD test are used in the study process.The dynamic behaviors such as dynamic shear modulus,damping ratio,dynamic deformation and dynamic pore water pressure of the saturated loess are analyzed.The influence of initial complex stress conditions on liquefaction behaviors of the saturated loess are proposed.Combined with several cases of seismic liquefaction in loess region during historical earthquakes,the characteristics of liquefaction of the saturated loess caused by earthquakes are determined.The evolution laws of the dynamic pore water pressure,as well as the interaction mechanism between the dynamic residual deformation and the dynamic pore water pressure of saturated loess,are discussed.The macro and micro mechanisms of seismic liquefaction of the saturated loess are clarified.In addition,the physical process of seismic liquefaction of the saturated loess is obtained.On this basis,the anti-liquefaction properties and solidification mechanism of saturated lignin-modified loess are studied.The anti-liquefaction indicators of saturated lignin-modified loess on account of the performance-based seismic design concept are proposed.The creative achievements under the above-mentioned researches are summarized as follows.(1)The dynamic stiffness of the saturated loess is significantly reduced,and the damping ratio of it increases significantly in the large dynamic shear strain.Compared with unsaturated loess,the dynamic shear modulus ratio of saturated loess in different regions is less different.The dynamic shear strain at which it begins to decay is significantly reduced compared to the unsaturated soil,and the decay rate is significantly accelerated.Under cyclic shear load,the saturated loess produces the "liquefaction" phenomenon of dynamic stress attenuation,dynamic strain,and dynamic pore water pressure increase.However,due to the dissipation of dynamic pore water pressure caused by closed pores in loess,the maximum dynamic pore water pressure is difficult to reach the effective confining pressure.The physical index has a significant influence on the dynamic characteristics of saturated loess.The higher the dry density,the higher the plasticity index and the smaller the silt content,the more difficult the saturated loess is to liquefy.(2)The dynamic characteristics of the saturated loess are significantly affected by the initial stress state.The dynamic characteristics of saturated loess under complex stress conditions are significantly different from those under simple stress conditions.The anti-liquefaction strength of the saturated loess decreases with the increases of the initial principal stress direction angle due to the pre-shear stress.However,the dynamic horizontal strain and the octahedral shear strain increase significantly with the increases of the initial principal stress direction angle.The dynamic pore water pressure ratio increases in general.With the increases of the coefficient of intermediate principal stress,the liquefaction resistance of the saturated loess increases,the dynamic radial strain increases from negative to positive rapidly,whereas the octahedral shear strain decreases gradually.Moreover,the maximum dynamic pore water pressure ratio reaches the maximum at b=0.5.The consolidation deviatoric stress ratio has a great influence on the dynamic strength of saturated loess at the initial stage of cyclic shear.The maximum dynamic pore water pressure decreases with the increase of the consolidation stress ratio and dynamic strain.(3)The seismic liquefaction disasters of the saturated loess are mainly low-angle slip in slope area.It is characterized by large-scale soil flow or mudflow,low intensity of triggered seismic motion,significant site ground motion effect and uncommon soil emitting.The seismic ground motion effect of the loess site has a significant influence on the liquefaction process of soil.The peak value of surface ground motion of saturated loess site is obviously lower than that of unsaturated loess site under the same condition.It is a period corresponding to the maximum spectral value of the acceleration response spectrum is significantly higher than that of the unsaturated loess site.Moreover,due to stiffness and viscosity of the saturated soil,the surface ground motion does not have obvious amplification or even attenuation compared with the input ground motion.Therefore,under the premise of ensuring the stability of the soil structure,it is more conducive to the accumulation of dynamic pore water pressure,which has a significant promotion effect on soil liquefaction.(4)The structure of the loess is much more broken when it liquefied.The edge of the pores is more angular and the number of large pores is obviously increased.Meanwhile,the connectivity among the pores tends to good.The liquefaction process of the saturated loess can divide into three stages including structural adjustment,channel development and liquefaction failure from a microscopic perspective.The dynamic pore water pressure of the saturated loess shows accumulation characteristics during the whole cycle shearing.Moreover,it has obvious hysteresis with respect to the dynamic strain.The hysteresis period decreases with the increase of vibration cycles.Pore volume compression caused by soil skeleton deformation under cyclic shear load has a controlling effect on the accumulation of dynamic pore water pressure.The cumulative growth modes of dynamic pore water pressure are influenced by the physical index and structure of loess,which can be divided into three modes: rapid growth in the early stage of deformation,continuous growth,and rapid growth in the late stage of deformation.(5)The seismic liquefaction mechanism of the saturated loess is mainly described as the change on-resistance of soil skeleton under cyclic load and the adaptive response of pore water to the deformation of the soil structure.The alternating effect between weaken of the strength of the loess skeleton and the increase of the pore water pressure caused the softening of the soil during cyclic shearing.It shows an approximate flow status until the structural strength of the soil is partly or completely lost.According to the average effective stress of the saturated loess during liquefied,the liquefaction mechanism of the saturated loess can divide into the deformation-dominated type and pore water pressure-dominated type.The heterogeneity of the loess leads to a gradual failure behavior in the liquefaction process.(6)The dynamic stiffness and dynamic strength of the saturated lignin-modified loess are enhanced significantly.Furthermore,the increase of the dynamic deformation and the pore water pressure of the lignin-modified loess are obviously inhibited.The significant improvement of the anti-liquefaction strength of the lignin-modified loess is due to pore filling and cementing of the lignin,thinning of the electric double layers of the loess,reinforcement of the lignin fiber,adsorption of fine particles and free water by the lignin fiber,and ion exchange between lignin and loess.The lignin-modified treatment method can completely eliminate the liquefaction potential of the loess foundation with the design seismic acceleration A?0.30 g.