Investigation On The Influence Of Freezing-thawing Cycle And Drying-wetting Alternation On The Permeability Anisotropy And Spatial Variability Of Loess

In order to reflect the characteristics of the directional variation in particle size composition of loess from south to north,loess samples representing clayey loess,loess and sandy loess were collected from Xi'an,Yan'an and Mizhi,respectively for further test study.Particle Size Distribution tests and Triaxial Permeability tests were conducted to analyze the spatial distribution of particle size composition and permeability anisotropy.It can be obtained that as the geographical location varies from south to north,the fine particle content of loess decreases and the coarse particle content increases gradually.The characteristics of particle size distribution changes from "double peak" to "single peak".Aggregates(coarse particles surrounded by clay particles adhered to them)in the "double peaks" loess interlocked with each other,which forms larger vertical dimension of the inter-particle voids.However,the coarse particles contact with each other directly in "single peak" loess,which lead to similar dimensions of the inter-particle voids in all directions.This makes the permeability coefficient for the "double peak" loess smaller than that for the "single peak" loess.The anisotropy of the permeability for the "double peak" loess is more obvious,either.By comparing the apparent structural characteristics and porosity variation characteristics of the undisturbed loess and remolded loess before and after freezingthawing,drying-wetting and coupling action.The results showed that: the porosity of loess in three regions changs most strongly in the cycle of 1 to 2 times,in which the horizontal and vertical porosity changing ratio of " double peak " loess are similar.However,the vertical porosity changing ratio of " single peak " loess are significantly greater than the horizontal ones.Freezing-thawing cycle has little damage to loess' s apparent structure,but in the process of drying-wetting alternation and coupling action,cracks gradually develop and connect into a network,resulting in the complete loss of soil structure and integrity.Triaxial Permeability tests were conducted to determinate the permeability coefficient of loess in three regions before and after freezing-thawing,drying-wetting and coupling action.It can be obtained that the permeability coefficient of the undisturbed loess is greater than that of the remolded loess,and the vertical permeability coefficient of the undisturbed loess is higher than that of the horizontal one initially.In the initial stage of freezing-thawing,drying-wetting,the permeability coefficient of loess in three regions changs most strongly,and then gradually stabilized.The anisotropy of horizontal and vertical permeability gradually weakened,and the permeability differences between the undisturbed loess and remolded loess gradually decreased.Multiple linear regression analysis were conducted to analyze the influence weight of various factor on permeability in the coupling action.The results indicate that dryingwetting alternation plays a leading role,freezing-thawing cycle plays a facilitating role,and the effect of cell pressure and cycle times can be ignored.Based on the test results,an empirical formula for predicting loess permeability coefficient in initial state,freezingthawing and drying-wetting condition is developed for reference of practical engineering.The research results are expected to provide references for the geological disasters prevention and design of structures and buildings in the loess region.