Study On Prediction Of Collapsible Deformation Of Loess

Loess collapsibility has always been the focus and difficulty of loess area research.Under the overlying load and water immersion,a series of engineering disasters will occur in loess,causing serious economic losses,such as foundation settlement,surface collapse,building collapse and so on.Collapsibility coefficient is not only an index of loess collapsibility,but also a key parameter to evaluate the collapsibility grade of site.However,the results of field and indoor tests are often quite different,and there is no reasonable theoretical or empirical prediction formula,so it is difficult to evaluate the project.In order to find out the reasons for the difference between the results of in-situ immersion test and indoor test,and obtain the empirical formula for predicting the collapsible coefficient of loess,the collapsible loess of Xi’an Hancheng intercity high-speed railway is taken as the research object,and the 30m exploratory well is excavated for sampling.The basic physical indexes and particle analysis curves of loess at different depths are made in the laboratory Collapsibility and layered collapsibility,collapsibility coefficient measured by indoor high pressure consolidation test,as well as the yield pressure of the structure under natural and saturated compression conditions.The results of the in-situ flooding test and indoor test were analyzed,and the following results were obtained.（1）The difference of load effect between in-situ and indoor is the main reason for the difference of collapsibility prediction.According to the existing"building code for collapsible loess area"（GB 50025—2004、GB 50025—2018）,Q₃loess layer is the main collapsible soil layer,and Q₂soil layer has slight collapsibility or no collapsibility.According to indoor experiments,Q₂loess still has great collapsibility under high pressure,and the self weight load on site is far less than the load causing the maximum collapsibility.Although the immersion depth of the site is far beyond the collapsible depth of loess specified in the specification,the total collapsible amount of the site is far less than that of the indoor experiment.（2）High pressure consolidation test is needed to determine the collapsibility coefficient of deep loess.The load corresponding to the maximum collapse coefficient of loess is controlled by the yield pressure of loess structure under the condition of natural water content.The structure yield pressure of deep loess is high,so the high pressure consolidation test can show the complete relationship between collapsibility coefficient and load.（3）An empirical formula for predicting the collapsibility coefficient of loess is established.The linear equation is used before the yield pressure of saturated loess structure,and then the normal equation is used to establish the empirical relationship curve between load and collapsibility coefficient.The physical meaning of the parameters in the empirical formula is analyzed,and the correlation formula between the parameters and the basic physical indexes of loess is established.（4）The experimental results of high pressure consolidation in three places of Loess Plateau are used to verify the availability of empirical formula.The empirical formula is validated by using loess from Heping Town,Jingyang,Shaanxi and Pucheng area of Shaanxi Province.The error between the maximum collapsibility coefficient estimated by empirical formula and the measured results is within 7%.The empirical formula can be used to predict the collapsibility of loess.