Study On Engineering Characteristics Of Compacted Loess Under Leaching Action

With the rapid development of urban construction in the hilly and gully areas of northwest China,many loess high-fill projects have emerged.After full compaction,the collapsibility of high-fill loess under unidirectional compression has been eliminated,showing good mechanical properties.However,because of the long service life of such projects,it is likely to encounter the long-term leaching effect such as the change of underground water level or continuous rainfall.In practical terms,the saturated compacted loess is affected continuously by leaching,thereby the strength weakening and deformation of soil appeared gradually.In addition,the leaching process occurs only when the soil is saturated or collapsible.So,the strength weakening and leaching-induced deformation are relatively slow,which means the development of such engineering diseases is often concealed.Therefore,it is necessary to study the leaching-induced deformation characteristics of compacted loess in order to prevent and control similar problems in advance.This paper takes a certain high-fill loess in northern Shaanxi as the research object.The experimental and theoretical research contents and achievements are as follows:?1?By using the independently improved stress-controlled triaxial apparatus,macroscopic leaching tests were carried out on the compacted loess under different stress combinations and leached time.The results showed that all the leaching samples were deformed,the strength weakened and the permeability coefficient decreased.Moreover,the changes of leaching-induced deformation,strength and permeability coefficient have the characteristics of timeliness and stage.During the leaching period,the cohesion decreases gradually,while the internal friction Angle increases only slightly.Confining pressure can promote the compaction effect from leaching stress level,but it limits the increase range of leaching-induced deformation,strength weakening and permeability decreasing.The leaching stress level affects the magnitude of leaching-induced strain and the weakening degree of ultimate normal stress.When leaching stress level is small,the permeability coefficient is inversely proportional to it,but when leaching stress level is large,the permeability coefficient increases at the later stage of leaching.The hydraulic gradient of each leaching sample is less than the corresponding critical hydraulic gradient,indicating that the seepage force will not cause seepage failure.Through nonlinear regression analysis,the relationship between permeability coefficient,leaching duration factor and stress combination was established,which was used to quantitatively study the permeability change.?2?The microscopic properties of the samples were studied by scanning electron microscope.The results show that the pore size,probability entropy,fractal dimension and micro void ratio decrease,and the number of pores increases during leaching.The clusters number,probability entropy,fractal dimension and connection coefficient increased,while the clusters size and curvature coefficient decreased.The decrease of pore and cluster size indicates that the leaching-induced deformation is a compaction process under the action of pressure and loss of soluble salt.Cluster size change rate is defined to analyze the intensity of microstructure change.In addition,through a series of primary selection and KPCA methods,the main microscopic parameters which affect the macroscopic leaching-induced volumetric strain and cohesion were determined;And them are pore diameter,pore fractal dimension and curvature coefficient,pore probability entropy and cluster fractal dimension,respectively.?3?Through acid-base balance method,the salt ions and total soluble salt content of leaching compacted loess were measured and analyzed.During leaching,the ion concentration and soluble salt content decrease periodically.The easy,medium and insoluble soluble salts with the highest initial and residual contents were Ca?HCO₃?₂,CaSO₄·2H₂O and CaCO₃ respectively.Confining pressure influences the smoothness of the curves for ion and soluble salt content,and s influences the shape and hysteresis of the curves.To evaluate the difference of soluble salt loss between different stresses and leaching process,the maximum and average difference indexes and the soluble salt ratio were defined respectively.Among them,the maximum difference indexe is inversely proportional to leaching stress level,the average difference indexe is directly proportional to leaching stress level.In terms of strength,ultimate normal stress is proportional to the relative soluble salt content.The intensity of the initial weakening is determined by leaching stress level.The three weakening characteristics show that the change of soluble salt is the internal cause of leaching-induced deformation and strength weakening.Based on the test data,the strength parameter formula of the combination of soluble salt and stress is established.?4?By analyzing the relationships of the consolidation parameters and failure parameter to the soluble salt and stress combination,the revised formula of the Cambridge model is obtained.In order to verify the applicability of the modified model to leaching compacted loess,the macroscopic leaching test data for each group of stress combinations was calculated and analyzed one by one.The results show that the strain calculated value is relatively closed to the experimental values.And the revised model can be used to predict and analyze the leaching-induced deformation of the compacted loess.