Failure Mode And Stability Evaluation Of River Embankment Improved By Cement-Soil Mixing Column

Being improved by cement-soil mixing column is a widely and effective method to treat soft soil foundation.At present,the stability of embankment improved by cement-soil mixing column is often based on the assumption of circular failure mechanism.However,in practical engineering,the failure mode of mixing pile is complex.Accurate evaluation and calculation of the stability of embankment improved by cement-soil mixing column is the key to engineering design and related development.Referring to the project of soft soil treatment of Baishan Navigation Lock,Anhui Province,the stability of column treated river embankment has been systematically studied by a series of centrifugal tests,numerical simulation and field tests.The main contents and results are as follows:(1)The N=50g centrifugal model tests are designed to study the stress and failure characteristics of river embankment improved by cement-soil column.The results show that the embankment produces obvious cracks and sliding body when the centrifugal acceleration reaches 70-80 g.The entrance of the most dangerous sliding surface is about 1/2 away from the shoulder of the first grade slope.According to the stress and failure characteristics,the foundation is divided into three areas: compression shear area,bending shear area and shear area.The bending shear area may be the first to fail,and the failure mode of columns is mainly bending shear failure.The columns at different positions under the embankment are not destroyed at the same time,and the failure process has sequence.(2)The cementation damage of cement-soil materials are studied.The application of cementation damage of cement soil in numerical software is realized by USDFLD and ABAQUS,and the numerical model of slope improved by column is established.The calculation results of the prepared cement soil model are very consistent with that of triaxial tests,compared to the other models such as Mohr-Coulomb model.The results of this numerical model are also consistent with that of centrifugal tests.Besides,considering the strain softening of cement soil will lead to a more accurate results.(3)Varieties of failure modes with their conditions of river embankment improved by column are studied using ABAQUS.The evaluation,design and calculation method of slope stability suitable for practical engineering are put forward.Various failure modes can be existed in the river embankment improved by column,such as compression,shear,bending,inclining,sliding of columns,extrusion of soil and excessive settlement of foundation.The shear and bending failure of column is the most common failure mode.According to the load density and practical requirements,the stability evaluation method basis of river embankment improved by column is given,which also provides guidance for designs.The calculation method of safety factor of river embankment improved by column in bending and shear failure is proposed.The sliding surface of the whole embankment is divided into a three-stage wedge,and the real safety factor is solved.(4)A series of field monitoring tests are carried out to study the actual effect of column on the river embankment,and verify the proposed methods.At the end of the filling,the maximum deep horizontal displacement of the soil is about 28 mm,revealing the good reinforcement effect of columns.The pile-soil stress ratio is stable at about 2.5 after filling,showing the characteristics of composite foundation.The failure trend of the test section is bending and shear of columns.The safety factor of the river embankment is 2.05,which is a safe and stable result.This result is compared with the numerical one to verify the accuracy.