Mechanical And Hydraulic Properties Of Soil-bentonite And Consolidation Behavior Of Cutoff Wall

Pollution of soil and groundwater is serious in China today.Soil-bentonite(SB)cutoff wall is a kind of barrier which can effectively impede the transport of pollutant in groundwater,and has a good application prospect.However,the understanding of the mechanical properties,as well as the consolidation behavior of the SB cutoff wall is still insufficient,and most of the practices in the design,construction and quality assessment processes depend on experience.Researches are mainly focused on the laboratory element tests,and field test data are scarce.In this paper,the mechanical and hydraulic properties of soil-bentonite and the consolidation behaviors of SB cutoff walls are analyzed,depending on the aspects of the laboratory element tests,full-scale tests and the theoretical calculation:(1)The influences of different test methods on the measured hydraulic conductivity,compression index and strength parameters of wall materials were studied.When the consolidation pressure is relatively large,flexible-wall permeability test can obtain lower hydraulic conductivity,due to restraining the side wall leakage.The compression index obtained by one-dimensional consolidation is greater than that by isotropic consolidation test.The strength parameter obtained by quick shear test is less than that by undrained tri-axial compression test.Dry-wet cycles will cause the overall expansion and shrinkage of samples,and increasing consolidation stress can effectively resist shrinkage deformation.Saturation degree of samples decreases with the increase of matric suction,and has little relation with stress.The hydraulic conductivity of the sample's core part changes little after 3 dry-wet cycles.(2)The effect of the stress path on the stress-strain relation of the wall material has been studied.It is found that the larger the slope of stress loading path,the smaller the volumetric strain,the faster the shear modulus reduces.The stress-strain relations under the same kind of"consolidation-linearly loading" paths can be normalized as one curve.In proportional loading process,there is a good linear relationship between the shear strain and volumetric strain.According to the test results,a modified non-linear constitutive model with four moduli was proposed,and the calculation results under drained loading conditions are closer to the measured data than those of the Duncan-Chang model and the modified Cambridge model.(3)Long-term stress state and deformation of the soil-bentonite cutoff walls were studied under field condition,and the consolidation behavior of the wall was analyzed.The forming process of the cutoff wall can be basically divided into sedimentary stage,primary consolidation stage and secondary consolidation stage.Due to the low permeability of the backfill,the consolidation process takes several months to complete.Lateral deformation is the dominant form of consolidation deformation.During the consolidation of the wall,the ground soil on both sides will reach limit equilibrium,and sliding wedges will occur,which has a significant impact on the values and distributions of the stress and deformation of the wall.Except consolidation deformation,there is a large shrinkage deformation in the shallow portion of the cutoff wall.(4)A modified method was proposed for calculating the undrained shear strength of the soil-bentonite cutoff wall based on CPTU tests,which was adopted to analyze the in-situ undrained shear strength distribution of the cutoff wall in the field test.The reason for the measured non-monotonic dissipation curves in the pore pressure dissipation tests has been explained with the non-uniformity of the backfill.By comparing different methods,it is found that the method proposed by Sully et al.(1999)and the one-dimensional consolidation theory can obtain the closest consolidation coefficient and hydraulic conductivity to the laboratory test results,respectively.(5)A numerical method for determining the stress in the cutoff wall based on the mechanical equilibrium analysis of horizontal slice element was proposed,and the validity of the method was verified by analyzing the cases in the literature and in the field test.Analysis of parameter sensitivity shows that the horizontal stress is significantly affected by the internal friction angle of the ground soil and has a geometric similarity in distribution.An analytical solution was given under simplified conditions,and was compared with the numerical solution.(6)Numerical calculation was carried out to simulate the primary consolidation process of the soil-bentonite cutoff wall in the field test.It is proved that the lateral squeezing mechanism is close to the reality.The effect of consolidation on the service life of the cutoff wall was analyzed by numerical calculation of pollution movement,and suggestions for the design of the cutoff wall were provided.