| Soil-bentonite(SB)cutoff wall is a barrier for underground contaminant containment,which can effectively slow down the migration of contaminants and protect the surrounding environment.This technology has been put into engineering applications abroad for decades,and is currently being promoted in China.Due to the semi-fluid nature of the SB backfill material,SB cutoff walls will go through a period of consolidation after backfilling.The changes in stress distribution and the deformation of SB cutoff walls during consolidation process will influence its performance in contaminant containment.Understanding to the consolidation behaviors of SB cutoff walls was developed through long-term in-situ monitoring cases and theoretical studies.However,the deeper consolidation mechanisms are still unclear.In addition,the existing methods for stability analysis of slurry trenches under complex scenarios are too complicated for hand calculation.To this point,this paper applied finite-element analysis and developed multi-procedure models to simulate the construction and consolidation process of SB cutoff walls.Insight studies on the consolidation mechanisms of the SB cutoff walls and stability analysis of slurry trenches were presented in this paper based on these numerical models.Analytical solutions for consolidation and a new method for stability analysis of slurry trenches were also proposed.These studies could provide guidance for the design and construction of SB cutoff walls.(1)Based on the assumption of rigid sidewalls,a plane-strain finite-element model of the SB cutoff wall was established to simulate its consolidation process when the sidewall friction existed.The simulation results were used to analyze the consolidation mechanisms in detail.The sidewall friction,consolidation stress,excess pore pressure,and effective stress of the wall interacted with each other during the consolidation process.Due to the rapid growth of effective normal stress on sidewalls,the sidewall friction could increase rapidly in the early stage of consolidation,which caused the vertical consolidation stress to decrease rapidly.The decrease of excess pore pressure was caused by the decrease of vertical consolidation stress and consolidation seepage,and the former reason had a major impact.Horizontal drainage was dominant of the total water discharge during the consolidation process,while vertical drainage contributed a small part of it only when the wall was shallow and thick.(2)Two definitions of the degree of consolidation of SB cutoff walls were proposed.One took into account both the contribution of the decrease of vertical consolidation stress and the contribution of consolidation seepage.It focused on describing the ratio of the current decrement of the excess pore pressure to the initial vertical consolidation stress.The other focused on describing the ratio of the excess pore pressure dissipated due to consolidation seepage to the current vertical consolidation stress.Using the results of the finite-element model,the degrees of consolidation under both definitions were calculated.Due to the different emphasis,the calculation results of the two definitions show respective characteristic.(3)Three analytical solutions for SB cutoff walls' consolidation process under different assumptions were proposed.These solutions were verified with the results of finite-element models.Analytical solutions ? and ? can predict the excess pore pressure and the degree of consolidation very well when the sidewalls are rigid and the sidewall friction is not considered.Analytical solution ? can also predict the transverse distribution of the excess pore pressure.Compared with solution ?,the prediction of solution ? for the excess pore pressure when the sidewalls are rigid and the sidewall friction is taken into account has made a great progress.(4)Taking Bucknell University's field research of a full-scale SB cutoff wall as a study case,the whole process of the construction and consolidation of this wall was simulated.The finite-element model for this simulation was established by improving the former model with rigid sidewalls.It took into account the sidewall friction,the deformation of adjacent soils and more detailed in-situ conditions.The field monitoring data were used to verify this model,and the results show that the model can well predict the changes of earth pressures and pore water pressures with time at different depths.Thus,the validity of the multi-procedure simulation and the methods for determing parameter values were verified.Compared with the previous simulation results under the assumption of rigid sidewalls,the basic consolidation mechanism did not change,but some special results occurred due to the change of scenarios.(5)Based on Rankine's theory of earth pressure,a new approach for stability analysis of slurry trenches was proposed.Scenarios with soil stratification and variably distributed surcharges can be analyzed by hand calculation when the proposed approach is used.The proposed approach was verified by the fact that it gives the same factor of safety as that found by Coulomb force equilibrium methods in the same scenario.(6)A three-dimensional finite-element model was established to simulate the excavation of the slurry trench,based on which the stability of the trench and the stress distribution of adjacent soils were studied.The results show that as the length of the excavated section increases,the factor of safety gradually decreases.Therefore,controlling the length of each separated excavation can help with the stability of the trench.The factor of safety under three-dimensional condition was much greater than that of plan-strain condition,indicating that the two-dimensional stability analysis is more conservative.Further analysis on stress distribution of adjacent soils shows that there are horizontal and vertical "soil arching",and "downward stress transfer" mechanisms in the adjacent soils.Moreover,alternation excavation can effectively avoid large changes in stresses or large displacements of the adjacent soil. |
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