Study On Constitutive Models For Soils With Degradable Solid Skeleton And Their Applications

Soils with degradable solid skeleton are widely used in the various fields of geotechnical engineering.Soil skeleton degradation can be divided into physical degredation and biochemical degradation according to the mechanisms.Physical degradation refers to the particle distribution change and the solid mass loss of soil under the actions of external forces,such as the particle breakage of dam aggregates,the breakage of railway ballasts,and the hydraulic erosion of soils.Biochemical degradation refers to the transformation of active constitutes of soil skeleton into liquid and gas phases under the action of chemical or biochemical action,such as the biodegradation of municipal solid waste(MSW).If the results of skeleton degradation cannot be properly evaluated in actual engineering,serious geotechnical engineering problems may be caused,such as the crack of concrete face and large settlement of earth-rockfill dam caused by the aggregate breakage and the serious environmental geotechnical disasters in landfills caused by the rapid biodegradation of MSW.A proper constitutive model is the basis for evaluating the effects of soil skeletondegradation on the mechanical behaviors of soils.The dissertation conducted a systematic research on the constitutive models for skeleton-degradable soils.First of all,the influences on the stress-strain behaviors of two major changes caused by the skeleton degradation,namely,particle distribution change and solid mass loss,were studied,and then a general constitutive model framework for skeleton-degradable soils was proposed.Then,two typical skeleton-degradable soils,particle-breakage soil and MSW,were selected as the research focuses.The skeleton degradation characteristics and stress-strain behaviors of them were discussed and their constitutive model were developed based on the model framework.The achievements of this research are summarized as follows:(1)The results of triaxial tests and numerical simulations of discrete element method(DEM)indicated that the increase in nonuniform coefficient of granular assemblies made the normal compression line(NCL)and the critical state line(CSL)translate downward in the e-lnp plane.Accordingly,the stress-strain behaviors of granular assemblies in shearing changed from the behaviors of first hardening and then softening,and mainly shear dilation to the behaviors of hardening(no peak strength),mainly shear contraction.The results of one-dimensional compression tests of MSW samples of different ages indicated that the solid mass loss caused by the biodegradation made the compression lines of MSW move downward in the V-lnp plane.The slope of the compression line of MSW at the old age was lower than that of MSW at the young age,indicating that the compressibility of the former was lower.(2)A constitutive model framework for skeleton-degradable soils was proposed by introducing two parameters of k_d and p_b related to the skeleton degradation in the conventional constitutive model(i.e.,modified cam clay model and UH model).The parameter of k_d was introduced to reflect NCL translation caused by the particle distribution change;thus,the model framework can describe the influences of the particle distribution on stress-strain behaviors of soils.The parameter of p_b equates the solid mass loss to additional stress applied to soils,enabling the model framework to describe the deformation and compressibility variation caused by solid loss.The framework can be extended to a constitutive model for a specific soil through establishing the relationships between the external variables related to the skeleton degradation and the above two parameters.(3)A constitutive model for particle-breakage soils was developed based on themodel framework.First,the relationship between particle breakage index B_r and the model parameter k_d was established to realized the extension of the model framework.Second,a new variable for describing the breakage evolution rule of soils was proposed to overcome the limitations of the plastic work in describing the particle breakage evolution.By comparison,the model can well predict the stress-strain behaviors in triaxial tests of different particle-breakage soils.(4)The mechanical behaviors of MSW are different from those of conventionalsoils under the effects of the reinforced components.In the conventional triaxial compression tests,MSW exhibited continuous hardening behaviors and usually cannot reach failure or critical state.However,in the drained constant deviatoric stress tests,MSW samples became unstable at high stress ratios,manifested by the softening and rapidly increase in axial strain and plastic volume dilation,and finally physically collapsed.The continuous hardening and instability behaviors of MSW are both related to the effects of reinforced components.Because of the special stress paths and deformation characteristics in the drained constant deviatoric stress tests,the reinforced components in MSW cannot play an effective role to sustain the external loads,which led to the instability.In the field investigation of Guangzhou Xingfeng landfill,the high pore gas pressures,which were 40-50 k Pa higher than the static hydraulic pressure at the same depth,was measured near the sliding surface in the waste mass.Under the effect of increasing pore gas pressures,the stress paths of the MSW near the sliding surface were close to the constant deviatoric stress path,indicating that the instability of MSW may be one of important reasons for the sliding of waste mass.(5)A constitutive model for MSW was developed based on the model framework.The relationship between p_b and biodegradation degree of MSW was established to describe the influences of biodegradation.A critical stress ratio considering shear hardening,denoted as M_F,was introduced in the model to describe the influences of the reinforced components on the stress-strain behaviors of MSW.By comparison,the proposed model can well predict the stress-strain behaviors of MSW in the conventional triaxial compression tests,the instability stress ratio of MSW in the drained constant deviatoric stress tests,and the vertical settlement in the long-term one-dimensional compression tests of MSW.(6)A coupled numerical simulation for MSW of biodegradation-skeleton deformation-multiphase migration was conducted through a finite element software Open Geo Sys(OGS).The proposed constitutive model for MSW was realized in finite element analysis by the secondary development of OGS.In the calculation example of single MSW layer,the simulation results can reflect the influences of biodegradation rate on the settlement.In the calculation example of two dimensional MSW slope,the simulation results can reflect the different influences of biodegradation and high gas pressure on the deformation of the slope.The deformation mode of the MSW slope obtained by the numerical simulation is close to the actual case of the waste mass in landfills.