Solution Of Cylindrical Cavity Expansion/Contraction In Structured And Unsaturated Soils And Its Application In Geotechnical Engineering

In geotechnical engineering,the process of pressuremeter test and pile installation can be generalized as the expansion of cylindrical cavity,while the deformation of surrounding soils/rocks during tunnel excavation and drilling can be regard as the contraction of cylindrical cavity.Consequently,the research on the cylindrical cavity expansion/contraction problem is helpful for the theory analysis of the aforementioned geotechnical engineering problems.At present,most of the existing research assumed that the soil is saturated and reconstituted.However,the natural soil is structural and unsaturated,and their mechanical behavior is significantly affected by the soil structure and moisture status.Therefore,the analytical method is used first in this paper to present the solutions of cylindrical cavity expansion and contraction in saturated structured soils,as well as that of cylindrical cavity expansion in unsaturated soils under drained(constant suction)and undrained(constant moisture)conditions.Subsequently,the improved sub-incremental explicit integration algorithm was adopted to develop the UMAT subroutine of the Cemented-CASM model,and a numerical solution for the cylindrical cavity expansion and contraction in unsaturated soils was further proposed.Taking the contraction problem as an example,the numerical solution was verified by comparing with the analytical solution.This numerical method will provide a foundation for the cylindrical cavity expansion and contraction under complex boundary conditions.Finally,the theoretical solutions were applied to predict the results of lateral pressure tests,analyze the pile installation,and calculate the unloading deformation of drilling surrounding rock.The research in this paper have filled the gap in the theoretical study of the cylindrical cavity expansion and contraction problems,and provided a theoretical basis for the related geotechnical engineering problems.The main research contents include the following aspects:(1)Based on the structured cam clay model,an undrained solution for cylindrical cavity expansion was derived considering the soil structure and its degradation.In the elastic region,Hooke’s law and small strain assumptions were adopted to obtain the expressions of effective stress and pore pressure.In the plastic region,the SCC model was employed to consider the influence of soil structure and its degradation.Combining with the large strain(logarithmic strain)theory,the effective stress governing equation of the soil around the cavity was established by using the Lagrangian analysis method.Taking the stresses at the elastic-plastic boundary as the initial values,the variations of stresses around the cavity was solved.Comparing with the solution in reconstituted soils,the influence of structure and its degradation was analyzed;meanwhile,the effects of structural parameters were evaluated.(2)Adopting the Cemented-CASM model considering soil cementation,an undrained solution for the cylindrical cavity contraction in structured soil was proposed.On the basis of illustrating the differences between the cylindrical cavity expansion and contraction,an elastic-plastic solution for the cavity contraction in structured soils was obtained by directly deducing the elastic-plastic stiffness matrix,and then combining with the hardening rule and the large strain theory.Compared to the traditional method,the present method is more concise.By comparing with existing solution,the present solution was verified,and the mechanical response characteristics of cavity contraction in structured soils was discussed.(3)Based on the unsaturated modified cam clay model,the cylindrical cavity expansion solutions under drained(constant suction)and undrained(constant moisture)conditions were established.Firstly,by introducing auxiliary variables,the equilibrium differential equation under the Euler system is transformed into the Lagrangian system,and then the drained solution of cylindrical cavity expansion in unsaturated soil was established;subsequently,The constitutive equation reflecting the relationship between specific volume and suction was introduced to drive undrained solution for cylindrical cavity expansion in unsaturated soil.Finally,parameter analyses were conducted to discuss the influence of unsaturated hydraulic characteristics on the cylindrical cavity expansion process.(4)Numerical solutions for the cylindrical cavity expansion and contraction in structured soil were proposed through secondary development of finite element analysis software.The numerical model was established in the ABAQUS/CAE,and the UMAT subroutine of the Cemented-CASM model was developed by using the improved sub-increment step explicit integration algorithm.Taking the cylindrical cavity contraction as an example,the numerical solution was verified by comparing with the theoretical analytical solution.This numerical method will provide a foundation for the cylindrical cavity expansion and contraction under complex boundary conditions.(5)To illustrate the applicability of the theoretical solutions,it was applied to the analysis of related geotechnical engineering problems.Considering the pressuremeter test and pile installation as the expansion of cylindrical cavity,the solution presented in this paper was used to predict the stress around the cavity.Considering the deformation of surrounding soils/rocks during tunnel excavation and drilling as the contraction of cylindrical cavity,it was used to analyze the wellbore pressure and excess pore water pressure at the wellbore wall.By comparing the calculation results with the existing measured data or numerical calculation results,the applicability of the theoretical solution in practical engineering was verified.