Study On Cavity Expansion Theory And Numerical Analysis Of Self-boring Pressuremeter Test

Self-boring pressuremeter(SBPT) has been effectively utilized to determine in-situ properties of soils in terms of overcoming soil disturbance to a large extent,especially in saturated soft soil for determination of permeability characteristics and undrained shear strength.However,it is recognized that the undrained strength value and horizontal coefficient of consolidation derived from SBPT are different from those obtained by good laboratory tests and other in situ tests.In order to find the reasons of inaccuracy,the possible factors are analyzed by using finite element methods and quantified by comparing with analytical solutions of cylindrical cavity expansion that are useful interpretation methods which involves important assumptions about material behavior,pressuremeter geometry and drained conditions.Many other geotechnical engineering problems also rely on cavity expansion theory for analyzing,such as pile driving,CPT and anchor working mechanics. And the theory has been progressively refined over the past years.Many research results indicate that it is important for study on cavity expansion theory to choose the reasonable yield criterion and to consider the strain-softening and shear dilatation behavior of geomaterials.Though achievements have been made on the theoretical study,the cavity expansion in non-brittle softening and dilatant soil is in the stage of development.Thus the other purpose of this paper is to present cylindrical cavity expansion analysis in dilatant and elastic-plastic soil,taking non-brittle softening and dilatancy behavior of geomaterials,and the effect of intermediate principle stress on soil strength into account.The main investigations consist of the following portions:1,The current use of fundamental mechanics in developing rational interpretation methods for deriving soil properties from SBPT is reviewed,and the sources of inaccuracy and modified methods are analyzed and summarized.2,Considering the effects of intermediate principal stress on material yielding strength, SMP yielding criterion is selected in analysis of cylindrical cavity expansion.Meanwhile, combining Bolton's simplification on the stress-dilatancy relationship and Bolton model to reflect soil shear dilatancy and strain softening behavior this paper analyzes the problem of cylindrical cavity expansion in typical quartz sand(Ottawa sand) by discretizing the plastic zone to determine stress field,strain field,limit cavity pressure,and variation of friction angle, dilatancy angle and void ratio.It is noted that this analysis is applicable to cavity expansions from zero initial radius and finite initial radius simultaneously.The impacts of intermediate principal stress on cavity expansion are examined by comparing the present solutions with results based on Mohr-Coulomb criterion.And the effects of initial state and critical friction angle on ratio of plastic radius,limit pressure,dilatancy etc.are explored.A set of charts have been provided for use in direct estimate of limit pressure and ratio of plastic to cavity radius as a function of soil state(relative density and initial stress state) for soils with the similar properties to Ottawa sands.3,The extended spatial mobilization plane theory(SMP) is adopted in consideration of the effect of intermediate principal stress on soil shear strength.Damage softening parameter, taken as degradation grads of material mechanics capacity,is introduced to depict the soils softening behavior after yielding.Rowe's Stress-dilatancy equation in combination with cohesion degradation expression by damage softening parameter is rewritten.It can simultaneously consider the bonding component in dilatant response of elastic-plastic soils and softening behavior due to structure degradation for cemented soils during cavity expansion.The cavity expansion problem is formulated in small strain in the elastic zone and large strain in the plastic zone.Based on the extended SMP criterion and stress-dilatancy relation,the governing equations of axisymmetric problem in the plane strain condition and the partial differential equations for the boundary-value problem of cavity expansion in frictional cohesive soils are established.Solutions of radial and hoop stresses and strains around an expanding cavity are obtained by recursive computations.The significance of consideration of the effect of intermediate principal stress is demonstrated by a comparative study between the present solution and current solution based on Mohr-Coulomb criterion. Also the influence of damage softening parameter,cohesion and friction angle is examined by a parametric study.The proposed solution presented here can be of interest for the interpretation of pressuremeter tests or pushing-in piles carried out in cohesive-frictional materials under drained condition.4,Numerical simulations of self-boring pressuremeter loading tests using ABAQUS are performed to assess the influence of limit length of SBP and strain range on undrained shear strength derived from pressuremeter curves.The numerical models are built where soils behavior is depicted by modified Cam model,and low coefficients of permeability are chosen to control undrained conditions during expansion.The pressuremeter curves are obtained from numerical simulation of SBPTs for ratio of length to diameter L/D values of 6,10,15, 20 and various overconsolidation OCR range from 1 to 20.According to Gibson's analysis, the undrained shear strength using the least squares method is determined from the several chosen strain ranges.The ratio between the undrained shear strength obtained with infinite L/D and the value with various L/D is defined as correction factor,which are obtained from several strain ranges.It is observed that strain range over which the pressuremeter curve is fitted has little influence on derivation of the correction factors,while it has significant influence on undrained shear strength.And further,strain ranges of deriving undrained shear strength are proposed for different overconsolidation ratio by comparing finite element results for L/D=∞with analytical solution(Cao et al.) based on modified Cam clay model.5,Based on elastic-perfectly plastic Drucker-Prager model,analysis of holding tests using FEM is carried out to illuminate the applied conditions of commonly used a closed-form solution proposed by Randolph and Wroth through investigating the effects of pressuremeter geometry,the strain rate,permeability coefficient and the cavity strain level on partial drainage during cavity expansion,distribution of excess pore pressure,dissipation of excess pressure during SHT.Moreover,based on the results of numerical analyses,strain rate is proposed and the values of time factor T₅₀ provided by Randolph el al.to estimate horizontal consolidation coefficient are modified in view of the effects of soil permeability and strain rate on partial undrained.Since stress relaxation during SHTs exists and significantly influences the decay of excess pore pressure,the value of T₅₀ deduced by current solutions that ignore the soil theology can be inaccurate.The numerical analysis is performed considering stress relaxation during SHT based on coupling Drucker-Prager and time hardening theology model,where theology parameters are determined by clay rheological test. It is concluded that consideration of soil theology during SHT can speed the decay of excess pore pressure which is the combined results of the emergence of stress relaxation and changing distribution of excess pore pressure that tends to slow down dissipation.For lower coefficient of permeability,soil theology has more significant impact on decay of excess pore pressure.The influence of soil theology is assessed by computation of theology parameters variation within the range by the clay rheological tests for the case that s_u=48kPa,I_r=156.