Instability Of Soft Soil In Passive Zone And The Analysis Of Its Stability Control

As China enters the era of high speed,the large-scale construction of infrastructure such as highways and wide roadbed,the instability caused by the erosion flow of soft soil in the passive area of roadbed has been paid more and more attention by experts and scholars in the industry.Scattered piles as compensation of soft soil subgrade is widely used in highway and high-speed railway in soft soil area,because of its advantages such as better deep reinforcement effect,settling control stability and reasonable economy.However,the stability control theory of scattered pile subgrade lags behind the engineering practice,so it is urgent to carry out systematic research on its working mechanism and improve the relevant analysis theory and design method.Based on the research of stability problem caused by lateral plastic flow in soft soil area under load at home and abroad,this paper focuses on the shear instability mechanism of soft soil and its stability control.The laboratory test,theoretical analysis combined with numerical simulation technique,constructs the synergistic model of scattered pile as compensation to soft soil,the load transfer model of scattered pile,put forward the judgement on the basis of the plastic zone of soft soil,reveals the instability mechanism of soft soil roadbed in passive area and stability control method of scattered pile,and the practicability of relevant theories are proved combined with engineering example.The main research contents and achievements of this paper are:（1）Based on the ratio of the average undrained strength of soft soil foundation to the previous consolidation pressure was constant 0.22（Mesri,1989）,and the calculation method of undrained shear strength was simplified by considering the evolution law of the undrained strength of soft soil with depth.（2）Based on the cooperative design principle of roadbed scattered piles,this paper uses Boussinesq elastic stress theory and mathematical algorithm to gain the additional stress field of the weak part between scattered piles,proposes a method for judging the critical load of soft soil foundation.Based on the theoretical analysis of undrained strength（S_u）,the paper also complete the elastic-plastic theoretical analysis method of shear flow of soft soil under subgrade,reveals the instability problem of passive zone caused by plastic flow,and indicates the mechanism of"reducing settlement"of soft soil compensated by scattered piles because of the significant reduction of additional stress of weak part between piles.Furthermore,the paper analyzes the influence of the passive slope ratio,the setting mode of the anti-pressure platform and the hard-shell layer on the stability of the subgrade under loading of the soft soil foundation by the algorithm.Meanwhile,the mechanism of the adverse landform of the slope toe and the area index（105 m²～141m²）in the passive area,is correlated to improve the analysis method of the additional stress field of the soft soil subgrade.（3）According to the design principles of limit state,the stability concept model of scattered pile as compensation to soft soil was proposed that scattered piles sharing load（stress concentration）can inhibit shear plastic deformation of subgrade,and puts forward the bearing capacity computation method of subgrade scattered piles,which can be used in the soft soil roadbed to design and check the spacing and length of scattered piles.（4）Based on unit cell analysis model and combining with the theory of"soil arch"of subgrade cylindrical shear displacement proposed by Marston,this article derives the analytical solution of cylindrical micro-element differential control equations of shear displacement.And this paper introduces the ideal elastic-plastic model of cylindrical shear displacement,establishes the stress diffusion analysis model of the gravel cushion,and derives the evolution law of the load with single ratio ofλ₀ on the bottom surface of the scattered pile（h=0）and the baseλ_b,after the diffusion of the gravel cushion on the top of the cap of the scattered pile.Based on the theoretical analysis of load transformation of scattered pile shear displacement and base load transformation of cylindrical shear displacement in the subgrade.Based on the theory of cylindrical elastic-plastic state analysis,the typical three-layer system of the lime-soil layer under roadbed and the filling in the middle and upper lime-soil roadbed are further established,the applicability of the method to check the foundation structure of roadbed scattered piles is verified with an engineering example.（5）This paper systematically indicates that stability-control standard of roadbed loading（[VD]=5mm/d、[VS]=10mm/d）and the convergence-control standard of roadbed preloading settlement.The stability monitoring during the filling period is mainly used to control the filling rate and ensure the stability of roadbed.The main unloading time and the construction time of surface layer are monitored during the preloading period.（6）Based on the consolidation expression of equal load preloading proposed by Zen Guoxi（1981）,the paper puts forward the stability index of the increment of horizontal displacement of soft soils in the passive zone during graded loading and the increment of embankment settlement,and derives the general formula of subgrade under load preloading,preloading,and overload preloading associated with settlement and sedimentation rate.Combining with the relevant control standards,which can guide the engineering practice.In this paper,the design of construction drawing and the convergence checking method of settlement rate in preloading period are put forward.