Study On Mechanism And Engineering Applications Of Building Underexcavation And Reinforcement In Loess Regions

The loess-based subsoil is extensively distributed in the western regions of China,characterized by its unique stress features and collapsibility because of its vertical structure,large porosity,and fine particles etc.These attributes might lead to uneven settlement,wall cracks,and potential tilting or even collapse of buildings during usage,posing a severe threat to the regular operation and safety of these structures.Rectification of building inclinations combined with foundation reinforcement stands as an effective approach to address these concerns.This approach involves researching relevant technologies for rectification in loess areas,reinforcing the original building foundations post-rectification to enhance stability,thereby ensuring continued safe usage of the structures.This not only saves costs and minimizes resource wastage but also holds strategic significance in reducing carbon emissions and promoting sustainability.Currently,the building tilt correction method is mainly divided into three categories:lifting method,forced landing method and comprehensive tilt correction method.Among them,the horizontal subsoil extraction method for rectification has been widely applied in engineering practice because of its clear principle,easy operation and economic saving.However,research on the mechanism of this method is still in its initial stages,particularly with regard to the unique soil conditions in loess areas.There is a lack of systematic theoretical research and engineering applications.This thesis,starting from the common occurrence of building inclinations in loess areas,employs a combination of theoretical analysis,field experiments,numerical simulations,and various research methods.Building upon the existing research results,it systematically investigates the mechanism of the horizontal subsoil extraction method for rectification in loess areas and the pile foundation bearing capacity when reinforcing the foundation with micro-piles.The primary research contents are as follows:(1)Based on the Mohr-Coulomb failure criterion,an analysis was conducted on the deformation and failure modes of the inter-hole soil throughout the entire process of horizontal subsoil extraction for rectification.By establishing a correspondence between the failure modes of the inter-hole soil and the stress state of the soil,the deformation of the inter-hole soil was categorized into four stages: elastic-plastic critical state,plastic expansion stage,failure-reshaping stage,and ultimate compaction state.Using the deformation characteristics of each stage,a comprehensive method for calculating the entire process of settlement in the inter-hole soil during horizontal subsoil extraction for rectification was developed.Building upon this method for calculating settlement over the entire rectification process,two models were proposed: the "tunnel" model for calculating settlement during rectification with a single-layer horizontal hole and the "truss" model for calculating settlement during rectification with double staggered holes.These models established calculation formulas for rectification settlement values using different subsoil extraction methods.Through numerical analyses,it was observed that the calculated rectification settlement quantities under different subsoil extraction methods aligned well with actual results,demonstrating good applicability.(2)An in-situ experiment was designed and conducted in loess areas to study the factors influencing the ground settlement in horizontal subsoil extraction and the impact of excavation hole parameters on the stress redistribution of surrounding soil.The experiment verified and analyzed the damage,failure,and reshaping processes between horizontal subsoil extraction holes in loess-based subsoil.Results from the in-situ experiment demonstrated the following:Under certain upper structural loading conditions,the influence of the excavation holes on the redistribution of surrounding soil pressure conforms to the failure mode of the inter-hole soil and the failure-reshaping mechanism.This validation confirms the theoretical correctness and reliability of the four-stage inter-hole soil failure model.Time,upper loading,hole diameter,and inter-hole spacing significantly affect the ground soil settlement.Results from the range analysis indicated that the impact of hole diameter and inter-hole spacing on settlement quantity is relatively equal.(3)Through numerical simulation analysis,the primary influencing parameters of the horizontal subsoil extraction method for rectifying building inclinations were examined,including base pressure,hole diameter,hole length,inter-hole spacing,and depth of overlying soil.Grey correlation theory was applied to conduct sensitivity analysis on the rectification settlement quantities for each parameter.The results indicated that the sensitivity order of factors influencing the effectiveness of the horizontal subsoil extraction method for rectification is as follows: base pressure,hole diameter,hole length,inter-hole spacing,and overlying soil thickness.Therefore,in the design of subsoil extraction rectification,enhancing the rectification effect can be achieved by increasing the base pressure,enlarging hole diameter,extending hole length,and preferably designing circular hole sections whenever possible.(4)The research focused on analyzing the bearing capacity of existing foundations reinforced by micro-pile steel pipes.The study combined theoretical calculations and static load tests and involved establishing a pile-soil interaction model.This model,utilizing the load transfer method,calculated the vertical bearing capacity of the piles.The investigation scrutinized the bearing characteristics,axial force on the piles,lateral frictional resistance,pile top settlement,and the associated failure modes.Analyzing the parameters in the load transfer model revealed the rules governing the impact of relevant parameters on bearing capacity.Considering the small diameter,high slenderness ratio,and susceptibility to instability of micro-pile steel pipes,a bearing capacity calculation formula was developed,incorporating stability influences into the load transfer-based bearing capacity calculations.Additionally,it was observed that micro-pile steel pipes have an optimal critical pile length.As the pile length increases,there exists a critical value for the pile’s bearing capacity.(5)Taking an example of rectifying the incline of a certain ancient pagoda,as well as rectifying the incline of a high-rise building and subsequently reinforcing the foundation with micro-pile steel pipes,in the northwest loess region.This study validated the proposed calculation methods.It comprehensively analyzed the reasons behind the building inclinations,provided detailed insights into the overall rectification and reinforcement strategies,critical construction technologies,and the comprehensive monitoring methods throughout the entire process.Upon completion of the rectification,a thorough evaluation of the rectification and reinforcement’s effectiveness was conducted.The maximum inclination of the inclined buildings met the regulatory requirements.