Study On Uneven Settlement Mechanism Of Soft Soil Foundation In Karst Area And Foundation Underpinning Of High-rise Buildings

"Nine-storey terrace,from the base soil",the ancients attached great importance to the importance of building foundation.Today,with the construction of more and more nine-storey or ten-storey or even 100-storey platforms,the requirement of foundation for engineering projects is getting higher and higher.All kinds of unfavorable geological phenomena,such as karst caves,landslides,soft soil,etc.,are often encountered,which will bring about influence and damage to the foundation of high-rise buildings,uneven settlement and even collapse,which will bring great economic losses and negative social impact.A building in Duyun,Guizhou Province,has subsided since the beginning of construction because of the unknown engineering geological conditions.Before underpinning,the maximum settlement of the building is 166.01 mm in the Northeast corner,while it rises 2-3 mm in the southwest corner of the building.This phenomenon of "seesaw" indicates that the uneven settlement of buildings is extremely serious.Therefore,this paper takes this as a case,finds out the stratum information under raft foundation through drilling data,and obtains the physical and mechanical properties of rock and soil combined with laboratory tests,and then analyses the causes of uneven settlement of the high-rise building.In addition,using automatic static level,IBIS-L topographic micro-deformation monitoring system,strain gauge and other monitoring means to obtain the building settlement,deformation,stress and other changes.According to the deformation characteristics and uneven settlement reasons of the building,a specific construction scheme is adopted,and a foundation underpinning technology combined with anchor static pressure pile and SJP material cement slurry is put forward.Secondly,the settlement characteristics of buildings under natural conditions are analyzed by numerical simulation method,and the settlement and deformation characteristics of buildings after foundation underpinning are also simulated.Finally,the effect of foundation underpinning is comprehensively evaluated by monitoring data,quantitative calculation and numerical simulation results,and the settlement mechanism of high-rise buildings is analyzed combined with the settlement process of buildings.The main research results are as follows:(1)Through drilling and other exploration data,it is found that there is a certain scale of dissolution zone(cave group)and soft soil under the raft foundation of the building,and the caves in the northeast of the site are densely developed,and the soft soil also shows the characteristics of thick northeast and thin southwest.(2)The physical and mechanical parameters of soft soil and bedrock in the study area are obtained through basic physical and mechanical tests,which provide basic parameters for building settlement calculation and numerical simulation.(3)According to the results of field investigation and the monitoring data of buildings,it is shown that:(1)from January 2017 to June 2018(before foundation underpinning and reinforcement),the maximum settlement of buildings appears at JC-10 monitoring point in the Northeast corner,and there is a phenomenon of seesaw rising on one side of settlement,and the settlement in the eastern part of buildings is much higher than that in the central and Western parts;The cracks on the north side wall tend to W,and the cracks on the south side wall tend to E;furthermore,the farther away from the maximum settlement point,the cracks grow,the longer the length,the wider the width,the smaller the inclination angle,and the higher the seepage ratio;thirdly,the maximum strain of the shear wall appears in the straight line of the maximum differential settlement point(northeast and southwest corner);fourthly,the apparent displacement of the building is at the end of the building.Before foundation underpinning and reinforcing project,it is in a growing trend.After foundation underpinning is completed,the monitoring points remain stable.(4)Combined with formation information and monitoring data,the cause of uneven settlement of the building is related to the development of underground dissolution zone and cave,the uneven distribution of underground soft soil and the change of underground water level.Groundwater has the most obvious influence on settlement and settlement rate.(5)In view of the geological structure and settlement reasons of the building,SJP cement grout combined with anchor static pressure pile is applied to the foundation underpinning technology,and good engineering results have been achieved.(6)Flac3d simulation of natural settlement of buildings found that: the displacement and deformation of the roof of the building has magnification effect,the closer to the upper part of the building,the greater the settlement of the foundation soil,the closer to the surface settlement,the greater the settlement of the eastern part of the building;Flac3d simulation of building foundation underpinning reinforcement,the smaller the difference of settlement between East and west sides of the foundation;Uniformly,the plastic zone mainly occurs at the junction between the East and west sides of raft foundation and rock and soil.In addition,there is a certain range of plastic zone at the corner of the East and West sides.In addition,there is a large compressive stress at the bottom of steel pipe pile.(7)Combined with numerical simulation,quantitative calculation and later monitoring data,the building remains stable after foundation underpinning reinforcement,reflecting the good effect of anchor static pressure pile and grouting lifting reinforcement scheme.In addition,combined with the principle of effective stress,the settlement process of buildings is analyzed.