Study On Process Stability Of Deep Foundation Pit Support In Karst Area Of South Hunan

Influenced by dissolution,the rock-soil interface fluctuates unevenly in the karst development area,and the red clay overlying has special engineering properties such as strong water sensitivity and swelling and contraction.Deep foundation pit engineering in Karst area,especially "half rock and half soil" deep foundation pit engineering,has special geological conditions.It is often not easy to calculate structural loads from earth pressure theory,and its instability process presents typical progressive failure characteristics.Based on a deep foundation pit project in a karst area in southern Hunan,this paper establishes a numerical model by ABAQUS on the basis of studying the water sensitivity characteristics of red clay strength and engineering geological conditions,discusses the stability changes in construction process,and puts forward the control measures of process stability.The specific research contents and results are as follows:(1)The physical mechanical properties and main mineral compositions of red clay at rock-soil interface were studied through laboratory tests: red clay has high water content,high liquid-plastic limit and high shrinkage;red clay has strong water sensitivity,cohesion decreases linearly with the increase of water content,and internal friction angle decreases power function with the increase of water content;through X-ray diffraction test,its main mineral compositions are quartz.Montmorillonite,kaolinite,illite and goethite.(2)Establishment of stability evaluation model of foundation pit process: analysis of deformation characteristics and foundation pit stability of foundation pit and supporting system in excavation process;combined with water sensitivity characteristics of red clay strength,the influence of water content increase on foundation pit stability is discussed;through simulation of rainfall and supporting pile-soil flow process,the mechanism of foundation pit instability is revealed,the shear strength of red clay is reduced by rainfall,and the foundation pit level is also discussed.With the increase of displacement,the flow of soil between piles results in stress transfer and redistribution.With the development of the flow soil,the sliding surface through the slope is gradually formed in the foundation pit.(3)Propose process stability control measures:surcharge soil at foundation pit bottom?additional anchor cable?the two excavation.By controlling the horizontal displacement of the foundation pit through the base heaped load,and simulating the influence of different heaped load section sizes on the stability of the foundation pit,the optimum heaped load height of the foundation pit is determined to be 6m;the reinforcement measures of adding anchor cables are adopted to ensure the stability of the foundation pit construction process,effectively control the horizontal displacement of the foundation pit,and improve the stability of the foundation pit;the influence of heaped load excavation process on the stability of the foundation Qualitative influence.The process stability control measures have achieved good reinforcement effect and ensured the stability of the foundation pit in the construction process.Studying the process stability and stability control of deep foundation pit in karst area of "half rock and half soil" is beneficial to enriching the process stability research system of deep foundation pit engineering in Karst area.Accumulating the cases of process stability control of deep foundation pit engineering in karst area can provide reference for similar projects of deep foundation pit in Karst area.