Study On Problems Of Flow Deformation Induced By Construction Of Deep Foundation Pit In Soft Soil Area And Its Countermeasures

In the eastern coastal areas of China,especially in the lower reaches of the Yangtze River,soft soils are widely distributed and have a large thickness.Because soft soils have high water content,high compressibility and poor permeability,they are prone to thixotropy and their engineering geology is very poor.In recent years,studies have found that flow deformation is likely to occur during the construction of foundation pits.However,the deep foundation pit project is a complicated temporary project.The safety factor of the support structure is low.Due to the deformation of the soft soil during the construction of the pit,the ground settlement around the foundation pit and the deformation of the building can be easily induced.This paper takes the underground continuous wall and bored cast-in-place pile supporting structure engineering in the deep soft soil distribution area in Nanjing as an example to analyze the environmental geotechnical engineering problems existing in the construction of the supporting structure.For example,base groove collapse and bore hole shrinkage cause the soft soil flow deformation around the foundation pit,and then induce the cracking problem of buildings.On the basis of this goal,with the aim of reducing the disturbance of soft soil and not causing flow deformation,the support technology using SMW method piles was proposed.The advantages of the SMW construction method pile as support structure for deep foundation pit in soft soil area are analyzedto solve the problem of soft soil flow deformation in deep foundation pit construction in soft soil area.In this paper,relying on the deep and special-shaped foundation pit engineering of the Nanjing Youth Olympic Games,the SMW construction method pile is adopted as a deep foundation pit supporting structure in the soft soil zone.The actual engineering construction inspection shows that the deformation of the soft soil around the foundation pit is very small and completely controlled in the safe range.This successful case shows that SMW construction method piles have a small impact on the surrounding construction of the deep foundation pit supporting structure and have a good applicability.The main research results of this paper are as follows:1.The paper reviewed the research data,summed up the engineering characteristics of soft soil,the deformation of soft soil and the research status of deep foundation pit in deep soft soil.During the construction of deep foundation pit in soft soil area,many environmental geotechnical engineering problems have been induced due to the flow deformation of soft soil,such as the ground settlement around the pit,ground collapse,and the long-distance effect of soft soil flow deformation,resulting in the existing building deformation and damage problems.2.This paper analyzes the main environmental geotechnical engineering problems existing in the construction of underground continuous walls and bored cast-in-place piles commonly used in deep foundation pit supporting structures in soft soil areas.In the construction of foundation trench excavation of underground continuous walls,due to high water content in soft soil,poor engineering properties,and insufficient self-stabilization,the problems of collapse and intrusion of trench walls are likely to occur,and serious over-excavation occurs.Similarly,in the construction of bored piles for foundation pit support in soft soil areas,shrinkage and hole collapse often occur.The soft soil is disturbed by the construction and the structure is damaged.In combination with over-excavation,soft soil flow deformation is easily induced,causing damage to existing buildings around the pit.Therefore,the effectiveness and advantages of the methods used in underground continuous walls and cast-in-place piles commonly used in deep foundation pit supporting structures in soft soil areas have been challenged.3.The paper takes the J3 area of the deep and special-shaped foundation pit engineering of the Nanjing Youth Olympic Games as an example.The soft soil is widely distributed in the project site and its thickness is very large.The depth of the foundation pit is 12.7m,and the shape is irregular.There are high-rise buildings and various pipelines and roads around the pit.The environmental conditions are complex,and the deformation control is very demanding.In order to reduce the disturbance to the soft soil and the environmental effect of the flow deformation induced surrounding excavation,the excavation type SMW pile method is used for the excavation in this section.Through FLAC 3D numerical simulation analysis,it shows the applicability of the method in this area.At the same time,the foundation pit project was successfully completed,which further proved the feasibility and effectiveness of this support technology.