Calculation Of Passive Earth Pressure And The Deformation Behavior Of Foundation Pit Under The Condition Of Pit-in-pit Reinforcement

Foundation pit engineering continues to develop in a deeper and more complex direction.The existence of underground auxiliary structures such as basements and elevator shafts has made more and more pit-in-pit appear in foundation pit projects in recent years.However,for some soft soil areas,the traditional cantilever support combined with internal support often fails to achieve better results,reinforcing the soil in the passive zone of the foundation pit becomes one of the effective methods to control the deformation of the enclosure structure.At present,most of the relevant researches only separately study the pit-in-pit and the reinforcement of the passive zone.There are few researches on the construction status of the reinforcement of pit-in-pit,and the theory lags behind the engineering practice.Therefore,it is of great engineering significance to study the deformation behavior and force of the foundation pit under the condition of pit-in-pit reinforcement.Based on the limit analysis method,this dissertation derives the passive earth pressure formula under the pit-in-pit and its reinforcement conditions,and then uses the MIDAS/GTS software to carry out corresponding research on the deformation behavior of the foundation pit retaining pile under this condition.The main research results are as follows :(1)Use limit analysis method to calculate the passive earth pressure of finite soil like pit-in-pit,use MATLAB and MAPLE calculation software to derive the calculation formula of finite soil passive earth pressure and its corresponding slip angle,revise the traditional calculation method of earth pressure;In addition,the force between the pile and soil is taken into consideration,the calculation formula of the passive earth pressure and the slip angle considering the force between the pile and soil is obtained,and compared with the unconsidered one,it is found that under the same calculation parameters,the passive earth pressure will increase significantly and the slip angle will decrease after consideration.(2)After considering the reinforcement of the pit-in-pit,the passive earth pressure of the limited soil is further optimized,using a combination of limit equilibrium method and limit analysis method,the calculation method of passive earth pressure and limit slip angle under the condition of pit-in-pit reinforcement is deduced.A calculation example is used to verify that the pit-in-pit reinforcement can greatly increase the passive earth pressure within the depth of the reinforcement,move the resultant point upward,and improve the anti-overturning stability of the enclosure structure.(3)Using MIDAS/GTS software to establish a two-dimensional finite element model,it is found that the reinforcement effect is the best when the reinforcement is set on the side of the outer pit surrounding pile on the platform;There is a most suitable size for the reinforcement(reinforcement depth 7H/6,width 3B/5)to achieve the best reinforcement effect.The increase in the degree of reinforcement affects the reinforcement effect mainly in the control of the lateral displacement of the outer pit retaining pile;In addition,by constructing an orthogonal table,using range and variance analysis,it is concluded that the various factors are arranged according to the sensitivity level as follows: distance between the reinforcement and the outer pit>reinforcement depth> reinforcement degree> reinforcement width.(4)Establish a three-dimensional finite element model and compare it with the two-dimensional model.It is found that the lateral displacement of the pile at the center of the pit side of the three-dimensional model is smaller than that of the two-dimensional model,but the soil reinforcement will reduce this difference;The influence range of the inner and outer pit angle effect is about 3 times of the corresponding excavation depth,accounting for about 60% of the corresponding side length.Based on this range of influence,the traditional skirt reinforcement form is optimized,the soil within a certain range close to the pit corner is not reinforced,and this kind of reinforcement can achieve the purpose of reducing the project cost under the premise of effectively controlling the deformation of the inner and outer pit retaining piles.