Study On Passive Earth Pressure And Deformation Control Calculation Theory In Foundation Pit With Skirt Reinforcement

In the era of rapid development of national economy and acceleration of urbanization,people have made unprecedented efforts to develop underground space.The foundation pit engineering is developing in a deep and large direction,and the surrounding environment of the foundation pit is becoming increasingly complex,so the difficulty of support is significantly increased,especially in soft soil areas.As a kind of bad geology,soft soil is widely distributed.According to statistics,the cost of soft soil reinforcement in foundation pit engineering of this area accounts for 26.6%?58.5%of the total supporting cost,and the cost of soft soil reinforcement in projects with slightly larger construction scale can reach tens of millions of levels.Moreover,soft soil area is also the frequent occurrence point of engineering safety accidents,and soft soil has a great impact on engineering construction both in terms of safety and economy.Although most of the skirt reinforcement projects in the passive zone of foundation pit have been successful at present,the design and calculation theory still stays at the experience level and lacks theoretical support.In the calculation theory of deformation control of foundation pit,the influence of factors such as width,height,soil properties of lateral and below the reinforcement mass on the m value of skirt reinforcement can not be reflected;In the aspect of passive earth pressure calculation theory,the existing classical earth pressure calculation theories are all put forward for semi-infinite isotropic body.There is no relevant earth pressure calculation theory for the case of semi-infinite non-isotropic body formed by skirt reinforcement in the passive zone of foundation pit.The earth pressure theory of limited range reinforcement non-isotropic body will be a brand-new topic of earth pressure theory,and this topic has important practical significance to make up for the blank of calculation theory in this field.The purpose of this paper is to establish the calculation method of passive earth pressure and deformation control theory of foundation pit with skirt reinforcement with sufficient theoretical basis,strong applicability and high reliability,and to provide a comprehensive and systematic solution to the problem of skirt reinforcement in passive zone of foundation pit.In the aspect of parameter test of skirt reinforcement soil-cement,the physical and mechanical parameters of soil-cement are obtained by in-situ test and indoor test,and the correlation formula between in-situ test index and indoor test index is established.On the basis of fully understanding the failure mechanism of passive zone of foundation pit with skirt reinforcement,the analytical formula of passive earth pressure calculation of foundation pit with skirt reinforcement is derived.In the aspect of deformation control calculation theory of foundation pit with skirt reinforcement,the analytical formula of equivalent m value of skirt reinforcement mass is derived,and the deformation characteristics of foundation pit with skirt reinforcement are deeply studied.The main work and research results are summarized as follows:(1)The sliding failure mode of the passive zone of foundation pit with skirt reinforcement based on?angle theory is proposed.The results of finite element numerical test show that the sliding failure mode based on?angle theory is reasonable and correct.It is found that the shear strength parameters of soil-cement have an important influence on the potential fracture surface in the passive zonr of foundation pit with skirt reinforcement.When the potential fracture surface forms change from passing through the reinforcement mass to bypassing from the bottom of reinforcement mass,the soil-cement cohesion ratio and the tangent value ratio of internal friction angle have critical values,while the soil-cement stiffness parameters(elastic modulus and Poisson's ratio)have little influence on the potential fracture surface forms in the passive zone of foundation pit with skirt reinforcement.The concept of critical width of limited range reinforcement mass and its solution method are put forward.(2)The calculation method of passive earth pressure of foundation pit with skirt reinforcement is put forward.Based on the block limit equilibrium method,the calculation analytical formula of passive earth pressure of foundation pit with skirt reinforcement is derived,and the Mohr stress circle method is used for the theoretical verification.This method avoids the problem that it is difficult to determine the values of c₁ and?₁ of cement soil of limited range reinforcement mass,and is reliable for limit equilibrium analysis.The passive earth pressure of foundation pit with skirt reinforcement is controlled by?angle.When the width of skirt reinforcement mass does not exceed the Rankine passive fracture surface passing through the pile bottom,the skirt reinforcement basically has no contribution to improving the passive earth pressure;When the width of skirt reinforcement mass exceeds the Rankine passive fracture surface passing through the pile bottom,the ultimate earth pressure in the passive zone of the soil layer below skirt reinforcement mass will be higher than that of Rankine passive earth pressure.The larger the?angle,the better the influence.Under the condition of the same amount of reinforcement mass,the?angle of reinforcement mass arranged in vertical rectangle is larger,and the greater passive earth pressure can be provided.From the perspective of passive earth pressure,vertical rectangle is better than horizontal rectangle;Compared with horizontal rectangle reinforcement mass and top-stepped reinforcement mass,bottom-stepped mode is not conducive to increasing?angle,so it should not be advocated only from the perspective of passive earth pressure.(3)The deformation calculation theory of foundation pit with skirt reinforcement based on rigid pile rotation theory is put forward,and the analytical formula of equivalent m value calculation of skirt reinforcement mass is derived.This method can consider the influence of factors such as width,height,soil properties of lateral and below the reinforcement mass,etc.,and does not need to obtain shear strength parameters of soil-cement.And it changes the dominant status of engineering experience,which has important guiding significance for the practice of skirt reinforcement in passive zone of foundation pit.Furthermore,the modified analytical formula for calculating the equivalent m value of reinforcement mass considering the influence of the replacement ratio,elastic modulus and shear strength parameters of reinforcement mass is proposed,and the adjustment coefficient related to the elastic modulus and shear strength parameters of reinforcement mass soil-cement is given,and the calculation method of equivalent m value of reinforcement mass in stepped type mode and multi-layer soil in front of reinforcement mass and the calculation method of horizontal reaction coefficient when the reinforcement mass reaches the supporting pile on the other side is also put forward which further strengthens the applicability of the deformation calculation theory of foundation pit with skirt reinforcement.(4)On the premise of the same amount of reinforcement mass,the effect of horizontal rectangle reinforcement mass on foundation pit deformation control is better than that of vertical rectangle reinforcement mass;From the point of view of controlling the deformation of foundation pit,there is little difference between adopting the bottom-stepped reinforcement mass and the top-stepped reinforcement mass;Compared with horizontal rectangle and stepped type reinforcement mass,horizontal rectangle reinforcement mass has the best effect on controlling foundation pit deformation,but when the width of reinforcement mass reaches a certain value,the effect of stepped type reinforcement mass on controlling foundation pit deformation is basically equal to that of horizontal rectangle reinforcement mass,but the amount of horizontal rectangle reinforcement mass is greater than that of stepped type reinforcement mass.When considering economic factors,stepped type reinforcement mass can be preferred.Compared with vertical rectangle and stepped type reinforcement mass,when the width of reinforcement mass reaches a certain value,the effect of vertical rectangle reinforcement mass on controlling foundation pit displacement is even weaker than that of stepped type reinforcement mass,and its economy is poor.Therefore,it is not recommended to adopt vertical rectangle reinforcement mass from the aspect of controlling foundation pit deformation.(5)There is a critical value of elastic modulus of reinforcement mass,which will not affect the maximum horizontal displacement of supporting piles after exceeding this critical value.The critical value of elastic modulus of reinforcement mass is related to the height of reinforcement mass,and the greater the height of reinforcement mass,the smaller the critical value,while the critical value of elastic modulus of reinforcement mass has little to do with the width of reinforcement mass.The influence of Poisson's ratio of reinforcement mass on the maximum horizontal displacement of supporting piles is limited.In a limited range of influence,the greater the height of reinforcement mass,the smaller the influence of Poisson's ratio on the maximum horizontal displacement of supporting piles.There is a critical value for shear strength parameters of reinforcement mass,and when this critical value is exceeded,the maximum horizontal displacement of supporting piles will not be affected.The critical value of shear strength parameters of reinforcement mass is related to the height of reinforcement mass,and the greater the height of reinforcement mass,the smaller the critical value.However,the critical value of shear strength parameters of reinforcement mass has little relationship with the width of reinforcement mass.(6)The total displacement of reinforcement mass consists of rigid body displacement and absolute deformation of reinforcement mass,in which the absolute deformation of reinforcement mass includes compression deformation and shear displacement.The ratio of rigid body displacement to total displacement is about 24%?55%,compression deformation is about 5%?19%,and shear displacement is about34%?63%.It can be seen from the proportion range of each component of total displacement of reinforcement mass that the compression deformation of reinforcement mass accounts for the smallest proportion,and the proportion of shear displacement is roughly equal to that of rigid body displacement.(7)Based on in-situ test,field sampling and indoor test,the correlation formula between indoor test indexes of soil-cement of skirt reinforcement(unconfined compressive strength,elastic modulus and shear strength index)and in-situ test indexes(static penetration resistance,standard penetration test hammer count and heavy dynamic penetration test hammer count)is established,and the correlation degree is significant on the whole,which shows that it is feasible to predict indoor test indexes of soil-cement by in-situ test indexes.(8)There is a certain optimization space for the width,height and form of the reinforcement mass in the passive zone of the foundation pit with skirt reinforcement in A plot project of Wuhan dream special zone,and the reasonable optimization measures should be comprehensively determined in combination with the control standards of specification and the requirements of surrounding environment protection.