Research On Basal Heave Stability Of Foundation Pit Bottom In Loess Area

In this paper,the safety calculation of the basal heave stability of foundation pits is further improved by analyzing various methods of stability and safety factor of foundation pit.Considering the existence of the weak underlying layer at the bottom of the basement,the calculation method of the basal heave stability of the foundation pit is verified by the example,and the concept of the critical thickness of the solid soil layer at the bottom of the foundation pit is also obtained.By using the two methods of limit equilibrium method and numerical analysis method,the obtained safety factor is analyzed,and the feasibility of numerical analysis to calculate the basal heave stability of foundation pit is verified.Finally,based on the grey correlation analysis method,the sensitivity of each factor affecting the stability of the foundation pit is analyzed.The following research results have been obtained:(1)Based on the circular sliding method in the limit equilibrium method,The influence of the ultimate bending moment value of the supporting structure and the layered soil parameters on the basal heave stability coefficient of the foundation pit is considered.Through the actual engineering example,the calculation results of the calculation formula of the basal heave safety factor of the foundation pit derived from this paper are compared with the calculation results by other calculation methods,and the calculation method has certain rationality.(2)There are also some weak underlying layers in the bottom of the foundation pit in the loess area,and the calculation of the basal heave safety factor of the foundation pit with weak underlying layer in the technical specification of the foundation pit is simple.This paper will use Prandtl method.The ultimate bearing capacity model is used as the basis of calculation,and combined with Meyerhoff and Hanna's study on the ultimate bearing capacity of the foundation with soft substratum,it is deduced that when the bottom of the soil excavation has a soft substratum,the ratio of solid soil layer H to base width B is different under the subsurface,the calculation method of the basal heave stability is more accurate,it is verified by the example.It is continued to analyze that there is a critical thickness value of solid soil layer.Exceeding this thickness will not consider the influence of soft substratum on the stability of foundation pits.(3)This paper combines the finite element difference software FLAC3 D to simulate the foundation pit,and obtains the safety factor of the numerical analysis method through software analysis,compares it with the safety factor obtained by the limit equilibrium method,and gradually analyzes the value of each influencing factor.The influence of the basal heave of the foundation pit is analyzed: weight of soil is reduced,the soil cohesion is increased,the internal friction angle in the soil is increased,the external overload of the foundation pit is reduced,the insertion ratio of the support structure is increased,and the lowermost the change of the influence of the track support from the bottom of the pit to the bottom of the pit,the increase of the ultimate bending moment value of the supporting structure and the deepening of the embedded depth will improve the stability of the foundation pit.The safety factor obtained by numerical analysis is not much different from the safety factor obtained by the limit equilibrium method,and it meets the requirements of the specification,thereby verifying the rationality of the numerical analysis method and providing a certain reference for the design of foundation pit.(4)This paper adopts the grey correlation analysis method to analyze the sensitivity of each factor affecting the stability of the foundation pit,and compares and calculates the degree of association about each influencing factor,and then analyzes angle of internal friction and cohesive force that is the most sensitive in the basal heave stability of the foundation pit.The depth of the embedding and the insertion ratio of the supporting structure and the severity of the soil layer have little effect on the stability safety factor of the foundation pit.However,the surface of the pit is overloaded.The effect of the stability of the foundation pit is minimal.