Study On Mechanical Mechanism And Optimal Analysis Of Gravity Anchored Foundation For Large Section Pipe Jacking

Compared with the traditional open excavation technology,the pipe jacking technology has the advantages of small excavation area,better control of surface deformation,fast construction progress,less interference to traffic,no precipitation and so on.However,in coastal soft soil area,the low bearing capacity and high compressibility of soil make the jacking force of large-section pipe far exceed the allowable jacking force of traditional back wall.In view of the above engineering geological conditions,gravity anchorage foundation is designed based on the principle of "self-gravity + anchorage force",that is,the combined structure of foundation floor,composite foundation(full reinforcement of high-pressure jet grouting piles),bolt and steel bull leg work together,which can solve the problem of insufficient stiffness of traditional reaction device during the construction of rectangular pipe jacking with large section in coastal soft soil area.In this paper,a numerical model is established to simulate the construction process of gravity anchorage foundation with the help of finite element analysis software,combined with the field measured data,to study the change law of stress and strain of gravity anchorage foundation,analyze its mechanical mechanism,and carry out optimization analysis on this basis.The theoretical results can provide guidance for the design and construction of similar pipe jacking projects in the future.The main research contents and conclusions are as follows:(1)Basic theoretical research on gravity anchoring of large cross-section rectangular pipe jacking.The overturning moment and eccentric moment caused by the uneven distribution of the jacking reaction force will cause the sliding and overturning of the foundation slab;The main influencing factors of anti slip include the friction force and passive earth pressure between the foundation slab and the composite foundation;The main influencing factors of anti overturning include the gravity of the foundation slab,the pressure of the box culvert,and the anchoring force of the anchor rod;The self gravity of the structure and the anchoring force of the anchor ensure the stable operation of the entire structure;The displacement mechanism mainly analyzes the horizontal and vertical displacement of the structure,and the shear strength of composite foundation is particularly important.(2)Study on the mechanical mechanism of gravity anchoring foundation for large section pipe jacking.The construction numerical simulation model of gravity anchor foundation is established using finite element software to simulate the engineering situation of gravity anchor foundation during pipe jacking construction;The deformation of gravity anchor foundation is positively correlated with the jacking reaction force,and the greater the jacking reaction force,the greater the deformation amplitude;The self gravity and anchoring force of gravity anchored foundations are particularly important,which limit the deformation and displacement of the foundation floor and ensure sufficient friction to balance the jacking force;The passive earth pressure provided by the composite foundation can balance the remaining jacking reaction and play a complementary role;Gravity anchored foundations will lose friction due to upward warping and deformation of the foundation floor.(3)Optimization analysis of large section pipe-jacking gravity anchorage foundation.On the basis of the above research on the mechanical mechanism of gravity anchorage foundation,the advantages of prestressed bolt and uplift pile in working mechanism and anchorage force are used for optimization analysis.The comparison results before and after optimization show that the application of prestress of anchor rod is helpful to further control the rotation and warping deformation of foundation plate.The uplift pile can provide sufficient bearing capacity and uplift force for foundation floor,and balance the remaining horizontal load through the pile stiffness.