Theoretical And Experimental Study On Consolidation Of Soil Around Permeable Pipe Pile

Jacked pile foundation has the advantages of stable quality,low cost,low construction noise,small vibration and high bearing capacity.It has been widely used in foundation construction in soft soil areas.However,compared with bored pile,pile driving produces the squeezing effect,generating the excess pore water pressures,and as such the adjacent piles and structures can be adversely affected(vertical uplift,horizontal displacement or soil failure).In view of the shortcomings of jacked pile,the permeable pipe pile proposed by the author’s team can effectively accelerate the dissipation of excess pore water pressure,thus reducing the adverse impact of pile sinking.In addition,the permeable pipe pile can make the ultimate bearing capacity of the pile foundation play in advance,thus ensuring the construction progress and reducing the project cost.Different from the traditional pile foundation,the permeable pipe pile has a complex pilesoil interface,which makes it difficult to solve the consolidation problem of the soil around the pile through the traditional separation variables method and integral transformation method,and it is difficult to implement the optimal design of the drainage hole of the permeable pipe pile.In order to better explain the mechanism of permeable pipe pile,on the basis of previous research,firstly,the mathematical model of the pile-soil interface of permeable pipe pile is established by using semi-permeable boundary conditions,and the modified parameters are solved by back-analysis method.Then,the modified parameters are applied to the newly proposed consolidation theory of local permeable pipe pile,and the effect of local permeable pipe pile on the dissipation of excess pore water pressure and the optimization of drainage hole are studied.Finally,in order to verify the correctness of the theory,the influence of permeable pipe pile on the dissipation of excess pore water pressure and the timeliness of bearing capacity is studied through model test and field test,and the model test results are compared with the theoretical calculation.The main research work and innovation achievements are as follows:(1)In view of the problem of mixed drainage boundary formed by the arrangement of drainage holes in the pile body,the influence of drainage holes siltation on the permeability is compared,and the method of equivalent treatment of the boundary by using the modified parameters of the pile-soil interface is proposed,which makes the theory of consolidation of permeable pipe piles more perfect and closed to the real working condition.The pile-soil interface is changed from impermeable boundary to the mixed boundary of permeable boundary(drainage hole-soil boundary)and impermeable boundary(pile-soil boundary)by the arrangement of drainage holes in the pile body,and the drainage holes are distributed in three dimensions on the pile body,which makes it difficult to apply the conventional consolidation solution method.Therefore,in this paper,the permeable pile-soil interface is generalized as an impeded drainage boundary.And based on the results of numerical calculation,the modified parameters of the pile-soil interface are determined by using the back analysis method,and the fitting formula of modified parameters is obtained,which provides convenience for consolidation of permeable pipe pile foundation.(2)Aiming at the problem that the mechanical properties of pile body are reduced due to the drainage holes arrangement of pile body,according to the distribution law of initial excess pore water pressure caused by pile sinking(increasing along the depth)and stress distribution law of pile under compression(decreasing with the depth),the method of local arranging drainage holes in pile body is put forward,which can not only reduce the influence of arranging drainage holes on the mechanical properties of pile body,but also meet the function of accelerating the dissipation of excess pore water pressure and to the ultimate bearing capacity in advance.The modified pile-soil interface parameters are applied to the pile-soil boundary conditions of local permeable pipe piles.The boundary transformation method is used to solve the problem that the traditional separation variable method or integral transformation method cannot directly solve the mixed drainage boundary problem of local permeable pipe piles,and the semi-analytical solution of single pile foundation consolidation is obtained.The solution degradation and finite difference method are used to verify the correctness of the solution,and the influences of permeable section,foundation soil and pile geometry size on the consolidation of permeable pipe pile foundation are discussed.(3)In view of the phenomenon that the ultimate bearing capacity of jacked pile will increase with the dissipation of excess pore water pressure in the soil around the pile,the model test and field test of permeable pipe pile prove that the permeable pipe pile can accelerate the dissipation of excess pore water pressure and play the role of the ultimate bearing capacity of the pile in advance.In order to study the effect of permeable pipe pile on the dissipation of excess pore water pressure and the bearing capacity of the pile,the model test of permeable pipe pile was designed to measure the consolidation of the soil around the pile and the bearing capacity of pile.It was found that permeable pipe pile can accelerate the dissipation of excess pore water pressure.And with the acceleration of excess pore water pressure,the ultimate bearing capacity of the pile also plays a role in advance.The results of model test and theoretical calculation are compared to verify the validity and accuracy of the theoretical solution of permeable pipe pile.In order to overcome the problem that the centrifuge test may lead to error amplification under acceleration and that the model test cannot reflect the real ground stress field,the pore water pressure and soil pressure of the soil around the pile,the axial force of the pile shaft and the ultimate bearing capacity of the pile in different resting periods are measured through the field test,which verifies that the permeable pipe pile can accelerate the dissipation of excess pore water pressure and play the role of the ultimate bearing capacity of the pipe pile in advance.