Numerical Simulation Of The Construction Process Of Long Spiral CFG Piles And Study On The Variation Law Of Pile-soil Parameters

Based on the background for the problem of saturated sand liquefaction caused by the construction of long spiral CFG piles and in view of the problem that dynamic construction process for the long spiral CFG piles is difficult to simulate,the construction process is divided into two stages(drilling process and concrete pouring process)and a numerical model for the construction process of long spiral CFG piles is established based on the coupled Eulerian-Lagrangian method.Through the numerical simulation and model test of the dynamic construction process of the long spiral CFG pile,the influence of the construction and process parameters of the long spiral CFG pile on the saturated sand foundation is studied,and dynamic responses,changes of pore water pressure,void ratio of saturated sand foundation,horizontal displacement and the data of model pile consists of pile diameter and pile length are obtained.The rationality and accuracy of the simulation results are proved by comparing with the field test data and calculation results of the theory of cylindrical cavity expansion.The main conclusions of the study are as follows:(1)Based on the coupled Eulerian-Lagrangian method,numerical models were successfully established for the long spiral CFG pile single-pile drilling process,pouring concrete process,and the construction process of group-piles.(2)The presented numerical results prove that the vibration load generated during the construction acts on saturated soil in the form of irregular reciprocating shear forces,which leads to a large excess pore water pressure in the soil and an increase in soil void ratio.Both the excess pore water pressure field generated during the construction and the soil pore ratio after the construction show a parabolic distribution in the vertical direction.And the horizontal displacement response of the soil mass caused by construction is small.(3)During the construction process,the excess pore water pressure at a point inside the soil presents a phased change,which corresponds to the drilling process and concrete pumping process during the construction process.The accumulation of excess pore water pressure will occur during the pile group construction process and the direction of pile group construction is the direction of pore water pressure accumulation.(4)With the increase of drilling speed,the fluctuation of pore water pressure becomes more unstable,the peak value of pore water pressure increases,the pile diameter of the model pile increases,and the pile length slightly increases.The change of the drill pipe’s own parameters will affect the peak value of pore water pressure,but it does not affect the change trend of pore water pressure.The increase of pile spacing makes the response of excess pore water pressure smaller,and the model pile length and pile diameter approach the design value.(5)The variation trend and peak size of the excess pore water pressure obtained by the simulation are consistent with the experimental results.The peak size of the excess pore water pressure are consistent with the calculation results of the cylindrical hole expansion theory within the range of 3-9d from the pile core.The pore water pressure is obviously lower than the theoretical calculation value.(6)Some measures are proposed to avoid liquefaction of saturated sand,such as changing soil conditions(reducing groundwater level,reducing saturation),optimizing drill pipe parameters(adopt appropriate drill pipe rotation speed,reduce squeezing effect of construction to soil),controlling construction technology(jump with single pile,jump with multiple piles,jump with piles and rows).