Research On Calculation Method Of The Pre-bored Grouted Planted Pile Based On Frictional Capacity Of Pile-cemented Soil Interface

The pre-bored grouting planted(PGP)pile is a new type of composite pile foundations which consist of a pre-stressed high-strength concrete(PHC)pile and the cemented soil layer surrounding the pile.It is widely used in the soft soil area of southeast coast of China because there is no squeezing effect and slurry pollution during construction.In the PGP pile,the PHC pipe piles,square pile and nodular pile are mainly adopted as the core pile,the bearing capacity of PGP pile is controlled by the frictional capacity of pile-cemented soil-soil interface.The behavior of PGP pile and the frictional capacity of cemented soil-soil interface has been investigated through field tests by many researchers.Nevertheless,the research on the frictional capacity of pile-cemented soil interface is very limited which is important to analyzing the load transfer mechanism and the bearing capacity of PGP pile.Comparing solid pipe piles and square piles with the same side surface area,the concrete material required for the square piles are relatively reduced by 22%,hence,the square piles are more economical.However,there are few studies on square piles,the researches on the frictional capacity of square pile-cemented soil interface have to be done to guide engineering practice.In this paper,a series of three-dimensional pile-soil interface shear tests were conducted using the group’s self-developed specific shear test apparatus to investigate the frictional capacity and the failure mode of square pile-cemented soil interface,and a calculation method was proposed to analyze the load-displacement response of the PGP pile.The main research work and the conclusions are listed below:1.The effect of varying the side length of the square pile and the curing time of the model pile on the frictional capacity of the square pile-cemented soil interface was investigated.The test results show that the maximum pile top load of the square pile-cemented soil interface shows a trend of increasing at first and then decreasing with the increase of the side length of square pile;the maximum skin friction of square pile-cemented soil interface decreases with the increase of side length of square pile;the maximum skin friction of square pile-cemented soil interface increases with curing time;the effect of cemented soil strength on the normalized maximum skin friction(maximum skin friction/cemented soil strength)is not significant.The increase of the side length of square pile makes the failure mode of square pile-cemented soil interface change from progressive failure to brittle failure,this is because of the stress concentration occurring at the corners of the square pile and the decrease of thickness of the cemented soil around the square pile which hamper the frictional capacity of square pile-cemented soil interface.The residual skin friction of square pile-cemented soil interface decreases with the increase of side length of square pile when the progressive failure occurs;The residual skin friction of square pile-cemented soil interface is close to each other when the brittle failure occurs.2.The effect of varying the cement content of cemented soil on the frictional capacity of the square pile-cemented soil interface was investigated.The test results show that the maximum skin friction of square pile-cemented soil interface increases with the increase of the cement content which is because the cemented soil strength increases with the increase of cement content.The failure mode of square pile-cemented soil interface is also changed from progressive failure to brittle failure.The effect of core pile shape on the frictional capacity of the core pile-cemented soil interface was investigated by comparing the interface shear tests when the core piles were pipe piles and square piles.The test results show that the scale effect of the pipe pile-cemented soil interface shear tests is consistent with the square pile-cemented soil interface.Under the same equivalent diameter of core pile,the maximum skin friction of pipe pile-cemented soil interface is smaller than the square pile-cemented soil interface due to the stress concentration occurring at the corners of the square pile.The effect of the roughness of the square pile surface on the frictional capacity of the square pile-cemented soil interface was investigated by varying the coating density and grain size of the sand coated on the square pile surface.The test results show that the maximum skin friction increases with the mean grain size of coating sand on the square pile surface,and increases slightly with the density of coating sand on the square pile surface.3.By analyzing the skin friction and the relative displacement of the square pile-cemented soil interface,the results show that the shear process of square pile-cemented soil interface experiences three stages: elastic stage,brittle failure(progressive failure)stage,stable stage.The three-fold line model can be used to describe the shear relationship of the interface during brittle failure.This section presents a calculation method for the analysis of the behavior of a PGP pile considering the influence of the variation of the pile elastic modulus along the depth and the variation of the frictional capacity of layered soils.In this approach,a three-fold line model,based on the shear test results,is used to simulate the shearing behavior of the core pile-cemented soil interface;a hyperbolic nonlinear model,based on the field test results,is used to simulate the behavior between the skin friction and the relative displacement developed along the cemented soil-soil interface;a nonlinear model,based on the field test results,is used to simulate the PGP pile base load-displacement responses.The reliability of the proposed calculation method is verified by comparing with the field test results.This method can provide the basis for the settlement calculation of the PGP pile.