Study On Vertical Bearing Capacity Of Tapered Concrete Screw Pile In Composite Foundation Engineering

The tapered concrete screw pile composite foundation is a new kind of ground improvement. Piles in this composite foundation are screw piles which have frustum cone-like bodies. Nearly no studies have done on the bearing capacity of tapered concrete screw piles and the pile composite foundation composed of the new kind of piles before. This paper will discuss the calculation methods of the vertical bearing capacity of tapered concrete screw piles. Laboratory model tests and 3D numerical simulation have been carried out to verify the methods and to study on the influencing factors of variable cross-section screw pile and the pile composite foundation. The principal research results are as follow:1) On the basis of Terzaghi’s theory of ultimate bearing capacity of deep foundation, the derivation process of computing method of optimal screw-pitch is showed. Based on B.Hanson and Vesic’s theories of ultimate bearing capacity of soil foundation, this paper establishes calculation methods of the ultimate bearing capacity of tapered concrete screw piles when the screw-pitch is greater than the optimal screw-pitch. Considering the characteristics of local shear failure, it is sure that the results of the calculation method based on the theory of Terzaghi are closer to test ones. When the screw-pitch is smaller than the optimal screw-pitch, considering the differences of the shear strength of soil and pile-soil, this paper establishes the calculation method of the ultimate bearing capacity of variable cross-section screw pile. The theoretical calculation methods of the ultimate bearing capacity of variable cross-section screw pile proposed in this paper are in good accord with the test results and the numerical simulation results.2) Indoor scale model tests are carried out to study on the vertical bearing capacity of tapered concrete screw piles and to compare that with the conventional screw piles and the conventional straight piles. The results show that under the same conditions and the same loads, the stock utilization of the variable cross-section screw piles is about 1.26 times that of the conventional screw piles and about 2.97 times that of the conventional straight piles. It’s found from the analysis of the axial force diagram that the pile friction of tapered concrete screw piles is larger than that of the conventional screw piles and the conventional straight piles. The ratio of tip resistance of variable cross-section screw piles to the bearing capacity is 25.8%～35.1%, and the ratio is 47.7%～82.6% for the conventional straight piles. The results also show that the screw-pitch has a remarkable effect on the vertical bearing capacity of variable cross-section screw piles. The bearing capacity of screw piles which screw-pitch is 27mm is greater than the ones which screw-pitch is 15mm and 40mm.3) According to a subgrade improvement engineering in Xi’an-Chengdu railway project, 3D numerical simulation is carried out to analyze the influence factors and regularities to the bearing capacity of the pile composite foundation. The results show that the pile diameter should be controlled between 400-600mm. The rational space of piles should be four times of the pile diameter. The effective length of piles is about 15m. The pitch and the width of screw thread have a significant impact on the bearing capacity of piles, and the optimal pitch is about 220mm. And the rational width of screw thread should be controlled between 100-200mm. The thickness of screw thread has a relatively smaller impact on the bearing capacity of piles, and should make it equal to the width of screw thread to keep the quality of piles. Strength of soil among piles is positively correlated with bearing capacity of composite foundation, and negative correlated with pile-soil stress ratio. The cushion with larger rigid should be used to improve pile-soil stress ratio and the thicker one can have effective regulations on pile-soil stress ratio. But the thickness of cushion should not exceed 400mm.The research of this article can provide guidance and reference for the design calculations of the vertical bearing capacity of tapered concrete screw pile composite foundation, and can contribute to a comprehensive understanding of the characteristics of the new kind of composite foundation.