Study On Vertical Bearing Capacity Of Screw Piles With Variable Diameters In Composite Foundation Engineering

The variable diameter screw pile composite foundation is a new type of foundation improvement.The piles in this composite foundation are composed of two parts:the large diameter upper and the small diameter lower parts.As the pile body with screw thread,the vertical bearing capacity and the economy of the piles have great advantages compared with the smooth straight piles.There is little research on the vertical bearing capacity of the variable diameter screw pile composite foundation.In this paper,the calculation method of the vertical bearing capacity of the vertical bearing capacity of the variable diameter screw single pile and composite foundation is based on the foundation treatment engineering in Zhangjiakou-Huhehaote passenger rail line project.Numerical simulation have been carried out to verify the theoretical calculation methods and to study on the main influencing factors of variable diameter screw single pile and composite foundation.The principal research results are as follow:(1)Through the method of numerical simulation,the vertical bearing capacity under the condition of different foundation of the variable diameter screw pile is 2.2?2.6 times of smooth straight pile,and the amount of material is only 43.4%of smooth straight pile.The vertical bearing capacity of the variable diameter screw pile is 90.8%-94.4%of the smooth straight pile,but the amount of material is only 43.4%of the conventional screw pile.The axial force of pile body decreases gradually from the top of the pile to the end of the pile besides the section of variable diameter,and the pile top load is mainly assumed by the side friction resistance of the pile.So the variable diameter screw pile belongs to the friction pile.(2)Based on the limit equilibrium theory and the Mohr Coulomb strength theory,the calculation methods of the best screw-pitch,the vertical bearing capacity of variable diameter screw single pile and composite foundation are deduced.Through the theoretical calculation and numerical simulation calculation to comparative analysis,in the actual engineering,the best screw-pitch is about 200mm,and the results of the vertical bearing capacity about the theoretical calculation method are considerably close to numerical simulation method.(3)Based on the Midas GTS numerical simulation methods,the main influencing factors of variable diameter screw single pile and composite foundation are analyzed.The results show that:the upper pile length ratio of the total length of the pile of the variable diameter screw pile have great impact on the vertical bearing capacity,the greater the proportion led to higher vertical bearing capacity.The best diameter of the upper and lower part piles is 500mm/300mm.The rational width of screw thread and the thickness of screw thread are 100mm.The space of piles should be controlled between 3-4 times of the pile diameter.The length of pile should not be more than 16 meters.The cushion with larger elastic modulus should be used to improve pile-soil stress ratio and the vertical bearing capacity.The thickness of cushion should be controlled between 200mm?600mm.The bearing capacity of composite foundation is directly proportional to the strength of soil between piles,and we can improve the strength of the soil to improve the bearing capacity of composite foundation.In this paper,the research results of the vertical bearing capacity of variable diameter screw pile composite foundation can provided guidance and reference for the related engineering design calculation.