Study On The Bearing Capacity Of Pull-out Pile Of Transmission Tower Foundation In Loess Region

With the rapid development of national economy,the scope of transmission line construction is more and more extensive.As a key part of the transmission line project,the pile foundation of the transmission tower occupies most of the proportion of the whole project both in construction cost and construction period.Ordinary reinforced concrete cast-in-place pile with straight hole is the most widely used foundation of transmission line pile in China.However,according to China's existing technical specifications,the size of the foundation design is more conservative than that of foreign countries,and the cost of foundation construction is much higher than that of foreign countries.If the foundation is too large,large-scale excavation will inevitably cause damage to vegetation and soil erosion,which is not conducive to the implementation of national energy conservation and emission reduction measures.Therefore,it is of theoretical and practical significance to strengthen the research on the basis of transmission lines.In this paper,the tensile ultimate bearing capacity test of different forms of pulled pile foundations(equal-section piles and bottom-expanding piles)in loess region was carried out,and the following work was mainly done and the research results were obtained:(1)In this paper,the failure pattern of pile foundation is summarized,and the law of lateral friction resistance is analyzed.The calculation method of the ultimate bearing capacity and the test design of the ultimate bearing capacity are expounded in detail.(2)Through the analysis of the ground crack distribution mode and the profile crack distribution mode when the mechanical perforated foundation and the manual cut foundation are damaged,the cross section foundation of the mechanical cut presents the typicalcomposite shear surface failure mode,and the surface crack is radially distributed;Due to the existence of horizontal tension,the manually excavated foundation with enlarged bottom presents different section failure forms from that of the mechanically formed foundation,and the ground cracks are mainly distributed radially along the loading direction.(3)Through the test data analysis,found that two types of piles under drawing load does not appear to strengthen its load-displacement relationships,at the same time reflects the phenomenon of cooperative work of piles and soil under drawing load,when reach the limit load based damage of surrounding soils,two types of artificial excavation foundation has good resistance to deformation ability,from loading to destroy the work of piles and soil ability is better.(4)The strain analysis shows that the main reinforcement under the pull load is not uniformly stressed,and the steel reinforcement of mechanical bored piles is generally in the elastic working stage with small plastic deformation.As for the manually excavated foundation,the strain analysis shows that the manually excavated pile foundation with enlarged head makes the strain change at different positions different from that of the mechanically perforated foundation.The yield phenomenon exists in some main reinforcement data,but the strain data at most measuring points along the depth direction shows that the reinforcement is mainly in elastic working state.(5)The results show that the bearing capacity calculated according to the failure theory of cylindrical shear plane and the failure theory of composite shear plane is too conservative and has a large gap with the measured value.The ultimate bearing capacity determined by the standard calculation method is also greatly deviated from the test.Based on the measured values of the test,the pull out coefficient of the loess geomorphology of the test site is calculated reversely,and the values of the pull out coefficient of the foundation of two types of different piles are recommended,which can provide an important reference for the design of the pile foundation in this loess region.