Research On Bearing Performance Of Assembled Steel Pipe Pile Used In Transmission Lines

In the context of building industrialization,constructions of transmission lines,which is especially difficult in mountainous areas,have caused considerable inconvenience due to the difficulties in access to construction equipment,slow progress in foundation building and poor disposal of spoil.In this paper,a research on assembled steel pipe pile is proposed to solve these problems.As transmission line pile foundations are subjected to pulling force from wires in the upper tower and influence from wind and snow,the top of foundation will withstand downward pressure load,uplift load and horizontal load from the upper structure,of which the uplift load is control condition in most basic design of various transmission line towers,the research on the ultimate uplift capacity of assembled steel pipe piles is a necessary condition for the design of assembled steel pipe piles.While the construction method and structure of this type of pile is different from existing piles,ultimate uplift bearing capacity should be analyzed based on current theory in view of the typical soil conditions in mountainous regions.The main contents of the research on the ultimate uplift bearing capacity are as follows:(1)Through the large-scale interface shear test,the shear behavior of the pile-soil interface under the condition of clay and gravel clay was studied respectively,and the cohesion and friction angle data of the pile-soil interface were obtained,which provided the test parameters basis for the subsequent finite element numerical simulation.Firstly,for the typical soil in mountainous areas,hard plastic clay and gravel clay were selected as the soil conditions.The large-scale interface shearing instrument was used to conduct the direct shear test of the hard plastic clay and gravel clay,and shear test of the interface between the soil and the grouting body.As the experiments have been carried out,both the shear behavior and the physical and mechanical properties of the pile-soil interface was analyzed by shear stress-displacement curve and the normal stress-shear stress curve respectively,providing a test basis for the numerical simulation parameters.The main conclusions are as follows:(a)In the direct shear test of soil,the shear strength-shear displacement curve of gravel clay gradually appears as work hardening type with the increase of normal stress.(b)In the interface shear test,the shear failure happens at the interface between the soil and the grouting body rather than between the grouting body and the steel plate.(c)The shear strength-shear displacement curves of the interface between the clay and the grouting body,the gravel clay and the grouting body both show the transition from a polygonal line to a hyperbolic shape as normal stress increases.(2)Through the model tests of assembled steel pipe piles under vertical pull-out load,the pull-out bearing characteristics of assembled steel pipe piles were explored.According to the test data,the P-S curve was drawn to explore the law of the uplift load changing with the displacement.The strain changed of the pile during uplifting was measured by the FBG sensor arranged on the inside of the pile.According to the strain distribution of the pile,the variation of the axial force along the depth and the law of the variation along the depth of the pile were inverted.The main conclusions are as follows:(a)It is observed that the load of model piles varies along the track of“L”with the trend to increase under clay and gravel clay conditions.(b)Under certain uplift load,the axial force distribution of piles decreases along the depth,and the rate of reduction of axial force increases gradually.(c)Generally speaking,whether in clay or gravel clay,the distribution of lateral friction of piles is large in the middle and lower regions,and small at both ends.The phenomenon of maximum lateral resistance occurs near the double diameter of piles above the pile bottom.The lateral friction of each part of pile increases along with the uplift load increasing.Meanwhile,the lateral friction increases and gradually develops from top to bottom along the pile.(3)The three-dimensional finite difference models of assembled steel pipe pile were established by FLAC 3D software,and the ultimate uplift bearing capacity of assembled steel pipe pile was explored by using the physical and mechanical parameters of pile-soil interface obtained from large-scale interface shear test.The rationality of the numerical model was verified by comparing the results of model test and numerical simulation.Then,through parametric analysis,the effects of length-diameter ratio,grouting thickness,rock-socketed length and anchor on the ultimate uplift bearing capacity of assembled steel pipe piles were explored.The main conclusions are as follows:(a)The ultimate displacement increases with the increasing of pile length.The ultimate uplift bearing capacity of piles increases exponentially with the increasing of pile length.(b)With the increasing of pile diameter,the ultimate displacement increases linearly.The ultimate uplift bearing capacity of piles increases linearly with the increasing of pile diameter.(c)With the increasing of grouting thickness,the ultimate displacement increases gradually.The ultimate uplift bearing capacity of assembled steel pipe pile is linearly related to the thickness of grouting body.It is suggested that the thickness of grouting body should be less than 300 mm,and the thickness of grouting body should be about 100 mm for pile foundation with conventional pile diameter.(d)With the increasing of the length of rock sockets,the ultimate uplift bearing capacity of pile foundation increases gradually,but the ultimate displacement is not very different from each other.The relationship between ultimate uplift bearing capacity and the length of rock sockets is linear.However,when the length of rock sockets is longer than 4D,the gap between segments is larger than 1mm.Therefore,it is suggested that the length of rock-socketed pile is about 4D.(4)Based on the results of parametric analysis of numerical simulation and existing codes,a multi-parameter method for calculating the ultimate uplift bearing capacity of assembled steel pipe piles was proposed.The analysis results were obtained by establishing finite difference models whose parameters modified by large-scale interfacial shear test.The influences of length-diameter ratio(L/D),grouting thickness ratio(t/D),rock-socketed depth ratio(h_r/L)on the bearing capacity of assembled steel pipe piles were taken into account by using multi-parameter fitting technology.The comprehensive influence coefficient ξ was introduced to modify the design code method of transmission tower foundation.Comparing the revised standard calculation method with the numerical simulation results,it is found that the uplift bearing capacity of pile foundation is more consistent with the actual situation by the revised method which has a certain degree of safety and affluence and is more suitable for design.