Experimental Study On The Uplift And Horizontal Bearing Characteristics Of Roots Foundation

Based on the good uplift resistance effect,root foundation has been applied to the foundation engineering of transmission tower.Making full use of the uplift and horizontal bearing capacity of root foundation can effectively save the cost of foundation engineering.However,there are few studies on the influence of vertical arrangement spacing of root key,pile length,pile diameter and soil between root key layers on uplift and horizontal bearing capacity of root foundation.It is urgent to carry out more in-depth,more intuitive and efficient experimental research.This paper designed and carried out the indoor model test of the root foundation based on the combination of strain test technology and particle image velocimetry technology.The response characteristics and failure modes of pile top displacement,pile axial force and lateral friction resistance during the whole process of loading to failure were analyzed.The full-scale field tests were carried out for the geology of the western region.As for the geology of the western region to carry out field footage tests,we further analyzed the influence of the root key arrangement of root type foundation single,double layers on its load transfer law and the force characteristics of the root key by means of steel gauge and strain gauge testing techniques.The numerical simulation full-scale test pile model was established,the numerical model was checked many times through the field test data to verify the reliability of the model,and the indoor model test results were verified and expanded.In this paper,the vertical uplift and horizontal bearing characteristics of root foundation and the mechanism of pilesoil interaction were explored by combining indoor model test,field full-scale test and numerical simulation.The main research results are followed:(1)The influence and mechanism of the factors such as the buried depth and spacing of the root key on the uplift bearing capacity were studied.The asynchrony between the normal resistance of the root key,the side friction resistance of the pile and the effect of the bearing depth of the root key were discussed.It is suggested that the vertical spacing of the root key should be 3.5L ～ 5L(L is the length of the root key).When the vertical displacement control requirement of the pile top is strict,3.5L can be taken.(2)According to the test,the uplift bearing capacity of the root foundation was related to the aspect ratio,the displacement of the pile top and the uplift bearing capacity of the lower root key decreases with the increase of the aspect ratio.Increasing the pile diameter and pile length could expand the range of pile-soil interaction in the outer normal direction and exerted the resistance of deep soil to improve the uplift bearing capacity,and the effect was more significant when the pile length was increased.The horizontal bearing capacity of the root foundation was related to the aspect ratio.The pile section below the effective ’ embedded ’ section of the pile body had no contribution to the horizontal bearing capacity.Increasing the pile diameter could effectively improve the horizontal bearing capacity of the root foundation.(3)The test showed that the plastic failure type of the soil around the pile was related to the vertical spacing of the root key,which was mainly divided into four types :"Types of the through destruction","Types of interaction destruction","Types of individual destruction"and ".When the buried depth of the first root key was shallow,the overlying soil was prone to disturbance and plastic failure,which led to the weakening of the uplift bearing capacity of the lower root key.When the pile top displacement control requirements were more stringent,a reasonable increase in the number of root key layers could greatly improve the uplift bearing capacity.(4)The finite element numerical model was established and verified with the indoor model test results.The relationship between the bearing characteristics of the root foundation,the root bond length,the root bond density and the physical and mechanical properties of the soil around the pile was analyzed.The load-displacement distribution of the root foundation under different conditions was counted.To ensure the effective play of the uplift bearing capacity of the root foundation,according to the displacement control requirements of the pile top of the transmission tower foundation,it was proposed that the length-diameter ratio should not be greater than 15.