Experimental Study On Uplift Resistance Of Nodular Square Pile Group Piles In Sandy Soil

Prestressed concrete bamboo square pile(" bamboo square pile " for short)is a new type of pile developed recently.Based on the common square pile with equal section,the structure and mechanical mechanism of the pile are changed by setting bamboo joints along the pile body.Compared with the traditional equal-section square pile,the bamboo square pile has the advantages of higher uplift bearing capacity and better economy.At present,bamboo square piles have been put into use in some projects in coastal areas,but on the whole they are still in the stage of technical development and application research.There are few studies on their pull-out properties,let alone those related to pile group pull-out.However,in our practical engineering application,pile foundation is generally in the form of pile group,single pile and pile group have great differences in their working characteristics when subjected to uplift load.The joint action between pile group and soil forms a load-bearing system,and the whole process of stress is very complex.This paper studies the pull-out mechanism and bearing capacity of bamboo square pile group under uplift load in sandy soil through model experiment of bamboo square pile group.The main research contents are as follows:(1)The mechanical mechanism of square bamboo pile under uplift load and the calculation method of bearing capacity of square bamboo pile are studied.(2)Study and discuss the loading device and loading scheme of the indoor model test;A group of square piles with equal cross section and square piles with bamboo joints were designed and tested.The experimental results show that the ultimate uplift bearing capacity of single bamboo joint square pile in sand soil is about 45% higher than that of common square pile.When the load is small,the axial force transferred to the lower part of the pile body is less,and the position of the maximum lateral friction resistance of the pile body is higher.With the increase of load,more axial force is transferred to the lower part of pile body,and the position of maximum lateral friction resistance moves down.Under the same uplift load,the earth pressure around the bamboo segment square pile is higher than that of the ordinary square pile.(3)Using the method of indoor model test,the bearing characteristics of bamboo square pile group under the conditions of different pile spacing and different pile number were studied.The experimental results show that the load-displacement curve of bamboo square pile group in sandy soil is steep,and the average ultimate uplift bearing capacity of single pile is less than that of single pile.With the increasing of pile spacing,the superposition of stress between foundation piles decreases,and the ultimate tensile strength of pile group increases linearly.Under the same pile spacing,with the increase of the number of piles in the bamboo square pile group foundation,the interaction between the piles becomes more serious,and the ultimate uplift bearing capacity of the pile group decreases.In a 5-pile group foundation,the central pile is affected by four corner piles at the same time,so its pull-out performance is poor,and the values of pile axial force and lateral friction resistance are less than those of corner piles.(4)By collecting the field static load test data of the single pile with square bamboo joints and comparing it with the indoor model test results,the pull-out mechanism of the single pile with square bamboo joints was analyzed and explored.The analysis results show that the model test results of the single pile are similar to the field static load test results,which indicates the improvement effect of the anti-pulling performance of the bamboo slabs.