Model Tests Study On Bearing Behavior Of Precast Micro-pipe Piles In Soft Soil

The precast micro-pipe piles foundation is suitable for all kinds of conditions of soft soil with a wide range of application prospects.It can meet the requirements of transmission line construction of SGCC and bring great economic and environmental benefits to adopt micro-pipe pile foundation for the transmission line.At present,there are still few researches or applications for micro-pipe piles in practical engineering.Therefore,it is necessary to study the bearing behavior of precast micro-pipe piles so that some relevant data can be provided when it is applicated in foundation engineering of transmission line.Based on equation of motion of vertical load and flexural differential equation of horizontal load,the similarity principle of the model of precast micro-pile piles is given.Based on the loading features of application of the transmission tower foundation,the slow loading method is adopted to conduct a compression,uplift and lateral loaded test of precast micro-pile piles,and the bearing behavior of precast micro-pipe single pile and pile group in compression,uplift and lateral load are analyzed.The main results can be made as follows:(1)In pipe piles producing industry,the minimum diameter is 300 mm.According to relevant parameter of PHC prototype pipe pile with a diameter of 300 mm,the scaling factor(λ_L)of model /test is set as 1/10,the size of model pile is designed as length of 1.1 m,diameter of 30 mm,and wall thickness of 70 mm.For making model piles,epoxy resin is poured between 2 PVC.In this way,the requirement for similarity of elasticity modulus can be meet.(2)The compressive,uplift and lateral loaded precast micro-pipe piles test system is made from steel model case with size of 1.5 m×1.5 m×2 m,compression,uplift and lateral test loading device.In compression test,lever analogy is adopted to exert load.In uplift and lateral loaded test,the pulley is adopted to transfer load.A single pile,2 × 2 group pile and 3 × 3 pile group were designed for the compression,drafting and horizontal loading test.The pile spacing of the 2 × 2 pile group is 2.5 d,and pile spacing of the 3 × 3 pile group is 3d.Pile group uses elevated pile cap.(3)Through the compression tests of single pile and pile group,it is shown that the compressive load-displacement curves of single pile and pile group drop steeply,the ultimate compressive bearing capacity of single pile,2×2 pile group and 3×3 pile group are 190 N,835N,and 1660 N.The result of compressive ultimate bearing capacity estimated by the specification method is verified by the result of test.The Pile group effect coefficients of 2×2 pile group and 3×3 pile group are 1.09 and 0.97.This indicates that the compressive bearing capacity of pile foundation in pile group is almost the same as that of single pile.However,as the piles increases,the efficiency of ultimate compressive bearing capacity will decrease.When the single pile and pile group are exerted with ultimate load,due to the shear failure of the soil at the depth of 30 cm above,the side fiction decreases while the fiction of the low half part still keeps large.When the side friction is fully played,the relative displacement of the pile soil is about 1 to 1.6 mm.(4)Through the uplift tests of single pile and pile group,it is shown that the uplift load-displacement curves of single pile and pile group drop steeply,the ultimate uplift bearing capacity of single pile,2×2 pile group and 3×3 pile group are 170 N,480N and 1000 N.The validity of compressive ultimate bearing capacity estimated by the specification is verified by the result of test.The pile group effect coefficients of ultimate uplift bearing capacity of 2×2 pile group and 3×3 pile group are 0.706 and 0.654.This indicates when the pile group is uplift,the ultimate capacity of foundation pile in group pile is less played than single pile.As the piles increases,the efficiency of uplift capacity decrease.When the pile group is totally uplift from the soil,it is shown that the soil between the piles isn’t brought out on the pile group.This indicates the foundation soil is partly destructed.the relative displacement of the pile soil is about 2 to 3 mm.(5)Through the lateral loading tests of single pile and pile group,it is shown that when the single pile are laterally loaded,the load-displacement varies slowly.The first half part of the curve varies slowly.However,when the lateral load is more than ultimate bearing capacity,displacement excursion starts to appear in the curve.The ultimate lateral bearing capacity of single pile,2×2 pile group and 3×3 pile group are 110 N,390N and 600 N.The pile group effect coefficients of ultimate lateral bearing capacity of 2×2 pile group and 3×3 pile group are 0.886 and 0.606.This indicates that the bearing capacity of pile group is less played than single pile,as the piles increase,the efficiency of lateral bearing capacity decreases.When the single pile and pile group is laterally loaded,the moment of single pile and foundation pile increase at first,then decrease from upside to downside.The depth at which the moment is maximal is about 30 cm.The moments at depth of 70 cm below are almost 0.When the lateral load of single pile is equal to the average lateral load of foundation pile,the moment of pile can be described as follow: the moment of back row in pile group<the moment of front row in pile group<the moment of single moment.When the precast mocro-pipe single pile is laterally loaded,The varational law of moment of pile body in elastic stage analyzed by m method is the same as the result of test.When lateral load is small than ultimate bearing capacity,The result calculated by p~y curve method is the same as the result of test.