Bearing Behaviors Of Micro Cast-in-Place Pile With Steel Tube Under Shaft Load

Micro cast-in-situ piles with steel tube is made up by steel pipe and fine concrete or mortar which is grouted into pipe installed in Φ130~220mm-size holes that are dogged by a small rag.The pile is widely used in landslide treatment and foundation pit support,both in reinforcement for foundation or pile foundation of existed buildings owing to its advantages of mini diameter,convenient construction,high bearing capacity and little disturbance to soil.At present,more researches mainly focus on horizontal loading conditions and less studies on vertical loading.In most cases,the pile just be equated to ordinary rock-socketed pile when projected to vertical loads,and a separate and systematic study is little.Based on the collection and sorting of previous studies,this paper uses the method of numerical simulation and theoretical analysis to deeply study the bearing behavior of the pile under vertical loads.The main research contents and achievements are as follows.(1)Analyzing the characteristics,application scope and key technology of construction of micro cast-in-situ piles with steel tube,clarifying the mechanical properties of steel pipe and core concrete in CFST columns and the calculation method of bearing capacity.(2)The large-scale finite element software ABAQUS is used to carry out the static analysis of the micro cast-in-situ piles with steel tube.By changing the bedrock modulus,socketed depth and steel pipe wall thickness,the change of load-bearing capacity and load sharing ratio is discussed.It is found from analysis results that the pile has large slenderness ratio though,it belongs to the end bearing pile.Besides,bedrock modulus and steel pipe wall thickness have a great impact on the bearing capacity,but the influence of socketed depth is small.In addition,the proportion of load sharing depends not only on the steel pipe wall thickness,but also with the bedrock modulus and socketed depth.(3)Using Buckle and Risk modules in ABAQUS to explore the possible instability of micro cast-in-situ piles with steel tube in soft silt layer.Firstly,the buckling characteristics of micro cast-in-situ piles with steel tube are analyzed,and then the stable bearing capacity of the pile is calculated.Finally,the factors which influence the stability coefficient of the pile are analyzed.It is found that the influence of boundary conditions is the greatest,followed by pile diameter and soil around pile,Steel strength and concrete strength have little effect.(4)Combined with the hoop function of CFST column in structural engineering and the Winkler foundation model of ordinary cast-in-place pile in geotechnical engineering,considering the restraint effect of steel pipe and surrounding soil on core concrete.Then numerical simulation,theoretical derivation and curve fitting are applied to explore calculation methods of stable capacity of micro cast-in-situ piles with steel tube,and a formula that can consider material plasticity and initial geometric defect of structure is obtained finally.(5)By analyzing the application of micro cast-in-situ piles with steel tube in the foundation reinforcement in Jinshazhou,Guangzhou,including the reinforcement ideas and construction steps,as well as the field test of bearing capacity,an impression on its bearing behavior is deepened.