Experimental Research And Theoretical Analysis On Mechanical Characteristics Of Piles With Expanded Branches Or Plates

The pile with expanded branches or plates is a kind of new-type pile foundation developed at the beginning of the 1990s. It has already used widely successively in many provinces and districts of our country now, and made the good economic and social benefits, because of its higher bearing capacity and lower settlement. At present, this new technology has the remarkable results in the practical project, but it is still at technological development, popularization and application stage on the whole, lacks systematic researches in theory, unclear about the characteristics of compressive resistance and pulling resistance and the destruction mechanism of expanded branches or plates, and still lacks reliable and practical bearing capacity calculation formulae of compressive resistance and pulling resistance. So, in order to find out the working mechanism, failure mode and capacity characteristic of the pile with expanded branches or plates further, and provide the rational design considerations for the project, this paper, on the basis of the in-situ static loading test, stress measuring pile body and the loading experiment of indoor model of expanded plates, analyzes and studies the load transferring rules and the failure forms of the pile with expanded branches or plates under the loads of compressing and pulling. Then, this paper offers the amendment and improvement to the existing ultimate bearing capacity calculation of expanded branches or plates, and gets some conclusions.1.Improving the theory on the compressive resistance bearing capacity of piles with expanded branches or plates.Through the test data of static load tests and stress measuring pile body ofsingle pile, calculate the resistance of every expanded branch or plate, the frictional resistance on the sides of the piles and the resistance on the tips of the piles. And get that at the later loading stage, the frictional resistance of pile has researched its limit and the increasing load is mainly bared by the expanded branches or plates who undertake more than half total load; and the resistance of expanded branches or plates has the nature of the end bearing capacity, so it can be ascribed to the frictional end bearing pile with multiple supporting points. It still points out that the Q-S curve of compressive resistance presents as a little S type slowly changing curve. It is the first time to introduce the "the exertion coefficient of frictional resistance of pile sides_ ξ" that is suitable to the piles with expanded branches or plates, making the amendment and improvement for the existing ultimate bearing capacity calculation of compressive resistance of expanded branches or plates, and it more suitable to the project design.2.Setting up the theory of load transferring of piles with expanded branches and platesThe results of tests show that the load transferring function of resistance of expanded branches and plates, the resistance of tips of the piles and the frictional resistance of sides of piles can be simulated by using the hyperbola model. The simulative formulae are given out and the result of simulating is good. According to that, the load transferring is firstly applied in the simulating calculation of Q-S curve and axial force of pile body. In the practical projects, the existing testing materials can be used to calculate Q-S curve and ultimate bearing capacity of piles with higher bearing capacity, and this has realistic significant for reducing or substituting partly the in-situ pile load test.3.Setting up the theory of pulling resistance bearing capacity of piles with expanded branches or platesThis paper, for the first time, adopts the method of burying steel stress gagues to measure the stress of pile body when the piles with expandedbranches or plates bear pulling loads, and get that at the earlier stage of pulling force, the actions of the expanded plates of piles are not remarkable, but with the load increase on the top of piles, it becomes greater, when closing to limit load, the expanded plates will bear more than 40% total load. The pulling resistance bearing capacity of expanded plates brings into play sequentially from upper part to lower part. For the first time, it provides the experience-reduced coefficient of pulling resistance of side resistance (a) and the practical formula of pulling resistance ultimate bearing capacity of piles with expanded branches or plates. Because it is simple, practical, objective and reliable, the formula can popularize and apply to project design. Results of tests show that the relative lower design position is the main reason that the action of expanded plates exerts later. It is suggested to discuss the relationship between the position and bearing's exertion of expanded branches or plates further in the future.4. Conducting the strength tests and analysis of expanded plates.It is the first time to use the YAW-5000 computer controlling electric liquid servo pressure testing machine to conduct the loading tests. Testing the total load -displacement curve of expanded plates, show that the failure form of expanded plates belongs the oblique compression failure under the testing conditions. According to the experimental results, for the first time this paper sets up bearing capacity calculation formulae of expanded plates considering the factors such as the concrete strength, the ratio of the diameter of expanded plates to that of pile (D/d) , and the ratio of height to width (h/b1) etc, and suppling a gap on the strength design and calculation of expanded plates. And according to the results using finite element method, it is pointed out that the rational region of the ratio of height to width is 1.5≤h/b1≤2.8.5.Analyzing the piles with expanded branches or plates and the foundation by finite element methodDiscussing the bearing and deformation characteristics of single and group piles with expanded branches or plates by using two-dimensional axialsymmetric finite element method and three-dimensional finite element method respectively. Putting forward the corresponding suggestion on design of the critical vertical interval of expanded branches or plates, horizontal distance of piles and the methods of laying piles.