Study On Uplift Loading Mechanism Of Single Enhanced Prestressed Concrete Pipe Pile

Enhanced prestressed concrete pipe pile was invented on the basis of irregular-shaped prestressed concrete pipe piles by Zhejiang Tianhai pipe pile Co., Ltd in 2005. The circular rib and lengthwise rib were added into its bodywall, which save a large amount of material, and increase its rigidity, shaft resistance as well as branch resistance. Similarly, vertical compression and uplift capacity have been significantly enhanced, too, so it called"enhanced pipe pile"for short. The mechanical bond express joint and specific sealing material were used when in pile connection substitute for traditional welding, compared with conventional prestressed pipe pile, it has character of reliable connection, convenient construction, high strength and low comprehensive cost and so force. At present, it has been applied in all over the country, achieved good economic and social benefits.The enhanced prestressed uplift pipe pile is a very complex mechanics system, unlike conventional prestressed pile and squeezed branch pile. Up to now, it is still on the exploring stage in used for uplift pile, lacks profound theoretical analysis and reliable design basis, which are urgent problems to be solved in further research and make it applied widely. In this paper, based on in-situ static uplift loading test of conventional prestressed pipe pile and enhanced prestressed pipe pile, the working performance of enhanced pile was further studied by means of theoretical analysis, numerical computation. The main contents include as follows:(1)In veiw of the field static uplift loading test of four piles (including: two enhanced prestressed concrete thin-wall pipe piles and two conventional prestressed concrete thin-wall pipe piles) with same external diameter, the distortion behavior and load transmission mechanism were analyzed comparatively. The results show that the U ?δcurves of enhanced pipe piles were more gently, their ultimate bearing capacity were more 56 percent higher than the latter.(2)The load transferring models of conventional pipe pile and enhanced pipe pile were established, selection and calculation of their parameters were discussed comprehensively. According to the method of coordination harmony displacement, the simulating calculation of U ?δcurves and axial force of these two kinds of pipe piles were figured out by use of VC++ language. The calculated U ?δcurves were conformed to test results, therefore, this method is practical and feasible. In the practical projects, the existing testing information can be used to calculate U ?δcurve and ultimate bearing capacity of enhanced pipe piles, and this has realistic significant for replace or partially replace the in-situ load test. (3)Based on in-situ test results, the FEM software MIDAS/GTS was used to simulate the behavior of the conventional pipe pile and enhanced pipe pile, the result show that influence sphere of soil around enhanced pipe pile ware more obvious than the former, a cylinder surface was formed beside circular rib. Horizontal displacement of peripheral soil was a cup-shaped along the axis, decrease in the radial. The load distribution along the enhanced pipe pile was significantly different from the conventional pipe pile, which showing step-like nearby circular rib. It is apparent that circular rib play an important role to resistant pile from pulling out.(4)Analyzing the calculation models of these two kinds of piles in homogeneous sandy soil with standard length by three-dimensional finite element method, which was established by means of load transferring method, the results indicated that: their influence sphere were showing "inverted trumpet", but the later was more distinctive. The displacement of pile top and peripheral soil of enhanced pipe pile was significantly less than conventional pipe pile in each load case. Axial force of circular rib was decreased from top to bottom, the same as its additional stress, so its time-effect was significant.(5)The factos, such as: the distance of circular ribs and number of lengthwise ribs of enhanced pipe pile, were changed to numerical calculation, results revealed that loading capacity and influence sphere got smaller with the increase of distance of circular rib, failure modes of sparse circular rib were different from the close ones. The effect on bearing capacity of lengthwise ribs was inapparent, but which can effectively improve the pile rigidity, efficiently reduce circular rib's punching failure. Four is optimal number of lengthwise ribs by analysising its stress fields and displacement fields.