A Study On Load-Transferring Mechanism And Engineering Application Of Cast-in-Place Pile With Branches And Plates

The cast-in-place pile with branches and plates (abbreviated to 'CPBP'hereinafter.)which improved on the cross-section of columnar pile, has high bearing capacity and low settlement. High economic and social benefits are obtained from application of CPBP in China over the past ten years or so. CPBP has significant influence on eliminating many technical defects and improving the bearing capacity character of cast-in-place pile. However, theory develope more slowly compared to application of CPBP , there are still different opinions on load transmission character, bearing capacity mechanism, stress-strain character, failure character, determination of optimum space of branches/plates, and calculation of ultimate bearing capacity of single pile. In order to analyse connotation of high bearing capacity and low settlement, and explore design theory of CPBP, load transmission mechanism, stress-strain character, settlement character and failure character are systemically studied based on static loading test of foundation pile and other in-situ test data. Thus the design scheme is optimized,and the formula for calculating ultimate bearing capacity of single pile is derived from the failure character of stratum near branches/plates.a) Analysis of pile driving mechanism of CPBP. Through analysing the structure and the process of driving plie, it is indicated that branches/plates not only make the bearing capacity area increased, the bearing capacity of single pile raised, but also make the end resistance reduced. It is the thrusting-expanding effect that makes the shear strength of stratum increased,thus the bearing capacity of pile effectively raise.b) Discussion on load transmission mechanism of CPBP. Significant time and space effect are indicated in the process of load transmission based on engineering in-situ test. Along with load transfering from head to end, skin friction and branches/plates resistance raise till the maximum value, then fall down with further improvement in settlement, finally maintain the residual shear strength.There are interaction in skin friction, branches/plates resistance and end resistance, that is, the upper and lower section resistance interact on each other.c) Study on stress-strain character with specific equative friction method.The distribution character of skin friction, branches/plates resistance, and end resistance are respectively indicated not only on the work condition but also the maximum condition. It further proves that CPBP belongs to the frictional end bearing pile with multipoint resistance.d) Conclusion of the optimum space of branches/plates and piles for pile group design through combinating theory analysis with engineering test results. Only limited number of branches/plates can significantly increase the bearing capacity of pile.e) Establishment of formula for calculating ultimate bearing capacity of single pile. It is indicated that the residual strength differ depending on stratum,which cause different ultimate bearing capacity of single pile, furthermore, it is also size effect of branches/plates and improvement in stratum near branches/plates that have fluence on ultimate bearing capacity of single pile, finally, the synthetic revision coefficient of branches/plates resistance is ruled in the formula for calculating ultimate bearing capacity of single pile.f) Significantly good effect on engineering application of CPBP. Many actual examples are analysed, which show that the bearing capacity of CPBP is about two times as the bearing capacity of columnar cast-in-place pile, and CPBP also save construction period and investments.