Reliability Study On The Bearing Capacity Stability Of Long-short Pile Composite Foundation

Composite foundation is an effective way of foundation treatment.Among it, the long-short pile composite foundation is widely applicated inengineering, the experience is rich, but related theoretical research is yet to beperfected. People have mastered some practical design calculation and analysismethods of composite foundation, but reliability analysis mainly focuses onsingle pile composite foundation, the research on reliability of long-short pilecomposite foundation is little. Based on existing researches, this paper studiesthe reliability on bearing capacity stability of long-short pile compositefoundation. The main research and conclusions are as follows:(1) This paper introduces the reliability research status in domestic andforeign geotechnical engineering and composite foundation, reliability theoryand calculation methods (including JC method, optimization method(geometric method) and monte carlo method, etc.). The statistical principle of soil parameter, localized correlation theory and correlation distance calculationmethod of soil also instructed in this paper.(2) The pile in composite foundation are summarized in a variety ofdestruction form, on the basis of examples, this paper put forward two possibledamage modes of the long-short pile composite foundation: model l, the longpile, short pile and the soil all damage along the slip surface; model2, soil slidaround the long rigid pile along the slip surface. And also give the calculationprocessing method for different damage modes.(3) Use the circular arc sliding method to establish limit state reliabilitymodeles, and then deduce the limit state equation of long-short pile compositefoundation on the two failure modes.(4) Take the field test data of a long-short pile composite foundation ofone high-rise in Taiyuan to calculate stability bearing capacity reliability indexof the long-short pile composite foundation combined with optimizationmethod. Spatial correlation of the localized parameters is considered in thecalculation. Use Excel to deal with the data, call programming solver to searchthe most dangerous sliding arc, and solve the factor witch influence thereliability. Finally, determine the damage way of composite foundation inthis paper by comparing the results of two models, and analyze the reasons.The results summarized as follows:①Before considering the correlation of soil, reliability index is2.5or so,a relatively low reliability, and after, it rise to3.7or so, high reliability. The latter conclusion is consistent with the final test results of this project, witch shows that this nature cannot be ignored in the calculation.②Considering different load distribution and sliding arc In the two models, take the smaller reliability one as most likely form of destruction. Data comparison shows that, the destroy form of the fundation we study in this paper is model1, the long pile, short pile and the soil all damage along the slip surface, reliability index is3.66～3.66, afety factor of1.75, dangerous slip surface position is that:x is about0.75～1m, z between1m to1.5m, h in the range of5m to5.5m.③Parameters as cs,φs, γs, load p, and shear strength of pile τp have a greater influence on the variation of reliability.