Experimental Research For The Long-short-piles CFG-PHC Integrated Composite Foundation

Long-short-piles composite foundation is a new type of foundation developed in civil engineering in recent years. This new type of foundation has good technical and economic benefits for decreasing foundation subsidence and coordinating the relation between ground load capacity and deformation, which has been proved by engineering practices. However, research on it is not sufficient, and there is not unanimous conclusion for its load sharing discipline, capacity determination, deformation development mode and so on. Secondly, limited by economic and technological conditions, presently there have been lots of experiments to mainly single piles or single-pile-composite foundation, but these experimental achievement cannot fully reflect the work mechanism and the bearing capacity of long-short piles composite foundation, particularly of multi-piles integrated composite foundation. The recent situation has limited the engineering application of long-short-piles composite foundation. Thirdly, CFG piles are the kind of earth-taken piles while PHC pipe piles are earth-pushed piles. To a certain extent, these two piles make up for deficiencies each other, which is possible to reduce the influence to surrounding environment. Therefore, there is necessary to research the large in-situ experiment of long-short-piles composite foundation combining different type of piles, for developing its interrelated theory and promoting its engineering application.This thesis has researched long-short-piles CFG-PHC integrated composite foundation by in-situ experiment and theories analysis. Its main conclusions are as follow:(1) In long-short-piles integrated composite foundation, long piles'pile-soil stress ratio(110-130) is obviously bigger than short piles'pile-soil stress ratio(30-45). The stress ratio of CFG to PHC remains about 3.0-4.0, which should be concerned with this composite foundation's designing parameters.(2) In long-short-piles integrated composite foundation, long piles bear more load, its load sharing ratio is 65-70%. Then short piles are followed, its load sharing ratio is 15-25%, and the soil bears less load, its load sharing ratio is 10-15%, (3) Soil under the cushion cap causes outward horizontal shifting, and closer to cushion cap, soil near piles block is limited stronger by piles.(4) According to the current basic theories on composite foundation, by calculating, the long piles'pile-soil stress ratio and the short piles'pile-soil stress ratio are 100-110 and 45-50,.which is close to the results of the situ experiment.(5) The thesis suggests that the design procedure of loading capacity should be amended, suggests "working degree combining method", and suggests that working degree of long piles and short piles are 0.6-0.8 and 0.2-0.3, the strength exertion degree of soil among piles is 1.4-1.8. Combined the situ experiment, these are verified and prove that the suggested values have certain compatibility.(6) The thesis conducts sensitivity analysis to the influence factors of load capacity, considers that the diameter of long piles is the most sensitive factor and both the distance between long piles and the distance between short piles are the second sensitive factor.