| As a common method of soft ground improvement,composite foundation technology has been widly used to improve soft ground .In coastal deep soft soil region,with the improvement of amoumt and size of constuction and the reason of engineering properties of soft clay,using common ground treatment technology to improve soft ground has become more and more difficult.It is very important to look for new ground treatment technologies which should be credibility in theory,reasonable in economy and feasible in construction for the construction of coastal region.For improving deep soft soil in Nansha area,cement-soil long-short pile composite foundation new technology and tapered pile composite foundation new technology have been put forward in paper.By the means of in-situ tests,in-lab model test and theoretical and numerical analysis, engineering properties of Nansha soft soik effective pile length of composite foundation in deep soft soil,mechanism and displacement theory of cement-soil long-short pile composite foundation in deep soft soil,compacting effect of tapered single-pile and mechanism of tapered single-pile and tapered pile composite foundation have been studied in the paper.The main contributions are as follows.(1)The Nansha Area lies in the south of the Zhujiang Delta. The form circumstance and tiny structure of Nansha deep soft soil have been summarize by analyzing a great deal of investigation and test date.Base on carefully analyzing and studying on physics and mechanics indexs of Nansha deep soft soil, the typical characteristic of high water content,high viod ratio,high compressiblity,low intensity,low permeability and low bearing capacity have been found.Moreover,some relationships amonge physics and mechanics indexs which can help engineers to choose suitable physics and mechanics values have been gained by the means of statistics and regression analysis.(2) Base on different hypothesises,such as no load of soil top among piles under rigidity foundation, having load of soil top among piles under rigidity foundation and having load of soil top among piles under feasible foundation ,several calculation formulas for calculation effecitive pile length of composite foundation have been gained by theory analyse and formula deduction. The rationality of several methods has been comparably studied which can offer theory support for design of pile length of composite foundation in deep soft soil.(3) Base on static load test,in-situ tests and in-lab model test, mechanism and improvement effect of cement-soil single pile composite foundation under cushion have been studied.moreover,a calculation method has been acquired thinking of the displacement relationship between pile,soil and composite foundation.(4)Base on static load test, the mechanism of load transfer of cement -soil long-short pile composite foundation and the stress of pile and soil among piles with the change of load level of composite foundation have been studied in the paper. Moreover, a computational formulas for bearing capacity characteristic value and bearing capacity coefficient of cement-soil long-short piles composite foundation which have though of the relationship of pile,soil and composite foundation.Finally,according to the mechanism of long-short pile composite foundation,the overall displacement of long-short pile composite foundation is the summation of displacements of different distincts.(5)Base on in-lab model test of tapered pile,the compacting effect and mechanism of tapered piles with different taper angles and same section pile have been study.Then, according to the displacement compatibility equations of composite foundation,a formulas for pile-soil stress ratio of tapered pile composite foundation has been achieve.Finally,according to result of in-lab model test,it has been study that the mechanism of tapered piles and same section pile composite foundation by the method of comparable study and it has been found that the pile-soil stress ratio of calculation and test is very close.
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