Study Of Engineering Characteristics Of Composite Foundation With Long And Short Piles For Deep Soft Ground

The use of composite foundation with long and short piles is becoming more and more popular in practical engineering. The design and its reasonable evaluation of such kind of foundation have been deeply concerned by engineers. However, quite few reports were presented on the behavior of this kind of foundation. Perfected theory is not yet formed. In this paper, the engineering characteristics of the composite foundation were studied by using FEM and field tests of a practical engineering. The main work includes:1. The advancements and the applications in practical engineering of the long-short pile composite foundation are introduced. And the advantages and shortcomings of the composite foundation are analyzed. A new method for calculating settlement is given.2. Software ABAQUS is simply introduced. It is used to analyze one-dimensional consolidation problem and an in-situ test of long-short pile composite foundation. The results show that ABAQUS can be used as a good tool in numerical analysis of geotechnical engineering problems.3. A great amount of three-dimensional elasto-plastic analyses were performed on the behavior of composite foundation with long and short piles, the effects of some factors such as length and stiffness of long and short piles and the thickness of sand cushion on the consolidation behavior of the composite foundation were carefully studied. The following conclusions were drawn:(1) There exist optimal values for module of short pile, the length of both short and long piles, and the thickness of cushion. They should be reasonably determined in the design of the composite foundation.(2) The deformation and pore pressure of sub-layer are affected little by the module of short piles. So we can enhance the bearing capacity of foundation by increasing the module of short pile without worrying about the increase of the settlement of sub-layer.(3) The increases of the length of short or long pile in the composite foundation will result in transforming load to deep soil layers. The deformation of sub-layer will be obviously increased with the increase of the length of long pile.4. A case study was made by using ABAQUS and the results were compared with field measurement. Stress ratio of pile to soil, soil pressure and pore pressure in the process of construction are analyzed. It confirms again the feasibility of ABAQUS in geotechnical analyses.The results indicate that the behavior of composite foundation with long and short piles is affected by many factors. So it is suggested that ground improvement methods for the actual project should be analyzed carefully and the optional design should be conducted for the purpose of economy and reasonability.