Research On The Strength Characteristics Of Cement-sand-soil And Settlement Of Mixing Pile Composite Foundation

With the development of the society and the transportation, more and more subgradeconstructions are built in wet-weak areas. In addition to the traditional ground treatments, thecomposite foundation which has been proved as practical is increasely applied in wet-weakareas. As the most representative composite foundation, the cemented soil mixedpile composite foundations are applied in a growing number of areas. Since the distribution ofsoil layers varies in different areas, according to the railroad project from Lanzhou to theZhongchuan Airport, this thesis firstly studied the piling forming mechanism and thereinforcement effect of the cemented soil mixed piles utilized in sand stratum; Secondly, thethesis compared the cemented soil mixed piles utilized in sand stratum with which in loessstratum. The major work and conclusions of the present thesis are as follows:1）Through analyzing the reinforcement mechanism of the sand-cement mixed piles, theresearch shows that it is the mutural enhancement and the combined effect of sand and soil aswell as cement-hydrate that make the sand-cement has the advantage, over the bess-cement,of higher strengh.2) The indoor mixture ratio tests and the unconfined compressive strength tests lead tothe conclusion, that is, like the bess-cement, factors including the two ash (fly ash and thecement) content aw, the maintenance period T, the proportion of the two ash (fly ash and thecement), the stir intensity, curing styles,etc could influence the strength of thesand-cement.The strength of the cemented soil improves as the cement conten increases. Andthe strength will also be enhanced when the cemented soil is stirred more evenly. The testsalso shows that the water storage is more conducive to demonstrating the strength of thecemented soil.3) The simulation study of both the single pile composite foundations and the group pilescomposite foundations of the sand-cement mixed pile was conducted by the finite differencemethod of FLAC3D. Having a larger elastic range, the settlement of the sand-cement mixedpiles is superior to that of the bess-cement mixed piles. The distribution of the axial forces aredifferent in the single pile composite foundations and the group piles composite foundations.As the pile body goes deeper, the axial force increases firstly and then decresses. The nutralpoint, which moves up when the load increases, is the point from which the increasing axialforce becomes decrease. Because of the pile pushing and the large mat layer, the axial force ofthe central pile varies much slightly, the variation of the side pile comes second and that of thecorner pile is the most obvious in the group piles composite foundations. Since thedistribution of the side frictions are different in the single pile composite foundations and thegroup piles composite foundations, the depth of the nutral point is the critical point at whichthe pile lateral friction changes from a minus sign to a plus sign. The pile laterl friction of thegroup piles composite foundations is close to0when the scope of the pile body is from-10m to-2m. Due to the minor relative displacement of the pile-soil, the central pile has littlechange in the pile laterl friction while the side pile and the corner pile vary more obviousbecause they are close to the border, have a larger extent to dissipate the stresses and therelative displacement is much bigger.