Characteristics Of Composite Ground Of Stone Columns

Soft soils are of large void ratio, high moisture content and compressibility, and low permeability, and have poor engineering properties. In order to satisfy the requirements for the bearing capacity and settlement of actual engineering, it is often necessary to improve the soft soil foundation. The composite foundation is a kind of artificial foundation in which parts of natural soil are improved or replaced, or some enhanced materials are installed. The natural soil and the enhanced columns bear the loading together.Stone columns are one of the columns which were widely used in composite foundation first. There are still some problems in current theories regarding the loading transfer, settlement and consolidation of stone columns in the composite foundation. These theories can give some calculation methods for the settlement and consolidation of composite foundation with stone columns on the basis of certain assumptions only for simple boundary condition and uniform soil. In this thesis, a series of field tests were performed to an oil tank foundation, and the bearing capacity, settlement and consolidation of the composite foundation with stone columns were analyzed. A three-dimensional FEM method was applied to discuss the settlement, bearing capacity and load transfer characteristics of composite foundation with stone columns. On the basis of some assumptions, the analytical solution to consolidation of composite foundation with stone columns was proposed, which takes into consideration the stress concentration, the varying upper load, and nonlinear characteristics of soft soil.The test and calculation results show that:(1) The stone column has higher composite modulus than soft soil, thus the composite foundation with stone columns has less settlement than the natural foundation. The construction of stone columns enlarges the boreholes, which compact the soil around the columns to some extent. With the dispassion of excess pore water pressure due to the disturbance of construction, the strength of surrounding soil increases. The permeability coefficient of stone columns is much greater than that of surrounding soil, and the installation of stone columns provides a horizontal drainage path, and changes the consolidation of natural soft soils. The pore water in soils flows horizontally towards the columns and then flows vertically through the columns, which decreases greatly the length of drainage path and the consolidation time. Consequently most of settlement occurs in the period of construction and the settlement after construction decreases. (2) The increasing length of columns and replacement ratio can make the settlement decrease. However, the change rate of the settlement of composite foundation decreases with increasing length of columns. Therefore it is not practical to reduce the settlement by increasing the length of columns simply. (3) Due to the adjustment of the cushion, the settlement of surrounding soil is larger than that of columns. Within the upper part of composite foundation, the load transfers from surrounding soil to columns, and the axial force of columns increases along depth. There exists some depth called neutral plane at which the settlement of columns and surrounding soil is equal. The pile-soil stress ratio increases with the increasing column length and decreasing replacement ratio, respectively. (4) The consolidation rate of composite foundation with stone columns is accelerated with the increasing column length and replacement ratio. The influence of replacement ratio on consolidation rate is larger than that of column length.