Study Of Settlement Computation Method Of Gravel Pile Composite Foundation Under Embankment Load

With convenient material, simple making and low cost, the gravel pile composite foundation has been widely used in construction projects. This kind of foundation treatment is suitable for China. However, relevant theory has not been advanced. Especially the settlement computation method for this kind of foundation treatment has fallen far behind the engineering practice. At present, there are two types of settlement computation methods for the stone column composite foundation. One is the numerical analysis method and the other is the analytic method. The numerical analysis method is of some advantages. But it is not convenient to apply in engineering practice since its application requires strict test technique and high quality of staff. As a result, a convenient analytic method is needed. Existing analytic methods are mostly aimed at the composite foundation under rigid base, in which the column and the soil has the same strain. Moreover, the radial deformation of the pile has not been taken into consideration. These methods are not applicable to the gravel pile composite foundation under embankment load. Since the embankment weight is a kind of flexible load, the compressive deformation of soil is more than that of piles in the stabilized layer. The pile would stab up and down, so the deformation of piles and soil would not be compatible. Under vertical load, the radial deformation of piles should not be neglected because the crushed stone is loose. With the suggested vertical and radial displacement model, this dissertation takes the interaction of soil and column into consideration and derives some analytical-functions for the stabilized layer of the composite foundation under embankment load, including the normal stress in pile and in soil in the foundation, the frictional resistance between pile and soil, the compressive deformation of pile and soil. Furthermore, a reasonable method for estimating the extra stress in sub-layer is proposed. Based on the Terzaghi consolidation theory and the Carrillo theorem, the differential equations for vertical, radial and total consolidation of gravel pile composite foundation under embankment are derived, as well as their analytic solutions. The equations are reasonable in theory and easy for engineers to apply. Engineering cases have verified their accuracy. According to analytic computation, the stone column composite foundation under embankment is studied to find the vertical and radical distribution of stress and strain in column and in soil of the stabilized layer and the distribution of frictional resistance along column side. Analysis shows that the frictional resistance on the column side is linear along the column and decreases gradually with replacement rate. The normal stress in column increases gradually with depth above the neutral depth z m and decreases gradually with depth below the neutral depth z m, also decreases with replacement rate. The normal stress in the soil around column decreases gradually with radial distance. In the vertical direction, the normal stress in soil decreases with depth above the neutral depth z m and increases below the neutral depth z m. In a typical element, the compressive deformation of column is less than that of soil. The deformation difference increases with radial distance and decreases with replacement rate. When the replacement rate is high enough, the difference of column and soil settlements on the top of the composite foundation becomes quite small. If the replacement rate is reduced, the settlement difference will increase. Settlement of the stabilized layer increases with the length of column, and settlement of the sub-layer decreases with the length of column. Total settlement of composite foundation still decreases with the length of column.