Abstract: Composite foundation aims at controlling settlement. Settlement calculation plays an important role in soft soil foundation design and construction stages. Based on the typical unit cell model of piles and soil, the relationship between composite foundation compression coefficient and elasticity modulus has been established. The factors such as time and consolidation have been taken into consideration. The settlements have been evaluated to verify the formula proposed in this paper. The formula is applied to the settlement deformation observation data of the soft ground distributed along the Suining-Ziyang expressway in Sichuan province. The formula's feasibility is further verified. The research shows that the calculation data are 10-20 times higher than the observed data. During the filling stage, the result calculated by the proposed method is in accordance with the observed results of the project. Based on these, the paper discusses primarily for the influential factors of the settlement such as pile diameter and the pile spacing. The best ratio of pile diameter and pile spacing is from 2 to 4.