Finite element method was used to calculate the additional tensile stress of different layers of the pavement and of steady broken stone-cement layer

which are both caused by the differential settlement between new embankment and old embankment for expressway’s extension project. Based on the results obtained by finite element method

and the requirement of differential settlement of subgrade in code and that of the function and structure of pavement

this paper put forward a control standard of variation range of cross-slope gradient of road bed’s crown after completion

less than or equal to 0.45 %

as the allowable differential settlement for expressway’s extension project on soft soil foundation according to design theory of control settlement. Finite element method was also used to analyze some cross-sections of Foshan-kaiping expressway extension project in China. The extension part of subgrade in this project would be treated by composite foundation of deep mixing pile. Through analyzing composite foundation of deep mixing pile with different construction parameters by FEM

the differential settlement of subgrade were respectively obtained. Combining with the control standard of differential settlement drawn in this paper and the design theory on control settlement

the optimum design parameters of deep mixing piles were obtained

i.e. pile spacing equal to 1.6m

length of pile equal to 8m

diameter of pile equal to 0.5 m. In practice

pile spacing would be 1.5 m

0.1 m less than the calculation’s result because certain non-concordance happens between piles and soil when bearing load.