The combined composite foundation improved by both sand drain and cement-soil mixing piles

is a new technique of ground improvement for soft soils. It can not only accelerate the consolidation process but also enhance the bearing capacity of the soft soil foundation. According to the configuration of the pile and sand drains

this type of composite foundation is simplified as an axisymmetric model with the cement-soil mixing piles located at the center of the unit cell. The water is assumed to flow into the sand drain from the center along the radial direction and the water flows into the sand drain is equal to that flows out of it

i.e.

the equal flow assumption at the soil-sand drain interface. In this way

the smear effect of the sand drain can be taken into account and the consolidation equation for this type of composite foundation is developed. Then analytical solution is obtained for the loading scheme which is applied gradually and the stress increment is distributed in a trapezoid form in the vertical direction. Furthermore

with degeneration of the obtained solution to the simple cases

the analytical solution under an instant loading is presented. Finally

the influences of the following parameters

including the ratio of influenced zone radius to sand drain radius

the loading period

pile-soil compression modulus and sand-drain’s permeability coefficient

are investigated. This analytical solution has a theoretical significance to improve the design and the calculation of this type of composite foundation.