A full scale deep mixing improved soft clay ground overlain by 6.0 m high reinforced embankment was constructed and monitored in order to study its deformation and consolidation characteristics. The local differential settlement between the deep mixing pile and its surrounding clay, which could induce downdrag skin friction, was observed from the full-scale test. The effect of local differential settlement and piezometric drawdown on the development of negative skin friction and the compression of the deep mixing piles are discussed, and an analytical method for long-term settlement analysis is presented. To avoid consequent undulation of the pavement at bridge approach, it is crucial that the lengths of deep mixing piles supporting the approach embankment must be varied smoothly. A method of proportioning the lengths and a typical layout of DMM piles for bridge approach embankment are presented.