Abstract: The deep excavation of an underground pedestrian passageway in Qianhai District of Shenzhen is presented in this paper. The site subsurface soil consists of artificial rockfill and the thick soft silty mucky soil is deposited at the bottom of the proposed excavations. The secant soldier pile with two rows of jet grout as the seepage curtain at the interface of existing and the proposed excavations was used as the excavation supporting structure. Two different approaches were planned in treating the soft soils at the bottom of the proposed excavation: one is to use high pressure jet grout; the other approach is to increase the length of the soldier piles; adjusting the location of internal supports; applying the prestress in the internal strut; and pile supported underground passageway. By using the later approach, an integral system consisting of pilecap and soldier pile supporting structure can be formed. After comparison, the results indicate that later approach is better than the first one in aspects of internal force induced deformation, stability performance and project cost. And the actual monitoring results also show that the performance of secant piles combined with pile foundation in thick silty mucky soils is stable, feasible and economical.

Key words: Thick Soft Soils, Deep Excavations, Pile Foundation, Secant Pile, Optimized Design