ABSTRACT
Singapore Downtown Line Stage 2, Contract 912 is the first station after the Jalan Gali Batu Depot. This contract comprised of a 1.8km long cut and cover tunnel including the Bukit Panjang Station. The tunnel depth ranges from 15-25m, making it possible for the use of cut & cover method.
This paper presents the several trials in the selection of temporary retaining wall systems for the cut and cover construction. The selection was very much dependent on the difficult soil conditions, presence of existing structures, stringent design criteria and construction practicality. Most of the walls chosen are of secant bored piles. The main challenges are at the station west wall, zone 4 and zone 6. At the station, the substructures lie directly beneath the existing light rail transit viaducts. At zone 4, the soil type changes drastically from poor soil to rock below the tunnel soffit. At zone 6, the tunnel lies beneath an existing expressway viaduct, with hard rock outcrop condition.
After several trails, the optimum design solution for each zone of tunnel and station was made. The different types of walls chosen include peanut/donut shaped walls, diaphragm walls, open cut with rock bolts at high rock head outcrop, soldier piles in grout mixed piles and secant bored piles. The final design solution satisfied the different conditions in terms of design and constructability.
