Some of the underground Rapid Transit System (RTS) stations or building basements are constructed by the top-down construction method. In this method, the underground retaining walls are first installed. In most cases, these retaining walls are concrete diaphragm walls.
This is followed by excavation to just below the roof slab level of the underground structure, with the retaining walls and struts supporting the soil at the sides. The roof slab is then constructed, providing a massive support across the excavation. Access openings on the roof slab are provided so that works thereafter could proceed downwards to the base slab level of the underground structure.
Upon completion of the base slab, the side walls are constructed and the intermediate struts are progressively removed. The access openings on the roof slab are then sealed and the ground is subsequently backfilled and reinstated.
1. Installation of Retaining Wall
The underground retaining wall, which is usually a concrete diaphragm wall, is installed before excavation commences.
2. Excavation &Installation of Steel Strut
The soil is excavated to just below the roof slab level of the underground structure. Struts are installed to support the retaining walls, which in turn support the soil at the sides.
03 . Construction of Underground Structure
The roof slab is constructed, with access openings provided on the slab for works to proceed downwards. The roof slab not only provides a massive support across the excavation, it also acts as a noise barrier.
04 . Construction of Underground Structure
The next level of slab is constructed, and this process progresses downwards till the base slab is completed.
05 . Construction of Underground Structure
The side walls are constructed upwards, followed by removal of the intermediate struts. The access openings on the roof slab are then sealed.
06 . Backfilling & Reinstatement
After the underground structure is completed, the soil is backfilled to the top strut level before the strut is removed. This is followed by completely backfilling the top of the underground structure and finally reinstating the surface areas.
Inconveniences of Top Down Construction Method
Noise is generated when the grab/trench cutter excavates the soil as well as during the panel concreting process. As it is vital that concrete is poured continuously into the panel till the whole panel is completely and homogeneously filled with the concrete, it is inevitable that work runs into the night when the panel is concreted so as not to affect its structural integrity.