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Top down final

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Top down final

  1. 1. Some of the underground Rapid Transit System (RTS) stations are constructed by the “top-down” method. In this method, the underground retaining walls are03 . Construction 04 . Construction first installed. In most cases, these retaining walls are concrete diaphragm walls. of Underground of Underground This is followed by excavation to just below the roof slab level of the underground Structure Structure structure, with the retaining walls and struts supporting the soil at the sides. Decking Decking While noise, dust and other inconveniences The roof slab is then constructed, providing a massive support across the Roof Opening Roof Opening excavation. Access openings on the roof slab are provided so that works thereafterStrut Roof Slab Strut Roof Slab are inevitable during construction, LTA will 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 Strut work closely with the contractor to keep intermediate struts are progressively removed. The access openings on the roof Strut slab are then sealed and the ground is subsequently backfilled and reinstated. them to a minimum. TOP-DOWNThe roof slab is constructed, with access The next level of slab is constructed, and CONSTRUCTION METHODopenings provided on the slab for works this process progresses downwards tillto proceed downwards. The roof slab the base slab is completed. Construction Site Hoardingnot only provides a massive supportacross the excavation, it also acts as a Divertednoise barrier. Road05 . Construction 06 . Backfilling & Proposed Underground of Underground Reinstatement Structure Structure Decking Reinstated Road TOP-DOWN 01 . Installation of 02 . Excavation &Strut Roof Slab Backfill CONSTRUCTION Retaining Wall Installation of Designed & produced by C2 Design Studio Pte Ltd Side Walls Completed Retaining Wall Steel Strut Strut Underground Structure Roof Slab to be Base Slab ConstructedThe side walls are constructed upwards, After the underground structure isfollowed by removal of the intermediate completed, the soil is backfilled to thestruts. The access openings on the roof top strut level before the strut is removed.slab are then sealed. This is followed by completely backfilling The underground retaining wall, which is The soil is excavated to just below the the top of the underground structure and March 2005 usually a concrete diaphragm wall, is roof slab level of the underground If you have any suggestions or feedback, finally reinstating the surface areas. installed before excavation commences. structure. Struts are installed to support please call our Customer Service Line: 1800 – CALL LTA 1800 – 2255 582 the retaining walls, which in turn support the soil at the sides.
  2. 2. INSTALLATION OF RETAINING WALL 03. 02 . Safety Measures Inconveniences- DIAPHRAGM WALL The Land Transport Authority (LTA) accords top priority to safety. Noise is generated when the grab/trench cutter excavates the soil Professional Engineers (PE) and Qualified Persons (QP) are engaged as well as during the panel concreting process. As it is vital thatThe following pictorial guide to carry out stringent checks on the temporary structures to ensure concrete is poured continuously into the panel till the whole panelillustrates the construction sequence that they are installed correctly and safely before the excavation can is completely and homogeneously filled with the concrete, it isof a typical diaphragm wall. This type proceed from one level to the next level. In addition, our engineers inevitable that work runs into the night when the panel is concretedof retaining wall was used for theconstruction of Kovan Station on the 04 . monitor the various stress and strain gauges installed on the temporary so as not to affect its structural integrity.North East Line (NEL). structures on a regular basis so as to be sure that the stresses fall within acceptable limits set by the design engineers. Likewise, instruments are extensively installed in the vicinity of the construction site to monitor vibrations, ground movements etc. This is so that the engineers are always in the know of the impact of the construction on the surrounding buildings and structures, thereby ensuring that they are safe. 01 .01 . 02 . 03 . 05.Construction of Guide Wall Excavation of Panel Installation of Rebar Cage Repetition of Process Guide Wall The crane lifts up the reinforcement-bar cage Process 2 - 4 repeats for the remaining soil in Bite and places it within the panel. between the panels till the entire length of the No.3 Bite diaphragm wall construction is completed. Concreting Pipe No.2 Bite Moving Off No.1 Panel Wall ~1200mm Bentonite Slurry Completed Diaphragm Wall Rebar Cage ~800 to Completed Panel 1000mm ConcreteA guide wall is constructed to set out the position of 04 . The grab/trench cutter cuts and removes the soilthe diaphragm wall. to form the panel. The excavation is stabilised by Concreting of Panel filling it with bentonite slurry to support the wall Concrete is poured into the of the excavation. panel to form the panel wall.

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