TOP DOWN CONSTRUCTION 3. 1. INTRODUCTION: Conventional buildings with underground basements are built by bottom-up method where structure floors are constructed sequentially from the lowest level of basement to the top of the super-structure. This conventional method, called the bottom-up method, is simple in both design and construction, but it is not feasible for large structures with limited time for construction, site constraints, deep excavation, land with higher saturated soils and loose soils. In the Top down method, the permanent structure is built from top to bottom of the basement along with the excavation. Top down construction is an advanced method and is mainly used for the structures like Metro Stations. In this method, basement concrete slabs act as lateral bracing for the perimeter wall system. Ground level and subsequent basement slabs are poured, with access holes left to allow excavation beneath. As each subsequent sub grade level is completed, the floors act as lateral bracing for the perimeter wall system. Perimeter wall which is generally referred to as Diaphragm wall (or D wall) is a single, safe solution for all the constructional problems encountered in underground construction. D walls provide effective earth retention and good control over ground water movement. It’s capable of carrying the superstructure loads, providing early utilization of the superstructure even before the completion of substructure of that building.
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Early restoration of the super structure is possible even before the completion of the building and the structures above ground can be carried out simultaneously with the structur es below ground. This greatly reduces the time for construction. Concrete diaphragm walls are more costeffective when they can simultaneously function as a groundwater cut-off and temporary soil retention system during the excavation phase of the project, and then as permanent underground walls with load-carrying capabilities for the finished structure.
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Inability to install external water proofing outsi de the walls. Potential water leakage at the joints. More complicated connections for the roofs, floor and base slabs. Access to the excavation is limited and limited space for the construction of base slab. Complex design involved.
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Easier and economical construction of roof since it can be cast on prepared grade rather than using bottom forms. The structural slab act as an internal bracing for the support of excavation, thus reducing the amount of tie backs required. The virtually vibration-free operation minimizes the potential for ground movement and its resulting detrimental settlement. It requires less width for construction area. Reduced environmental pollution associated with pre-construction earthworks Eliminates the need to underpin adjacent structures.
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Construct the perimeter wall, Construct piles. Place the steel columns or stanchions where the piles are constructed. Cast the floor slab of first basement level. Proceed to the first stage of excavation. Begin to construct the superstructure Cast the floor slab of the second basement level Proceed to the second stage of excavation. Repeat the same procedure till the desired depth is reached Construct the foundation slab and ground beams, etc. Complete the basement Keep constructing the superstructure till it gets finished.
Rakesh Kumar S., Design Engineer Any questions or suggestions are welcom e.
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