Introduction

Conventionally building having basements is usually built by a bottom-up method. In simple words, the stage of construction is from the bottom of the sub-structure to the top of the superstructure. Gigantic civil engineering projects usually have constraints of time and working space. So we have to follow the reverse of this conventional procedure. We call it the “Top-down construction method” which means we go from top to bottom. I will try to explain it through various aspects and photographs.

What is the reason for selecting Top Down Construction?

1- Distance between the boundary wall of the existing building and the building to be constructed is too close and the possibility of soil collapse is too high.

4- This method is usually preferred for tall buildings with deep basements, underground car parks, underpasses, and metro railway projects.

3- If the water table is too high in the area where the building has to be constructed.

2- This method is preferred for buildings having two or more basements.

Construction Procedure

Stage 1: Retaining wall, usually a diaphragm wall is constructed.

Stage 2: Excavation to just blew the roof slab level is done, with struts installed for support.

Stage 3: Roof slab is constructed, with an opening to allow machines to be lowered to excavate level below and for excavation material to be removed.

Stage 4:Excavation for each required level is then carried out until the base slab level is reached.

Stage 5: The station base slab and walls are then constructed. Openings that are not used for permanent access are sealed. Backfilling above the roof slab is done and the surface environment reinforced.

Stage 6: Station is fitted out.

Advantages
1– It allows early restoration of the ground surface above the tunnel  The temporary support of excavation walls are used as the permanent structural walls.
2– The structural slabs will act as internal bracing for the support of excavation thus reducing the number of tiebacks required  It requires somewhat less width for the construction area.
3– Easier construction of roof since it can be cast on prepared grade rather than using bottom forms.
4– It may result in shorter construction duration by overlapping construction activities.

Disadvantages

1-Inability to install external waterproofing.
2-More complicated connections for the roof, floor, and base slabs.
3- Potential water leakage at the joints between the slabs and the walls.
4- Risks that the exterior walls (or center columns) will exceed specified installation tolerances and extend within the neat line of the interior space.
5-Access to the excavation is limited to the portals or through shafts through the roof.
6-Limited spaces for excavation and construction of the bottom slab.
 
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