A Complete Guide for Top-Down Construction

The bottom-up method, which involves erecting substructure and superstructure levels sequentially, starting at the building’s base and working up, is frequently used to create buildings with subsurface structures.

In contrast to bottom-up construction, which uses extensive excavations to build permanent structures from top to bottom of the basement, top-down construction involves building from the ground up. In this instance, the retaining wall is cast top-down while being temporarily supported by the inside permanent framework. The higher-level slabs are cast before the lower-level slabs, acting as horizontal supports for the walls as the excavation progresses.

A novel method of earth retention that is becoming more and more well-liked in the building sector is the use of top-down construction. The two main types of urban structures where the top-down technique is used are tall buildings with substantial foundations and underground constructions like parking lots, underpasses, and subway stations.

Installing a retaining wall is the first step in construction; next, load-bearing components that will support the future superstructure are installed.

The basement columns, which are typically steel beams, are built and installed on the load-bearing components before any excavation is done. As load-bearing components, concrete piles or diaphragm wall panels are frequently utilised.

Advantages of Top-Down Construction

• The superstructure can be rebuilt before the building and the buildings above ground are finished, or it can be built concurrently with the underground components to speed up construction. 
• When utilised as a groundwater cut-off, a temporary soil retention system during excavation, and as permanent load-bearing walls for the finished project, concrete diaphragm walls are more cost-effective.
• Temporary support is replaced with permanent slabs.
• Since the roof may be cast on prepared grade without bottom forms, construction can be done quickly and affordably.
• In order to support excavation, the structural slab serves as internal bracing, which reduces the requirement for tiebacks.
• Operating with almost no vibration reduces ground movement, which results in settlement.
• When compared to conventional building, this style of construction requires a smaller excavation width. Consequently, the expense and pollutants are decreased.
• It provides the wall with a strong support system, which minimises movement.
• There is no need to support nearby constructions.

Disadvantages of Top-Down Construction

• The excavation work and the construction of the substructure must move more slowly and at a higher cost due to the restrictions set on the plant’s size and the restricted access.
• Waterproofing cannot be installed on the exterior of the walls.
• There is a chance that water will leak out at the joints.
• It could be required to leave gaps in the slabs for future excavation.
• The connections become more challenging as you descend to the foundation slabs, floors, and roofs.
• The excavation is difficult to access, and there isn’t enough space to lay a foundation slab.
• The permanent slabs require some vertical support in the interim.
• The permanent structure could support larger loads since the construction became stiffer throughout the interim phases.
• The design is intricate.

Sequence of Top-Down Construction

• Build the embedded retaining wall, which is often a diaphragm wall.
• Build the piles and the surrounding wall. The steel columns or stanchions must be positioned where the piles are being erected.
• The first basement level’s floor slab was designed with an opening so that equipment could be lowered to the level below and excavation waste could be removed. The current pace of superstructure construction must continue.
• Move on to the second stage of excavation after casting the floor slab for the second basement level.
• Till the necessary depth is attained, repeat the procedure.
• Complete the basement by adding the foundation slab, ground beams, etc.
• Additionally, the superstructure’s construction can be done in accordance with the design’s floor and height requirements.