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Buildings with underground structures are often constructed using the bottom-up approach, where sub-structure and super-structure levels are built one after the other, beginning from the building's foundation and moving towards the top.

Although this traditional approach, known as the bottom-up approach, is straightforward in design and execution, it is not practical for projects with limited construction time or site conditions (size of the location, legal issues, etc.). The answer to these issues is the top-down building approach.

Top-Down Construction

Top-down construction approach is the opposite of bottom-up construction in which permanent structures are built from the top to the bottom of the basement using deep excavations. Here, the permanent internal structure is used as temporary support for the retaining wall, which is cast in a top-down sequence. To serve as horizontal supports for the walls as the excavation continues, the higher-level slabs are cast before the lower-level slabs.

Using a top-down approach for construction is a unique approach to earth retention which has been gaining popularity in the construction industry. Tall buildings with deep foundations and underground constructions like parking lots, underpasses, and subway stations are the two main kinds of urban structures where the top-down technique is applied.

The construction starts with the installation of a retaining wall, which is followed by the installation of load-bearing elements that will support the future superstructure.

Before any excavation is done, the basement columns, which are normally steel beams, are built and placed on the load-bearing components. Concrete piles or diaphragm wall panels are typically used as load-bearing components.

Sequence of Top-Down Construction

Following is the typical top-down construction sequence:

1. Construct the retaining wall that is embedded (usually a diaphragm wall).
2. Build the perimeter wall and piles. The steel columns or stanchions need to be placed where the piles are being built.
3. Cast the floor slab of the first basement level with an opening to allow machinery to be lowered to the level below and to remove excavation waste. Commence the first stage of the excavation.
4. The superstructure construction needs to be carried out at the current rate.
5. Cast the floor slab for the second basement level and go on to the second stage of excavation.
6. Repeat the process until the desired depth is reached.
7. Build up the ground beams and foundation slab, etc., and complete the basement.
8. Further, the construction of the superstructure can be carried out as per the design height/floors.

Advantages of Top-Down Construction

1. The superstructure may be rebuilt before the completion of the building and the buildings above ground, or it can be completed simultaneously with the structures below ground, thus reducing the construction time. 
2. Concrete diaphragm walls are more cost-effective when used as a groundwater cut-off and temporary soil retention system during excavation and as permanent load-bearing walls for the finished structure.
3. Permanent slabs replace temporary support.
4. The roof can be built easily and cost-effectively since it may be cast on prepared grade without bottom forms.
5. The structural slab acts as internal bracing to support excavation, minimizing the need for tiebacks.
6. Virtually vibration-free operation lowers ground movement, causing settlement.
7. The excavation width required for this type of construction is less when compared to conventional construction. Thus, reducing the cost and amount of pollution.
8. It offers the wall a sturdy support system, reducing the movement that occurs.
9. No need to underpin neighboring structures.

Disadvantages of Top-Down Construction

1. Due to the limits placed on the scale of the plant and the restricted access, the excavation works, and the building of the substructure must proceed at a more gradual pace and at a higher cost.
2. It is impossible to install waterproofing on the outside surface of the walls.
3. There is a possibility of water leaking out at the joints.
4. To give access to future excavation, it may be necessary to leave holes in the slabs.
5. The connections get increasingly difficult as you go down to the roofs, floors, and foundation slabs.
6. Entry to the excavation is limited, and there is not enough room to build a foundation slab.
7. In the temporary situation, the permanent slabs need some vertical support.
8. Because of the construction's increased rigidity throughout the intermediate phases, heavier loads could be accommodated by the permanent structure.
9. The layout is complicated.

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