The Top-down method is mainly used for two types of urban structures, tall buildings with deep basements and underground structures such as car parks, underpasses, and subway stations. The method has been used for deep excavation projects where tieback or anchor installation was not feasible and soil movements had to be minimized. There is a possibility of saving the overall construction time.
The construction begins with retaining wall installation and then load-bearing elements that will carry the future super-structure. The basement columns (typically steel beams) are constructed before any excavation takes place and rest on the load-bearing elements. These load-bearing elements are typically concrete piles or diaphragm wall panels.
Top-down construction sequence
The typical construction sequence of top-down construction is as follows:
- Construct the embedded retaining wall (usually a diaphragm wall).
- Construct piles. Place the steel columns where the piles are constructed.
- Proceed to the first stage of excavation.
- Cast the floor slab of the first basement level with the opening to allow machines to be lowered to the excavated level below and for excavation material to be removed.
- Proceed to the second stage of excavation; cast the floor slab of the second basement level.
- Repeat the same procedure until the desired depth is reached.
- Construct the foundation slab. Complete the basement.
- Keep constructing the superstructure until it gets finished.
- The superstructure construction can proceed at the same time as the substructure
- Temporary propping is replaced by the use of permanent slabs.
- It provides a stiff support system for the wall, minimizing movement.
- Piling can take place from the existing ground level.
- The excavation works and substructure construction are slower and more expensive due to the restrictions on the size of the plant and the limited access.
- Holes may have to be left in the slabs to provide access for the subsequent excavation.
- Vertical support for the permanent slabs is required in the temporary condition.
- Inability to install external waterproofing.
- The stiffer construction during the intermediate construction stages attracts higher loads into the permanent structure.
References: Gaba A., Hardy S., Doughty L., Powrie W., Selementas D., Guidance on embedded retaining wall design, Ciria, 2017.
