Top-Down Construction Method
Top-down technology (downward) is the advanced method employed to substructure construction, other than the conventional method (upward).In this method, basement concrete slabs act as lateral bracing for the perimeter wall system. Ground level and first basement slabs are poured, with access holes left to allow excavation beneath. As each subsequent subgrade level is completed, the floors act as lateral bracing for the perimeter wall system.
Working space and construction duration: it is not required a large working space for foundation excavation and saving cost by eliminating to construct the retaining wall. Especially for public transport works as traffic tunnels, this method helps to soon re-established trafffic road. And the top-down method of construction enables a high-rise superstructure and its sub-basement to be built simultaneously (popularly for civil works have basements) —-> accelerate construction.It is not required of the temporary strutting system (Bracsing System) to support the basement walls during excavation and construction of basement. That is cost-saving for constructiion. Temporary strut system is often very complex problems of space and very expensive construction.
- Construction schedules can be compressed by saving time in construction of substructures and high-rise superstructure at the same time (of course, we have increase the cost of strengthening the lower part, and if the “savings” schedule can not cover the “cost” for safety strengthening, it’s not necessary to do quick, top-down first and then the high-rise superstructuresthe as seen in Hanoi. After construction of the ground floor, we can separate completely superstructure and underground construction. You can construct simultaneously the basements and the superstructure.
- Foundation problems (the phenomenon of sludge, groundwater …): attention that in dense urban areas of high-rise buildings, if open excavation (open cut) with diaphragm wall, deep foundation and water table to be lowered in the construction of the underground structures, adjacent buildings are not assured. (easily occur sliding roof excavation, subsidence, cracking …), Top-down construction method to solve this problem.
- Construction of the basement with groud floor was constructed at first can partly reduce the impact of bad weather. ( Based on: http://deltacorp.vn)