TOP DOWN CONSTRUCTION

    

Concurrent above and below grade construction

Conventionally, buildings with underground basements are built by bottom-up method where sub-structure and super-structure floors are constructed sequentially from the bottom of the sub-structure or lowest level of basement to the top of the super-structure. Though this conventional method, also called as bottom-up method, is simple in both design and construction, it is not feasible for the gigantic projects with limited construction time and/or with site constraints. Top-down construction method as the name implies, is a construction method, which builds the permanent structure members of the basement along with the excavation from the top to the bottom. 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. In this case the basement floors are constructed as the excavation progresses. The top/down method has been used for deep excavation projects where tieback installation was not feasible and soil movements had to be minimized. Top-down construction method which provides the significant saving of the overall construction time has been adopted for some major projects where time factor is of primary importance. The sequence 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 barrettes constructed under slurry (or caissons).

PROCEDURE

The typical construction procedure of top down construction is as follows

·         Construct the retaining wall.

·         Construct piles. Place the steel columns or stanchions where the piles are constructed.

·         Proceed to the first stage of excavation.

·         Cast the floor slab of first basement level

·         Begin to construct the superstructure

·         Proceed to the second stage of excavation; cast the floor slab of the second basement level.

·         Repeat the same procedure till the desired depth is reached

·         Construct the foundation slab and ground beams, etc. Complete the basement

·         Keep constructing the superstructure till it gets finished.

INSTALLATION OF RETAINING WALL

The underground retaining wall which is usually a diaphragm wall, is installed before excavation commences.

Excavation and installation of steel strut

The soil is excavated just below roof slab level of the underground structure. Struts are installed to support the retaining walls, which in turn support the soil at the sides

Construction of underground structure

The roof slab is constructed, with access openings provided on the slab for works to proceed downwards. The roof slabs not only provides a massive support across the .

Construction of underground structure

The next level of slab is constructed, and  this process progresses downwards till the base slab is completed

Construction of underground structure

The side walls are constructed upwards, followed by removal of the intermediate struts. The access openings on the roof  slab are then sealed.

Backfilling and reinstatement

After the underground structure is completed, the soil is backfilled to the top strut level before the strut is removed. This is followed by completely backfilling the top of the underground structure and  finally reinstating the surface areas.

STRUCTURAL MEMBERS REQUIRED FOR TOP-DOWN CONSTRUCTION (Top Down Construction- Part 2)