TOP DOWN CONSTRUCTION

By: The Civil Engineer on 19:56

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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.

STRUCTURAL MEMBERS REQUIRED FOR TOP-DOWN CONSTRUCTION

Design and construction principles for top-down method primarily call for two major structural elements.

·        Columns with sufficient capacity must be pre-founded in bored piles or barrettes to sustain the construction load and to utilize as part of bracing system.

·         Excavation for basement must be carried out with the support of permanent

           retaining wall so that basement floor slabs can be utilized as lateral bracing.

Diaphragm wall of 0.8m to 1.2m in thickness with sufficient embedment in firm soil layers is commonly used as a retaining wall whereas prefabricated steel columns known as Stanchions embedded in either large diameter deep-seated bored piles or barrettes are utilized as structural columns. Figure 1 illustrates the top-down construction method with utilization of stanchions and diaphragm wall.

Advantages and disadvantages

Advantages:

1. The shortened construction period due to simultaneous construction of the basement and the superstructure.

2. More operational space gained from the advanced construction of floor slabs.

3. The higher stiffness of floor slab compare to steel struts improves the safety of excavation.

4. It is highly suitable for construction for tall buildings with deep basements to be constructed in urban areas.

Disadvantages:

1. Higher cost (due to the construction of pile foundation)

2. Since the construction period of the basement is lengthened, the lateral displacement of retaining wall or ground settlement may possible increase due to the influence of creep if the soil layers are encountered.

3. The construction quality may influence because of worsened ventilation and illumination under floor slab.

4. It requires highly skilled supervision and labour force.

Conclusion

From the above report we can conclude that top down constuction has its suitability for certain kind of mega structures. It is suitable for structures with deep basements like undergrond rails, car parks etc. It is also very efficient way of doing two way construction to save time. Skyscrapers with deep basements in urban areas should be constucted using top down construction. But top down construction needs very efficient planning and designing and skilled supervision and labour force. Top down construction is the need of the hour as it is highly time efficient and is becoming popular and is coming more and more in practice with every passing day.