Top Down Construction Method

Open Cut Technique Open Cut Technique is a very old method of basement construction. This technique are used in open space area where there is no obstruction and no building or structure nearby. Furthermore, Open Cut Technique are economical compared to the other type of basement construction. Beyond 6m deep, uneconomical as it will involve a massive excavation work and will require retaining wall. This technique can be used in almost any soil. A sloped open cut excavation is the most cost and schedule effective. When the trench is very deep and expensive backfill materials are required, then a vertical cut at the toe of the slope supported by shoring may be effective.

Procedure i. ii. iii.

Excavation – to left soil for back fill purpose. If soil not suitable as back fill material, approve backfill material shall be mobilise. Construct bottom-up – construction stage start at lowest level. Backfill – the expose area to be filled by approve filling material up to ground level.

Advantages      

Allows continuous excavation, laying and backfilling operations. Minor breakdowns usually do not cause delays to all activities. The open trench needs only the design of the cut bank slope. Because there are no additional support operations and equipment, it is the economical choice. The open trench provides easy access to the work because equipment and construction materials are minimized. The open cut method is suitable for most ground conditions, except for oozing mud (soft mud or slime) and running sands

Disadvantages  

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The slope of the bank requires more excavation and backfill volume than the other options. The only bank support is the strength of the soil. If drying, flooding, or change of soil properties weakens the soil, then sloughing and collapse can happen with little or no warning. The sloped banks require a wider work area. The bank slopes may force the use of larger equipment because the distance to reach into the trench is increased and a greater volume of soil must be excavated and backfilled.

Cut and Cover Technique The cut and cover construction technique has been used for many years as a means for building underground transportation facilities. This method involves the installation of temporary walls to support the sides of the excavation, a bracing system, control of ground water, and underpinning of adjacent structures where necessary. Normally carried out in congested area (urban). The choice of using permanent or temporary retaining wall depends on the ground water conditions.

Procedure i. ii. iii. iv. v. vi.

Construction of diaphragm wall decking, and pin piles at left area. Construction of diaphragm wall decking, and pin piles at right area. Decking, excavation and strut installation at central area. Construction of main structure (down – up). Pavement restoration in section. Completion of underground station.

Advantages  

Cut and cover tunneling is very cost effective in shallow depth tunnel construction Effective when dealing with loose or weak material.

Disadvantages  

Leaves surface unusable during the construction process Requires large spaces for long periods of time, even when retaining walls are in use over open cut excavation

Top Down Construction Method

The most common method used for constructing underground basements are called top down method. Although this method is simple in design and construction but it is not feasible for projects which required limited construction time. This construction method builds the permanent structure members of the basements along with excavation form the top to bottom. Tall buildings with deep basements and underground structures such as car parks, underpass, and subway stations are mainly constructed using this method. Top down construction method provides the significant saving of the overall construction time.

Procedure i. ii. iii.

iv. v. vi. vii. viii. ix. x.

Retaining wall is installed Piles is placed as an intermediate columns to support the slab Proceed with soil excavation just below the roof slab level with margin for P.C.C. thickness. If roof slab is more than 3 meter level than structural strut need to be provide to support the retaining wall as per design. The floor slab of first basement level is cast. The superstructure is then constructed. After that, excavation is continued through the opening provided. Scaffolding and bottom formwork is fixed below openings. Reinforcement for the opening is done and proceed with concreting process. After concreting, water proofing is done to prevent water leaking. Backfilling need to be done in layers and lastly the surface is levelled.

Advantages  

Structures above and below ground can be carried simultaneously and thus saving time for construction. Cost for formwork can be saved since the permanent column and slab can be used to support loadings during construction.

Disadvantages 

Problems of excavating in dark & dusty situation even with artificial lighting

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Limited head room & working area during casting of concrete slab, boring of holes and inserted pre-founded column

Composited Technique The main difference is the construction of pre-founded columns along the perimeter area only instead of constructing over and across the proposed floor plan area. The band of supported slab over these pre-founded columns will give natural daylight & better ventilation through the middle or core areas at the proposed floor plan. This will facilitate easy excavating, haulage, loading and carting away of excavated materials as well as good access for bringing in new materials. This basically the combination of top down technique and bottom-up technique.

Procedure i. ii. iii. iv.

To install permanent or temporary wall. Excavation start up to the construction level. Construction start from lowest structure. The process of slab and side wall continue until super structure stage. This method not required strutting system. If using temporary structure as retaining wall, the temporary structure can be remove after the completion of substructure. The super structure only can start after sub structure is completed.

Advantages  

Cut and cover tunneling is very cost effective in shallow depth tunnel construction Cost for formwork can be saved since the permanent column and slab can be used to support loadings during construction.

Disadvantages 

This technique is introduced with the aim of overcoming the disadvantages of working in dusty and dark environment.

References

https://constructionduniya.blogspot.my/2012/02/top-down-construction.html http://www.diaphragmwallconstruction.com/topdownconstruction.aspx http://www.engineeringcivil.com/what-are-the-advantages-of-using-top-down-approach-inbasement-construction.html http://docshare01.docshare.tips/files/25957/259573946.pdf