Top-Down Construction System

Sukumaran

Start at ground level, usually then install piles which will later form the basement columns. Cast floor slab on ground, again the first one is usually at or just below ground level. Wait for it to gain strength and then excavate below slab and between piles down to next basement slab level - ie the first slab becomes the "roof". Cast next slab on excavated ground and repeat process until down to the bottom level.

At the same time, after casting the first slab you can start going upwards - concrete columns, steelwork or whatever forms the above ground structure.

Important to programming is to know when supporting structure is strong enough to take the next lift up (or down) and what temporary / permanent measures are required to accomodate this.

Hope this helps.

Sukumaran

Andrew is right. A few points to watch for:

The first slab you cast (ground floor) will probably need additional strengthening because its going to be your working surface. It needs to cater for construction loads, materials storage etc. The piles, whether temporary or permanent, need to cater for the additional loads, too. I suggest you ask the Engineer to add more rebar so that the slab can cater for 25 to 30KPa. That should cope with most normal events.
The new basement’s ground support system at the perimeter needs to be installed first. Your engineer may design a sheet pile or diaphragm wall system, depending on ground conditions and your local design codes. If the storey height of the basement floors is not too much he should be able to design a D-Wall system that can span about 6 metres of height without intermediate strutting. This can allow you to excavate in jumps of 2 floors at a time thus allowing you to get to the deepest levels as quickly as possible and reducing the time you need to keep openings workable for "mucking out". The intermediate floors can be cast later, bottom up. If a sheet pile wall, then you may not be able to excavate down in jumps of two floors at a time.
The temporary piles that you installed to support the ground floor and that later become columns must be designed as columns, because there will be a time when they’re taking all the vertical loads without the lateral support of the ground, as a pile does. Also, if you’re using steel H-piles, they will need shear studs welding to them to support the floors as you go down. Later you can encase the columns in concrete to become permanent columns or cut them out; depends on the structural design of the final basement.
Give careful consideration to your mucking out routes and methods.
Make sure there is enough ventilation. The construction plant will be diesel powered and the fumes are heavier than air, so you must keep pumping or blowing in fresh air for your site personnel and for the machinery to burn in their engines. Similarly ensure you have adequate lighting; for safety and working.
Make sure you carefully plan your escape routes in the event of and emergency. The conditions are not dissimilar to tunnelling and many of those considerations also apply to top-down construction.

Good luck,

Roger Herod

Hi guys,

TQ for the informations.

Just few more questions on the system:
a) Can we use normal machineries for earthwork or use special machineries to expedite the work ie. excavators, lorries etc.
b) Could you guys give me some reference for top-down construction method. ie. the books

Regards.

You can use ordinary diesel powered equipment, but you should make sure that the exhausts are equipped with scrubbers and spark arrestors. Don’t use petrol powered machines because of the greater carbon monoxide output. Any electrical ventilation fans that are intended to extract smoke in the event of a fire should be capable of working at high temperatures, say 250degC. These are about twice the cost of "normal" fans.

I don’t have any reference books to hand, but try Google!!

TQ Roger.

I manage to get 2 references from google search engine.

1) www.yurkevich.ru/pdf_publications/hi-tech.pdf
Development Top-Down Method Of Underground Construction Or Hi-Tech In Russian

2) www.lta.gov.sg/projects/doc/TopDown%20Final.pdf
Top-Down Construction

Regards.