Underpinning slab without interrupting mech. equip

[pwht1]

Hey all,

I'm doing an extension on a building that requires a courtyard/pathway thing installed at the 'basement' level.
However, that requires excavating around an external slab on ground which is housing mech plant that must remain operational during construction. The slab is only 100mm thick with a single layer 7mm mesh (SL72) and the excavation is 1.5-2m below the slab level.

Could someone please advise me on a suitable underpinning operation?

The foundation material will be a stiff clay with 150kPa bearing capacity. No need to worry about swelling.

 

[BAretired]

I'm not sure. I think I would like to consult with my consultant, Mike McCann before committing myself.

Hey, Mike what do you think?

BA

 

[Zambo]

I meant a pad either side of the slab so that the beam is simply suppoted on 2 pads and runs beneath and parallel to the edge beam shown on your sketch. But in plan we don't know where your excavation is apart from at the front of the slab.

If this excavation continues either side of the slab, also with a 2m high retaining wall, you could try constructing either side of the slab first and then using the new retaining wall either side as a support structure for the slab while you exavate and support under the slab.

 

[pwht1]

Zambo, apologies for the misinterpretation. Your proposal however is still inpractical, thanks for having a go though.

A few pointers:
How would the steel beams be installed beneath the slab? The beam would be 'fat' and expensive. Any driving mechanism used to push it through the soil would require a tremendous amount of force that could cause the beam to rupture the slab and take out the mechanical gear.
Yes the excavation goes around 3 sides of the slab (both short sides and 1 long one). Thought I already said that but no matter. The pads supporting the beams will still be compromised by the excavation.

If i was to go the underpinning path I would have the operation done in 900mm segments. That is excavate 900mm, fill with concrete, skip a 900mm section and repeat the first 2 steps then go back to the skipped section and underpin it... etc etc.

 

[Zambo]

Ah yes I've re-read your posts and you did mention excavation on 3 sides.

 

[BAretired]

So far, the only solution which appears to work is sheet piling. The poorly compacted fill below the slab doesn't make the job any easier.

It might be possible to install a pile at each corner and at one meter centers between, then use timber cribbing between the piles, dropping the cribbing down as excavation proceeds.

BA

 

[csd72]

Zambo,

If this is only a temporary situation then I would consider following the geotechs advice on temprary faces. A near vertical face and a 0.5m 1:1 batter would not take up much room.

I would get the geotech to confirm that for the specific application.

You can then pour a footing at the base and then place permanent precast walls around with raking props. Better still, do it in 1m steps underpinning style and then place no fines concrete between the soil and the concrete. I did a job in Toorak, melbourne this way where we used this method to underpin two adjacent houses and one adjacent embassy in this manner and we had no issues.

This will not form a waterproof wall though but is approprate for a 'wet' basement.

One way that you can really save money on temporary construction measures is to make them part of the permanent works.