Underpinning slab without interrupting mech. equip
Underpinning slab without interrupting mech. equip
(OP)
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.
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.






RE: Underpinning slab without interrupting mech. equip
Is this around one or two sides, or more of the slab? How close is the machinery to the ecge of the slab, what does it weigh, and is it on spring isolators, or mounted directly to the slab?
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Underpinning slab without interrupting mech. equip
The machinery is within 100mm of the slab edge and sits on small PFCs (channels), no spring isolators. Not sure on weight, but I think 200kg wouldn't be far off.
I've specified underpinning for 400mm deep footings with masonry walls before and seen it done. Both the footing stiffness and brick arching makes it simple. But this slab is only 100mm thick and the machinery is just another complication thrown in that makes me a little unsure.
Advice?
RE: Underpinning slab without interrupting mech. equip
I would underpin with CMU in 6 foot horizontal segments with 12 feet between underpin segments, and in three stages, until the entire slab was underpinned. This, coupled with the additional recommendation below, should provide adequate vertical stability until the underpinning is completed.
Thinking further, I would pour a slab at the bottom of each 4' section (or so) and design it and the CMU underpin sections as individual "L" shaped retaining walls, putting in the proper resteel. These individual slabs should be below the walkway level so you can pour a continuous finishing walkway slab.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Underpinning slab without interrupting mech. equip
1ft = about 320mm
3000 / 320 = 9.375 ft
or
2000 / 320 = 6.25 ft
Sizing can not be correct - ???
I worked in emperical ft
It's such a small area, move the machine, or google micro piling, I never used it but could be the answer
RE: Underpinning slab without interrupting mech. equip
Compare the cost to relocate the mechanical equipment against the cost to underpin and allow it to remain in place. Include the cost premium for working around said mechanical equipment in with the underpinning. It may very well prove to be less costly to relocate.
Mike - I think you miscalculated the Imperial units - 2m x 3m = 6'-7" x 9'-10".
Ralph
Structures Consulting
Northeast USA
RE: Underpinning slab without interrupting mech. equip
RE: Underpinning slab without interrupting mech. equip
Yea, you're right. Too many things on my mind I guess. Thanks.
Then sequentially do each end and half of the long side - four stages.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Underpinning slab without interrupting mech. equip
question: is sheet pile not applicable here? why?
RE: Underpinning slab without interrupting mech. equip
RE: Underpinning slab without interrupting mech. equip
If it was a regular footing, I'd use mass concrete instead of CMU (or blockwork as it's called in Aus). However, this is a bit more of a delicate job and I was looking for more robust options without blowing the budget...which is already a few $mil over.
RE: Underpinning slab without interrupting mech. equip
Then excavate to your formation depth and cast a footing for a retaining wall which will be the permanent support for the machine slab. Depending on the geometry your temporary support beams may have to be cut off and become part of the final retaining wall.
RE: Underpinning slab without interrupting mech. equip
I think you're suggesting cantilevering steel beams off concrete pads to support the slab. There is a overturning moment that will need to be transferred from the steel beams to the pads. The only way to resist such a moment would be to rely on the weight of the pads, which means they'll be massive. This can be done, although not a practical solution. However, what happens to those pads when I excavate down 2m to the bulk earthworks platform? They too will be compromised and I'll be back at square one.
I think the only solution is to drop a couple of bored piers down next to the slab and shotcrete the soil face. I'd prefer something not as expensive but I can't see any other way of doing it :(
RE: Underpinning slab without interrupting mech. equip
Screw piles might accomplish the same thing as bored piers and save a step (the concreting).
Ralph
Structures Consulting
Northeast USA
RE: Underpinning slab without interrupting mech. equip
BA
RE: Underpinning slab without interrupting mech. equip
As mentioned earlier I'd prefer some kind of low risk underpinning operation not bored piers. But I can't see any better (practical and cheap) method that can be achieved without risking damage/interruption to the plant equipment.
RE: Underpinning slab without interrupting mech. equip
BA
RE: Underpinning slab without interrupting mech. equip
RE: Underpinning slab without interrupting mech. equip
BA
RE: Underpinning slab without interrupting mech. equip
I'll sum it up for you.
Foundations are medium dense or very stiff to hard clayey and sandy colluvium/fanglomerate. Recommended allowable bearing pressures are 150kPa for strips and 200kPa for pads. Excavation shall not encounter permanent ground water but seepage is likely during rain. Temporary site excavations can be formed near vertical but the upper .5m should be battered at 1H:1V.
Want to know anything else?
RE: Underpinning slab without interrupting mech. equip
Hey, Mike what do you think?
BA
RE: Underpinning slab without interrupting mech. equip
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.
RE: Underpinning slab without interrupting mech. equip
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.
RE: Underpinning slab without interrupting mech. equip
RE: Underpinning slab without interrupting mech. equip
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
RE: Underpinning slab without interrupting mech. equip
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.