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.

 

[msquared48]

So you are doing a vertical 4 to 6 foot cut in stiff clay at the edge of an existing 4" slab, is that correct? And you are worried about supporting the mechanical equipment and slab during the installation of, well, whatever?

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

 

[pwht1]

Yes, and yes. We'll be excavating around 3 sides of this slab (it's only 2m by 3m in plan) and the other side is butting against an existing building.

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?

 

[msquared48]

So the plan is about 12.5 ' X 19 feet or so.

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

 

[STATICPH]

2 x 3m = 2000 x 3000mm

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

 

[RHTPE]

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

 

[delagina]

question: is sheet pile not applicable here? why?

 

[msquared48]

RHTPE:

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

 

[delagina]

anyone wants to answer a newbie engineer's question =)

question: is sheet pile not applicable here? why?

 

[pwht1]

No sheet pile is not applicable. It costs too much for such a small job, and is needlessly introducing another industry to the site. But thanks for having a go anyway

 

[pwht1]

Sorry, moving equipment is not an option - client demand.

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.

 

[Zambo]

What about casting a couple of foundation pads a metre or so (don't know the area in plan of your excavation) away from the machine slab and about -400mm below the soffit of the slab. Then excavate to expose the end of the slab and under it for about 300mm, push a beam in to support the end of the slab and jack off your new pads. Then repeat with a bit of a tricky hand dig job at midspan of your machine slab for a second beam.

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.

 

[pwht1]

Zambo, thanks for the input. But the process you're suggesting is missing a few things.

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

 

[RHTPE]

Screw piles might accomplish the same thing as bored piers and save a step (the concreting).


Ralph
Structures Consulting
Northeast USA

 

[BAretired]

Provide a sketch of what you are attempting to do.

BA

 

[pwht1]

Sketch attached BA.

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.

 

[BAretired]

Please clarify location...Australia? Do you have any geotechnical reports?

BA

 

[pwht1]

BA, correct Australia. Yes I have a geotech report, what would you like to know?

 

[BAretired]

I would like to know what it says. I am not a mind reader.

BA

 

[pwht1]

Haha I'm not going to post it on a public forum.

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?