Underpinning in Sand

[HD69]

I am needing to carry out some underpinning to a dry pack sandstone foundation wall as I need to drop the floor level in a basement, however the footings are founded at a higher level than the proposed slab level.

I cannot use the conventional mass pour method of underpinning (which involves excavating alternate sections in sequence to a pre-established depth below the footing and place concrete on each pit; then repeat the method until the entire affected area has been underpinned)as I am in sand and the sand will not be stable enough to allow for the segmental excavation.

Is there any way to underpin this wall safely or do I need to support the entire upper structure using needles and pins, then demolish the existing foundation wall and rebuild the footing and wall at the appropriate level?

 

[dhengr]

HD69:
Get a GeoTech guy involved to confirm the sand conditions and that this would work in your case. Open up the existing lower slab and excavate down to the bottom of the existing footing. You may want to leave some slab (or blocking to the slab) in place to provide some lateral support for the existing found. Then use a pressure grouting system to solidify the loose sand under and for some distance behind the existing footing. You want to end up with something akin to a soft sandstone, so you can work and excavate it to do the rest of your found. work.

 

[HD69]

thank you dhengr