propped retaining wall close to spread footing

[sarju115]

Hi All,

The client wants to have an underground water tank on the first floor. This is a new RC framed building. The tank has four columns close to the four corners of the proposed tank (approx. 2-3' away from the footing edge). The column footing are typically 5x5 or 6X6' spread footing at 5' depth. The tank is approx. 10' deep. No drainage is proposed for the tank walls.
I am thinking of designing the wall as simple span, with at rest pressure with 0.5 times the vertical foundation pressure as surcharge. Also adding the hydrostatic pressure below water level. Is this the correct approach? Does it adversely affect the column footings (settlement??)?

Thanks in advance,

Sarju

 

[aeoliantexan]

The tank design approach sounds reasonable, but how will you get it into place? Excavating near-vertically beside the footings risks failure of the footings. It may be possible if the soil is strong and the footings overdesigned, but you need a geotech to make that determination. The presence of groundwater makes the construction more risky and more difficult. You may need a braced excavation, and some settlement of the footings is likely unless you underpin them. You need good professional advice.

You probably know that the tank needs to be weighted or anchored to keep it from floating and pushing up the floor slab.

 

[sarju115]

Aeoliantexan,

GWT is very low according to construction in nearby areas. They'll construct the tank prior to the construction of footings. Braced excavation seems likely.

 

[hokie66]

I would put the footings down at the bottom of the tank.

 

[aeoliantexan]

sarju115,
I thought the building was already in place. If you can build the tank first, your approach looks OK. You can be a little more conservative with the wall design without spending a lot of money. But if you build the tank in an open excavation with sloped sides, backfill will be necessary. I would avoid supporting the footings on the backfill unless you have great faith in the contractor and the inspecting agency. It would be best to avoid the risk and build the footings at the basement level as Hokie66 suggested.