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# Rectangular Concrete Tank: Stability Forces

## Rectangular Concrete Tank: Stability Forces

(OP)
I've posted this in Structural already but really would like some foundation/soils people to comment too:

I am reviewing the sliding stability of a 16'x14'x17'-deep concrete tank. Essentially, it will need to behave as a retaining wall during a possible future installation of an adjacent tank. There will be full height soil on one side and full excavation on the opposite side of the tank.

When I check the sliding stability for this case I get a S.F. = 0.74. Increasing the slab doesn't add enough weight to increase the S.F. by much.

I am hoping to use the friction between the soil and the SIDEWALLS to help pump-up my S.F. > 1.5. However, I can't find any information concerning how to adequately calculate this resistance.

My guess would be to use the Friction Angle for Dissimilar Materials from AASHTO (typically used in calculating the vertical component of Coulomb lateral earth pressures) to get a corresponding friction coefficient for the walls. Then, multiply the linearly varying lateral soil pressure normal to the sidewalls of the tank to get a linearly varying friction resistance for the orthogonal direction.

Thoughts? Thanks!

### RE: Rectangular Concrete Tank: Stability Forces

Hi US Engineer
I do not know the subsoil underneath your deep concrete tank with the SF of sliding= 0.74, it seem to be the soil is unsuitable for the direct footing.
I think the pile foundation will be another option for your tank foundation.

### RE: Rectangular Concrete Tank: Stability Forces

A cross section will help clarify your case.

### RE: Rectangular Concrete Tank: Stability Forces

Can you fill the tank with water during construction? That will greatly increase your normal forces and resulting sliding resistance.

### RE: Rectangular Concrete Tank: Stability Forces

Your tank is not very large but is deep. I do not think it is very practical to think that the tank needs to be stable, on its own, during an adjacent, future excavation. In my experience, tanks are not designed for future, adjacent excavation. If an adjacent tank is to be built in the future, the contractor could install sheeting around the new tank area and brace this sheeting to the existing tank. With the first tank being only 14 or 16 feet long but 17 feet deep, the future tank's excavation would need to be a significantly sloped open cut if sheeting were not used. If there are adjacent underground utilities, the sloped open cut may not be possible. IMHO, designing the first tank to be stable during an adjacent tank excavation is a nice idea but expensive and not very practical.

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