Two story CMU building on a slope 1

[sengineering]

I'm working on designing a two story building (same as the attached sketch) & here are the conditions:

- The building is 72 ft x 32 ft;
- The lower level should be all concrete & waterproof, the top level is CMU with an A-frame roof;
- The building is in a flood zone & the first floor will be completely submersed in water when it floods;
- This is an industrial structure in a water plant.

I have not designed a structure like this before & I have concerns about the lower level;

What are the main things to look out for or be concerned about in my design?

Do I need drainage? Or a retaining wall (or other methods of stabilizing the soil) so the soil on the bottom of the building wouldn't be washed off over time?
Is it a concern for the passive pressure of the soil behind the building causing overturning or sliding down the slope?
What would you recommend for the foundation? A slab? Piles?...

I'd appreciate your advise and help!

 

[dik]

Build it vertical, not perpendicular to the slope.

Do I need drainage? Or a retaining wall (or other methods of stabilizing the soil) so the soil on the bottom of the building wouldn't be washed off over time?

You need to design for two conditions... one normal and one submerged. You need a retaining wall as well as drainage. You can stabilise the soil... best to talk to a geotekkie.

Is it a concern for the passive pressure of the soil behind the building causing overturning or sliding down the slope?

Active pressure pushing against the wall resisted by foundation and/or soil. Have to look at the pressure from a saturated soil being resisted by a weakened submerged soil.

What would you recommend for the foundation? A slab? Piles?...

Either, if properly designed will work.


Dik

 

[msquared48]

I would set the building back further into the slope, eliminating the need for the downslope fill at the basement. You will also be able to balance cut and fill better.

Mike McCann, PE, SE (WA)

 

[sengineering]

Thanks for the replies. I have limited real estate & can't move the building further in.

Are there any specific parts of the codes I need to look at for basement design?

 

[msquared48]

Yes. The IRC, ACI, and I believe there are some requirements for knockouts in the foundation wall to allow water to pass since you are in the floodplain. I can't remember the governing code though. Someone else will...

As for the downhill slope, I would consider using pile pile under the strip footing.

Mike McCann, PE, SE (WA)

 

[racookpe1978]

Doesn't he have the bathtub effect (empty swimming pool effect) due to the water pressures below during flooding - when the soil is saturated or underwater, but the basement/subfloor is empty?

 

[SlideRuleEra]

I would put this entire industrial building on a driven pile supported mat foundation. This will provide the following advantages:

1. Minimize the fairly complex cut and fill work to get soils suitable for a building foundation. Piling will virtually eliminate the changes of future differential settlement, which could lead to real problem in both the waterproof portion of the building and the CMU upper floor.

2. The weight of the mat foundation, together with pile designed to resist uplift (if necessary) will prevent flotation, as mentioned by racookpe1978.

3. The pile foundation will provide a suitable base for the necessary uphill (10' tall) retaining wall, and can be designed to resist the horizontal thrust from that fill.

4. The downhill retaining wall becomes optional. Don't build it, as shown in my marked up sketch (forming the foundation mat could be somewhat tricky). Or, more likely, use piling to support this retaining wall and to resist horizontal thrust from fill under the foundation mat.

5. The building is in a flood area, who knows what weather and soil conditions (moisture) will be. If the pile driving equipment can get to the site (that may be a big "if", but you mentioned piling as a possibility), foundation construction can proceed more or less independent of soil conditions. Quickly getting construction "out of the ground" when conditions are difficult is very important.

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[sengineering]

Thank you for the great answers. I ended up going with the piles as you recommended.

 

[hokie66]

SRE's solution is best. You may need some battered piles to resist the thrust from the uphill fill.

 

[SlideRuleEra]

Thank you, hokie. Thrust from the backfill is a major factor.

sengineering - Foundation and first floor design will be dominated by flood loading. A reference is FEMA P-936 "Floodproofing Non-Residential Buildings".

Buoyancy force (including an appropriate safety factor) is very high, >>1400 kips.

First story walls will not only have to be waterproof, but will also be subjected to significant hydrostatic pressure. The pressure (not just the force) will be different on some of the 4 walls.

In the same way, the floor (mat foundation) will have significant uplift force acting on it. The mat foundation should be much thicker than usual. Piling will almost certainly be subjected to uplift, particularly the interior piling (as compared to piling at or near the building perimeter). Don't even consider wood piling, use steel or concrete.

Hydrodynamic and Impact loading from moving flood waters and debris may also require some piles battered in a second direction.

See ASCE 7-10 for info on calculating the forces.

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[dik]

and just a caution... hydrodynamic loading can be substantial and also impact from objects in the water...

Dik