2 Level Basement on Steep Grade

[CANeng11]

We are currently looking at the engineering for a house from a designer that is built on a steep slope with essentially a 2 leveled walkout basement (see my rough sketch attached). The house is roughly 45 feet wide (into the page) and they are also showing an open to below area in the middle third of the wood diaphragm above the lower basement level.

Some of the issues we see with how it is currently designed include:
- stepping the footings up from the lower basement to the upper basement;
- the upper basement footings potentially being built on fill;
- the large loads imposed on the lower foundation wall from the surcharge of the upper footing, as well as the slab attached to the bottom - of the upper wall pushing on the top of the lower wall;
- potentially issues with the felxible diaphragm with the open to below.

I'm wondering if anyone had suggestions on the type of design we should be looking at here. Does the lower basement foundation wall need to be moved further than 10 feet away from the upper? We have been going around in circles a bit on the best way to proceed on this one and are looking for others opinions on the best path forward.

 

[Ron247]

One quick thought, either increase the 10' or make the upper basement footing deeper in the ground to minimize its influence on the lower level.

I would not touch this without a good geotech report. Around where I live, they rarely get a geotech report on residential. Someone will be a $250,000 commercial building on a decent lot with a geo-tech report and then later build a $2M house on an embankment on the lake without a geotech. Baffles me. Over the depth you are going, there could be greatly different conditions.

 

[JoelTXCive]

This project seems like a good candidate for a global stability analysis. The walls are close enough in proximity to each other that you probably need a geotech to look at the whole system.

When we have tiered walls like this we send the geotech some concept drawings and see what they say. They can tell you right off the bat if a shallow system will work, or if you need drilled shafts.

What is the backfill material?

 

[CANeng11]

There is a geotech report from the site that shows it is mostly sand and gravel and silt till down to footing elevation.

 

[hokie66]

Global stability is the biggest issue here. And that upper basement floor should be concrete, not wood.

 

[racookpe1978]

Sand and gravel, with the loads on steep slope?

Doesn't make sense: That steep an incline of sand and gravel"should have" tumbled downhill/eroded many years before the building was ever considered. Is there anything (clay mix, other rock ledge maybe) holding the hillside in place now?

 

[CANeng11]

The grade is approximately 30% (3H:1V). The borehole shows 200mm topsoil, gravel to 1.5m, gravelly silt to 3m, silt till to 4.5m and sand and gravel to 6m.

 

[jdgengineer]

If you don't have a geotech I would definitely get one. Personally, I wouldn't touch the project without one.

Regarding the terraced retaining wall, seems like designing the intermediate slab as a deep beam to span to the adjacent perpendicular concrete walls may allow you to have a wall restraint at the intermediate level.

You could also consider drilled piers below the upper wall. This will help remove the surcharge of the upper footing on the lower wall and could help form a tension-compression couple with the lower wall to take out the wall moment at that point. At the very least I would deepen the upper footing as mentioned above to reduce the surcharge on the lower wall and elimonates the upper footing from sitting on the backfill of the lower wall.

You could also consider deadman or tiebacks for the retaining wall if space behind the wall is available and after you run the calculations you can't get the wall to work without.

The wood diaphragm over the lower basement doesn't bother me, that's fairly common. If you made that concrete it may be more difficult from a seismic perspective at the daylighting side.

 

[SlideRuleEra]

I'm wondering if anyone had suggestions on the type of design we should be looking at here.
Does the lower basement foundation wall need to be moved further than 10 feet away from the upper?
We have been going around in circles a bit on the best way to proceed...

Here is another concept. A slope of 3H:1V (18.4^o) with sand/gravel should be stable (angle of repose >30^o).

Instead of backfilling for the the garage and upper basement floors, leave the slope "as-is" and design both as elevated floors. This supports all of the garage and part of the upper basement on relieving platforms, making the upper retaining wall "go away" and reducing the height soil behind the lower retaining wall.
This is a compromise proposal. Geotech problems may be reduced while an elevated garage floor becomes more of a structural issue.

I agree with the others that contacting a geotech is essential.

http://www.SlideRuleEra.net

 

[HouseBoy]

Often in this type of case, we have designed the room below the garage using precast planks with topping for the garage floor. As SRE mentions, it takes some of the load out of the foundation structure.

I also like the idea that others have suggested of lowering the footing at the rear of the garage. Drilled piers are common around here and that could solve the problem of bearing in fill at the front of the garage.

Ditto on the "get a geotechnical consultant" but it sounds like you may already have one (you reference "borehole"). One thing that might be an issue (perhaps relating to global stability) is - what's under the "sand and gravel to 6m". The conditions at the base of the slope might be important too and might also give some clues.

45 feet is (probably) too wide for the concrete walls to span horizontally so, concrete floors as diaphragms or otherwise transferring lateral forces seems like a good idea.

 

[racookpe1978]

My house is also on a significant slope, though "only" a 4:1 to 5:1 on a 1 inch of clay and the rest NW GA "crumbly rock" (Piedmont foothills).

What I found I needed was a curb above ground upstream to divert water running downhill from collecting against the uphill walls of the basement and garage. Our house looks a little bit like the sketch above: Garage uphill but on the ground itself (no open area under the garage), a ventilated crawlspace under part of the first floor as the hill gets lower, then a full basement under the rest of the first floor.

But just one basement level. Walkway & sidewalk in back of the basement exit are on "generic contractor's fill" probably dug out from the basement.