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Re: Stepped Retaining Walls into Mountain Side

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Upslope or downslope (where's the Garage located)?
Chuck Utzman, P.E.

Gerard Madden, PE wrote:
Stepped Retaining Walls into Mountain Side

I have been asked to come up with a better design for a new residence cut into a hillside/mountain. The foundation is the focus of the review and my recommendations.

In summary, the house is 4 levels, where each level is sort of split in half (i.e. one floor has a half flight of steps up to the next floor level) . Probably a little confusing. Basically a tiered cut into the mountain.

Anyway, the original engineer used cantilever retaining walls at the hillside and they range in height from 5 ft to 14 ft for the stem. The site is underlain in bedrock and the soils engineer called for drilled piers and grade beams for the foundation. The engineer did this where the house foundation sits on top of the soil, but used traditional cantilevered retaining walls on spread footings where it is cut into the mountain side.

In reviewing the calculations, the engineer seems to have designed these walls considering them as independent walls with the soil pressures from the soils report. However, it does not appear that he has accounted for the upper walls surcharging the stem of the lower walls as the upper wall bear on the retained soil of the lower walls.

The contracting is barking about his design because he is required to provide a 5 foot deep continuous key at the bottom of the wall to resist sliding. Since this is in bedrock, understandably he is questioning this.

I am wondering if the native soils are removed anyway due to the cuts for the residence down to and into the bedrock, that the earth pressures with be present at all. This is rock essentially being retained. I am also wondering if it is a good idea to just provide a concrete wall inclined with the cut of the hillside and use rock anchors to basically form off the hillside. The house was designed with a crawlspace achieve by using a ledger at the retaining walls and sitting on top of the grade beams at the toe of the house.

If anyone could chime in with some suggestions and perhaps a good resource on hillside foundations I would appreciate it.

Thanks,

-gerard

Santa Clara, CA