Tall Retaining Wall at foundation 1

[Inlander]

I've recently received a plan set for a new 3-story home that will have a 24' tall reinforced concrete retaining wall. The wall is located between the rear of the garage (at top level) and the house which is cut into the hillside (about 24' down from garage slab). The retained portion will be about 18' of cut and 6' of fill beneath the garage. The wall length/building width is 29'. It can be top restrained and will have proper drainage. I don't have the soil information yet. I could support the wall, at least at the ends by the side walls.

What are some reasonable options to make this happen (deadmen, tie-backs, counterforts, caissons)?

 

[STrctPono]

If I am understanding you correctly (a sketch would help) this retaining wall is mainly in a cut condition. If so, a top down construction approach seems most reasonable to me. I would be considering soil nails. Is the property line anywhere near the retaining wall?

I think I understand it better now, It sounds like for the lower 18' cut, a tieback approach would be reasonable and then for the upper fill portion, perhaps a MSE wall sitting above the tieback wall. You may have a different looking facing for the walls above and below. Unless of course you shotcrete the whole thing afterwards. The question is all the weight from the fill and the garage above. I think you will need to work closely with the geotechnical engineer on this.

As a side note, a conventional retaining wall supported at the ends with the side walls should probably consider the at-rest earth pressures in those regions. However, given the information you provided, it doesn't sound like a conventional retaining wall would be the most economical solution.

 

[BridgeSmith]

I'd say that designing it to span horizontally between the end walls could be the best option, if the end walls can provide adequate sliding restraint.

A row of concrete drilled shafts (sometimes called a secant wall or tangent wall) may also be an option. The advantage of those is that it doesn't require excavation to the dredge line to put in the wall.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[GC_Hopi]

Soldier pile wall is pretty efficient for 24 feet. As others have mentioned a conventional RC retaining wall is probably not economical but I am little confused since you stated the wall is between the garage and the house. Does this mean the wall will be part of one or both of these structures.

 

[Aesur]

There was another post recently regarding a similar wall, take a look - it's a good read and may have valuable information. Link

 

[Inlander]

Thanks for your input everyone. I like the idea of soldier piles if the loads get too high. I'm thinking of restraining it at the top and at a few feet below the top of the side retaining walls. I may add a T-beam at that level to transfer the load to each wall. It'll have to be done in three phases plus the garage slab. Soil nails make good sense, but at this point I don't know what kind of soil. It seems they might be a good alternative to laying the hillside back or installing shoring. Does this seem reasonable? It seems like a lot of work. But if it fails, the whole house is trashed.

I've attached a conceptual sketch of my vision of this retaining wall. I may add a turndown at the back of the T web to act as a deadman.

 

[Colostruct]

Inlander

I've designed a couple dozen similar foundations and each time it is different depending on the soils.

to consider:

Excavation may cause GLOBAL slope instability. The Geotech should run a slope stability analysis.
Excavation may cause LOCAL slope instability. Layback cut may not be feasible.

IF the slope may be GLOBALLY stable during excavation SHORT TERM shoring for worker safety (micropiles etc) may be apprpriate. IF GLOBALLY unstable tiebacks (soil nailing) may be appropriate. I have used permanent soil nailing with a shotcrete wall for permanent support. The foundation wall in front is isolated and non-retaining.

I just finished an 18 ft wall. The slope was determined to be stable globally so we did temporary micropile curtain wall (tiebacks would have been in the property next door). My structure is pretty similar to yours. But, I didn't do a shear key because the extra depth of the cut for the shear key really affected the shoring. Instead I did a footing that acted like a lateral beam and transfered loads to footings on the side that extended outside the building so they were loaded with soil. This was needed anyway for the top and mid-wall contributions.

One thing that happens to me every time I design one of these is that I get fixated on the larger retaining wall and forget, for a bit, that the side walls still have significant retaining and are, sometimes, trickier to design. Cantilevered side walls sometimes deflect too much for a building. So, I try and find locations for buttresses or counterforts. Sometimes we rely on passive restraint on the opposite side and transfer loads through framing. But, you can cross the line where that is appropriate at some point. And, often the restraint is questionable because of adjacent construction

This is a BIG design. I will spend a week on one of these foundations...maybe more. So much to consider and SO affected by slope and soil properties.