Retaining Wall on Ledge

[SteelPE]

I am currently designing a building with a basement. The walls of the basement to to be constructed as retaining walls and are 12 feet deep. The geotechnical report places ledge approx 10' below grade requiring our basement to be placed partly in ledge. The question has come up in regards as to whether or not we need a footing for this basement wall or if we can just connect the wall to the ledge. Other than actually attaching the reinforcing into the ledge, are there any other concerns with attaching the wall to the ledge? Is this method not recommended for any reason?

 

[KootK]

I suspect that you're fine so long as rebar end cover can be maintained at the bottom of the wall. I'm not clear on what this ledge is however. Can you elaborate? Also, you're retaining wall is not a cantilever wall, right? It is supported near the top laterally?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SteelPE]

The ledge is stone ledge, the type they have to blast through (hence being in the geotechnical report), not a concrete ledge.

The wall needs to be a cantilever wall due to the construction sequence.

 

[oldestguy]

I'd be very skeptical about such a plan unless you know much about the "ledge".
Generally there are crevices, weak planes' etc. in bedrock. These may be oriented in just the wrong direction. You should balance the cost and effort of a very detailed rock investigation, as with numerous core borings, lab tests, etc. along with specialized work against a conventional footing. Is there the need for special techniques, such as cemented in anchorage rods that could be "done wrong" etc.

 

[KootK]

Frickin' cool! If this happens, I'd love to see the details. I imagine that part of the impetus for this is to avoid the extra excavation into the rock that might be required for a conventional retaining wall.

1) I share OG's concerns on the dirt side. I'd think that you could get around much of that with some conservatism and a testing regimen applied to the in place rock anchors however. But, just as OG implied, that stuff costs money too so it's a trade off.

2) You'd want to give some careful attention to the detailing to ensure that the rock anchor tension gets transferred to shear in the wall reliably. I'd almost want a distribution member on the inside like a whaler but, obviously, that isn't going to happen.

3) If I understand the situation correctly, the tension/compression couple that you're going to form with the ledge has a lever arm of only about 2'. That sounds a little crazy. I'd think that you'd get enormous anchor forces due to the backstay effect and wind up with significant shear demand in your wall between the two levels of rock anchors (assuming that there are two).

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.