Retaining Wall Connection To Rock/Ledge

[engineer817]

I am designing a retaining wall that will sit on ledge. I have completed a full analysis and have a final design that requires a 5 foot heel in order to add enough friction force to prevent sliding. Has anyone ever doweled a footing to ledge to prevent lateral movement? If so, what values could I conservatively assume for the ledge such as allowable lateral bearing pressure from the dowels to the ledge? Any info would be greatly appreciated.

 

[jdonville]

engineer817,

If I understand correctly, you will grout rebar dowels into the rock and then pour your retaining wall heel around the dowels. The shear transfer will be limited by a) the shear strength of concrete, b) the shear strength of the rock, or c) the shear strength of the dowel/grout. You should check all three (with appropriate safety factors).

Alternately, you could use a shear key into the rock and avoid doweling altogether.

I would perform unconfined compressive strength tests on any rock cores in order to estimate the rock shear strength. Additionally, you could use the RQD from the rock cores to put together approximate Rock Class or RMR values to develop a design shear strength of the rock.

Good Luck.

Jeff

Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.

 

[BigH]

If this ledge is sitting such that you have a major slope below it - you should also check dips of rock bedding to ensure that you don't have a predisposition of creating (by adding the retwall loading) a ledge stability problem.

 

[engineer817]

We have designed the wall to work by the ledge/concrete interface friction force. We will throw a few dowels in for safety. Thanks for your replies.

 

[HouseBoy]

I have designed rock anchors to go at the heel and into the rock so that the tension force in the anchor produces the friction needed between the footing and the bearing surface.
5 foot heel doesn't seem like too much unless it is a difficult excavation.
Rock anchor depth will depend on the characteristics of the rock. You might want to extend as deep as the all is tall in order to engage soil weight equal to the DL you were going to need in the first scenario.