Shallow Embedment Rock Anchors

[BNGeo]

I've been asked (as the geotechnical engineer) to consider the use of shallow rock anchors for resisting uplift of a buried structure resulting from buoyancy of the structure (structure below groundwater level). I would typically design the rock anchors according to FHWA prestressed rock anchor methods, accounting for considerations of rock-ground bond strength, grout-bar bond strength, and rock mass uplift.

The project has massive bedrock, and the result is that rock mass uplift generally controls, and the rock anchors are somewhat lengthy (min 10 ft free-stressing length plus bond length into the rock).

I've been told that other project have utilized very shallow (say 24" long) anchors. What I would probably call rock dowels. However, such shallow dowels would have very low capacity due to the rock mass uplift consideration.

Has anyone used these shallow of dowels to resist buoyancy? If so, what is the basis for ignoring the rock mass check?

Currently my only thought is that we could use very shallow "dowels" with capacity estimated to be controlled by rock-ground bond strength, and we could then proof test all anchors to verify capacity. In that approach we really won't have any validation of the approach until construction, however.

 

[PEinc]

If the rock is very fractured (even if RQD = 100), your shallow rock anchors or dowels probably need to be closely spaced in order to mobilize sufficient rock for mass stability. Shallow anchors might need to be closely spaced so that their pull-out cones are close enough to ensure a large mass rather than a number of small, low mass rock cones.

http://www.PeirceEngineering.com

 

[BNGeo]

Thanks PEInc - the "goal" I've been requested to meet is something like 15 ft o-c. So, fairly widely spaced given the shallow depths.

 

[MTNClimber]

What are your anchor loads for the 15 ft spacing?

 

[BNGeo]

60 kips. Although at 60 kips the very shallow 2' long lengths will obviously not work for bond or for rock mass. The will require about 8' long bond zones at 7" diameter to achieve 60 kips. But that is assuming the anchors would be passive anchors and would have an end nut or wedge like a Williams spin lock anchor.

 

[MTNClimber]

Seems like you’re in the right ball park with those numbers. Just make sure you have some good rock core information in your borings.