Rock anchor uplift capacity?

[tocoadog]

Was wondering if someone had any insight on determining the uplift capacity on a rock anchor?

Doing some tower work, with shallow refusal (<6 feet), structural engineer wants pier and mat foundation recs. Well, to prevent uplift in the mat, rock anchors have been recommended. The only thing, I don't know how to determine the uplift capacity. The material is basalt, so fairly porous material. No other testing has been performed, except the cores with REC and RQD over 75 percent.

I am sitting down to review "Rock Foundations" by the COE, and I am planning to buy the rock and soil anchors by the Post Tensioned Institute, but I thought I would check here to get some quick insight or opinions on determining the uplift capacity.

What is the process you do or have done?
Thanks.

 

[rchanke]

I use a grout to rock bond stress of about 300-psi for basalts. Apply that stress across your bond area to yield your bonded anchor capacity. Check this against the yield strength of the anchor.

You get your hands on "Recommendations for Prestressed Rock and Soil Anchors" by PTI and all your questions will be answered.

 

[tocoadog]

Thanks for the quick response.

In terms of bond area, should I just assume an area? I think this is what I'm having problems with. Should I provide the uplift capacity or just the bond strength of the grout and rock interface?

And yeah, I agree, I need to buy the PTI book.

Thanks again.

 

[rchanke]

Normally you have a design capacity to work towards. Based on that and an estimated bond stress of 300-psi you will work towards what size of anchor bar to use. The size of the anchor bar will dictate the drilled hole diameter which in turn governs the bonded area (pie*dia*length). This brings you to constructability, what are the common sizes of anchor bar used within that load range, what are the common lengths they can be purchased and so forth.

 

[eric1037]

Check out Williams Form Engineering Corp.'s website for typical sizes of anchors.

http://www.williamsform.com

 

[PEinc]

Anchor length for tiedown anchors is not only a function of the bond stess. It is also a function of the required mass stability analysis. You may be able to bond an anchor to a minimal length of soil or rock, but that doesn't mean that the anchor will engage a sufficient mass of soil or rock to provide the required uplift resistance.