Uplift on Rock Foundation

[aggman]

Does anyone know where I could get some design information for the design of tension bores in rock. I have a customer that would like to use a 6" drilled bore in rock, place rebar in the bore, and fill it full of grout for tension anchoring for uplift, rather than just use the weight of the footing. I have seen this done before, but am not sure how to determine the actual uplift capacity of the bore. Any help would be appreciated. Also, the customer owns a limestone quarry, and the foundations are for conveyors. No geotechnical was hired by the owner, and I am using an allowable of 10000 psf for the bearing. The quarry floor stone appears very solid. Thanks in advance.

 

[cbosy]

Try the EM 1110, "Engineering and Design of Rock Foundations" by the Corp of Engineers. You can get it off the internet-go to "Engineering Manuals." Another good one is "Recommendations for Prestressed Rock and Soil Anchors" by the Post-Tensioning Institute. Last address I had for them was in Phoenix, Arizona.

 

[aggman]

Thanks cbosy,
I will check these out.

 

[cbosy]

Another thing I remembered where a structural eng. used them is "don't put all your eggs in one basket" use more than 1 at each column location. In that case a rock mass pulls out out the ground and it's shape depends on the # of anchors. You should insist that the rock be cored and get some advise from a good geotech. Just because the floor looks solid doesn't mean the the rock is competent. At least if the owner backs off and doesn't want to pay you've covered your _ss.

 

[Focht3]

The rock bearing is probably okay - what are you using for pullout resistance at the rock/grout interface? Will the core holes be smooth, or irregular? What kind of equipment will be used? How deep do they need to be? What is the rock jointing pattern like, and will it affect the anchor capacity?

Ask the owner hire a geotechnical consultant to help you address these issues - at least make a formal written request. Good posterior protection either way.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[aggman]

I had already asked the owner to get a geotech to look at the site, but he refused stating he didn't feel it was necessary for what he is doing. The drill rig is a typical drill rig that you would use to drill holes in the face of a high wall to load shots. To give you an idea of the loads we are looking at per pier.
DL = 4000#
LL = 5960#
WL = +/-10800#
EQ = ?? (determining this morning)
I have already decided to use a 30" x 30" pier, 2' tall. This still keeps bearing @ 3000psf, which I seriously doubt will cause a problem on rock. I threw out the 10,000psf because an old wise engineer once told me you could get that on about any type of rock confortably. The rock shows no visable joints, and the floor seems smooth and in good condition. I also contacted the owner yesterday evening to see if he had a concrete contractor hired for the job yet. He said no, but he had someone in mind. I am going to contact the contractor today and see if he has a certain type of grout that he would like to use, then I will contact the manufacturer and see if I can get the bonding strength to use in my calcs. Thanks for all your input, I appreciate your help as always.

 

[DRC1]

Williams Form Systems and Dywidag Systems International make rock anchor systems and can provide design information. Also, the bars can be tested with a center hole jack. Anchors are typically tested to 125- 133% of capacity. This way you can verify you have the capacity you need, or if not, add a few anchors.

 

[clayR]

DRC1,
I think you probably mean design vs capacity ie. the anchors are usually tested to 1.3DL (design Load). The bars are sizes so the design load is typically 60% GUTS (ultimate).

Grade 150 steel is used for post tension work and can be tested to 80% of Guts. The rebar grade 75 can be tested to 90% of yield, and could be a better choice for passive anchors, providing there is a concern about loading the foundation.

cheers

 

[cbosy]

Although you can test the anchor, you still need to look at "global stability" i.e., an entire mass of rock getting yanked out out the ground (e.g., a cone).

 

[Focht3]

Everyone -

Just a quick question about the proposed anchors and your personal philosophy. Should the anchors be prestressed, or the nuts "finger tightened plus 1/4 turn"? I don't see a need for tightening to 100% design load; but would 10% be prudent or desirable? Some other level?

I realize this question is fraught with unknowns. But it is quite different from designing tie-backs for a retaining wall. I don't have a firm opinion on this one - wondering what your thoughts are -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[BigH]

The parameters that need to be addressed:
1. Bond of steel bar to grout - usually not a problem
2. Bond of concrete to the rock
3. Shear failure of the rock in tension - Army Corps has the figures - it's been discussed - say use 30deg from vertical as cone from tip of anchor. Neglect, say first metre of rock - it is not that expensive to drill rock - in the fact that 1 or 2 m extra won't break the bank.

Next - do you use passive system or active. Passive means that the tension load is created from "rest" condition. Can have relatively "significant" movement in order to mobilize strengh. Active means that you would put some prestress on the rock - less movement of system to pull out to gain design load.

It is most likely that passive system is sufficient unless, of course, small uplift movements cannot be tolerated. This way, you can put in bar and embed in foundation. For higher loads, I would probably consider some prestress as in some passive systems, there might be a bit of "play" in the initial load take-up. For his light loads, I'd say passive.