Passive Ground Anchors

[DCBII]

We have some footings founded on rock that need to be anchored to the ground to prevent overturning. We'd like to core into the rock, clean the hole, and use a grouted rebar to engage the rock mass. All the literature I have seen suggests that post-tensioned anchors are typically used. Is this required or could a non-post-tensioned anchorage such as I've suggested be used? Is there any literature out there on this subject? The only time the anchors will see tension is when there's a design wind or earthquake load.

 

[PEinc]

This is a fairly common question for foundations under tension. If you use pre-tensioned tiedown anchors to their full tension design load, you place a lot of compression load on the footing and increase the bearing pressure when there is no tension loading. This often is not wanted and may increase the bearing pressure beyond allowable, requiring larger footings. If you lock off the tiedown anchors at some percentage of the tension design load, the bearing pressure is less but the tendons need to stretch more in order to pick up the full load. This stretch equates to footing movement which often is not wanted. If you use grouted dowels instead of tiedown anchors, they are not pre-tensioned and must stretch to pick up the tension load. However, you can install dowels with sufficient cross sectional area that the elastic stretch can be minimized. Both methods have been used. It's your call. Literature suggests tiedown anchors because the literature is being written by people involved in anchor design and construction. Nobody is writing about grouted dowels; but that does not mean that grouted dowels will not work. Don't forget to check the mass stability of the anchored footings. The required rock mass (with proper safety factor) may be more critical than the required grout to rock bond length.

http://www.PeirceEngineering.com

 

[oldestguy]

Your post suggests that you have info on what the "rock mass " is. But asking about literature suggests maybe not. The anchorage ability depends greatly on what sort of rock it is. For instance sedentary rock, such as limestone, may have many layers, not bound together. Igneous rock, such as granite is more likely one mass with occasional cracks. Allowable shear stress as well as rock unit weight is needed. Then comes the degree of weathering as well as potential submergence in defining that "mass". My old version of US Navy NAVFAC DM-7 has a section on rock anchorage. On-ine I suspect that reference also has it. Some soil mechanics texts also may have a section. Check with vulcanhammer.net.

 

[BigH]

If you have access to ICOLD 2014 proceedings - there is a paper on grouted dowels for uplift. Passive anchors will require some movement to develop the anchor capacity - and as PEInc says, it is the rock mass "cones" that may very well govern. Oldestguy's NAVFAC is correct.

 

[glass99]

What kind of grout do you guys use for grouted dowels?

 

[cvg]

you might find some useful information on williams form website. they use a polyester resin grout

http://www.williamsform.com/Ground_Anchors/Polyester_Resin_Rock_Anchors/polyester_resin_anchors.html

http://www.williamsform.com/Ground_Anchors/Spin-Lock_Rock_Anchors/spin-lock_rock_anchors.html

 

[glass99]

cvg: Thanks. Is the Williams grout similar to concrete anchor bolt material like Hilti HY 200 (i.e. an epoxy grout, and is also rated for use with "stone".)

 

[PEinc]

Grout for tieback anchors, tiedown anchor, and micropiles is most often plain Portland cement and water with a w/c ratio of 0.45 to 0.5, or 5 to 5.5 gallons of water per 94# bag of cement. Anything other than that is usually a waste of money.

http://www.PeirceEngineering.com

 

[cvg]

the polyester resin grout is specifically for rock bolting

 

[glass99]

PEinc: would you use a Porltland cement grout for a say 1" diameter steel rock anchor? A mini pile into soil would be a different story I assume.

 

[DCBII]

Thanks for the input everyone. I've located NAVFAC DM-7.02 and it seems to have what I'm looking for.

 

[PEinc]

I use Portland cement grout for thousands of tiebacks, tiedowns, soil nails, and micropiles. The only time I use a bagged grout mix is for smaller, dowel-type applications where there will not be a large grout mixer available. The tieback and tiedown anchors may have either deformed steel bar (threaded) tendons or multi-strand tendons.

http://www.PeirceEngineering.com

 

[glass99]

PEInc: what about into rock? Still portland cement grout? In a smooth bore hole into unfractured granite, does portland grout stick to rock?

 

[PEinc]

Yes. It even "sticks" to smooth, clean steel like pipe, shovels, and beams.

http://www.PeirceEngineering.com

 

[PEinc]

The Post-Tensioning Institute's Recommendations for Prestressed Rock and Soil Anchors contains tables for cement grout bond to granular and cohesive soils and to various rock types. FYI, for granite and basalt, the ultimate bond stress for rock to grout is listed as 250 to 450 psi. Per PTI Sec. C6.7.1, Rock Anchors, "The ultimate bond stress between the rock and the anchor grout can be approximated by using a value of 10% of the unconfined compressive strength of the rock, up to a maximum value of 4.2 MPa (600 psi)." Per PTI Sec. 4.11, Grout, "Cement based grouts are most commonly used for anchors. Polyester resins have been used in certain applications."

http://www.PeirceEngineering.com

 

[glass99]

PEinc: interesting that it works. It is a little surprising that cement would chemically bond in a reliable way to the smooth surface of rock without any kind of mechanical interlock, though apparently it does! 600psi is comparable with a low grade epoxy in a metal connection, which I consider to be a high strength bond.

 

[PEinc]

I haven't used 600 psi ultimate bond and don't know anyone who has. It's not normally needed for ground anchors.
If the drill holes are angles such that grout could run out of the drill hole, then one might use the resin cartridges. However, cartridges can be hard to mix if you are trying to spin a long, heavy ground anchor tendon. Pre-mixed grout is much more expensive than regular cement and water.
The bond for cement grout to smooth steel may not be the same as the bond for epoxy grout to smooth steel.

http://www.PeirceEngineering.com

 

[hokie66]

PEinc is correct. Portland cement grout is proven. That is what we use in bonded post-tensioning in structures as well, where it bonds to prestressing strands and ducts.

 

[RFreund]

I think I am just echoing here but what would you use for small loads? Say only a couple kips? Would you use a Williams product or would you cement grout in a 3/4" threaded rod?


EIT

http://www.HowToEngineer.com

 

[PEinc]

For a small load with small, short rods, drilled with a small diameter bit in good rock, I probably would use a pre-mixed high strength grout.

http://www.PeirceEngineering.com