Passive ground anchor design

[aeoliantexan]

My company likes to use passive anchors in soil or rock to restrain hydrostatic uplift or to support some retaining walls. I am familiar with PTI's guideline for design of post-tensioned anchors, but having difficulty finding guidelines for passive anchor design, other than the brief treatment in DM-7. Does anyone know of a useful document for passive anchor design?

 

[SlideRuleEra]

See:

"Ground Anchors & Anchored Systems" (304 pages) by the US Federal Highway Administration

"Quick Design Guide For Screw-Piles & Helical Anchors in Soil" (13 pages) by the International Society for Helical Foundations

http://www.SlideRuleEra.net

 

[jdonville]

One thing that gets fleeting attention in many treatments of this subject in my experience is tensioning of the tendon against the rock surface for rock anchors prior to casting the anchorage into the base mat or other foundation element. The way it has been explained to me is the tensioning limits the deflection required to engage the anchor as the bonded zone is already engaged.

 

[aeoliantexan]

Tensioning against the rock surface sounds like a good idea, but I would think it would be hard to protect against corrosion.

 

[jdonville]

You don't typically have corrosion worries with passive rock anchors as one side of the bearing plate is bearing on rock and the other is cast into your mat with the bare tendon and nut.

If you're still anal about corrosion, you can get DCP anchors with metallized bars and hardware, although this is unusual and thus relatively expensive. There are potential embrittlement issues with metallizing high-strength (150ksi Fu) tendons.

If you are anchoring to soil, you can sleeve the DCP tendon through the mat and then use a more traditional trumpet anchorage once the mat has cured. The nut and tendon can be covered with a cap and grease or can be cast into a pocket in the mat with a 2nd (usually non-structural) pour.

Or, you can choose to not tension the anchor and accept a bit more deflection for the anchor to engage. In this case you can have the corrosion-protected length of anchor project a bit into the mat and use a bearing plate secured by 2 nuts on the tendon also cast into the mat.

 

[aeoliantexan]

jdonville, your explanations were quite clear; thank you.

SlideRuleEra, I have searched the FHWA document pretty thoroughly without finding any mention of passive (unstressed)anchors. Can you please point the way? Or do you know of another source?
Thanks

 

[fattdad]

just google, "Brinch Hansen." He's done a lot of work on this topic.

f-d

ípapß gordo ainÆt no madre flaca!

 

[PEinc]

Examples of passive ground anchors would be soil nails and rock bolts. You could check for FHWA manuals on soil nailing.

http://www.PeirceEngineering.com

 

[SlideRuleEra]

Maybe FHWA "Soil Nail Walls Reference Manual".

http://www.SlideRuleEra.net

 

[aeoliantexan]

Thank you, my friends. Your suggestions will be helpful. I think you have confirmed my previous conclusion that there are no readily available detailed guidelines for what I call "passive anchors", generally grouted bars in rock or soil used to support a wall or prevent hydrostatic uplift of a slab or mat. My understanding is that soil nails reinforce a soil mass by tying the soil in front to the soil behind and also support a lightly-loaded facing. Rock bolts generally join blocks of rock together to form a stable mass. I think I can borrow from both those technologies plus post-tensioned anchors.

Our detail often is a bar grouted into a 4" or 6" hole in rock, occasionally soil, with a plate or hook at the top cast into the concrete structure. I am sure we have some that are well over 50 years old, and I have never heard of a failure. Lately we have been using epoxy-coated bars. I recently saw a design by another firm that had bars grouted in corrugated encasement similar to Class I post-tensioned anchor design. I am curious about the wide range between passive anchors protected only by grout and concrete versus PT anchors with "double corrosion protection". Lately we have been requiring pre-construction testing and learned that some of our rocks eg, marl, don't fit into PTI's list of expected ultimate bond strengths.

I would welcome more discussion.

 

[PEinc]

If you are going to use encapsulated (double corrosion protected) passive ground anchors, don't forget to use a trumpet attached to the back of the bearing plate. This trumpet is needed to double protect the bar length, with bare threads, that is above the corrugated plastic sleeve, just behind the bearing plate. That is the most susceptible area for corrosion. I would only use DCP encapsulated protection only in the most aggressive soil conditions. Otherwise, I would use either epoxy coated or galvanized nails and hardware.

http://www.PeirceEngineering.com

 

[aeoliantexan]

Good thought. I guess the trumpet can also provide extra protection for the tendon at the rock/structure contact where water and air can move and the grout column may want to crack.

Under what conditions would you provide an unbonded zone?

 

[PEinc]

Yes, that is exactly the purpose of a trumpet. Soil nails and rock bolts do not have unbonded lengths. Tieback and tiedown anchors have unbonded lengths to assure that the anchor's bond strength is developed behind the soil or rock that is loading the structure. Soil nails and rock bolts knit the soil or rock together as a mass, like resteel in concrete. You don't want any unbonded soil or rock mass. If you need to verify bond stress for grout to soil or rock, then I recommend using sacrificial anchors that do have an unbonded length and a limited bonded length with a known grouted diameter so that the anchor can be tested to bond failure, or to at least the ultimate design bond stress, without failing the tendon.

http://www.PeirceEngineering.com