Rock Anchors 1

[moe333]

I am designing rock anchors to resist seismic uplift of a steel water storage tank. The anchors will extend approximately 30-40 feet below grade into weathered granitic rock and I have several questions:

Is there a minimum unbonded length for the anchors? I saw 10' referenced somewhere. Is the unbonded zone filled with grout after the anchors are tensioned?

Is there a minimum grout cover between the steel anchor and the rock? How is corrosion dealt with? Is the grout cover increased to mitigate corrosion? Is an epoxy coating used? Is there testing that should be completed to evaluate corrosion potential?

Are the anchors generally tensioned to the design load, or some other load?

Thanks for your responses

 

[dik]

Is there a reason for tensioning the anchors? Then do you need the gap to develop the pre-tensioned load?

Dik

 

[Panars]

A free reference for ground anchors available online is the FHWA manual Geotechnical Engineering Circular No. 4 - Ground Anchors and Anchored Systems, FHWA-IF-99-015.

http://www.fhwa.dot.gov/engineering/geotech/library_listing.cfm

The unbonded length is usually 10 feet for bar stendons and 15 feet for strand tendons. This length prevents significant long-term reductions in the load. However, if rock is 30-40 feet deep, your unbonded length will probably extend down to the top of rock. The unbonded length is filled with grout, but there is a sleeve around the tendon to prevent the grout from bonding to the tendon in the unbonded length.

The above manual contains the answers to your other questions.

 

[moe333]

Thanks Panars,

The rock is actually at surface so my unbonded length would extend from the bottom of the footing to a depth of 10 feet.

I found a very comprehensive manual from the Post Tensioning Institute on soil and rock anchors. If anyone is interested:

http://www.post-tensioning.org/documents/

 

[Panars]

The PTI Manual Recommendations for Prestressed Rock and Soil Anchors is the primary reference for ground anchors in the USA. It is well worth purchasing. I gave a link to the FHWA manual instead because it can be downloaded for free and it repeats much of the information in the PTI manual.

 

[PEinc]

Is there a minimum unbonded length for the anchors? I saw 10' referenced somewhere. Is the unbonded zone filled with grout after the anchors are tensioned? PTI call for 15' minimum unbonded length. Most anchors are grouted completely in one stage. Two-stage grouting is not done too often. The unbonded length usually has plastic sheathingand greasing to allow stressing without obtaining capacity in the unbonded zone.

Is there a minimum grout cover between the steel anchor and the rock? How is corrosion dealt with? Is the grout cover increased to mitigate corrosion? Is an epoxy coating used? Is there testing that should be completed to evaluate corrosion potential? Tendons usually have a minimum of 0.5 inches of grout cover. There are several levels of corrosion protection depending on the corrosivity of tyhe ground or rock. Epoxy coatings are available for both bar and strand tendons. I don't recommend it for strand tendons. Sometimes there are problems with epoxy coated strands. PTI and FHWA publications include information on soil testing. There are tests for ph, resistivity, sulfates, sulphides.

Are the anchors generally tensioned to the design load, or some other load? According to PTI, anchors should be locked off at at least 50% of their guaranteed ultimate strength. I don't agree with this for several reasons. Usually anchors are locked of at between 75% and 110% of their design load. The lock-off load can depend on how much load you want to add to the foundation bearing pressure and how much elastic tendon stretch (structure movement?) you can live with if the tendon ever sees its full design load.

Don't forget that anchor length depends not only on the unbonded length and the required bonded length, but also on the total length required for mass stability in uplift. The cone of soil or rock (sometimes buoyant weight) must be heavy enough with a factor of safety to prevent the uplift. Frequently, expecially in ground conditions with good bond, the mass stability length is greater than the total of the unbonded length and the minimum bonded length.

 

[moe333]

Thanks again for the responses.

I am familiar with corrosion testing for soil (sulfate, pH, resistivity, chloride) but would you se the same suite of tests for rock? I guess you could crush it and test it, but I'm not convinced it's applicible for rock.

It is just my gut feel but I would think tensioning to 25-50 percent of the design load would be appropriate. Any thoughts on this?

 

[BigH]

I think that dik has a good point. Why are you tensioning the anchors. Why not just let them be passive anchors? You don't have to worry about lock-off loads, or relaxation with time. You need to have some 'movement' to engage the anchors but unless there is a strong need to minimize the movement that can occur during the engagement of the anchors, I think that the movement associated with passive anchors would be acceptable. See DM-7.02 page 170. I've seen some better representations but momentarily can't remember the source - but it was older than the FHWA manual listed.

 

[PEinc]

Strand anchors should have sufficient lock-off load to properly seat the wedges into the anchor haed. According to PTI, anchors should be locked off at at least 50% of their guaranteed ultimate strength. I don't agree with this for several reasons. Usually anchors are locked of at between 75% and 110% of their design load. The lock-off load can depend on how much load you want to add to the foundation bearing pressure and how much elastic tendon stretch (structure movement?) you can live with if the tendon ever sees its full design load.


If passive anchors are OK, then why not just install long dowels or rock bolts?

 

[ssiguy]

In response to BigH, anchors are post-tensioned to address fatigue. This is critical for cyclic or dynamic loads. If the anchor is passive, repeated load/unload would unravel its capacity fairly quickly. When it is tensioned, the anchor would not feel much load until that pre-tensioned load is exceeded.

http://www.soil-structure.com

 

[moe333]

ssiguy,

Thanks for the info and link. Is there a guideline for what the lock-off load should be? I discussed this with someone at dwydag who said 10% of the design load considering they are for seismic conditions that may only be seen several times in the life span. What about relaxation of the lock-off load? Is there an estimate I can use for this?

 

[ssiguy]

Your are welcome moe333. For active anchors, the idea is that there is sufficient pre-load in the anchor that it will never be exceeded by the applied load. That means the anchor is pre-loaded with the design load plus any accounting for losses. Besides the FHWA link provided earlier in the thread, there is a good online collection of publications by the "anchorman" here

http://www.theanchorman.com/

http://www.soil-structure.com