Rock Anchor for Gravity Wall 1

[hayeska]

Have a few questions regarding the use of rock anchors.

Gravity wall to be installed adjacent to and above rock cut. Owner wants to minimize footprint of wall. This creates more usable land/parking.

I would like to install vertical anchors to resist/reduce overturning and sliding.

My review of literature and multiple discussions on this site lead me to believe that a tensioned rock anchor would be best.

I understand the mechanics of the bond with the anchor, bond with the rock, pullout of the rock cone etc.

I would like to better understand the recommendation for the requirement of the unbonded length of the anchor. The unbonded length usually has plastic sheathing to allow stressing without obtaining capacity in the unbonded zone.

Is this only done for anchors that are tested? The thought being that one would want to be anchored deeper in rock that is more competent and not rely on the unbonded zone during the test?

I saw reference to this length as ten feet. This might be twice the bond length I need for the anchor.

With respect to overall stability, applying tension to the anchor compresses the wall, reduces overturning, sliding, and modifies the location of the resultant force (reducing pressure at the toe.).

The wall will be constructed in lifts with a few horizontal joints. The thread bar can be extended near top of wall and tensioned at that point.

All thoughts and comments welcomed.

 

[PEinc]

All tieback or tiedown anchors should be tested.
All tieback and tiedown anchors need unbonded lengths unless you are doing two-stage grouting where the second stage is grouted after testing the lower grouted section.
PTI calls for 15' minimum unbonded length for strand tendons. For temporary threaded bar tendons, the unbonded length can be as short as 10 feet.
Because your proposed anchors are tiedown anchors, you need to check the mass stability of the potential cone mass that could pull out. The length of anchor required to provide a cone mass with a safety factor often controls the overall length of the anchor.
Don't use too short a bond length. The extra cost to drill a few more feet of bond length is minor compared to the risk and cost of a tieback test failure and installation of a supplementary or replacement anchor. I probably would not use less than a 15' bond length. I certainly would not use a 5'bond length. I might use a 10' bond length only if the rock is very good and the design load is very low.
You should test the anchors before you build the wall. That way, it is easier to install a supplementary or replacement anchor if necessary. Once the wall is build, it can be hard and expensive to add anchors.
Get a copy of the Post Tensioning Institute's Recommendations for Soil and Roack Anchors. Also, check out Williams Form Engineering's web site and catalog on ground anchors.

http://www.PeirceEngineering.com

 

[hayeska]

PEinc,

This is a potential project and I have only done some preliminary work with respect to wall sizing and loads etc.

I intend to have the geotech engineer establish design criteria for the rock, RQD, bond stress etc. Have not met with them yet, so the following are my assumptions.

Based upon this information we would develop a preliminary design for the wall with an established drill depth to competent rock, bond depth, anchor spacing etc.

I assume we would then install a number of anchors (% ???) for testing to verify the design assumptions. If the test results were satisfactory, we would install the remainder of the anchors and then install the wall. I assume we would monitor the drill logs to look for inconsistencies when comparing to the test anchorages.

After the test loads are released, there would be no reason not to grout the remaining exposed threadbar in the drill hole. To allow for elongation, 15ft of the threadbar could be wrapped in plastic sheathing. I assume the unbonded length would be from the bonded length up into the gravity wall, depending on the depth of the drill hole; or should it be the last 15 ft of the bar?

I have the Williams catalog and will get a copy of the Post Tensioning Institute's Recommendations for Soil and Rock Anchors.

Thanks for your response.

 

[eea]

PEInc, thanks also for the great information on the William's website and catalog.

 

[PEinc]

hayeska, you wrote, "I assume the unbonded length would be from the bonded length up into the gravity wall, depending on the depth of the drill hole; or should it be the last 15 ft of the bar?" The bonded length should not extend up into the gravity wall. Otherwise, you could be testing the bond of the anchor within the concrete wall rather than in the bedrock and you would never know if the rock bond is acceptable.

http://www.PeirceEngineering.com

 

[hayeska]

PEinc,

For test purposes, the bonded length will be within the drilled hole. As an example, if I have a 20' drilled hole (to competant rock) and a 12' bonded length, the remaining 8' plus 7' into the gravity wall would give you the 15' free length. That is the 15' length I had referred to.

We propose to perform 3 test anchors with a specified bond length in rock. Verify the capacity of the anchors and then finalize the design of the wall.

The production anchors will be grouted full depth of drilled hole. They will be proof loaded, load released, and then the thread bar will be extended to the top of the wall and post tensioned. As an example, the last 15' of a 20' high wall would be sheathed to provide the free length.

I would imagine it would be best to have the last 15' as the free length since there would be no bond with the concrete and no dead weight to overcome when elongating the bar. Basically you would have to lift the wall to elongate the bar, or break the bond with concrete if the free length immediately followed the bonded length. I could also have the entire bar above the drill hole sheathed with plastic, but I don't think it is necessary.

This is what I had hoped to discuss; likely not presented well.

Thanks for your reply.

 

[aeoliantexan]

Why not design a reinforced concrete cantilever wall, either T-shaped or L-shaped? You can construct the footing with sleeves for the anchors; drill, grout, test, prestress, and lock off the anchors; then build the wall. Prestressed anchors increase the base sliding resistance and provide oveturning resistance for a relatively narrow footing with virtually no footing movement. If any anchors fail the testing, you can drill through the footing and add replacements.

An unbonded zone is necessary for prestressed anchors to accommodate movements during load transfer to the nut or wedges. It is crucial for strand anchors, because the wedges have to pull in far enough to bite the strands without loosing too much of the prestress load. Putting the unbonded zone within and below the footing gets the bonded zone deep, engaging a thick mass of rock. Putting the unbonded zone up in the wall seems like a waste of tendon length.

If your wall is to be close to the edge of the rock cut, be sure to consider the global stability of the wall, rock mass, and backfill. Consider the bearing capacity of the rock under the footing loaded by weight plus prestress force.