Latent Rock Anchors

[Abruzzese]

I work for a heavy/ highway contractor. We are bidding a job with an anchored soldier pile/ precast lagging wall. The designer has termed the rock anchors as "latent anchors". What is meant by "latent" in this context? Thanks.

 

[PEinc]

I am in PA and have been designing and building anchored walls in many states since 1975. I never hear of a "latent" anchor. Possibly they mean "passive" anchors which are not pre-stressed, such as soil nails?

http://www.PeirceEngineering.com

 

[Abruzzese]

No, they are definitely pre-stressed; locked off at 100% of Design Load.

 

[PEinc]

Anything on the plans that make them look latent? Ask a pre-bid question? Please let me know what you find out.

http://www.PeirceEngineering.com

 

[Abruzzese]

I think I figured it out.
Attached is the installation sequence. The permanent casing is socketed 30 ft. into rock. As shown in the anchor head detail, the casing is welded directed to the bearing plate, i.e. no "trumpet" is used. The strand anchor is pre-stressed against the casing, not the soldier pile. Therefore it provides a "latent", or hidden, tieback force against the wall when it is fully backfilled and loaded.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1706101330/tips/Latent_Anchor_Drawing_jboaxf.pdf[/url]

 

[PEinc]

Abruzzese, from the details you attached, the anchor looks like a micropile, probably designed for the same load in compression and tension. It looks to me like they may be trying to avoid cracking the precast lagging when the anchors are tested and locked off. Is this a fill wall, a cut wall, or a combination of both? I would like to see the lagging details if this is fully or partially a cut wall. Is there also temporary but left-in-place timber lagging to allow stacking of the precast lagging? Anchored precast lagging walls, especially tall walls, have been historically problematic unless the precast lagging is set (stacked) from subgrade up, after the excavation, tiebacks, and timber lagging are completed.
Where in PA are you and the project?

http://www.PeirceEngineering.com

 

[MTNClimber]

Interesting! I haven't seen that one before.

Anchored precast lagging walls, especially tall walls, have been historically problematic unless the precast lagging is set (stacked) from subgrade up, after the excavation, tiebacks, and timber lagging are completed.

How does a contractor try to hang precast lagging? Seems like a recipe for disaster.

 

[Abruzzese]

Thanks for the input guys/gals, the job is on Mount Washington in Pittsburgh. I think you're right, they want to avoid cracking the precast lagging.
Here are a few more details.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1706110889/tips/Latent_Anchor_Drwgs_2_dp1y13.pdf[/url]

 

[PEinc]

MTNClimber and Abruzzese, The Pittsburgh area (mostly Busway projects) has built heavy duty, tiedback, soldier beam walls with very thick precast lagging for landslide areas since the early 1980's. I bid several of them and, despite being told my prices were good, I was an outsider and was never successful. The construction of these walls usually involved making steeply sloped, open cuts as deep as possible to install soldier beams and stack the precast lagging. They would backfill to the tieback level, install lots of cribbing above the backfill level, install and stress the tiebacks, and then continue backfilling. Then to finish the rest of the lower portion of the wall, they would excavate and pour cast-in-place concrete lagging because they could not install precast. Result: ugly walls, cracked lagging near the tiebacks, unsafe excavation in landslide areas, very expensive. Thankfully times have changed, but apparently not enough.

EDIT: Abruzzese, I looked at your details and they confirm my suspicions about loading the tiebacks without needing lots of cribbing and cracking the lagging. I am also surprised to see tieback anchors called for so close to the bottom of the wall. Your wall needs to be built from the bottom up.
If the casing pipe is supposed to prevent the soldier beam from pushing back under the tieback test and lock-off loads, what keeps the casing from plunging downward and crushing the top perimeter of the bonded length's grout? If the outside of the casing pipe is not grouted for frictional capacity, the tieback load would need to be resisted by bearing on top of the bond length grout. A factored(?) 172 kip test load on the bottom area of a 6.625" x 1.0" casing pipe = 9,738 psi = 172,000#/(5.625" x 3.14 x 1.0"). Seems like a pretty high stress on the grout.

http://www.PeirceEngineering.com

 

[Abruzzese]

PEInc, the designer's sequence states to drill an 8" diameter hole 30 ft into rock, place grout in the hole, and plunge a 6.625" O.D. casing into the grouted hole. I guess the intent is for this 30 ft rock socket to resist the 172 kips. Remember, I'm just the 'dumb' contractor.

 

[DanielSgeo]

This is really neat I’ve never seen anything like this in the west.

 

[PEinc]

Abruzzese, the X-X (bonded) and Y-Y (unbonded) anchor details do not show any unbonded length for the anchor. Both details show bare strands grouted inside the corrugated sheath. There are no greased and sheathed, unbonded portions for the strands or for the corrugated sheath. Therefore, when jacking the strands, the jack pushes directly on top of the casing pipe which cannot move. Therefore, you will not be pre-loading the wall. The jacking load should never get to the lower 40 feet "bonded" length. The bare strand is grouted to the inside of the corrugated sheath which is grouted to the inside, smooth face of the casing pipe. When you stress the anchor all you are doing is stressing the bond between the encased strands and the smooth casing pipe. That is, you are trying to pull bonded, encased strands out of the casing pipe. Check the bond capacity of the strands' grouted, corrugated sheath to the smooth casing pipe for the anchor's length from the top of casing pipe under the jack to the top of 40' uncased "bonded" length. You have more than 30 feet of strand bonded inside a casing pipe with no unbonded length. The bond in psi between the grout and smooth casing pipe is much less than the bond in psi between the grout and the rock but you have significant, grout to casing, bonding area between the wall and the top of the 40 feet long uncased bond zone. When the completed wall tries to overturn or slide, the anchors will most likely act as soil nails bonded partially in soil and partially in 30 feet of rock (hence latent?). The extra 40' of rock bond seems to be overkill. Good intentions, bad design. Is this a PennDOT job? ECMS number?

http://www.PeirceEngineering.com

 

[Abruzzese]

PEInc...I see what you're saying, but I think it's a misprint on the call-out on the Unbonded Zone Detail where it says "bare strand". The strands in the unbonded zone have to be sheathed in plastic. That's the way the suppliers priced it.
Agreed, the 40 ft. bond zone is way overkill, especially for these light design loads.
It is a City of Pittsburgh job. Going to the bid opening now, wish me luck!