Anchor Rods 3

[DaveVikingPE]

What's the most common practice for applying tension during construction of an anchored sheet pile wall (waterfront bulkhead)? Anchorage is a steel anchor rod, with end nuts, attached to a concrete deadman.

Is it: a) tighten the end nut at the wale? b) tension the anchor rods by means of a turnbuckle somewhere between the wall and the deadman? or c)who cares because the wall isn't going to move that much.

I say it's "b" - use the turnbuckle.

 

[geomo]

Your anchored bulkhead is similar to temporary excavation support systems that also utilize tiebacks and sheetpiles. In those projects the standard practice is to apply a load, usually with a hydraulic jack, at the connection of the tieback to the sheetpiles. The deflection vs. anchor load is carefully measured (to 0.001 inch using a dial gauge) for load increments up to a specified load that is often 1.25 to 1.5 times greater than the design load. At that ultimate load the deflection vs. time is measured to test the performance of the anchor under sustained loading. Often the anchor is then unloaded and reloaded several times to further test anchor performance. Each anchor is tested at least once to the ultimate load and then the load is reduced to 0.75 times the design load and the anchor is "locked off".

The general testing procedure might be similar for anchored sheet pile bulkheads, but in lieu of a hydraulic jack the turnbuckle could be used with strain gauges on the anchor rod. You would also need to measure deflection of the sheet piles and possibly the deadman.

The need for such intensive testing is that, even when many anchors are used, the whole retaining system might not have enough redundancy to withstand failure of a single anchor. If a single anchor fails the whole bulkhead system can "unzip." Their are different degrees of failure, but what may seem to be insignificant movement could cause reorientation of loads thereby overstressing critical elements of the system.

You could overdesign the anchors with a higher margin of safety and feel better about not testing, but the cost of the overdesigned system might far exceed the expense of a thorough testing program.

 

[DaveVikingPE]

Thanks, geomo!

Question(s): assuming "tiebacks" mean grouted soil or rock or similar anchors, my understanding is that tension is applied at the sheet pile/tieback connection for these kinds of anchors as there's no place for a turnbuckle, really. In my case, the steel anchor rod's connection to the sheet pile is by an end nut and a plate (washer). The only way to apply measured tension to the anchor rod is by a turning the end nut with a torque wrench or similar tool.

In my case, and here's more information, I'm in an argument with the designer who's challenged my assertion that a turnbuckle at the midpoint between deadman and sheet pile wall is the proper way to go. He believes the all that's required is the end nut to which enough tension can be applied - by visual observation alone. He also asked me "...do you think the wall will move that much...?" He design load is 20 kips, how he's going to get that much without poper measurement is beyond me...

 

[ishvaaag]

PTI indicates for anchors irrespective of their strand or bar nature that "load shall be measured with a pressure gauge calibrated with the jack and accurate enough to read 100 psi changes in pressure". Hence it seems on the load part it seems to totally be unreliant on elongation measures; a jack (locked by screw, maybe) will be the way to apply tension to HS bars, and the nut only will be locking the measured force, hence tension.

 

[DaveVikingPE]

How does one attach a jack to apply the tension in the case I've described? I found good information (i.e., installation photos) at

http://www.earthanchor.com,

but it's related to their Manta Ray product, a driven anchorage system, rather than the two-piece anchor rod I've described. What I've found on other sites (e.g., the Corus Geotechnical site) is that postensioning the anchorage is by tightening the turnbuckle. Any recommended books, web sites with photographs of anchored sheet pile bulkhead construciton would be most appreciated.

 

[ishvaaag]

You can use a screw zone as long as required and 2 nuts preplaced on it. The external nut would hold the tension and the grip of the jack and needs not to rotate. Then the other may be used to lock the tension by rotating and adjusting it tightly against the bearing plate.

 

[DaveVikingPE]

Thanks, ishvaaag!

I'm not sure I understand that, though. Are you saying that the jack would apply tension to the anchor rod at the sheet pile wall? Would a turnbuckle even be used, then?

 

[ishvaaag]

Well, normally the plate would rest on the most external structural device able to sustain both the plate and the distribution of forces against the being compressed contained earth. Gyrating something (turnbuckle) to produce tension between plate and anchor will work as well if you have from where to proceed that way, and you can still measure tension either from a pressure gauge device at the plate as mandated by PTI -a rare device since interposed under the nut to measure the whole force- or maybe by use of Hooke's law on something over the HS bar or the turnbuckle position -which separates from what PTI asks for. So it is clear that PTI wants the applied tension, if not the locked, measured by a gauged jack.

 

[DRC1]

Tie rods & deadmen are typically used for walls without great height. Tie rods do not typically carry the same loads as gouted anchors, therfore the elongation is less.Deadmen tie rod systems are typically tensioned at the wale, but ussually to no specific value. You can compute the tension on the tie rod and compute the elongation. If the elongation will be enough to cause more movement of the wall than you would like, you can prior to backfilling, set the sheets with perhaps 75% of the computed deflection closer to the deadmen than your anticipated deflection. When loaded, the sheets will deflect close to the design location. However, I suspect you will find the deflection is not significant enough to worry about. Typically we drive the sheets install the deadmen and tie rods put the wale on and install the nuts, and if it si backfilled to the leve of the wale, draw the nuts up tight.
Good Luck!