Tiebacks

[Geotechnical76]

We are designing a wall with sheetpiles and tiebacks (anchored at both ends to sheetpiles). Another feature that is added to the site will be a new concrete bleacher with the bottom of footings about 4 feet from the ground surface. The anchors in the original design are placed about 2 feet from the ground surface. My question is how can we protect the tiebacks from sagging? Loads from these concrete blocks above the tiebacks will add additional stresses. One option is to use rock anchors by installing the tiebacks without sheetpiles at the other end and grout them in rock or soil. The other option is to use a concrete conduit and run the tiebacks through them. The first option will be expensive, while the second one will be cheaper. Does anyone have expereince with such siituations? How to calculate the additional stresses in the tiebacks? Any practical problems from usning the concrete conduits?
Thanks

 

[craigmcg]

As long as the soil is properly compacted and settlement isn't an issue, I wouldn't expect the stress increase to be that considerable. Normally a safety factor of 1.3 is incorporated into tieback stresses to account for such residual stresses. If you're really concerned, easiest way might be to simply increase the safety factor and tierod size accordingly.

Of course, all of this is dependent on the type of soil, magnitude of footing and loads, etc.

 

[PEinc]

If I am using thru-ties or tie rods with deadmen in any type of backfill, trench backfill, or embankment; I always install the tie rods in a 4" or 6" diameter, ungrouted, pipe sleeve where the tie rod rests on or near the invert of the pipe sleeve. Then, if the ground settles, the pipe will move downward but not enough to bend the tie rod.

There are relatively few documented cases of tiebck failures. However, unintended bending of tie rods (in addition to tension) is one of the leading causes of tieback tendon failure. I would not install a footing on backfill, or in an embankment, over an unprotected tie rod. If you know that a tie rod will be bent, it should be designed for the combimed stresses of axial tension and bending. However, I would rather prevent the bending than design for it.

 

[jdonville]

To summarize, you are designing back-to-back retaining walls (in a fill situation) that will act as the foundation for another future structure. Your initial concept is to use sheetpiles with some sort of anchor - whether through or grouted.

Have you considered using a back-to-back MSE structure to support the bleacher?

Jeff