Minimum Earth Cover Above Grouted Tie Back for New Retaining Wall 1

[Beanbag1]

Hello,
I am designing a new steel sheet pile (SSP) retaining wall with grouted tie-backs.
I plan to design the SSP as a cantilevered wall up to a certain height, backfill up to that height, install tie-backs 3-feet below that certain height, test the tie-backs, and then finish the backfilling operation up to the final finish wall height.
QUESTION: Is 3-feet of backfill above the tie-backs a sufficient height of overburden for testing the tie-backs? I am concerned about the wall failing inward as the pull-test apparatus pushes back on the wall. If 3-feet is not enough, what is the minimum height of earth required in order to test the tie-backs?
Thank you.

 

[PEinc]

Tiebacks are usually installed at approximately 10 to 45 degrees below horizontal. So, your anchors' bond lengths should be much more than 3 feet below that temporary (or original) ground surface. Increase your angle enough to provide a minimum of 10 to 15 feet of overburden above the anchor's bonded length. More depth may be required when performing high pressure grouting in order to prevent heaving the ground surface. Another consideration is checking that the height of soil behind the wall and above the tieback anchor provides sufficient passive resistance to prevent failing the wall and/or retained soils when testing the tieback anchor.

http://www.PeirceEngineering.com

 

[Beanbag1]

Thank you PEinc. I do plan on installing them on a 30 deg angle with the bond zone likely at least 30 feet in from the wall, so that should get me at least 15 feet of earth above the bonded zone (with wall installation point 3-feet below the surface). However, I hadn't considered the passive pressure resistance component, which now has me worried. I've seen pics of contractors installing tie backs with only a few feet of earth above them (at the wall location), so now I wonder if they installed some sort of temporary bracing during the testing process.

 

[PEinc]

The SSP will distribute the tieback test load and design load over a height of soil that is more than the height from top of wall to temporary subgrade elevation while installing and testing the tiebacks. There would be more concern if you were using soldier beams instead of SSP.
Most SSP walls are top-down construction where the SSP is driven from about original grade and then an excavation is made in front of the SSP. You seem to be describing bottom-up construction. Check to see if a tiedback SSP wall is the most economical type of wall. Do you have room to build a cheaper, gravity wall such as an MSE wall or a conventional concrete wall?

http://www.PeirceEngineering.com

 

[Beanbag1]

Very interesting. Thank you much for that clarification and question. Yes, I think I may look into other options.

 

[Panars]

I agree with PEinc that another wall type is probably more economical for a fill wall. An MSE wall is usually cheapest, but there are applications where you may not want to use them for other reasons (for example in a waterfront location.)
If you decide that the anchored sheet pile wall is still the economical choice for the fill wall, you should also look at using a deadman anchor rather than a grouted anchor. In the unusual case where I am designing an anchored wall in a fill situation, the deadman is usually cheaper because it can be installed by the general contractor. The grouted anchor typically requires a specialty subcontractor.

 

[PEinc]

Good response by Panars. Also consider if temporary sheeting would be required for building an alternate wall. Consider how much room you have behind the new wall for temporary sheeting or excavating for deadmen and tie rods.

http://www.PeirceEngineering.com