Helical Soil Anchors

[Sweever]

I am a structural engineer, not geotechnical, so some of my terminology may be off. I have project that involves the failure of a dry stack stone wall, approximately 9 feet high. The original drawings call for 6 helical soil anchors to stabilize an approximate 8 foot high soil elevation difference. The makeup of the wall is as follows, Sandy soil, 1x10 vertical wood planking, geotextile fabric, and then a wire mesh (though the existing conditions show a poly fabric mesh has been substituted), six soil anchors (helical with 18"x18" plate) spaced (3 approximately 2 feet down from the top of the soil and then 3 more 5 feet below the upper row), then the drystack wall.

Based on this, I would stay the drystack wall is not intended to retain soil. It should also be noted that the 1x10 wood framing is rotten (after 7 years of installation)

The upper row of soil anchors appears to have dropped approximately 3.5-4 feet lower than on the original design drawings and in two locations, the anchors appear to have pulled out 4-7" from the soil.

I have two questions,
1., is it normal to have this type of configuration to retain soil? Seems odd there is not some kind of structural wall or framing that the helical anchors tie into to retain the soil. Surely the wire mesh or poly mesh is not meant to retain the soil?
2. Would excessive rain (which happened the day before the collapse) cause the soil anchors to loose their structural integrity/capacity?

Thanks

 

[JAE]

How can 6 steel plates, poly mesh and geofabric hold back the soil?

OK - go re-read what I posted in the sixth post above. These very well could be [red]SOIL NAILS[/red] and not tie-backs.

With soil nails you don't need to have a large plate holding back a STRUCTURAL WALL.
The nails tie the ground together across the failure plane deep into the ground - some distance behind the front face of the wall.

I'm suggesting this because:
1. There is no structural wall in front
2. The "ties" are not attached to anything structural
3. The stone facing was not attached to or part of the "wall" system.
4. There are no big holes in the front wall facing

Basically it is an awkward type of geogrid reinforced wall...only using discete nails insteal of geogrid.

What might have happened is:
1. Over time the nails sagged out a bit
2. Or dirt material sluffed between the wood boards and the stone facing pushing the facing outward,
3. Or water possibly got in there and deteriorated the base soil or footing support of the stone facing such that it became unstable and collapsed.

 

[r13]

Do you see settlement or cracks on ground retained by the wall? I guess the anchors were no long enough to pass the potential failure zone (remember slope stability?), and shear failure of the retained soil has pushed the anchors and the stone facing out. Also, if rain water on the higher ground wasn't restricted, or able to found way to get behind the stone facing, surely it contributed to the problem.

 

[Sweever]

JAE, I appreciate your pointing out the difference. Unfortunately I don't really know if they are soil nails or ties. I am leaning towards soil ties as the drawings reference helical piles and indicate "chance" or approved equivalent. They also indicate "set anchor by torque, secure plate tight to slope, and load testing other than by torque is not required".

I should also not that the soil behind the wall does not visibly appear to have moved significantly outward, though I dont know if it was originally back sloped and is now vertical. The anchors themselves appear to have let go and moved away from the slope. Within two of the anchors, the plates now appear to be located where the stone wall would have been located. Therefore to me, the helical ties have released in tension, pushed out the base of the stone wall and caused the collapse.

What I would also like to indicate to my client is that the original soil stabilization design may have been flawed from the beginning. However, it sounds like if these are soil nails, this may not be a true statement? Or does the original configuration still not make a lot of sense. I would think with soil nails you would still need some structural wall? I just cant believe that anyone would think vertical 1x10 planking would be considered an adequate structural wall. An if they did, would you not need to have very tight spacing of the nails? every 12-16"?

Also I should note that the wall is a drystack stone wall, so water behind the stone wall should be able to easily drain out from the gaps int the stone. It did appear that there also might be a 2"-3" cavity of clear stone between the stone wall and the geofabric.

I did not see any cracks in the soil. I did go the upper property behind the wall in question and there was a noticeable, but only slight drop in the grade nearest to the wall which may indicate that the embankment is moving outward.

 

[r13]

1. From structural point of view, the soil anchor and 1x10 plank system seems weak, but not necessarily geotechnically. I am thinking "arching effect", but not sure it is applicable to this case, geotechnical guys can enlighten me here.
2. Even reinforced concrete retaining wall requires/recommended to have slopping face to ensure the plumbness. It is difficult for me to think the wall and stone facing were built vertically from the beginning. Did you ask the owner the reason for previous repair?
3. Yes, water can freely drain out from the stone joints, but given a long time of exposure to free water flowing behind the stone, on the way down, the water will erode materials behind the wall, evidently, the rodded wood. Also, the soil looks like been washed away from the anchors that seem protrude from the wall.

 

[JAE]

Sweever,
Understand your points. Here (attached) is the article outlining the difference. Even if they used AB Chance "anchors" they may have been nails (vs. ties). I guess the fact that there isn't a structural wall entity and your anchors appear to be simply holding back nothing but dirt leads me to nails - but perhaps I'm assuming the original designers knew what the heck they were doing.

 

[Sweever]

JAE- Thanks for the information.

 

[ozPE]

i have no clue about your wall, soil or how the helical anchors are connected etc... in general, although helical anchor suppliers publish torque-capacity correlation, you should not rely on them for structural applications, the correlation data should be used as an approximate guidance only. a static load test should be done to verify capacity. for a wall like this load test would probably be too expensive. plus this wall has a different failure mechanism than vertical helical anchors, even a load test may not be adequate.

 

[ozPE]

retired13,
if you don't mind me commenting, the arching effect happens best behind soldier pile-wood lagging walls. with the soil pressure the wood lagging deflects thus redistributing some of the pressure on them to the more rigid steel piles. soil arch develops between the steel piles.

 

[r13]

ozgurPE,

All comments are welcome, though I know some basics, soil behavior remains a big puzzle to me. Thanks, I shall look into it deeper.