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Helical Soil Anchors

Helical Soil Anchors

Helical Soil Anchors

(OP)
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


RE: Helical Soil Anchors

Sounds to me that the soil anchors were to simulate tie backs or dead men. May not have been extended back far enough beyond the failure plane to develop the passive pressure needed. Also, the diameter of the blades may not have been large enough to develop the needed lateral resistance. This scenario is not normal in my opinion.

The 2x10’s should have been pressure treated. I used PT 2x6’s on my wall at home and they are still in good condition after 30 years.

Yes, excessive rain could contribute too.

Mike McCann, PE, SE (WA, HI)


RE: Helical Soil Anchors

Sounds pretty sick. If any help from us, a photo or two might help in describing the insinuation.

RE: Helical Soil Anchors

1. Would need to see a picture but doesn't sound normal.
2. Hydro-static pressure from excessive rain is likely to have caused or contributed to the collapse especially if there is no back drain present.

RE: Helical Soil Anchors

These anchors are intended for mobile home tie down anchorage ... usually 600 to 1200# capacity, and not much depth of purchase.

Mike McCann, PE, SE (WA, HI)


RE: Helical Soil Anchors

You might check to see if these are helical "TIES" or helical "SOIL NAILS".

I just read an article on this - the TIES are usually used with a single or double helix at the end of the system, extending back beyond the assumed soil failure plane and typically tie the dirt beyond the plane to the face of a structural wall facing.

SOIL NAILS are typically long bars or pipes with multiple smaller helixes along their length. The idea here is that the nail extends across the failure plane and ties the two sides of the soil together so that the slippage along the plane is prevented. The nails have opposite directing tension on each side of the plan and are not intended to "hold back" a structural wall. In fact, they can be used with a simple shotcrete wall system or perhaps something like what you describe.

I'm not sure what to say about the drop in the anchors or the pull out. Just found the distinction described above as possibly helpful.

RE: Helical Soil Anchors

How the hell are we meant to understand the configuration of the wall when you just listed a load of items and no discussion on how they are placed or connected. We need a sketch or photo.

Also, I would love to see how the "anchors" are connected to the dry stack stone wall. In my experience you would need a very large plate on the front of the wall to spread the load. Otherwise the wall will pull off the anchor.

And a dry stack stone wall should NEVER be used as a retaining wall. There is no way of really assessing its structural integrity.

RE: Helical Soil Anchors

Love the intensity EireChch.

Agree- although the details are lacking from the OP, the system sounds slap-dash.

1. This is not a normal configuration- dry stacked stone is frowned upon these days and is not a predictable building medium.
2. The anchorage zone of your soil nails should be into moisture-stable material, so no, not if the nails were designed properly...they are almost certainly too short judging by the movement described.

All the best,
Mike

RE: Helical Soil Anchors

I don't like helical tie-backs and have never seen them keep tension over time.

Yes, water is likely a problem.

Yes, settlement is likely a problem.

Unlikely that the, "Design" was fully thought out.

Not something, I'd repair in like kind.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Helical Soil Anchors

Agree with most of the other respondents: doesn't sound standard or well-thought-out.

RE: Helical Soil Anchors

(OP)
My apologies for the lack of details, I was running off to a site. I have attached a photo below. Some clarifications.

The drystack wall does not appear to be retaining in any fashion. Its just a stone wall infront of an embankment that appears to have been stabillized with helical ties/anchors. The tie backs are not attached to the stone wall in any way. See picture below which shows the metal plates for the helical anchors. You can see how they are displaced vertically and have rotated.

The configuration and order of the components of the wall is as I described above and hopefully will be more clear with the photo.

I dont know anything about the helical anchors except that they have a 18x18x3/4" plate attached to them and the drawings indicate they should be torqued to achieve the design load. No indication of minimum depth of installation



RE: Helical Soil Anchors

Unfortunately, the photo doesn't help explain very much. How do you know the wall is failing? I see a pile of soil and some big rocks in front of the wall but don't see any clear evidence of wall failure. Maybe the failure is hidden behind the black fabric? Have wall movement measurements been made? Maybe the wall just looks ugly and was built ugly.

Helical anchors cannot be installed through rock without pre-drilling or coring holes. These anchors may be small diameter anchors that were installed wherever they could get the anchors in between the rocks. There may not have been any anchor displacements. The anchors may have never been installed in the locations shown on the original plans. The large 18 inch plates are used to span across any space between the rocks so that the anchors are bearing on and laterally supporting the adjacent rocks. Someone may have hung some geo-textile filter fabric and what looks like geo-grids to prevent soil from falling out from behind the rocks and through the spaces between the rocks.

it would be helpful to know if there are any inspection reports or testing reports from the anchor installation. the anchors

www.PeirceEngineering.com

RE: Helical Soil Anchors

You mention having the plans available, what was the purpose of those plans? Is this just a small section that was built like this due to a previous failure, or is the entire wall constructed in this manner?

RE: Helical Soil Anchors

(OP)
PEinc I don’t really understand your post. I know the wall has failed because the dry stack stone is lying on the ground in front. You can see the remaining stone wall in both pictures.

Also I mentioned in both my posts that the anchors don’t tie back the stone wall they appear to tie back the soil and the stone wall just sits out front. the way in which the soil was stabilize behind the stone wall does not make sense to me. How can 6 steel plates, poly mesh and geofabric hold back the soil?

Csigeo. Yes this was done to locally repair the wall that was bulging about 7 years ago

RE: Helical Soil Anchors

This situation reminds me of a Homeless Camp...

It looks really raunchy (fabulous new structural term here!).

I see what seems to be a structure beyond. Is there surcharge loading or trees just above the "wall", and I use the term loosely here?

Those plates look like the type used in soil nailing, but these should be combined with shotcrete and rebar, not a jumbled rock mass.

Mike McCann, PE, SE (WA, HI)


RE: Helical Soil Anchors

Ok so to me it looks like they tried to repair the previous failure using soil nailing and wood planks and then just matched the face of the wall without connecting anything. Were you able to see if there were large voids behind the wood planks anywhere? To me it looks like the repair may be holding but that the dry stack portion failed (probably from hydrostatic pressure from the recent rains) due to an insufficient design. It should have been shotcreted like msquared mentioned or tied in somehow.

RE: Helical Soil Anchors

(OP)
Yes this is what I am saying. It appears that they are using the 1x10 vertical wood planking as their « structural wall » in which the soil nails /anchors are attached to. There is no whaler to tie this vertical wood planking together. It makes little sense structurally how they were holding back the soil and the drawings were stamped by an engineer. I didn’t know if I was missing something and if this type of soil retaining system is common.

I did not see any large voids behind the wood planking though it was difficult to see. What really stood out was how the upper soil anchors had move down 3 feet or do and that they appeared to have pulled away from direct contact with the soil Would it be unusual for a contractor to install the anchors so dimensionally off from the drawing?

Above the wall is a wooden fence and yard. There is a tree maybe 25 feet from the wall on the upper property

RE: Helical Soil Anchors

Sweever, from the photos (without blowing them up), it looks to me like some of the 18" plates are bearing against stones and or gravel. Maybe I am wrong. Maybe what looks like stone to me is actually the boards you described. I really can't tell from the photos. The gravel under the 18" plates could be packing to provide full bearing beneath the steel plates that are not parallel with or bearing directly against the stones or boards. If there are no stones remaining around the anchors, the anchors could easily have bent downward a few feet due to additional wall collapse. In any case, if I were trying to fix this "fugly" looking wall, I would consider first thickly shotcreting the front of whatever is still standing there and then, starting from near the top of the "wall," install weep holes through the shotcrete and some drilled and grouted soil nails. (Yes, these would be more expensive than helical anchors, but how have the helical worked so far?) After most of the wall is shotcreted and nailed, I would clean up some of the soil and fallen stones at the base of the wall before shotcreting and nailing the remaining, lower portion of the wall. Consider shotcreting and nailing some of the remaining, leaning stone wall to the left of the failed area. I would not use more helical anchors. I have nothing against helical anchors. I have designed them for many projects.

www.PeirceEngineering.com

RE: Helical Soil Anchors

(OP)
I am not trying to fix the wall, just provide an explanation as to why it failed. To me the way the original engineer tried to retain the soil was bound to fail from the start.

RE: Helical Soil Anchors

The wall primitive restraining system provided did not provide a structurally adequate interwoven global retaining matrix to constrain the soil mass for the forces seen.

"Would it be unusual for a contractor to install the anchors so dimensionally off from the drawing?" Absolutely if it was not permitted or inspected...

Mike McCann, PE, SE (WA, HI)


RE: Helical Soil Anchors

Quote (Sweever)

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 SOIL NAILS 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.

RE: Helical Soil Anchors

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.

RE: Helical Soil Anchors

(OP)
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.

RE: Helical Soil Anchors

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.

RE: Helical Soil Anchors

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.

RE: Helical Soil Anchors

(OP)
JAE- Thanks for the information.

RE: Helical Soil Anchors

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.

RE: Helical Soil Anchors

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.

RE: Helical Soil Anchors

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.

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