Guy Anchors / Passive Resistance in Uncontrolled Fill

[JDvmi03]

Hey folks. Just like the title says:
I have a large site with a field of several feet of uncontrolled fill. You get a tree trunk here and there, occasional bricks/debris, large chunks of concrete, but mostly a fine SP.
My problem is there are a bunch of deadman guy anchors in that fill, and they've been there for at least 8 years, so I didn't get a great reaction when saying that it shouldn't be relied upon for foundation support.

Does anyone have any suggestions on shear strength parameters that can be used for analyzing lateral/passive resistance for guy anchors?

I included a photo from test pits. This is the best overall representation I could find.

 

[thaidavid40]

Don't know about applicable design parameters you might use, but looking at the photo, and reading your description, I personally wouldn't use anything greater than the weight of the fill within the volume of the helices. I wouldn't trust in any form of stress distribution or dispersal beyond the diameter of the helices. Just a personal, conservative opinion - that's all.
Dave

Thaidavid

 

[TLHS]

That's crazily conservative, though. Even in incredibly loose sand, the distribution of load becomes the most significant part of a pullout resistance.

I'd probably treat it like a reasonably loose material, see if the numbers work with a conservative safety factor. If it doesn't, do a pull-test on the anchor. Most contractors who install guy anchors are setup to do a test like that for verification.

 

[Lomarandil]

Here's an ambiguous anecdote for you, but on a transmission line project I was involved with years ago, we specified helical guy anchors in soil which was described to us by the geotech report as being essentially useless. We accordingly designed the anchors pretty conservatively.

We had a pretty upset contractor when he was unable to install the helical anchors as designed due to unexpectedly high resistance.

Possible moral of the story is to agree with TLHS -- a pull test may be your best option.

 

[JDvmi03]

I agree that a pull test of some sort would be ideal. However in the sector I work within, there rarely is the schedule or budget for a pull test. Generally we're looking at SPT testing almost exclusively.

I'm now looking at a different site now with a guyed tower (single tall mast and 3 anchors at 120 degrees radially), and there is about 20' of uncontrolled fill at the site and each of the anchors and the mast are shallow drilled shafts bearing within the fill. N-values within the fill range from 0 - 50bpf, mostly on the low side.

Intuitively the tower has been standing for years, and there's some phi and/or cohesion that is providing lateral-passive resistance for the drilled shaft guy anchors.

Undocumented/uncontrolled fill has become one of the most difficult issues I deal with at a multitude of sites.

 

[L0k]

There is no representative shear strength in the uncontrolled fill.
I do a conservative approach to remove all the non engineered fill, then backfill with suitable material & compacted in maximum 200mm lift.