Utility Pole Causing Entire Length of Wall to Fail? 1

[psychedomination]

Hi there,

I am dealing with a project where a utility company has a complaint from a homeowner that a utility pole with a stay cable installed near their boundary wall caused the entire length of it to fail.

Some context: the existing wall is not a proper retaining wall and is approximately 8' tall. It has no foundation but is built on rock; it is an old stone wall that is only connected with mortar, see below :

The homeowner hired an engineer to visit the site, who came to the conclusion that the entire stretch of wall (atleast 40’) is failing solely as a result of the utility pole, with the reasoning that due to the pole leaning at the same angle of the wall, it is the sole culprit.

I’m not sure I fully agree with this, although the pole may have contributed partially, I would have thought that this would be mainly a local effect and not cause the entire 40’ run of the wall to fail. The wall is only connected with weak mortar joints, so if the pole rotated, causing the earth to push against the wall, I would have thought it would be a local failure (crack along the mortar joints). I would've also expected to see some heaving or other evidence of movement in the area, which I didn't see.

The utility pole is anchored into the earth with a bust expanding anchor, see typical image below :

With this said, take for instance that the bust expanding anchor is a 2’ square. What would the zone of influence be for the impact that this would have on the earth. Would it be correct to assume a 45 degree influence line? See sketch in plan?

There is a lot of vegetation/trees etc on the top of the wall, I’m thinking this is more likely the culprit to the wall starting to overturn. Normal rotation of the wall should be normal for active earth pressure but I guess this is starting to pass the reasonable rotation amount. Sorry for the long post but wanted to include all relevant information, any thoughts would be helpful.

 

[XR250]

I agree that is "may" have some local influence. That wall was destined to fail regardless (at least the straight sections).

 

[IRstuff]

How old are the bushes? Seems to me that it could more likely that the root structures are pushing the wall, given the initial pull from the anchor moving the nearest section of wall outward.

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[dik]

for a gravity wall, it looks awfully thin for 8' high... the hydro pole hasn't helped... are there any photographs of it earlier?

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[psychedomination]

@XR250 Yes, the wall was definitely going to fail. It seems to me that the client is trying to get the utility company to fully replace it for them. This is why I am trying to ascertain the influence zone that the pole could have impacted.

@IRstuff Unfortunately not much historical information has been provided. Essentially the utility company used to have a pole embedded in the wall. This ended up causing the wall to break locally (perhaps a 4' wide section), which the utility company replaced. The utility company installed the new pole in the new position shown in the OT. I agree that the bushes seem to be the most likely culprit. Proving this with the information given is the tricky part.

@Dik I fully agree, this wall was definitely not built to the local building code but I assume this wall may potentially predate it. Unfortunately, I don't have any before photos. That would make this process a lot easier. I'll note the lack of before and after photos in my report. It's quite difficult to provide a definitive answer without them.

 

[KootK]

1) With respect to the proximate cause of the wall rotation, I agree with the homeowner and imagine that it went something like this:

a) That pole stay was put there to resist some tension on the other side.

b) The stay anchor is obviously pushing against the wall locally and probably caused it to rotate locally.

c) Because the wall lacks stability in general, local overturning begot global-ish overturning in sort of a lateral version of a P-delta moment amplification.

2) The wall lacking robust stability is, obviously, a pre-existing fault of whomever designed and/or built the wall.

 

[XR250]

I agree that the bushes seem to be the most likely culprit. Proving this with the information given is the tricky part.

My experience is that the roots from bushes tend to reinforce the soil mass and help maintain stability. It is the roots of large trees that seem to be an issue.

 

[hokie66]

The effect of the pole on the wall is nonsensical. That wall, like millions of other poorly constructed walls, was destined to fail. It took the pole with it. I would question the integrity of the engineer who said the pole had caused the wall to fail.

 

[Retrograde]

with the reasoning that due to the pole leaning at the same angle of the wall

To me that says that the wall failing has caused the pole to lean. I agree with hokie66.

 

[canwesteng]

Finally we've got a structural version of the chicken and the egg problem... did the pole cause the wall to fail or the wall cause the pole to fail? Any photos of the wall before the utility pole? Because I would be inclined to believe it would be tilting even before the pole was in place.

 

[Settingsun]

Option 3 is the pole was installed to match the lean of the wall, to avoid the base encroaching upon the path.

 

[LittleInch]

Where is the soil level behind the wall?, I.e. a cross section

Also it looks like there is a slope looking along that photo?

The pole may be the straw that broke the wall but it looks rather weak to me. Go 50:50 to replace it cost.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Ussuri]

The pole, along with the tension stay, look to me like they are actually holding the wall up.

 

[GeorgeTheCivilEngineer]

Presumably this is a civil matter so they need to prove it's been caused 'on the balances of probabilities'.

From the very limited information available, I find it unlikely that the anchor would have caused failure well away from the direct zone of influence of the anchor. Masonry is relatively flexible along long lengths so if it was the anchor you'd expect leaning near the pole and the wall near the camera to be vertical. Therefore it is equally plausible that the wall shifted, causing the anchor to move and making the pole lean. This would also explain why the entire length of the wall is leaning outwards a similar amount.

But hey.... energy companies should be making bank at the moment.

Edit - I think you;'d need more information of the construction of the wall near the pole and away from the pole to be able to draw any further conclusions.

 

[psychedomination]

Thanks for the responses so far everyone. They are very helpful.

The homeowner hasn't shared any before photos, I'm sure he would be reluctant to share(if it shows evidence of the wall leaning prior to the pole installation), as it appears they are trying to get the utility company to replace the entire wall.

@LittleInch the soil is two feet below the top of the wall. Soil height from road level ends up being around 8'. The wall is sloping down a hill. It starts off at like 4' then gets deeper to 8' as it gets closer to the pole.

Yes, perhaps it would be reasonable for the utility company to split some of the repair costs but that is why I was trying to figure out what would be a reasonable influence line for the pole forces pushing against the wall. The utility company could have fixed whatever that length is, and let the homeowner repair the rest.

 

[EcoGen]

Interesting question OP and thanks for the well written post with accompanying sketches! A couple comments/questions that I have:

1) What does the base of the pole look like close up? If it was installed plumb and rotated afterwards, there may be signs of cracking in the pavement surrounding its base. Also is the wall actually touching the pole or not? The photo makes it look like there is a gap.

2) What degree tilt do you have at the influence zone versus another location of the wall like 50 to 100 ft away? If angle of tilt is similar, then maybe the effect of the pole could be considered negligible on tilt.

3) What kind of tension forces in the cable anchor are we talking about here? If you calculate the horizontal component and then try to distribute it over the wall influence area, what kind of surcharge psf do you get? If this surcharge is much smaller than the the 8 ft deep soil pressure (you can assume 30 to 45 psf/ft), there is less reason to believe the pole is doing much to the already inadequate wall.

4) Also if the retaining wall didn't even exist, the huge 7 or 8 ft cube mass of soil by itself may even be enough to take the horizontal anchor load. Not sure how you would go about checking the soil stability for that purpose though.

EcoGen Consultants LLC
Structural Engineers
ecogenconsultants.com

 

[BridgeSmith]

The pole, along with the tension stay, look to me like they are actually holding the wall up.

That's what I thought. Unless it's an optical illusion, it looks like the wall is leaning on the pole. If you haven't already, the first thing I would suggest doing is putting a level on the pole and then 8 or 10 places along the wall, and see which way it's out of plumb, and by how much.

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