Is this much mount footing really necessary?

boondox

Chief Engineer, RedNeckTech Industries
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I am going down to the high desert in San Diego county this week to help my step son get the footings ready for a 6kW ground mount array. Class c soil I think. They want 12 * 2' diameter, 3' 6" deep holes with 2 inch schedule 40 galvanized pipe for the footings. That comes out to 5+ yards of concrete. Seems like over kill to me. But it will be fun, we are renting a skid steer with an auger. Taking the van and plenty of beer. Should be good times. But jeez, that is a lot of concrete!
 

400bird

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Think of the surface area in the wind. I think it needs lots of heavy concrete to hold the array down to the ground when the wind gets behind it
 

boondox

Chief Engineer, RedNeckTech Industries
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Yeah, sure. Still seems like a lot to me.
 

400bird

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Yeah, I thought about ground mounts for a while. Until I realized I'd have to dig those holes by hand. No thanks.

I'm not going to second guess the engineer, but you could probably get away with half that concrete for 99% of the time. But, they need to plan for a bit more safety than that. 99% of the time still leaves 3 days of the year (365 x 0.01 = 3.65 days) where it might blow away.
 

LywWyr

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I've seen some short (11ft) decorative light poles , and airport wind socks with foundations like the ones you have. They have rebar cages and grounding conductors that are attached to the cage and metal poles for lightning protection. The depth and diameter of the excavations seem seem typical. Are you mixing on-site?
 

boondox

Chief Engineer, RedNeckTech Industries
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Yeah, I think the odds are that the panels would blow out way before the mount would pull out even with half the concrete. But we will do it as they call for.

We will be bringing in a cement truck. My poor little cement mixer would probably self destruct trying to mix that much. My back wouldn't like it.

Still figuring out the best way to set the verticals. Seems like putting together the bottom of the rack (Verticals and the two long horizontals) and then supporting it over the holes in place might be the way to go. Then you know it is all going to line up.
 

LywWyr

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That's too much for me to mix. Those days are gone. I would turn to dust and blow away.
On 12" diameter holes I like to make 2x4 wood wedges(1-1/2 - 6" thick ) and use them to pin the pole in the center of the hole. I use 3 per hole and it leaves adequate room to get the mud around. Great time to install lightning protection as well. 20 ft of #4 bare copper wound in a flat pancake and tossed in the bottom of the hole, then stubbed up out of the mud then bonded to the vertical pole above grade works good . I prefer this to pounding ground rods.
The foundations settling is more of a concern most the time.
Good luck.
 

niktak11

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How many panels? I did six 10" diameter 46" deep footings for my 4.5kW 12 panel array.
 

AndyRonLI

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So this is the difference between an engineered solution and a prescribed solution. The prescribed solution is one that is very conservative, and can be used without any sort of engineering verification. Thus these types of code "requirements" are generally just accepted. You could have hired a professional engineer, and had him look at the soil, the site and do some detailed calculations as to what is really required to meet code. It would likely involve less concrete, and possibly a different style of foundation. But generally the engineer will charge you for the work. If you save 10 or even 20% in concrete costs would that cove the fee? Minimizing concrete, might involve re-bar. It might involve a square hole, necessitating a backhoe rather than an auger. Sometimes, the simple but overkill solution, is really the way to go.
 

boondox

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@AndyRonLI I think you are exactly right. The county will accept it, the rack manufacturer is happy and no engineer was hired. You make very good points about simplicity VS complexity.

Still seems like a metric crapload of cement but I think you are spot on about why they call for it.
 

AndyRonLI

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Also remember that codes don't really care where things are. If those panels go lose in a storm in a suburban or urban environment, they would make quite a mess. In a rural environment the damage is likely confined to the owners property. So the assumed risk is always worst case. The rule gets written... Thou shall not fly away not as, thou shall not fly away and cause damage.
 

OzSolar

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Check out this racking from Osprey. I'd be curious to know what others think. I'm getting ready to order it for a 13KW ground mount array for the reasons you mentioned. It might blow over in the first wind but after talking with a buddy who's a retired civil engineer the concept was solid and one he used before.

Hard to believe the something the size of your fist can replace all that concrete but apparently it does. Here's a video that shows it.

 

AndyRonLI

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Check out this racking from Osprey. I'd be curious to know what others think. I'm getting ready to order it for a 13KW ground mount array for the reasons you mentioned. It might blow over in the first wind but after talking with a buddy who's a retired civil engineer the concept was solid and one he used before.

Hard to believe the something the size of your fist can replace all that concrete but apparently it does. Here's a video that shows it.

So it becomes a question of code. Does you locality simply recognize it? Or, as I suspect, would the need an engineers report. Notice the phrase about loading the cable to the value in the engineering requirements. No question there are a lot of clever ways to anchor stuff out there. But this may not be "allowed" as a DIY solution. Never heard where you couldn't pour your own sono tubes.
 

rcrracer

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Do you know for a fact it is physically possible to dig in Socal dirt using ordinary means? Usually the ground is hard as concrete. Does spinning your car tires on the dirt leave black rubber marks? Tires squeal when they spin?
 

Bluedog225

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Those earth anchors are pretty slick. I’ve used a couple. You can use a big rod and and engine hoist to set.

I wonder if you could weld a plate or bar on the bottom of those schedule 40s and ballast them with bags on concrete. Would be a lot easier and weight is key here.
 

boondox

Chief Engineer, RedNeckTech Industries
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Do you know for a fact it is physically possible to dig in Socal dirt using ordinary means? Usually the ground is hard as concrete. Does spinning your car tires on the dirt leave black rubber marks? Tires squeal when they spin?

Well, we will find out. It is fairly sandy, being in the high desert. We are renting a skid steer with an auger. By Friday mid day we will have a better idea. I sure as hell hope so!
 

OzSolar

Started out w/nothing & still got most of it left
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So it becomes a question of code. Does you locality simply recognize it? Or, as I suspect, would the need an engineers report. Notice the phrase about loading the cable to the value in the engineering requirements. No question there are a lot of clever ways to anchor stuff out there. But this may not be "allowed" as a DIY solution. Never heard where you couldn't pour your own sono tubes.
Osprey mentions that this method is often considered a temporary structure so no engineering is required. (shoulder shrug)
 

Bluedog225

Texas
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Best of luck. I’ve stood a skid steer up on its hind legs trying to push an 8” auger into the dry clay around here.
 
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