Ground mount recommendations

[Maitake]

Didn't see this until today.
Sinclair designs makes a ballasted rack. I pour the ballast blocks.
They can also provide panels.
Glacial till here, not friendly to drilling or pounding in supports. Boulders fist size to car size all jumbled together.
Emgineer 775 has some video of the drive in version.

 

[Canxane]

A pounded post has almost no pullout strength, which is the most important thing for a solar rack. If there is any kind of permitting involved it's hard to imagine that getting approved.
I have a pounder that fits on the back of my tractor and in my area where there's lots of rocks it just doesn't work. If I'm lucky enough to find a spot to get a post in, the chances of it being plumb are almost zero.

In fact, pounded steel posts in most soils have greater pull out resistance than augured and cemented. We are talking about posts like this https://wheelermetals.com/store/products/d/19844/pre-cut-pipe-post/four-and-one-half-pipe-post-10-ft If you put this 5 feet in virgin ground it will NOT come out. Many, many engineering studies conclude that pounded steel pipe posts in undisturbed gground are stronger than augured and concreted. The NRCS and many federal programs now prefer the stronger pounded pipe.

There are obvious exceptions depending on your location and soil type.

 

[Zwy]

Since my last post I have scrapped all previous design ideas. Since then I have purchased a 2.5T mini excavator so now pouring a solid level slab is now an option. I have also since ordered 2 Tamarack 5 panel single pole ground mounts.

I am going to level two spots and pour 5’x5’x2 1/2’ thick pads. Haven’t decided if I’m going To weld a 1’x1’ flat bolting flange to the bottom of the posts and concrete anchor them down or just stick the posts in the concrete yet?

Add some rebar and tie it to a mounting flange. Nice ballast, you're on the right track.

 

[MarkSolar]

Many, many engineering studies conclude that pounded steel pipe posts in undisturbed gground are stronger than augured and concreted. The NRCS and many federal programs now prefer the stronger pounded pipe.

I'm not able to find any engineering studies that analyze pullout for pounded posts other than in the context of fence posts. Can you provide a link? I'm familiar with NRCS guidance on pullout for fence posts where the concern is pullout for corner and brace posts, but not familiar with them making recommendations for solar where uplift is an issue.

 

[Canxane]

I'm not able to find any engineering studies that analyze pullout for pounded posts other than in the context of fence posts. Can you provide a link? I'm familiar with NRCS guidance on pullout for fence posts where the concern is pullout for corner and brace posts, but not familiar with them making recommendations for solar where uplift is an issue.

I am not aware of a solar specific study either. The pounded pipe or tube is used in place of concreted footers for many styles of steel buildings which do withstand all engineering requirements for uplift. Large agricultural buildings for equipment and hay storage, which are basically just a roof (wings) on stilts, and have tremendous uplift are either bolted or welded to pounded pipe (and sometimes pounded i-beam and railroad track steel). It is fast becoming the norm - especially in remote areas. No concrete truck required. It is faster and cheaper and supposedly stronger in those heavy uplift applications. If it can hold a roof down on a hay barn without sides, it can certainly hold a solar panel down.

 

[MarkSolar]

I am not aware of a solar specific study either. The pounded pipe or tube is used in place of concreted footers for many styles of steel buildings which do withstand all engineering requirements for uplift. Large agricultural buildings for equipment and hay storage, which are basically just a roof (wings) on stilts, and have tremendous uplift are either bolted or welded to pounded pipe (and sometimes pounded i-beam and railroad track steel). It is fast becoming the norm - especially in remote areas. No concrete truck required. It is faster and cheaper and supposedly stronger in those heavy uplift applications. If it can hold a roof down on a hay barn without sides, it can certainly hold a solar panel down.

Ground screws and helical foundations come with engineering documents for permitting that indicate the pullout strength as a function of torque. So they can use a large torque wrench when they're installed to verify the pullout strength. Did your system come with engineering documents for permitting? Those documents should specify the size of the pipe selected as a function of soil type, frost depth, bearing load, uplift and lateral loading.

Besides uplift there is also issues with load bearing and frost heave. Ground screws and helical foundations have to extend 3-4' below the frost line to resist the force of an ice lens trying to lfit it out of the ground. If one vertical support heaves and the others don't, that would damage the mounting system and the panels. We have a 42" frost depth so my ground screws had to be 7' in the ground. They also are closed on the ends so they can take the bearing load, particularly in snow areas. We have to design for 40psf snow load on top of the static load. In my case that was 20,000# distributed over 10 ground screws so 2000# each. An open pipe only has a direct bearing surface equal to the cross sectional area of the wall thickness. So it's dependent on the soil friction to keep it from sinking.

I have seen piles used for buildings but not open pipes. The piles require core samples for a soil engineering analysis to determine the depth and surface area of the piles. Even small buildings have piles that extend 15' or more into the ground. For larger buildings the ground is drilled before the piles are driven and then grout is injected to bond the pile to the sides of the soil. There are other techniques for piles, but they all require an engineering analysis of the specific conditions.

 

[Canxane]

Ground screws and helical foundations come with engineering documents for permitting that indicate the pullout strength as a function of torque. So they can use a large torque wrench when they're installed to verify the pullout strength. Did your system come with engineering documents for permitting? Those documents should specify the size of the pipe selected as a function of soil type, frost depth, bearing load, uplift and lateral loading.

Besides uplift there is also issues with load bearing and frost heave. Ground screws and helical foundations have to extend 3-4' below the frost line to resist the force of an ice lens trying to lfit it out of the ground. If one vertical support heaves and the others don't, that would damage the mounting system and the panels. We have a 42" frost depth so my ground screws had to be 7' in the ground. They also are closed on the ends so they can take the bearing load, particularly in snow areas. We have to design for 40psf snow load on top of the static load. In my case that was 20,000# distributed over 10 ground screws so 2000# each. An open pipe only has a direct bearing surface equal to the cross sectional area of the wall thickness. So it's dependent on the soil friction to keep it from sinking.

I have seen piles used for buildings but not open pipes. The piles require core samples for a soil engineering analysis to determine the depth and surface area of the piles. Even small buildings have piles that extend 15' or more into the ground. For larger buildings the ground is drilled before the piles are driven and then grout is injected to bond the pile to the sides of the soil. There are other techniques for piles, but they all require an engineering analysis of the specific conditions.

Not an engineer. Glad you like your system. Just offering anecdotal evidence to the thread author for alternative ground mount options. I would hazard it is faster and stronger than building a home brewed wooden array or other cobbled together rack. If you live in an area which requires extensive permitting, you should pay for and use an engineered solution. If you don't require extensive permits - consider speaking with a agricultural fence builder.

 

[WYtreasure]

Contact someone who builds fences (check craiglist). Your basic corner post fence is a 4 or 5 inch schedule 80 or higher steel pipe that is pounded into the ground and the "h braces" are welded. The same guy can pound the posts wherever you want, at whatever height you need. He will also weld the cross members. It is a basic job for a fence builder. You can then stick any rail system you want on it. Much faster and cheaper than any other options, not to mention must stronger. No concrete, the posts are level and straight. Stick some geotextile cloth under it and dump a load of gravel on the fabric to keep the weed pressure down under the array. Then you don't need to mow or do any maintenance underneath. Looks better too.

The pounded posts, and all the welding was under $500 labor for the entire day. The adjustable rack lets me change the tilt.

Depending upon the depth to which posts were pounded, that setup could even work in Wyoming. We had 75 mph winds, more than once this Winter. Used oil field pipe is readily available in my area.

 

[BillG]

Here is a pic of my rotating array V4.0 design. I drilled a hole 36" deep and sunk the 10' post in concrete. It is extremely sturdy. This array can hold six panels in landscape orientation, though I have them here in portrait because I wanted 120VDC. What I'm finding is the by rotating my arrays East and West over the course of the day, I can more or less double output. Most importantly, with the arrays facing SE, I can start my wellpump at 7:30am.

 

[WYtreasure]

 

[BillG]

how hard is it to turn the array? Is there some kind of bearing in there? Assuming you have a sort of telescoping pipe assembly (smaller diameter fixed in the concrete, larger diameter that can spin?) How high do you think one could mount a system like yours above ground?

 

[Canxane]

Depending upon the depth to which posts were pounded, that setup could even work in Wyoming. We had 75 mph winds, more than once this Winter. Used oil field pipe is readily available in my area.

The vertical pipes are 10 feet in length. The 5 1/2 OD heavy wall pipes are 5-6 feet down. Has anyone ever tried to pull out a ground rod? Try pulling these out. They are then welded together horizontally and on the diagonal brace locking them into a unit that is not easily displaced. They have held up to very extreme weather including a close miss to a tornado. More worried about the aluminum iron ridge XR1000 rails than the pipe structure.

 

[WYtreasure]

The vertical pipes are 10 feet in length. The 5 1/2 OD heavy wall pipes are 5-6 feet down. Has anyone ever tried to pull out a ground rod? Try pulling these out. They are then welded together horizontally and on the diagonal brace locking them into a unit that is not easily displaced. They have held up to very extreme weather including a close miss to a tornado. More worried about the aluminum iron ridge XR1000 rails than the pipe structure.

Sounds like a perfect world for you and maybe me. All bets are off when it comes to tornadoes, panels just shred away anyhow.
Having a mount already in place to attach replacement panels is the best we can hope for. I'm betting you have such a mounting system.

Hopefully the panels don't ruin the wiring to the rest of the system when they depart the area during a tornado.

 

[Jack Rabbit Off Grid]

OMG, missed that, could that be a typo? That would be a 5 ton slab to hold down 5 panels. If it were 2.5 inches thick it would be about 750# which might be a bit light by itself but there's also the weight of the panels and racking so it's probably about 1000#, which seems like it would be close to adequate.
My seat of the pants calculation says large 5 panels are about 90 sqft, a 60mph wind on a vertical flat wall is about 10psf, so that would be 900#. For 120mph wind it's about 40psf so that would be 3600# force on a wall. The uplift will be less than that amount but don't know the angle so can't make a guesstimate.

Yes I did post 5’x5’x2.5’. I was going by another install that I found online. After you guys pointed out the weight I ran those numbers in a concrete calculator and yes something like 8280# per pad. So yes I have to rethink this.

your pretty close to panel square feet. Mine come out to be 92sqf and I want them at 40* tilt. my local wind load code is a min 110mph. Tamarack rates the rack I purchased @115mph. So maybe 5’x5’x1’ thick? That would be 3325# per pad.

 

[Jack Rabbit Off Grid]

My guess is Jack Rabbit is down under (Aus) with a 2.5 digger mate mini excavator

Nope Central Oregon. I purchased the machone from a Japanese importer. They called it a 2.5t operating weight machine. I think it’s 5400#.

 

[JustPractical]

You don't mention what climate you're in. If you have frost depth, you either need piers that extend below the frost line or make one floating slab for the whole array to ride on top of. Piers in really rocky soil means you'll probably have to hire someone with a rock drill who can put in ground screws to the proper depth.
If you don't have frost, you can use either precast concrete ballast, or poured in place ballast. They aren't connected together so they will rise and fall differently if you have frost heave, which will damage your panels and/or mounting system. So you would only be able to use them in warm climates.
There are all kinds of ground mount companies, the biggie is Iron Ridge which is expensive. I used Sunmodo, cheaper and more than adequate for residential applications. In all cases treated lumber could work, but for an array your size the warping and twisting of treated lumber may become a problem because it may damage your panels or the other mounting components. It's also more involved grounding the panels if you use lumber because the entire mounting structure doesn't have electrical continuity. I used ground screws but if I had it to do over I would pour a slab and set the mounting bolts in the concrete when it's poured. We're on glacial till and it was very difficult getting the ground screws in plumb and deep enough.

Mark - did you use the ground screws with the SUnmodo or create footings? I have rocky soil, and I don't think the ground screws will make it far. Sooner just use an excavator and make 24x24 footings that go down 5.5'. It's more concrete but less drilling labor.

 

[Maitake]

Beware trenching cost for a long run in glacial till. I'm over $10/ft just for dirt work and it's not finished.
400' trench and five 10 wheelers of boulders went out, same amount of sand came in.

 

[MarkSolar]

Mark - did you use the ground screws with the SUnmodo or create footings? I have rocky soil, and I don't think the ground screws will make it far. Sooner just use an excavator and make 24x24 footings that go down 5.5'. It's more concrete but less drilling labor.

I used ground screws. I agree, if you have rocky soil try something else.
If you dig footings 5.5' deep in rocky soil with an excavator big enough to get through the rocks, you're going to end up with a hole 5' wide. Then you'll have to form up the footing and backfill it without knocking it out of plumb or worse. If I did it again I'd attach it to a concrete slab and get bifacial panels to take advantage of the reflection off the concrete.

 

[scoharri]

I have the exact same problem. I borrowed a tractor mounted post auger to make the holes for my 10 panel rack. Well that went horribly wrong. Could only get maybe 10” into the earth before the rocks would stop me.


Better luck to you.

A ballast system would have been a good choice in this case.

 

[MarkSolar]

A ballast system would have been a good choice in this case.

As long as it doesn't freeze in his location a ballast system will work. But if there are multiple ballast boxes and they heave unevenly, it will damage the rack, panels or both.