Help Ground Mount – On Solid Sandstone

[sappstter]

I’m looking to do an installation in my backyard on a 30° sandstone slope, see attached pic. I’ve talked to my local building department and I’m okay to do so and not limited by setback and won’t need structural engineering if I keep the system mounted ~18” above finished grade. This is ideal as I want it mounted as low as possible for aesthetic purposes. If I use a per-engineered system then no structural PE stamp is required, I just need to provide the details. I’m looking at a 24 or 28 panel array either 4x6 or 4x7 layout.

My issue is the standard IronRidge details call for 24 x 12” dia. X 5 ft deep piers. I have no way of getting any sort of drilling rig to the slope due to the pool and limited access on the other side of the house. There is no way I’m going to be able to dig down that deep into the sandstone, a 2 man auger isn’t going into the sandstone, I can’t jackhammer a hole that deep. We had some backhoes in the yard before the pool was put in to move the retaining wall and they had a really hard time breaking up the sandstone to dig it out.

Any suggestions for securing to the hillside? I was thinking of possibly post bases secured with wedge anchors or epoxied anchors since it wouldn’t be too hard to drill smaller diameter holes into the sandstone.


The area outlined in red is where I plan to put the panels, it is similar to the rocky area to the left just has a few inches of dirt over it:

[organic farmer]

Go visit your nearest concrete plant. Talk to them and tell them you need a 'site-work' contractor, experienced in punching through sandstone. You will likely be put in contact with a crew who does this stuff all the time.

[bcroe]

If that tree is going to shade it much, you might want to
move the location. Bruce Roe

[sappstter]

If that tree is going to shade it much, you might want to
move the location. Bruce Roe

The tree doesn't seem to shade over the fence fortunately.

[peakbagger]

In my local granite, I just drill holes with a rotary hammer, blow out the holes with an air compressor and the use smaller diameter tubing to blow the dust out of the hole and then mix up some epoxy.and drop in threaded rod. Fastenal type places sell drop in anchor kits that are basically a two part container with epoxy and hardener in separate capsules, drop down the hole and then pound in the rod and the two containers break and it mixes in place and usually sets in 24 hours. Note the assumption is that the sandstone is fairly solid and not flaking. Usually in that case just drill deeper to get into solid material. No need to fill the entire hole with epoxy, as long as you have 6 os so inches of epoxy at the bottom that encases the rod it going to hold a lot of tension load. Most of the load on the slope will be side to side load (shear load) so you do not need a lot of tensile strength in most cases unless you are cantilevering loads. BTW most folks overtighten anchor bolts, with epoxy type anchors use nuts with nylon inserts or other means of thread locking and just snug them up.

[Mike 134]

I’m looking to do an installation in my backyard on a 30° sandstone slope, see attached pic. I’ve talked to my local building department and I’m okay to do so and not limited by setback and won’t need structural engineering if I keep the system mounted ~18” above finished grade. This is ideal as I want it mounted as low as possible for aesthetic purposes. If I use a per-engineered system then no structural PE stamp is required, I just need to provide the details. I’m looking at a 24 or 28 panel array either 4x6 or 4x7 layout.

My issue is the standard IronRidge details call for 24 x 12” dia. X 5 ft deep piers. I have no way of getting any sort of drilling rig to the slope due to the pool and limited access on the other side of the house. There is no way I’m going to be able to dig down that deep into the sandstone, a 2 man auger isn’t going into the sandstone, I can’t jackhammer a hole that deep. We had some backhoes in the yard before the pool was put in to move the retaining wall and they had a really hard time breaking up the sandstone to dig it out.

Any suggestions for securing to the hillside? I was thinking of possibly post bases secured with wedge anchors or epoxied anchors since it wouldn’t be too hard to drill smaller diameter holes into the sandstone.


The area outlined in red is where I plan to put the panels, it is similar to the rocky area to the left just has a few inches of dirt over it:

They are calling for piers because it's based your ground mount being on soil, not rock. Perhaps a chat with your local building dept explaining your plan to bolt to the limestone will solve your issue.

[sappstter]

In my local granite, I just drill holes with a rotary hammer, blow out the holes with an air compressor and the use smaller diameter tubing to blow the dust out of the hole and then mix up some epoxy.and drop in threaded rod. Fastenal type places sell drop in anchor kits that are basically a two part container with epoxy and hardener in separate capsules, drop down the hole and then pound in the rod and the two containers break and it mixes in place and usually sets in 24 hours. Note the assumption is that the sandstone is fairly solid and not flaking. Usually in that case just drill deeper to get into solid material. No need to fill the entire hole with epoxy, as long as you have 6 os so inches of epoxy at the bottom that encases the rod it going to hold a lot of tension load. Most of the load on the slope will be side to side load (shear load) so you do not need a lot of tensile strength in most cases unless you are cantilevering loads. BTW most folks overtighten anchor bolts, with epoxy type anchors use nuts with nylon inserts or other means of thread locking and just snug them up.

Thanks, I was just speaking to someone about something similar but what you described is what I had in mind.

[sappstter]

They are calling for piers because it's based your ground mount being on soil, not rock. Perhaps a chat with your local building dept explaining your plan to bolt to the limestone will solve your issue.

Building department wants me to figure something out and submit it to them for review, they said pre-engineered system details are the easiest way to go. I found a local contractor who have me some suggestions with core drilling and sinking the metal posts. I'm going to sketch up some details and review with the city, the city didn't seem overly concerned with what I wanted to do given the height that I want to mount the panels. They said they wouldn't consider it a structure as far as maximum lot coverage and lot setback requirements, but want to see details. I guess we'll see how much they scrutinize the support structure plans.

[bcroe]

use nuts with nylon inserts

My opinion, I do not use nylon insert locking nuts outdoors,
because they may not be rated for long term sun exposure.
Instead I use an additional nut, locked against the first.
Bruce Roe

[peakbagger]

I will buy that. I have seen some structural locking nuts with inserts somewhere in my travels.