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Ground rod in shale

JimBeau's picture

I have my new house dried in and I'm ready to have temporary electric service installed.  The problem is that the house is built on solid shale and driving the grounding rod may be a problem. Is it legal by code to simply lay the grounding rods in the conduit trench before backfilling (it's underground service)?  Can I try to drive them horizontally between layers?   I'm also worried about making a good earth connection. Any tips or tricks?


The pix below show the "soil" conditions. In 021 I'm setting the footer forms. In 036 we're digging the frost footer.

(post #102580, reply #1 of 14)

it's legal if you've hit "rock bottom", and the trench is at least 30" deep.


Are you putting rebar in the footers?  That may be your best best for a grounding electrode.

(post #102580, reply #2 of 14)

How bout hammer drill with bit extension to create the path.

(post #102580, reply #3 of 14)

Ham radio operators bury their ground rods horizontally I don't see why you cant too. They, the ham guys, do it to really get a good ground for their radios. A trick to improve contact between the rod and earth ground is to add a thin layer of soil over the ground rod then pour a half pound of CuSO4 crystals over the dirt and water it in with a hose. Tamp the soil, then continue to fill the trench with soil till level with surrounding area. Don't get the soil too wet cause then tamping it down just means creating a mud hole, use common sense here. Keep the ground rod close to the "main disconnecting means" and tie neutral and ground at that point only.

(post #102580, reply #4 of 14)

Take the 20' of Conduit trench closest to the foundation and overdig it 6" deep and 1' wide.


Sprinkle 10 lbs Calcium Chloride (de-ice salt) on that 20 sqft trench part.


Set up a light sprinkler and wash most of the salt down into the Shale. Pour 3" conrete, lay 20' of bare 2-0  on it, lay some bare 12 ga  across the 2-0 about 12" OC. Pour 3" more concrete.


Backfill at least that part of the trench with good moisture holding soil. Don't ever let it get bone dry.



SamT


SamT
A Pragmatic Classical Liberal, aka Libertarian.

I'm always right!
Except when I'm not.

(post #102580, reply #5 of 14)

I don't have a code book at home but someone here can look it up.


Regardless of the code a good solid ground is necessary not only for safety, but for surge suppressors to work properly.  If the soil is dry like it appears to be in the photos then I'd question that you could get a good ground buried in a shallow trench.


There are a number of things you can do.



  • Hammer drill down to where the ground is damp every day of the year.

  • Use a very long length of copper ground wire snaked all over, especially under downspouts.

  • Run the ground wire some distance to a wet, shady, low spot.

  • Use chemical enhancers.

I recommend that after the ground is placed you have an electrician come and test the ground with the proper instrument.  If it is not sufficient, you have to do more.  There is no reason you should guess that it might be ok.


I'm skeptical of the calcium chloride idea.  It might work, but the chemical is quite corrosive and you may lose the wire or ground rod over time.  I don't know for sure, but it appears to me to be a risk.  If it were me I'd use a chemical enhancer made for the application so you'd know it would be safe, durable, and code-approved.

(post #102580, reply #6 of 14)

Let me me more clear than my previous post:  there are many things that may work, in some circumstances.   Some of them may work better than the NEC. But as far as the NEC is concerned, you can only lay the ground rods horizontal (and there would be two, at least 6' apart), if the trench is at least 30" down, and rock prevents you from intalling vertically.


Rather than ground rods, the NEC also allows you to use >20' of 1/2" rebar in the footer as a ground, or >20' of #4 copper wire.  This "Ufer" ground will probably work better than ground rods. 


 http://www.psihq.com/iread/ufergrnd.htm 


http://www.iaei.org/subscriber/magazine/01_d/johnston.htm

(post #102580, reply #8 of 14)

"Rather than ground rods, the NEC also allows you to use >20' of 1/2" rebar "

Much more than ALLOWS it.

Rather they require the ufer ground if it can be accessed.

The ground rod is treated as a backup if none of the primary electrodes are available. And then it either needs to be tested or second electrode used.

. William the Geezer, the sequel to Billy the Kid - Shoe

(post #102580, reply #11 of 14)

I'm sorry if I gave the impression that code should't be followed.  I was suggesting ways of obtaining an effective ground when the soil conditions are difficult, but code must be followed.

(post #102580, reply #7 of 14)

Jim,


       Try to find out what the local electricians are using on difficult sites. Drive or bury what is the norm and then have a meggar test done by a experienced electrician. You want to be under 10 ohms.


      If you do not reach that level you will need to add more to your ground field. This can be done in several ways. A chemical can be added but I suggest you use a product called power fill or equivalent.


       Do not attempt to bury a bunch of ground rods or copper in one small area. It will cancel the whole affect. If you are able to drive rods they need to be spaced about 8' feet apart.


       Copper plates cad welded to a solid ground wire in a trench with power fill will get you where want to be in most cases.


                  Good Luck!

(post #102580, reply #13 of 14)

>  You want to be under 10 ohms.


That would be nice, but the NEC only requires you to get to 25 or less. 


 


 


-- J.S.


 

 

 

-- J.S.

 

(post #102580, reply #9 of 14)

There are a handful of methods that might work. Unfortunately, some of the suggestions made so far show an excess of thinking, and a lack of knowledge.

You really need to talk this over with an electrician, or the inspector, in your area. See what the local practices are. In this town, you would have been required to place the ground in the new foundation.

So STOP NOW... and visit city hall!

If they let you use a ground rod, use that backhoe to make a trench a good 8 ft long and at least 30" deep. Get a ten-ft. rod, and bend it so that the 'short' leg almost reaches the surface. Then pour 2" of cement around the rod, and bury.

(Some might argue that the cement is optional, but the improvement of the rod is so great, with so little extra effort, that I recommend it).

(post #102580, reply #10 of 14)

Thanks for the input. Just a few clarifications.


The house is precast concrete on gravel footers therefore no rebar (the footer form shown in the pic is for the fireplace). I thought about using the rebar in the basement slab but the house has infloor radiant (6" 2b stone, vapor barrier, 1" foam insulation, 6" concrete) so that option is out.


The shale drains extremely well. No standing water in the foundation dig since May '06


Based on all the above posts here's the plan ..... I have yet to finish the french drain trench which will be 42" deep. It will run from the corner of the house where the meter base is located downhill approx. 50 ft. to a wooded valley. The valley has much different soil conditions, mostly clay. I'll drive a few test probes to see where the shale gives way to clay and plant the grd. rods there, then use the french drain trench for the grounding wire back to the house.


I have to make an appointment with the electrical inspector to sign off on the temporary service installation so I'll run the plan by him when I talk to him.

(post #102580, reply #12 of 14)

More info from another thread. http://forums.taunton.com/tp-breaktime/messages?msg=44740.2


I like to look at what works, code, how much$.


So I think a undersized hole with driven rod would get it. But if you get technical I suppose you would have to get it engineered for the site with a stamp.

(post #102580, reply #14 of 14)

Do you happen to have a steel well casing? That's what I used, a 225' ground rod.

Scott.

Always remember those first immortal words that Adam said to Eve, “You’d better stand back, I don’t know how big this thing’s going to get.”