Hi Bob, Bill,
Last October I drilled in a field of ground electrodes in
preparation for the tower I'll be putting up this spring. I also
drove in a few the "old fashioned way" just to have a comparison
between the methods.
The electrodes that were placed in drilled holes were 3 strands
of "00" 7 strand copper cable. We used a hollow stem 8 inch
diameter drill. The hole was drilled to depth, then the cable
was inserted down the inside of the stem. We then poured the
homemade GEM material down the center of the stem as the stem was
gradually pulled out of the hole. This was a lot easier to do
than it sounds. The result was a fairly uniform backfill all the
way around the electrode. I placed six 10 foot and four 20
electrodes in fairly rocky alkaline desert soil (Tucson, AZ)
which has a hard pan layer (caliche) at a depth of 12 feet.
We "pull tested" the electrodes immediately after removing the
bit and finishing the backfill. This was done before any water
was applied to the dry GEM. It was not possible for a human
being to cause even the slightest discernible vertical travel
when attempting to pull any of the electrodes out of the hole.
So, my impression is that the rod to earth mechanical contact
using GEM as backfill is at least pretty good.
The driven rods were standard 5/8" copper clad steel 10 feet
long. There are a total of three of these.
I then began a regular regime of measuring the resistance to
earth of each electrode. The measurements were made using the
three terminal fall of potential method. The meter is an earth
resistance test set designed for the purpose. Enough time has
passed that I can now report on the results of the measurements.
Here is the executive summary.
1. The initial measurements of the driven rods yielded an
average for the three of 160 ohms. These measurements were
taken the day following the day they were driven. These rods
have improved slightly over the period of the test and now
they are averaging about 140 ohms. They appear to be
asymptotically approaching a final value of 120 ohms.
2. The 10 foot drilled electrodes initially averaged 140 ohms
(dry). When watered in, they fell immediately to about 80
ohms. Over the period since then, they have been steadily
improving. When I fit a curve to the data, they appear to be
asymptotically approaching a final value of about 22 ohms.
they should achieve 23 ohms at almost exactly one year.
3. The 20 foot drilled electrodes initially averaged 93 ohms. I
suspect these were significantly lower because the soil below
the caliche layer actually had some (_very_ little) moisture
in it. I think that the gypsum in the GEM absorbed enough of
this moisture to affect the readings before I could make the
first measurements. When these were watered in, the average
fell immediately to 45 ohms. In the time since then, they
have also steadily improved. Curve fitting shows them to be
asymptotically approaching a final value of about 6 ohms.
They should get to 7 ohms in almost exactly a year from the
day the holes were drilled.
I'm now estimating that the system when all tied together with
the subgrade conductors will measure between one and two ohms at
the central reference point. If it is significantly different
than this, I'll post the measurements at that time.
Drilling is not economical for a single electrode. But the cost
wasn't too bad for 10 of them. An alternative, if you know that
the hard layer is of limited thickness (no more than a foot or
two), is to excavate a hole down to the hard pan and then place a
small shaped charge on the hard pan to knock a hole through it.
This is a bit noisy but not at all dangerous if done correctly.
73, Eric N7CL
>From: Bob Wanderer <aa0cy@nwrain.com>
To: <towertalk@contesting.com>
>Date: Thu, 15 Apr 1999 19:27:49 -0700
>
>Angle or even horizontal will work.
>
>You can drill out the hard layer and backfill, although the
>rod-to-earth contact will not be as "tight."
>
>Because of the material, you may want to consider
>conductivity-enhancements such as GEM or coke breeze.
>
>73,
>Bob AA0CY
>ex Senior Applications Engineer, PolyPhaser Corp.
>
>----------
>From: Bill Aycock[SMTP:baycock@hiwaay.net]
>Sent: Saturday, April 10, 1999 7:05 PM
>To: Tower Talk
>Subject: Re: [TowerTalk] Ground Rods
>
>
>In all the recent discussion, I have not seen anything connected
>to my particular problem.-- I have a rock (sandstone, I think)
>layer about 6 ft down at the tower base site. An 8' rod driven
>into this stops -dead- with from 1 to 2 ft of rod above ground.
>
>Most specs and codes that I have seen ask for the top of the rod
>to be sub-surface, and to have clamp contact of the wire to the
>rod. This does make for difficulty in inspection, but thats what
>they say.
>
>I plan to drive my tower-base safety rods at an angle, to use
>all the rod without hitting the rock. Doing this will give me
>the same contact area (length) as if I had driven it in with no
>interference.
>
>I found this layer when I was driving the house service entrance
>ground several years ago. The local utility forman said to just
>leave the remainder above the surface (it was next to their
>pole) and tie in with a clamp. They apparently saw similar
>things with some frequency.
Snip...
--
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