Extracting an electric grounding spike

[Paul Colby]

Non-relevant background

So, in the dim time, we had a microwave link installed for our internet service. The dish was on a mast on the second story (about a foot square, no idea what frequency). The installers put in a grounding 1/2 inch diameter rod. I remember because of more than an hour of sledgehammering outside my window.

Okay, years pass. Several IP providers later. The dish is in the dustbin of history and we want the 1/2 a foot bent rod removed to make room for the new construction.

The actual question

Hours are spent extracting an 8 foot long 1/2 inch diameter copper rod.

Here's the mystery. When we started it was quite simple to rotate the rod through a full 360 degree. Easy in fact. Yet when pulled on it wouldn't budge. Okay, so the only thing keeping the rod in the ground is friction with the dirt. Why is the frictional traction force per unit area in the longitudinal direction so much greater than that in the circumferential direction?

 

[.Scott]

It's not rod-shaped anymore.
Copper is malleable. All that banging put a bulge or bend in it.

 

[Paul Colby]

It's not rod-shaped anymore.
Copper is malleable. All that banging put a bulge or bend in it.

I think the chances of that are likely 1. The great majority of the rod was still rod-shaped after extraction which doesn't mean it was that way in the ground. A bowed rod would press the rod's surface against the soil increasing the friction. I still don't see why is it harder to extract a bowed rod than rotate it?

 

[jrmichler]

Why is the frictional traction force per unit area in the longitudinal direction so much greater than that in the circumferential direction?

When twisting a rod, the wrench is a lever. The lever ratio is the distance from the rod to the hand on the wrench divided by the rod radius of 0.25 inches. A typical lever ratio would be about 8 inches divided by 0.25 inches equals 32. The straight pull force would then be 32 times larger than the force to twist the rod.

A large radius bow in the rod will not have a large effect on the force to either twist or pull it.

 

[Paul Colby]

A large radius bow in the rod will not have a large effect on the force to either twist or pull it.

That would be my intuition. The force needed to rotate the rod was quite small. Once in motion, the slipping friction is smaller than the static friction. The strategy was to pull while rotating. This failed at first. So we flooded the shallow hole with water. After much effort, we were able to get enough water down the length of the rod to slow, very slowly work it out.

I did not notice a marked decrease in the effort needed to rotate the rod. Nor did there seem to be a great difference between the lubricated and nonlubricated torque needed for rotation.

The bending of the rod may well play a big role, I just don't see why.

 

[Paul Colby]

A typical lever ratio would be about 8 inches divided by 0.25 inches equals 32.

Well, one strategy we tried was to use a shovel to pry the bar up. Biting into the rod with the shovel edge, we were able to pry on it without slipping. This is a much bigger lever than an 8" wrench.

 

[arydberg]

Non-relevant background

So, in the dim time, we had a microwave link installed for our internet service. The dish was on a mast on the second story (about a foot square, no idea what frequency). The installers put in a grounding 1/2 inch diameter rod. I remember because of more than an hour of sledgehammering outside my window.

Okay, years pass. Several IP providers later. The dish is in the dustbin of history and we want the 1/2 a foot bent rod removed to make room for the new construction.

The actual question

Hours are spent extracting an 8 foot long 1/2 inch diameter copper rod.

Here's the mystery. When we started it was quite simple to rotate the rod through a full 360 degree. Easy in fact. Yet when pulled on it wouldn't budge. Okay, so the only thing keeping the rod in the ground is friction with the dirt. Why is the frictional traction force per unit area in the longitudinal direction so much greater than that in the circumferential direction?

I would grab the rod with a pair of vice grips. Then use a car sissors jack with a board underneith to pull it out. Hold the vice grips parallel to the ground.

 

[Baluncore]

Yet when pulled on it wouldn't budge. Okay, so the only thing keeping the rod in the ground is friction with the dirt. Why is the frictional traction force per unit area in the longitudinal direction so much greater than that in the circumferential direction?

Soil adheres to the rod. When you rotate the rod you shear the soil, but not at the soil to rod interface. The soil coated rod therefore becomes variable in diameter like many hour glasses. You may need to continuously lift the rod as you rotate it, so it screws it's way upwards.

A similar problem is wherever the rod punches through, or passes between two rocks of any size. The edges of the rocks are pushed downwards and outwards as the rod is driven in, then when being extracted the rocks rotate back to pinch and lock the rod. Rotating the rod cuts grooves in the rod that effectively lock the rod, like in a collet.

Take a look at the extracted rod for indications of what mechanism was preventing it's extraction.

I usually clamp a chain to the rod, then lift it out with a front end loader. To extract a rod or a metal post can take several tonnes of uplift, often with some variation in the pull direction. It is interesting to note that it is often easier to extract a post set in concrete than a steel post driven into soil.

 

[Paul Colby]

Soil adheres to the rod. When you rotate the rod you shear the soil, but not at the soil to rod interface. The soil coated rod therefore becomes variable in diameter like many hour glasses. You may need to continuously lift the rod as you rotate it, so it screws it's way upwards.

This is quite plausible. It’s also consistent with flooding the rod with water. Eventually the dirt clods clinging to the rod would come free. The rod came out pretty clean.

 

[Baluncore]

The rod came out pretty clean.

That suggests the rod was in a stone free clay. The presence of water changes clay from a particulate grain into a grease. The water moves between the sheets of phylosilicate and makes it easier to shear the individual grains.

Ion exchange may help. Using an acid, to provide H+ to drive more stable K and Ca ions out of the clay, may help make the clay more fluid. Free rotation of the rod might help the liquid propagate down the hole.