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Magnet Extensometer Q&A

Question: What grout should we use with the magnet extensometer?Should the grout be weaker, stronger or the same strength as the surrounding ground ?

Answer: In soft ground applications, you might use a ratio of 6.6 water : 1 cement : 0.4 bentonite. This provides a 28 day compressive strength of about 4 psi. You should also consider using telescoping joints. These will accommodate settlement that would otherwise break the access pipe. They will also improve compliance to the surrounding ground.

In a hard clay, you might use a ratio (by weight) of 2.5 water : 1 cement : 0.3 sodium bentonite. This provides a compressive strength of about 100 psi and a modulus of about 10K psi. The strength and modulus of the grout are controlled mainly by the ratio of water to cement.

Question: What happens to the legs of the spider magnet over time? Does the spring force weaken, does the metal rust, and what is the effect on measurements?

Answer: There seems to be some confusion over the purpose of the sprung legs. Some people theorize that the legs "couple" the magnet to the surrounding soil. While the legs may provide some "coupling" effect, their true purpose is to hold the magnet in place while the borehole is grouted. After that, we don't care about the spring force of the legs or their resistance to corrosion.

The soil mass moves up or down and carries the magnet (and the grout and the pipe) along with it. We recommend using a soft grout, but unless the borehole has a large diameter, there is little chance that the grouted hole will become a pile, regardless of its strength.

We have strapped magnets directly to inclinometer casing with telescoping couplings, grouted the hole, and seen perfectly good settlement measurements - without the use of spider magnets at all.

Question: In terms of radius of curvature, how much deformation/bending of the access tube can be accommodated before problems will be encountered with the readings?

Answer: We recommend that you incorporate telescoping sections within the pipe. This will prevent buckling or curvature from occurring in the first place, so this question will never come up.

If you must calculate a radius value, consider the size of the probe, 5/8 inch in diameter by 8 inches long, and the inner diameter of the pipe you will use, which varies with the wall thickness of the pipe.

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