Geologists at work

cpt truck

A local gas station has been undergoing remodeling, but one day it had geo-specialists pay a visit. On the left is a small drilling rig, and on the right is the typical panel truck used for cone penetrometry testing (CPT). It’s propped up on stout legs to make a stable platform, and as I peeked underneath I could see the penetrometer shaft sticking into the ground like an ovipositor.

CPT consists of pushing a steel shaft with a standard cone-shaped tip straight into the ground. Sensors on the truck measure how much power it takes to do this in the tip measure the pressure, which varies as the tip penetrates different types of soil. Sensors in the tip also measure the electrical conductivity of the soil, pore-water pressure, and other things. In deep ground like this part of Oakland, a CPT tip can be pushed hundreds of meters down, but here they’re probably going down no more than 10 or 20 meters, just deep enough to see if the underground fuel storage tanks will be stable there. In chronically wet ground, empty tanks have been known to rise out of the ground during, say, earthquake shaking. Learn a little more on my site.

This is hardhat work, but it’s not very dirty. Geotechnicians can get steady and varied work doing CPT with no more than a high-school education. The crew of this truck included a woman, too.

By the way, I have entered the “Blog Your Way to Antarctica” contest, which runs through September. Please see my entry here, and if you like the idea, give me a vote—the earlier the better. I know it would mean a break in this blog, but I’d make it up to you.

3 Responses to “Geologists at work”

  1. Andrew Says:

    Thanks, Erik. That makes more sense than measuring raw power, which would always rise with depth. My linked page makes that clear (should have read it before writing this!).

  2. Erik Says:

    Actually they usually don’t measure power, but pressure on the cone tip and drag on a friction sleeve behind the cone tip as it penetrates, in addition to those other things. The ratio ratio of tip pressure and sleeve friction can tell you soil type, less than 1% is probably sand, 3-10% or greater is probably clayey.

    They can also can have a geophone at the tip so you can do down-hole seismic velocity tests during penetration.

    CPT gives a continuous profile (usually every couple of cm) of all the things it is measuring so it can be very good for finding thin soil layers that even a very careful driller might miss in a boring. It’s also very fast; you can easily push over 500 ft/day if you don’t have to stop for seismic testing. You do have to be careful in gravelly soils because the results can be very misleading, and if there is too much gravel or anything coarser then you can actually damage the cone tip.

  3. Naomi Schiff Says:

    Anyone who uses the word “ovipositor” in a nonbiological simile gets my vote to go anywhere he wants!

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