#74 Digging Deep at Ragged Mountain

January 24, 2008
Drilling cores to determine the geology underneath the site of the new dam at Ragged Mountain provides a window into another world and the perspective of geologic time.

 
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This show originally aired on January 17, 2008 on “The Rivanna Rambler,” a weekly public affairs show airing every Thursday at 11:55 a.m. on WTJU 91.1 FM or wtju.net.

On a bright afternoon during a warm spell earlier in the month, I was part of a group of Ivy Creek Foundation visitors gathered on a hillside above the wooded valley below the Ragged Mountain Dam. We were there to take a look at the drilling operation, part of the geotechnical studies being undertaken by the Rivanna Water and Sewer Authority in preparation for expansion of this drinking water reservoir.

Almost at the top of the service road that leads to the caretaker’s house, we park behind a series of trucks and get out with our guide, Chuck Kent, who works for the Service Authority and is overseeing the project. On the downhill side of the road, there’s a steep track that weaves through the woods, barely visible but for rumpled leaves and clumps of fresh earth here and there, and several men in cold weather work clothes are climbing up to greet us. No one is wearing hardhats, so we know that the drilling ops are over for the day, and though we wanted to see the equipment in action, there’s still plenty to look at and learn.

The track through the woods is where the drilling rig has traveled down the slope to the valley where an unnamed stream, tributary to Moores Creek, originates as the outflow from the existing dam. Orange flags are barely visible as they climb the hill on the other side of the valley, describing roughly where the dam will be. Chuck explains that the drilling contractor was hired to conduct seismic tests and drill cores to verify the integrity of the underlying bedrock. Knowing what lies beneath this section of the forest will, in part, determine structural requirements the dam that will eventually raise the pool elevation another 45 feet.

And here’s the drilling bit, a rough-looking piece of hardware on the end of a steel drill pipe. It’s three inches in diameter with diamond cutting surfaces and a hollow center through which the core is extracted. The cores themselves are collected in a special wooden box with troughs marked to show the sequence of the pieces. We get to inspect the last core, a series of four-foot lengths, some of which are comprised of shorter sections fit together where a discontinuity, or fault, in the substrate caused the core to separate. Keeping them in order is essential for understanding the structure and layering of the underlying rock. Fort the most part, this drilling operation revealed that down to 40 to 60 feet, it’s mostly unconsolidated. Deeper, the drill found granite bedrock. The longest core was 248 feet of drilling angled from the surface where the sides of the dam will be anchored into the sides of the stream valley. Below the foundation of the dam itself, cores were dug to about 120 feet. Each drill hole was then tested with pressurized water to see where the faults are, if any.

I lift one 12-inch section – testing its heft. It is dense and dusty gray, spotted with lighter crystalline structures, the history of this land over eons of time. In places, the core is stained red with iron ore. Other sections show lines of fracture, fault lines where water has intruded and weakened the rock. After the cores are analyzed, the Water and Sewer Authority will keep them, laid out in their special boxes, during the design and construction of the new dam, to refer to as needed.

As we marvel over these artifacts from earth beneath our feet, the contractors fire up a compressor. The day will dim fast on this January afternoon – and the last step in the process – filling the drill holes with grout – has to be completed. Like stitching up after an operation, the hole in the earth must be plugged, lest it allow rainwater or snow to infiltrate and compromise the existing bedrock. As we anticipate the change that is coming to this land over the next five years, it’s good to remember that it took millennia for other changes to take form in the land. It all depends upon how you choose to look at it.

Copyright 2008 Leslie B. Middleton

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