By Lee Roberts
SILVER POINT, Tenn. — A drill rig crew from the U.S. Army Corps of Engineers Baltimore District extracted core samples here March 14, that are being closely examined by Nashville District geologists at Center Hill Lake near what is known as the Saddle Dam. However, the exploratory drilling is revealing more than just the condition of rock formations; it's also bringing out the benefits of the two districts partnering together to explore the movement of water seepage through the karst geology in the area.
Nashville District geologists have long known the area around the Center Hill Dam is riddled with caves, sinkholes and open bedding layers in the rock. The Baltimore District's partnership fills a void and helps the Nashville District reduce costs and provides flexible drilling capabilities to district geologists who are painstakingly putting together the pieces of the sub-terrain puzzle.
Exploratory drilling can be expensive and time consuming work, said Melanie Leslie, the Nashville District's project geologist responsible for connecting the dots between the drill holes to capture an underground picture to help in the decision to install a barrier wall at the Saddle Dam or a roller compacted concrete berm just downstream.
The best value of partnering with the Baltimore District drill rig team "is in the fact that if we discover something wrong and we need to move over 10 feet and do some more exploratory work here or there, we can do so without costing more money or spending time on additional paperwork required to get the new holes in," Leslie said. "And time is money."
Drill Rig Operator John Blackson and Helper Steve Seager are drilling several hundred feet deep just downstream of the Saddle Dam and retrieving needed core samples for the Nashville District.
Nashville District Geologist Sarah Mackey is also working alongside on the collection and analysis of core samples and said it's been a great working relationship between districts.
"It's great working with the Baltimore District," Mackey said. "I think it's a great opportunity for both districts… in case they don't have work in their district it gives them opportunities… and they get experience on different projects."
Mackey said she is learning a lot on the site. Earlier in the day when the drill hit a void in the rock, the Baltimore District mixed environmentally-safe green-yellow dye and inserted it into the drill casing to see if and where the water would make its way downstream. She said the dye did exit at several rock formations downstream, which indicate a correlation between the elevation of voids they are looking at and seepage pathways.
The two districts are planning to continue working together through the summer.
"We will continue our exploratory holes," Mackey said. "We will probably find the same voids at the same elevation. They'll use this data to see how deep they'll need to excavate to hopefully cut off these seepage paths (during construction)."
Mackey said it's also been a great experience for her working in the field on this exploratory project.
"It's a good hands-on opportunity, especially for a geologist to actually get to work with rock and do what you've been trained to do," Mackey stressed.
Blackson said they are pumping water from the lake for drill operations and this particular drill rig moves around the steep slopes of the site using tracks much like a bulldozer. But the work is repetitious, he said, and so his team works hard to maintain a safe operation as they drill deep below.
The Baltimore District drilled 35-40 holes around the site on previous trips to Center Hill Dam in 2007 and 2009, but it's early on this trip, and they have "poked" just four so far.
"It takes time," Blackson said. "The T-3 is a bentonite layer -- (clay mineral) it's green and sticky. It's nasty. You can tell when you hit it. As soon as you hit it you go through it and you're done."
The T-3 bentonite layer comes from volcanic ash millions of years ago. Blackson explained that although the T-3 is much softer than the surrounding rock, the clay quickly "gums up" in the heat created by the rotating drill head.
Leslie added that the Nashville District does not have its own drill crews, which makes this partnership all that more valuable.
"Baltimore will come down here, and will just immediately get started. You don't have to go through all the red tape… it saves us money in that way. They are very good. They are very responsive," Leslie said. "If I wish to do something extra like the dye testing that has been going on today, they have no problems with doing that. They've been a great crew to work with."