- Describe the conditions of the REOstone quarry site.
Ted Kniazewycz: Prior to the flood, the REOstone rock quarry had a 50-foot-wide natural solid rock barrier that separated Richland Creek from the 500-foot deep quarry. During the flood, the water got so high and put so much pressure on the levee wall that it quickly eroded and spilled like Niagara Falls, filling the quarry in five hours.
We were called a week after the flood and were very interested because we’d watched this unfold on a live news broadcast, never dreaming we would be involved in this project.
The owner took us as close to the site as we could get, and what we saw was a 42-acre lake with 7 billion gallons of water connected to what used to be Richland Creek. What we couldn’t see was that not only did the levee breach the high wall of the quarry, the existing creek bottom was completely gone.
The force of the water eroded not only five feet of depth of the solid rock channel, it also eroded the rock ledge that supported the channel bottom. The levee could not be repaired in the same location because the supporting ledge no longer existed.
- How did GS&P’s solution change as the project progressed?
Ted: We were initially brought in to present the Rogers Group with permanent dam options. They first thought we could build a concrete dam, but as we evaluated the situation and discovered the fissure we realized a dam wasn’t possible. We explained that what they needed was a levee, not a dam, and recommended using the available natural resources at the site.
Once the water level went down the full measure of the fissure was exposed: 20-feet wide and 150-feet deep—a feature that had been there for hundreds or thousands of years. Since there was nothing we could do to fill or span the fissure, we had to come up with a different solution. Our role shifted to design a levee that housed the destroyed sewer system, protected the quarry from catastrophic flooding, and created a new path for Richland Creek.
- When you assessed the area, what emerged as the most urgent issue?
Ted: As a team, it was a lot. The Cochrill/Basswood trunk sewer line that was inside the high wall washed out, spilling raw sewage throughout 7 billion gallons of water, and creating a critical environmental situation.
Ken Stewart: And in addition to the 42-inch diameter sections of gravity sewer pipe that washed away, there was also a double barrel, 30-inch diameter aerial crossing—a gravity sewer that sits on concrete piers—that collapsed. So in an instant the city lost two large trunk sewer lines that served as the drainage basin for the west side of Nashville.
Ted: The first priority was to cut off the sewer flow. Metro Water Services contracted to install temporary pumps to bypass the section of sewer line that failed, and within a week stopped the raw sewage from flowing into the quarry, the creek and the Cumberland River.
Next, Rogers Group stopped the creek from flowing into the quarry so they could start draining the water. They placed a temporary earthen berm, or barrier, using whatever material they had at the site. Once the sewer lines were closed off and the berm was in place we quickly got started.
- What was complicated about the levee and the new sewer design?
Ted: The temporary pump system put in place by Metro Water Services was extremely expensive. It was costing an average of $125,000 a month to operate and required 24-hour monitoring. It was exposed, which increased the chances of failure. The safest place and best fit for the new sewer lines was inside the levee and under the new creek bottom, meaning maintenance wouldn’t be an easy task.
Ken: The large diameter shot rock boulders used to build the levee are roughly three feet in diameter and weigh about a ton each. Trying to dig through that material to fix a sewer line problem would be a nightmare. The other line would be underwater and underground, but there was no other acceptable option. To protect the new section of the Basswood trunk sewer located within the levee from damage during construction, the pipe was encased in concrete. This prevents the lines from floating in a flood event, and greatly reduces the potential of a future leak into the environment.
For the trunk sewer line in the levee we essentially created a monolithic structure, which is a 300-foot long section of sewer put together in 20-foot joints of pipe. The advantage is that the pipe joints can’t flex or deflect in any way.
Ted: We made it as stout as possible in the event that if a pipe failed the encasement of concrete would act like a pipe protecting the environment.
- After looking at several options to reconnect the sewer lines, GS&P recommended an inverted siphon system. Why?
Ken: Metro Nashville Government was adamant that we were not to install another aerial crossing sewer line like the one that was washed away. One alternative was to install a gravity sewer that would have gone under James Avenue and Briley Parkway and impact an NES substation, which could have doubled or tripled the cost of the project. That gave us only a few options, and we ended up with an inverted siphon.An inverted siphon is pretty rare; it’s the first one I’ve designed in my 13 years in engineering. Typically we use gravity sewer or force main, but because both sewer lines were now underground we used an inverted siphon to transport the sewage flow under the creek and then uphill to rejoin the traditional gravity system. An inverted siphon combines gravity flow with a short section of U-shaped pipe. Liquids flowing in one end are forced up and out the other end.
- What kinds of natural materials were available on site and how were they applied?
Ted: Since Rogers Group owned the quarry and all the material sites close by, they wanted us to use as many natural materials as were readily available.
We ended up using clay as a core, armored with shot rock, which is a variety of rock types and sizes used to protect structures from erosion. Clay is very impermeable to water. It stays somewhat moist, but can be compacted to achieve high densities that reduce water to a slow migration over time.
- What were some of the more unique challenges you encountered?
Ted: The location of the utilities provided constraints to the levee layout because of Tennessee Valley Authority (TVA) easement restrictions. There can only be a certain amount of fill placed under the power lines, but we needed a slope and creek bottom to bury the lines. The toe of the levee needed to be at least 10 feet from the fissure, which was running at a 45 degree angle towards the TVA easement. This was a critical element as the levee had to fit between these two constraints and the pinch point was where the levee was at its greatest height.
Also, when we started the final levee construction, we had to take out the temporary levee, so there was a period of time when the quarry was highly exposed; a significant rain event would have flooded it. Luckily it didn’t, which was extremely fortunate.
- This project’s complications were compounded by public concern over the location of Richland Creek. How did you address those concerns?
Ted: On the levee side, we were able to keep pushing the project forward because there were no other options. There was a very vocal group from the Richland Creek area that wanted the creek to go back to the way it was before. We had many conversations and meetings to explain that we couldn’t put it back where it was because that didn’t exist anymore. Once we showed them pictures of where the creek used to flow, we were able to explain how the creek we were going to put back would be environmentally improved. The use of graded rock allowed for the smallest possible footprint for this levee configuration.
It also provided overbank area from the relocated creek channel that could be used for habitat pools and trees and other plantings to enhance the creek and overall stream development. It would be the same length or longer, have natural trees and native plantings. We wanted to make it as pristine as a creek could be.
- Which aspect of this project had the most impact on you?
Ted: I’ll never forget that first time I went to the site and saw the quarry full of water. The wind was blowing pretty stiff, and there were white caps on the water and ducks floating in it. It was literally a huge lake. I’ll always remember watching the transformation as the water was pumped out. It was pretty amazing.
Ken: You don’t run across this kind of project very often. The scope and urgency made it extremely challenging. We have deadlines with other projects, but nothing of this magnitude with so many site concerns related to timing and flooding.
- What are you most proud of in this whole project as far as your personal involvement?
Ted: This was such a unique project. Just the fact that we pulled it off is a great feeling. The client was very stressed because his asset, the quarry, was exposed for a long period of time. He was really counting on us to get the job done. Even though they are a construction company, they couldn’t design it themselves. They could’ve called many engineering firms to help, but they called us. That is very gratifying.
Ken: The teamwork was amazing. This is the first time I’ve collaborated with Ted and Diane and the transportation group, and they jumped in and helped wherever needed. It just feels good to be a part of a group like that. Working with three different surveyors to gather and organize information so we could produce an accurate design didn’t leave much room for error, but we worked together and made it happen.
Diane Regensburg: It was also very rewarding to know that we exceeded the owner’s expectations by designing a project that addressed all the site constraints and allowed for construction of the new levee utilizing resources readily available to the owner. Our ability to thread a design solution through the site that used readily available clay and shot rock was a bonus that they did not expect.
Most of all, being able to solve a problem caused by the Nashville flood was extremely rewarding. I had personal friends and colleagues who were greatly devastated by the flood, and I spent many hours helping them recover. Being able to also use my engineering skills to address this problem was professionally fulfilling. I’m very proud of what we accomplished.