The engineer stood his ground. After facing an hour-long barrage of questions from Westsiders -- not all of the happy -- dam engineer Dr. Dan Marks had not budged on this recommendation: the road across Lake Auman Dam should remain forever closed to vehicular traffic in order to protect the integrity of the structure.
A crowd of nearly 200 attended the presentation, which capped a day of activities involving Marks and members of the Board and the Dam Committee of the Seven Lakes West Landowners Association [SLWLA].
Marks offered three reasons that he was recommending permanent closure of Longleaf drive across the dam:
1. Conditions encountered during last summers repairs to the dam were much worse than anticipated.
2. Even after the repairs, the dam's core will continue to deteriorate, though at a much slower rate.
3. Impact loading caused by vehicles traveling through the emergency spillway -- a 150-foot long, 4-foot deep dip on the east side of the dam -- could hasten deterioration of the dam's core and threaten the integrity of the repairs in precisely the area where the worst core deterioration has occurred.
"I do empathize with those of you that are inconvenienced by closing the road," Marks said. "But I can't as a professional engineer advise you to open that road, with the possibility that those three things could cause a failure."
"I understand the inconvenience," he said. "I would probably feel the same way, because I love water. I love dams. I would love to get up every morning and and drive across that dam with its view of Lake Auman. But I also know that my big old Ford Lariat with all my stuff totals out about 5000 lbs. Why should I take the risk of ruining the best thing in my life to save five minutes?"
The decision whether to reopen the road does not fall, ultimately, to Marks, but rather to the SLWLA Board of Directors. Director Mick Herdrich said at the meeting's conclusion that the Dam Committee would study the information provided by Marks, consider options, and make a recommendations to the Board about next steps. In the past, Herdrich has left open the possibility that the committee might recommend seeking a second opinion for another engineer.
A dam primer
Marks began his presentation with a review of Lake Auman Dam's structure and a primer on how it is supposed to work, and how it had begun to deteriorate before the remediation.
To drive home the importance of its structural integrity, he noted that the dam is classified by the State of North Carolina as a "high-hazard dam," meaning that a failure of the dam would almost certainly cause significant property damage and possible loss of life.
Lake Auman, he explained, drains into the small Willow Creek, which meanders down toward Jackson Springs. Should Lake Auman Dam ever fail, Marks said, there would be twelve feet of water in Jackson Springs Presbyterian Church.
The dam is built of earth, with an exterior of silty sand -- the kind of soil Seven Lakers find in their yards -- and a core of Triassic clay. Though that sort of clay is used in making bricks, the clays in Lake Auman dam have not been fired into solid bricks, but rather compacted with earth-moving equipment.
Unfortunately, Triassic clay can become suspended in water, and does not readily settle out. All dams either leak or seep water, Marks said. As water seeps through the porous outer surface of the Lake Auman Dam, it passes through the clay, saturating it into a putty-like substance, and, very gradually, carries some of the core away.
As a result, prior to the remediation, the water level inside the dam had begun to rise, saturating the soils on the face of the dam, and threatening its structural integrity.
The repair
The remediation undertaken last year involved passing a vibrating steel beam through the top of the dam, ramming it as far as it would go into the core, and filling behind it with a slurry made of clays and cement. The result is a sort of replacement for the original core that will stop the passage of water high in the structure, forcing the seeping water toward the bottom of the dam.
The process began on the east, or left side, of the dam and proceeded for two weeks with better than expected results -- that is, the depth that the vibrating beam was able to penetrate averaged eighteen feet -- less than the twenty feet Marks had anticipated. The depth the beam can penetrate is determined by the firmness of the soil it encounters.
But then, as the process neared the west -- or right -- side of the dam, the beam began to penetrate lower and lower into the core, reaching depths of forty feet or more, and indicating that the core deterioration in that section was far worse than Marks had anticipated. It is this much worse than anticipated deterioration that provides one of Marks' reasons for keeping the the dam road closed.
Marks and the contractor addressed this development by extending the slurry wall approximately 100 feet further toward the right side of the dam than originally planned, reaching depths on the far right side that were more in the 20-foot range.
The risk
Marks said he could not yet explain why the core deterioration in the right side of the dam was so much worse than elsewhere in the structure.
One possibility is that the clays used in that portion of the core were less suitable. He said he had been studying photographs taken during construction under a microscope and could definitely conclude that the soils used on the right side of the dam were a different color, suggesting a different composition.
Another possibility is that the impact energy created by vehicles traveling through the emergency spillway dip caused the putty-like clay in the core to more rapidly become suspended in and carried away by seepage water.
He said he planned, with the support of the Association, to continue to study the dam, taking core samples, in order to better understand the problems on its right side.
Marks did say that the dam is probably in better shape now -- after the remediation -- than it has likely ever been.
"It's like open heart surgery," Marks explained. "Once you've had it, you're better off than you have been in years. But that doesn't mean you don't have to be careful."
The Questions
Marks fielded dozens of questions, many of them aimed in one way or another at a central concern: "Is there anything the West Side can do to reopen the road across the dam?"
Given the engineer's concern about the impact on the dam caused when vehicles travel through the dip that is the emergency spillway, that became the focus for some questioners.
One resident asked whether the dip in the roadway could be replaced by culverts running under a flat roadway. Though Marks' technical reply likely went well over the heads of most in attendance, the ultimate answer was "No." Culverts cannot carry the amount of water that the state requires the emergency spillway to handle. Marks explained that it must be able the carry away the amount of rainfall that would occur in a 100-year precipitation event, without having water overtop the rest of the dam.
Another asked whether the roadway could be redesigned, using state of the art materials and techniques, in such a way that the impact of vehicles passing through the spillway dip would be absorbed by the roadbed instead of passed through to the dam's core.
"Today, there's just about not anything in engineering you can't do if you have the money," Marks replied. Pressed for what it might cost, the engineer said, "I'm not prepared tonight to give you cost estimate or a pavement design."
Marks also noted that any roadway design would have to be approved by state engineers before it could be implemented.
Director John Goodman asked Marks whether he had consulted with other engineers on his recommendation not to restore the roadway over the dam.
"I have talked with no less than twelve to fifteen dam safety engineers from California to New Jersey," Marks replied. Referring to communications from state officials that recommended the Association follow Marks' advise, he said, "The only reason that your dam safety engineer in North Carolina can't tell you to not open the road, is that he doesn't have the jurisdiction or the authority to advise you as a private engineer."
One resident asked Marks to estimate how many years allowing vehicular traffic might take off the life expectancy of the dam.
"Let's take 100 years as the expected life of the dam," he replied. "We are approaching the midlife of the dam. I would venture to say that that one factor [traffic impact] might account for a 20 percent reduction in life. That's just based on a lot of engineering experience." Marks said good data on the impact of roadways crossing earthen dams is hard to come by.
Marks mentioned during his presentation that the Dam Committee had explored remediation options that cost as much as $3 million — six times the cost of the option ultimately chosen. One landowner asked why the Board hadn't selected a more expensive option, if that would have been a better repair.
"We did not select the cheapest route," Marks replied. We went through a very detailed evaluation form and ranked the options. Cost was only one factor. The alternative selected was not the cheapest."
In addition to those looking for a way to reopen the dam to traffic, the meeting attracted a number of members who were already convinced that it should remain closed.
Bob Kieling compared the dam to a battleship that had taken a hit from a torpedo. "We had to close some compartments to save the ship. Those compartments need to stay closed."
Former SLWLA President Mary Anne Fewkes stresses the liability the each member of the Association would face if the dam failed.
"One thing that has not been mentioned is the fact that we all own the dam," Fewkes said "and, if anything happens, we are all liable . . . . We received a letter from the state saying that they would support what the dam engineer recommends. That we have in writing from the state. If anything happens to that dam, someone will dig up that letter and ask, "Why would you put a road over that dam if if your engineer told you not to do it?' We are all liable."
