Looking south from the top of the nearly six-mile-long, 125-foot-tall embankment, one can see the distant skyline of Dallas. To the north, 587,000 acre-feet of water impounded in Lewisville Lake stretches nearly to the horizon. Keeping the two separate is the function of the 60–year–old Lewisville Dam, which serves the vital duties of flood control and water supply for the region.
Recent fixes will help ensure the dam keeps working as intended.
Last year, many North Texans were alarmed by a Dallas Morning News story that warned of serious problems, including a 161–foot–long embankment slide on the upstream side of Lewisville Dam. On The Lewisville Texan Journal’s Facebook page alone, over 800 people shared DMN’s article. Record rainfalls in 2015 brought record-high lake levels and likely contributed to the damage, according to Corps officials.
By the time the public learned about the slide, the U.S. Army Corps of Engineers had already let a $6.4 million contract for its repair. Now, a year later, officials wanted to show off the repairs that have been done.
The repairs cost $8 million.
Fixing the slide
Corps project engineer Aloysia Brown, who managed the construction, described some of the challenges of the project.
“We had a lot of rain in the beginning of the construction that continued throughout construction,” said Brown. “And we also had other obstacles— ways to get our construction on-site.”
She explained that a temporary bridge had to be constructed in order to get some oversized equipment to the site.
“Pretty close to construction completion, we had another small slide happen – very small – about 1,100 yards of material,” she said. “We went ahead and replaced that.”
According to officials, crews first built a cofferdam around the site on the upstream side, then excavated all of the failed material before starting the repairs. Brown said the Corps took samples to look into the type of soil material that failed. Because the soil used contains a high amount of clay, which expands and contracts with moisture, the Corps replaced the failed material with lime-stabilized soil taken from an area of Corps property downstream of the dam.
Sarwenaj Ashraf, an engineer with the Corps, explained that the lime helps the soil, but that it was not originally used in the construction when the dam was built. Lime, added to soil, can decrease its plasticity and its tendency to swell with moisture.
That new soil was then packed in lifts. Rather than dumping all the material at once into a pile, it was built up and compacted in place layer by layer for better cohesion. The upstream surface of the dam was then re-covered with limestone riprap – small boulders that prevent the lake’s wave action from eroding the soil on the surface of the dam.
Afterwards, the Corps completed a new access road and ramps. The new access road from the crest to the upstream side of the dam allowed access to the slide area for repairs and will remain in place, allowing easier access for maintenance and future repairs.
About 500 to 600 feet of new asphalt pavement on top of the dam’s crest sits atop a new road bed. That road bed was excavated about five feet into the surface, then filled with compacted soil. Brown explained that a geomembrane sits below it, in attempt to regulate the amount of moisture moving through. A fiberglass grid is sandwiched between the asphalt layers. The Corps hopes this new road will crack less and thus allow less water to trickle in.
“If this works out like we think it is, we’re just going to continue— as we go through and replace asphalt, we’ll just do it with this method,” said Tim MacAllister, chief of operations for the Corps’ Texas lakes.
“I think that the whole team out here has done a great job,” said Brown. “I see this project as a major success.”
Brown said the work looks beautiful, works great, and should be around for a long time.
“We took our time and we did it right; that’s what is beautiful about it,” said Brown. “Everybody understands the importance of this project. When you look downstream and see the City of Dallas, and then see all this water in front of us– I think everybody on this team understood the importance of this project,” she said. “That’s why so much effort was put in to make sure it was done right the first time.”
Although the slide repairs are complete, much more work remains on the books for safety modifications to the dam. Owing more to the consequences of any dam failure than the chances of one happening, the dam is rated as high risk. Over 400,000 people and $22 billion in property would be endangered downstream if it were to fail.
For years now, the Corps has been studying the dam’s problems. In September, it released its plans for the 33,000 foot-long structure. The Lewisville Texan Journal wrote in September about the proposed modifications, which include construction of berms, relocation of municipal water pipes, and work on the spillway. (For details of the work, see our story from Sept. 29.)
The City of Lewisville has already budgeted for the relocation of its water lines at the dam to move them south of Jones street. The Corps is requiring that they be further away from the toe of the dam to reduce risk.
Corps officials stress at every opportunity that the dam continues to function as intended.
Another project on the books, likely to begin in 2020 is an $8.7 million project to repave Jones Street where it runs behind the dam through LLELA. That project, paid for mostly by the Federal Highway Administration, would widen the road and repave it all the way to the river, adding adjacent hike and bike trails.