On August 17, 2015, a significant storm hit the Omaha, Nebraska, metro area with nearly 4 inches of rain, which affected an already-saturated portion in the heart of the city. Heavy rains near 108th and Charles streets caused an adjacent roadway embankment to cave in, creating a 30-foot drop just a few feet from the edge of a traffic lane. The city was forced to close 108th Street for public safety concerns.
The next day, city officials brought in our firm to develop a short-term plan to stabilize the roadway embankment and to potentially reopen 108th Street.
About 600 cubic yards of soil and broken concrete were brought in to stabilize the embankment. But because the soils were saturated, and because the drainage structure had a 1.5-foot crack under the southbound lane, the decision was made to keep the road closed and develop a permanent solution. Time was of the essence since 108th Street provides access from a residential area to one of the main east-west routes in Omaha.
We evaluated several long-term drainage solutions to carry 320 cubic feet per second of water through the 25 feet in elevation difference while dissipating the energy and velocity before the water outlets into the downstream channel. The solution we created included several improvements, such as adding a larger broken-back pipe, adjacent and upstream storm improvement inlets, and new curb and gutter to redirect flows through the system rather than over the embankment.
As we created our design, we had to take several aspects into consideration:
- A deep, incised channel with more than 30 feet of elevation drop required accurate topographic surveying. We used 3D scanning to do this quickly and safely.
- We were working with an existing system that was significantly undersized, which led to major scouring and overtopping during intense storms.
- 108th Street carries 10,400 vehicles per day, which were detoured for just more than three months.
- We had to coordinate with Metropolitan Utilities District (MUD) to temporarily cap an 8-inch water main pipe and a 6-inch gas main pipe, and we worked with communications companies to temporarily support multiple fiber optic lines during construction.
- We worked with the U.S. Army Corps of Engineers to secure 404 permits for emergency repairs and permanent improvements.
- Our solution had to account for future widening of 108th Street.
- We had to design a broken back 72-inch reinforced concrete pipe (RCP) storm sewer to drop flows 25 feet to the channel bottom. The structural design of the impact stilling basin and internal baffles within the sloped portion had to dissipate velocities.
- Helical piles had to be incorporated at pipe bends to support collars from settlement and velocity forces of stormwater runoff.
- We needed to incorporate Flexamat into the roadway embankment for stabilization to account for sump overflows during heavy rains.
We submitted final plans to the City of Omaha on September 18, 2015, only one month after the embankment caved in and the road was closed.
Construction began 10 days after the final plans were submitted and was completed by November 25. The road reopened to traffic nearly three months after it was initially closed. The area was graded, seeded, and sodded early the next spring.
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