The American Bottom Levee Project
URS designs cutoff walls to cure seepage woes in Wood River levees
By ALAN J. ORTBALS
After six years, URS’s role in the levee restoration project is winding down. With its design work completed, all that’s left is some consultation with the project engineer, AMEC, as the final pieces of construction are undertaken.
URS first went to work on the levee project for Madison County in 2008, shortly after the Federal Emergency Management Agency had announced its intent to decertify the levee system protecting the American Bottom. That original assignment, according to URS senior project manager Richard Bird, was to take soil borings and analyze samples in preparation for the work to come later.
When AMEC Earth and Environmental, Inc. was selected as the project engineer by the Southwestern Illinois Flood Prevention District Council, URS was brought in as a subcontractor.
Since then, URS’s job scope has been focused on problems in the Wood River Drainage and Levee District, specifically, designing solutions for under-seepage.
“Uncontrolled seepage has been observed emerging on the slope of the levee and also at the toe of the levee in both the Upper and Lower Wood River levees,” Bird said. “Our job was to develop recommendations for remediation of seepage in areas not covered by the Corps.”
A variety of methods have been used in the restoration project including seepage berms and relief wells but, in this instance, it was determined that cutoff walls would also be required. A cutoff wall is a vertical seepage barrier constructed through the levee to connect to an underlying impermeable subsurface stratum. To create the cutoff walls, a trench will be dug under bentonite or cement-bentonite slurry along the riverside toe of the levee and then filled with either a mixture of soil and bentonite or hardened cement-bentonite slurry.
There are two cutoff walls that will be constructed in the Wood River levee. There’s a shallow (20 to 30- feet deep) 1,600-foot-long cutoff wall to be built at the riverside levee toe near the Alton marina. The shallow cut off wall will intersect a continuous clay layer to create an impermeable defense against seepage.
A deep cutoff wall will be constructed in an area Bird refers to as the corner where Wood River enters the Mississippi River. This area lacks the clay level found in the upper Wood River area so this deep wall has to go all the way down to either bedrock or glacial till. Bird explained that the alluvium of the Mississippi River is underlain with glacial till in certain areas. Millions of years ago, ice glaciers advanced as far south as present day St. Louis. These ice sheets were in the neighborhood of 1,000 feet thick and carried material with them. This “glacial till” is a mixture of clay, sand, silt and boulders and is very compact.
This deep cutoff wall will be about 2,000 feet long and 110 to 140 feet deep, Bird said. Work on cutoff walls is expected to begin early this fall and be completed in the first half of 2015.