A slurry wall is constructed
by excavating a 3-foot-wide trench up to 80 feet deep along the length
of the levee and filling the trench with slurry, which is a mixture
of soil, cement, clay and water. Once hardened, the slurry mixture
creates a watertight barrier to prevent seepage through and under
the levees during a high flood event.
The 19 miles of slurry wall constructed so far on the American River
are much deeper than originally designed. Seepage through the levees
was the main concern following the record flood of 1986, and engineers
planned slurry walls up to 30 feet deep to control this through-levee
seepage. High river flows in the near-record storms of 1997 caused
floodwater to seep under the levees and pointed to the need for deeper
slurry walls to control this under-levee seepage. As a result, slurry
walls are now as deep as 80 feet.
After the floods of January 1997, the Corps and its partners reassessed
the levee strengthening design because of damage to levees previously
believed to be strong. As a result, they revised the levee designs
to include deeper slurry walls and to completely cut off the gaps
near bridges and deep utility line crossings. This work will ensure
that the levees can safely pass flood events much higher than the
record 1986 flood.