Redwood Shores Levee Upgrade
BackgroundRedwood Shores is located on San Francisco Bay and is protected from the bay by a system of levees that were constructed many decades ago. The Federal Emergency Management Agency (FEMA) periodically updates Flood Insurance Rate Maps (FIRMs), and these FIRMs determine whether or not an area is considered a flood plain or a Special Flood Hazard Area (SFHA). Areas that are deemed an SFHA are then required to purchase very costly flood insurance. Law mandates that all federally insured mortgages in SFHAs require flood insurance. Since over 90% of mortgages are federally insured, (Fannie Mae, Freddie Mac, FHA, VA) most communities fall under this mandate and aggressively seek a cost-effective solution to avoid SFHA designation.
Project ScopeIn 2008 FEMA began updating maps in the San Mateo County area. These maps had not been updated since 1982. With changes such as new mapping technology, increased concerns about “Katrina-like” flood events and sea level rise, much of the levee system was deemed unacceptable. In fact, the preliminary map released by FEMA in April of 2008 classified Redwood Shores as an SFHA. In order to reverse the SFHA designation, the city along with San Mateo County officials started an intense effort to work with FEMA and the community to find a viable, economically feasible solution to increase the height, stability and overall integrity of their levee system. Adding to the city’s challenge, environmental hurdles appeared at every turn including breeding seasons for endangered species, various US Fish & Wildlife regulations and a very limited construction time window. After analyzing multiple options including steel and concrete, a substantial portion of the improved levees were designed with ShoreGuard Vinyl Sheet Piling. The cantilevered flood wall raises the effective level of the levees by adding 4-5 feet of freeboard. Additionally, the sheet piling also provides seepage protection and defends the levee from rodent damage.
PerformanceShoreGuard had several performance advantages over steel and concrete. First, steel walls need continuous, costly upkeep to prevent excessive corrosion. ShoreGuard is a corrosion resistant product and is virtually maintenance free. Secondly, steel and concrete require heavy equipment at the jobsite for unloading and staging. ShoreGuard is comparatively light and easy to handle which is valuable in environmentally sensitive areas. ShoreGuard and its high strength to weight ratio was a better choice than heavy concrete in the unstable soils of the bay. Two patented features also figured heavily in the material selection process. The CMI patented I-Beam Interloc design is optimized to provide lock integrity and maximum stiffness during high-impact installations. The advantages of the I-Beam Lock have long been recognized and specified by industry experts such as the USACE. In addition, the project utilized the patented ShoreGuard 24” box profile, which allows for faster installation and minimal seepage.
ConstructionDue to delays in the permitting process, Valentine Construction had only one month to install the sheet piling wall. During that month, intense storms forced a complete shutdown of the jobsite for almost a week. Teaming up with experienced subcontractor Hughes Construction, crews worked extended hours but they were able to complete the work by the hard deadline. One crew utilized an excavator mounted MKT vibratory hammer while the other used an ABI Mobilram unit. Crews were able to achieve over three times the production rate with the ABI unit compared to the excavator. Soil conditions were typical of the Bay Area comprised mostly of soft to firm clays and sandy clay loams.
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