47.5 MB - The Whole Building Design Guide

Slab-on-grade foundation on structural fill: Thisis considered to be the safest method to elevatea building in many flood hazard areas,except those where waves and high velocityflows may cause erosion. Structuralfill can be placed so that, when water risesup to the DFE, it will not touch the building(see Figure 2-24) and building access ismaintained. The fill must be designed tominimize adverse impacts, such as increasing flood elevations onadjacent properties, increasing erosive velocities, and causing localdrainage problems. To ensure stability, especially as floodwaters recedeand the soils drain, fill must be designed for the anticipatedwater depths and duration. A geotechnical engineer or soil scientistmay need to examine underlying soils to determine if thebearing capacity is sufficient to carry the added weight of fill, orif consolidation over time may occur. In addition, the effects oflong-term compaction of the fill should be considered, and mayprompt additional elevation as a factor of safety. The horizontalextent of fill from the foundation should be designed to facilitateaccess by emergency and fire vehicles, with a minimum 25-footwidth recommended. Designers are cautioned to avoid excavatinga basement into fill without added structural protection (andcertification that the design meets requirements for dry floodproofing),due to the potential for significant hydrostatic loadsand uplift on basement floors.Communities may require a registereddesign professional to certify that buildingselevated on fill are reasonably safe fromflooding. FEMA NFIP Technical Bulletin10 (2001) discusses criteria for thiscertification.Figure 2-24:Municipal buildingelevated on fillFillMAKING CRITICAL FACILITIES SAFE FROM Flooding2-59