FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

10 DESIGNING THE FOUNDATION Volume II
Figure 10‑1.
Closed foundation failure due to erosion and scour undermining; photograph on right shows a close‑up view of
the foundation failure and damaged house wall, Hurricane Dennis (Navarre Beach, FL, 2005)
10.2.3 Deep Foundations
Buildings constructed on deep foundations are supported by soils that are not near grade. Deep foundations
include driven timber, concrete or steel piles, and caissons.
Deep foundations are much more resistant to the effects of localized scour and generalized erosion than
shallow foundations. Because of that, deep foundations are required in Zone V where scour and erosion
effects can be extreme. Open/deep foundations are recommended in Coastal A Zones and in some riverine
areas where scour and erosion can undermine foundations.
10.2.4 Shallow Foundations
Buildings constructed on shallow foundations are supported by soils that are relatively close to the ground
surface. Shallow foundations include perimeter strip footings, monolithic slabs, discrete pad footings, and
some mat foundations. Because of their proximity to grade, shallow foundations are vulnerable to damage
from scour and erosion, and because of that, they are not allowed in Zone V and are not recommended in
Coastal A Zones unless they extend below the maximum predicted scour and erosion depth.
In colder regions, foundations are typically designed to extend below the frost depth, which can exceed
several feet below grade. Extending the foundation below the frost depth is done to prevent the foundation
from heaving when water in the soils freeze and to provide adequate protection from scour and erosion.
However, scour and erosion depths still need to be investigated to ensure that the foundation is not vulnerable
to undermining.
10.3 Foundation Design Requirements and Recommendations
Foundations in coastal areas must elevate the home to satisfy NFIP criteria. NFIP criteria vary for Zone V
and Zone A. In Zone V, the NFIP requires that the building be elevated so that the bottom of the lowest
10-4 COASTAL CONSTRUCTION MANUAL