Patricia Skinner, Baker, Eugene
Elevating or moving slab-built structures has been practiced for many years in a handful of states. It has been used to remove unwanted buildings from valuable sites, to relocate small businesses to higher traffic areas and to reposition buildings that violate local restrictions. It hasn’t been used often as a retrofitting tool for flood-prone homes, and until recently it hasn’t been available through Louisiana contractors.
The soils found in Louisiana floodplains make the methods described here difficult to practice in these areas. Natural floodplains have a higher clay content than soils in other parts of the country. When wet, this type of soil is hard to handle and difficult to compact for use in supporting a new foundation. Additionally, even a modest rain will saturate the surface layer of soil on the job site, and the heavy equipment operators will have difficulty driving equipment over it.
During the elevation process, the weight of the structure, slab and steel is supported on a reduced number of ground points. The bearing capacity of the soils in Louisiana floodplains is only 1,000-2,000 pounds per square foot. The bearing capacity is used to determine the number and size of crib bases and temporary support jacks; a low bearing capacity dictates larger and/or more crib bases.
Elevated slabs in other states or in other soil conditions are typically supported on a series of independent columns with 4 sq. ft. foundations. Elevated slab structures in floodplains require stronger foundations. The support columns must be tied together on a system of grade beams or set on a new slab (see foundation drawings, p.9).
Depending on the location of the flood-prone structure, it will be necessary to take into consideration one or more of Louisiana’s other natural hazards.
In coastal areas and along the banks of rivers and streams, where the velocity and wave action of water may be factors, the foundation and supports must be designed to withstand these hydrodynamic forces. In most of these cases, relocation of the structure will be recommended.
Parishes in northeastern Louisiana are in the fringes of the New Madrid Fault seismic zone. Construction in seismic zones may be regulated to improve resistance to earthquakes.
The southern third of Louisiana is hurricane territory, and any place in the state can be subject to tornadoes. The foundation design of an elevated building must meet the wind-loading specifications set forth in the standard building code as adopted by the local government.
One objective of the Denham Springs elevation demonstration project was to provide local examples of homes retrofitted with this technique. You don’t have to rely on photographs from remote projects or wonder if the procedure used in Michigan, Florida, Tennessee and elsewhere will work here in Louisiana.
In South Louisiana, you’re no more than two hours away from seeing the results. Comparatively, these homes are larger - up to 3,100 sq. ft. - and more complex than those in the Dry Creek and Tug Fork projects referenced below. Louisiana slab construction is different, and the soil conditions are more difficult. Photographs of elevated homes and additional estimating information are found in two publications of the U.S. Army Corps of Engineers’ National Flood Proofing Committee:
A Flood Proofing Success Story along Dry Creek at Goodlettsville, Tennessee, 1993.
Flood Proofing Technology in the Tug Fork Valley, 1994.
The elevation process is fully described in the Committee’s 1990 publication:
Raising and Moving the [1800 sq ft] Slab-on-Grade House with Slab Attached.
Thorough discussions of flood-proofing options, with technical and financial information, are found in the Committee’s 1993 publication:
Flood Proofing, How to Evaluate Your Options.
FEMA’s retrofitting resources include:
Design Manual for Retrofitting Flood-prone Residential Structures, 1986.
Engineering Principles and Practices for Retrofitting, 1995.
Drainage and water handling measures will add to the cost of the project, but will minimize weather-related construction delays and cost overruns.