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Differential Subsidence of Marshland Peat as a Geologic Hazard in the Greater New Orleans Area, Louisiana
Jesse O. Snowden, Wm. B. Simmons, E. Berkley Traughber, Raymond W. Stephens (1)
Since 1972, five New Orleans area homes have been destroyed by natural gas explosions. All of these homes were built on reclaimed Lake Pontchartrain marshland. This area is characterized by accumulations of marsh-grass peat, up to five meters in thickness. Loss-on-ignition analyses reveal that the peat is typically 70 to 80 percent organic matter by dry weight.
The marshland was reclaimed by canal drainage, lowering the water table as much as three meters, and the addition of land fill to compensate for initial subsidence. However, the drainage and land filling process itself causes three types of land subsidence: (1) primary consolidation of the drained peat and underlying clay strata, (2) secondary compression of the peat and underlying clay from the loading of land fill and drained peat, and (3) oxidation of the drained peat.
The low bearing strength of this material requires that buildings be supported by pilings driven at least 12 meters deep into more competent underlying clay units. Properly spaced pilings stabilize foundations, and, to a degree, retard subsidence directly under buildings, whereas maximum subsidence continues in adjacent areas not protected by pilings. This differential subsidence between buildings and adjacent land results in stress and structural failure of driveways, walkways, and, most importantly, the rupture of underground gas connections. This rupture releases gas into the highly permeable peat. Thus, gas may migrate laterally and accumulate under concrete slab foundations, creating an explosion hazard.
Differential subsidence of up to one meter has been measured, and the amount of differential subsidence correlates closely with measured peat thickness, the amount of water table lowering, and elapsed time since land reclamation. Thus, it is possible to predict areas of existing and future development that will experience hazardous differential subsidence.
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