ENGINEERING GEOLOGY OF THE NEW ORLEANS AREA
Courtesy of Roger T. Saucier and Jesse O. Snowden, College of Science and Technology, Southeast Missouri State University, One University Plaza, Cape Girardeau
(Originally published for the 1995 Geological Society of America Annual Meeting, New Orleans, Louisiana Field Trip Guide Book # 6a & 6b)
Introduction and Physiographic Setting
New Orleans has the flattest, lowest, and geologically youngest setting of any major city in the United States (Saucier, 1965). Indeed, the city and its environs have a maximum relief of only 7 m (23 ft), an average elevation of about 0.4 m (1.3 ft), and no surficial deposits older than about 3,000 years (Saucier, 1995).
The city of New Orleans was founded in 1718 as the southernmost port on the Mississippi River, where goods could be transferred to and from riverboats serving the vast interior Mississippi Valley. The city and adjacent communities lie wholly within the Holocene delta plain of the Mississippi River. The deltaic landscape is dominated by extensive tracts of freshwater swamp, intratidal fresh-to-brackish water marshes, and numerous lakes and bayous in interdistributary lowlands. "High" ground is limited to the narrow natural levee ridges that flank the present course of the Mississippi River and its abandoned distributaries.
The original city (Vieux Carre) was built entirely on the natural levee where maximum elevations are approximately 5 m (16 ft) above sea level. The levee thus provided a relatively dry and firm foundation for building. When augmented by a low artificial levee and a rudimentary drainage system, it also provided a measure of protection from flooding. The natural levee "strip city" was isolated by cypress swamps and grassy marshlands on the north, east, and west, and by Lake Pontchartrain still farther north. New Orleans remained restricted and isolated on the natural levee until the early 1900s when inventor-engineer Baldwin Wood designed a heavy-duty screw pump that made it possible to raise quickly huge volumes of water a short vertical distance. Drainage canals were dredged through the cut-over cypress swamps north of the city to Lake Pontchartrain, and Wood's new pumps were used to move the drainage water into the lake. Artificial levees were constructed to protect the newly drained land from flooding. By the 1920s developers were building on the reclaimed swamp, much of it near or slightly below sea level. Construction continued until shortly after World War II, by which time most of the old cypress swamp had been reclaimed and developed.
The remaining undeveloped land in Orleans and adjacent Jefferson parishes was chiefly the brackish-water remnants of Holocene interdistributary marshlands of the Mississippi Delta. Beginning in the 1920s much of this marshland was drained using the canal-and-pump system. While the cypress swamp environment is dominated by clay-rich sediment containing variable amounts of organic matter, usually in the form of buried logs and stumps, the interdistributary marshes are underlain by as much as 5 m (16 ft) of marsh-grass peat. These peat deposits commonly shrink to less than 1/3 their original volume when dried. Development of the marshlands thus presented new engineering challenges.
STRATIGRAPHY
Pleistocene Formations
Like most of south Louisiana, the New Orleans area lies directly over the Gulf Coast geosyncline where late Tertiary and Quaternary deltaic and nearshore marine deposits have accumulated to a thickness of about 12,000 m (40,000 ft). Alternating transgressive and regressive units consist almost entirely of unconsolidated sand, silt, and clay with abundant organic matter. Quaternary deposits comprise the upper 640 m (2,100 ft) of the sequence, and all but the uppermost tens of meters are of Pleistocene age.
The uppermost coast-parallel Pleistocene unit in south Louisiana is part of the Prairie complex (also called terrace or formation) which comprises the land surface north of Lake Pontchartrain. This unit in the Mississippi Valley area represents multiple environments of deposition and is time-transgressive, representing both the Wisconsin and Sangamon stages. As a consequence of several processes, including faulting and subsidence, the Prairie complex is exclusively a subsurface unit in the New Orleans area.
Because of its relatively shallow depth, the uppermost part of the subsurface complex was exposed to thousands of years of subaerial erosion during the Lake Wisconsin glaciation when sea level was lower than at present. Streams draining the uplands north and west of Lake Pontchartrain formed narrow entrenchments up to 21 m (70 ft) on the otherwise flat to gently undulating surface. Thanks to the availability of thousands of borings, it is possible to contour the erosional surface in considerable detail.
Compared to the overlying Holocene sediment, the Pleistocene sediment is generally much stronger (or denser in the case of noncohesive sediment), has lower water content, and is appreciably less organic. Characteristically, the upper several meters are moderately to well oxidized and desiccated as a result of the subaerial weathering and have distinctive colors and minor sedimentary structures. This preconsolidated "crust" assumes the role of "local bedrock" and has profound engineering implications. A second weathered horizon, attributable to an earlier Pleistocene sea-level lowstand, is found several tens of feet deeper in the Prairie complex and is an important factor in foundation design for extremely heavy structures such as high-rise buildings.
Marine Transgressive Deposits
The uppermost Pleistocene erosional surface was inundated by rising sea level, and the initial Holocene sediment was laid down in a nearshore Gulf environment beginning about 12,000 years ago. During the subsequent several thousand years, the Gulf shoreline moved north of the New Orleans area, and the eroded surface was completely blanketed with sediment. The materials accumulated to a maximum thickness of about 15 m (50 ft) and consist of mixed silt and sand of relatively low density with small amounts of clay but large amounts of shell.
Nearshore Gulf deposits continued to accumulate until about 5,500 years ago, at which time sea level was only about 1 to 2 m (3 to 6 ft) below its present level. At that time, a large barrier island complex formed along a roughly east-west line through the New Orleans area. Designated the Pine Island Beach Trend, the complex is a remarkably preserved ridge of well-sorted fine sand that is 5 to 8 km (3 to 5 mi) wide, about 55 km (34 mi) long, and has a mean maximum thickness of about 10 m (33 ft).
Because of local and regional subsidence, the ridge is now almost completely surrounded and buried by later Holocene sediment; but through large parts of the city, its surface is within 3 m (10 ft) of the surface. The sands of the ridge are competent enough to provide an excellent foundation for light structures. Consequently, the configuration of the ridge surface is extremely significant to local engineers and has been contoured using thousands of borings. The contouring reveals it is broken only by what is interpreted as relict tidal passes.
The Pine Island Beach Trend was apparently the active shoreline of the Gulf for about 1,000 years. During that time, sea level continued to rise slowly. The ridge prograded slowly westward under the influence of prevailing westward longshore drift, and the area north of the ridge steadily enlarged and deepened into a sound or bay.
Mississippi River Deltaic Deposits
During the time that the Pine Island Beach Trend was an active shoreline, the Mississippi River had little or no influence on the area. The river delta was located well to the west offshore from central coastal Louisiana, and no significant amounts of alluvium were transported into the New Orleans area. However, about 4,600 years ago that situation changed rapidly and dramatically as the Mississippi River diverted to a new course upstream in the alluvial valley and began extending distributaries into the area.
The initial effect of delta growth in the area was the deposition of fine-grained prodelta deposits in rapidly shallowing water on both sides of the beach trend. Soon thereafter, eastward advancing major distributaries encountered the western end of the beach trend, effectively marking the beginning of estuarine conditions in the area of Lakes Pontchartrain and Maurepas. During the subsequent one and a half millennia, deltaic sedimentation led to the formation of the St. Bernard delta complex (also called subdelta). By about 3,000 years ago the trunk channel of the river extended through the New Orleans area, and the river mouth was located as far as 145 km (90 mi) east of the city.
During the time the St. Bernard delta complex was active, sea level had reached its present level, and the New Orleans area was characterized by extensive tracts of interdistributary swamps and marshes between narrow natural levee ridges. Deposits of the former consist of extremely soft and compressible, high-water-content clays and silts with large amounts of organic matter, while those of the latter consist of relatively firmer, moderately oxidized and desiccated silty and sandy clays.
Depending on where delta lobes formed and how long they were active, active sedimentation from overbank flooding waxed and waned in the New Orleans area but remained substantial. The last major episode of sedimentation in the St. Bernard complex took place as recently as 1,000 to 2,000 years ago and is marked by the formation of the Metairie Bayou and Bayou Sauvage distributaries. These distributaries never carried more than a small percentage of the total river discharge and were active for probably not more than a few centuries; nevertheless, they formed a narrow natural levee ridge that became a major factor in the early settlement of the city. Even a ridge only 1 to 2 km wide that extends above sea level and has relatively competent soils is a premium resource in this part of Louisiana.
By about 1,000 years ago the Mississippi River became well established in its present channel through New Orleans. Since no distributaries were active and the natural levees along the trunk channel were well developed, relatively little flooding took place and sedimentation in interdistributary areas was minimal.
Subsidence
Moderate rates of subsidence are a natural consequence of the Holocene alluvial and deltaic sedimentation processes that produced the land on which metropolitan New Orleans is built. However, the artificial surface and subsurface drainage that accompanied urban development has significantly increased subsidence rates throughout the region. Amounts and rates of subsidence are also directly related to the sediment types found in the Holocene Mississippi delta complex.
Saucier calculated the average rate of regional subsidence in the New Orleans area to have been approximately 12 cm (4 in.)/century for the past 4,400 years. This figure is based on radiocarbon dates of peat deposits and does not include the estimated rate of sea-level rise during this period. On a smaller scale, the process is acting on individual sediment types and landforms at different rates. For example, natural levees and barrier island sands have high bulk density and thus subside faster than surrounding day and organic sediment.
According to Terzaghi, subsidence occurs as a result of three principal causes:
1) Primary Consolidation: the reduction in volume of a soil mass caused by the application of a sustained load to the mass, which squeezes water out from its void spaces.
2) Secondary Compression: the reduction in volume of a soil mass caused by the application of a sustained load to the mass, when the internal structure of the soil mass adjusts after water is squeezed out.
3) Oxidation of Organic Matter: the reduction in volume of a soil mass as chemical reactions occur that cause the organic matter to decompose into its mineral constituents.
When the groundwater level (water table) is lowered during artificial drainage, the material above the new water table is no longer buoyed up by the subsurface water. Therefore, an increased load is placed on all material below the new water-table elevation. Deep strata, both organic and inorganic, then undergo primary consolidation and secondary compression over a period of years. Additional compaction and subsidence are caused by primary consolidation, secondary compression, and oxidation in the material above the new water table. Thus, whether the volume change is due to primary consolidation, secondary compression, or oxidation of organic matter, the total amount of subsidence is directly dependent on the level to which drainage lowers the water table.
Relationship of Subsidence to Sediment Type
"When part of a delta complex is drained for urban development, subsidence greatly accelerates, but at different rates in each of the major deltaic sediment types:
1) Silty and sandy clays along the natural levee are affected the least. Natural levees represent the highest elevations in the Mississippi Delta, so most were not completely water saturated at the time of urban development. Further, as this relatively coarser sediment has a grain-supported internal structure, it is only slightly affected by dewatering of pore spaces. The same is true of barrier island and shoal sands (Pine Island Beach Trend) buried by the advancing delta.
2) Backswamp and interdistributary-trough clays, which underlie much of the cypress swamp in the New Orleans area, are subject to shrinkage upon drying as the internal structure of the clay is partly water supported. However, the low permeability of this sediment usually prevents it from drying to more than a few feet below the exposed surface, thus minimizing subsidence. The subsidence potential rises with increasing organic content because the presence of organic matter increases permeability and allows deeper drying. Buried logs and stumps in these deposits also provide pathways for moisture loss; and since they decompose when exposed to air, an irregular, hummocky surface is common after subsidence.
3) Peat deposits in the interdistributary marshes are highly permeable and have the greatest potential for subsidence when drained. Particularly large amounts of subsidence occur when the upper peat is left exposed to the atmosphere and shrinkage occurs. Desiccation of the highly organic soils results in extremely large capillary forces acting to compress the upper layers of soil. The compressive forces are much greater than those imposed by the overburden so that the soils are overconsolidated to the point of forming a stiff upper crust. The desiccation also allows oxidation of the organic matter to occur.
Although decomposition contributes to the volume change of the upper organic layers, the shrinkage caused by the large capillary forces associated with desiccation appears to be the dominant factor. On drying, Interdistributary marsh peat typically shrinks to 25 to 30% of its original volume.
Overview of Subsidence-Related Engineering Problems
in New Orleans
Prior to the mid-1950s, most of the construction in metropolitan New Orleans had been on the natural levees of the present Mississippi River and its former distributaries and in the reclaimed cypress swamps that border the natural levees. Most residential construction used raised-floor foundations supported by masonry pillars. Sometimes wooden pilings were driven to support the foundation pillars, but more often they were not. Most of these homes are still standing, although irregular subsidence requires periodic foundation leveling (shoring) in many neighborhoods.
An unfortunate coincidence was the widespread change to the concrete-slab foundation system by residential contractors at the time the marshlands in metropolitan New Orleans were developed. Some of the early construction in the reclaimed marshlands proved disastrous. The soft, spongy peat failed to support heavy concrete slabs, which simply sank into it, occasionally tilting and breaking in the process. Builders quickly learned, however, that if enough wooden pilings were driven 10 to 15 m (33 to 49 ft) through the peat into marine clays or sands below, the friction on these pilings would support the slab.
As had sometimes been the case in the cypress swamps reclaimed earlier, the drained land surface of the marsh is so low and hummocky that it is usually necessary to add up to 1 m (3 ft) of fill to level and elevate building sites. This is usually done on a lot-by-lot basis, and a variety of fill materials have been used ranging from broken concrete and asphalt to topsoil. Sand dredged from the Mississippi River or other nearby river channels is presently the common fill material for residential sites.
Differential Subsidence
The most serious engineering problems are related to the differential subsidence that occurs between houses or other buildings on pile foundations and the surrounding ground surface. "When houses or buildings are constructed using the slab-on-pilings technique, the foundation is usually stabilized, but the area surrounding the buildings often continues to subside. Many homeowners fill their yards with 10 to 20 m3 (13 to 26 yd3) of soil each year to compensate for this differential subsidence.
Hazards and Damage Due to Differential Subsidence
Major effects of subsidence include widespread damage to building structures; to sewer, water, and natural gas lines; and to streets, driveways, and sidewalks. Recent studies have also revealed failures of pile-supported foundations built over filled canals and other landfills. In some cases, oxidation and decay of subsidence-exposed piles have led to foundation failure.
As troublesome and expensive as subsidence-caused maintenance problems are, perhaps the greatest hazard is exploding natural gas. Gas and other utility service drops were buried in the soil, which is the usual practice. The stress created by differential subsidence was sometimes great enough to rupture gas lines, releasing gas into highly permeable drained peat. If the fill layer is less permeable than the peat, the gas may migrate some distance, eventually accumulating under a concrete slab foundation. Between 1972 and 1976 six homes were destroyed by subsidence-related natural gas explosions in Jefferson Parish. Fortunately, house-by-house replacement of rigid gas service lines with flexible lines and connectors appears to have ended the explosion hazard.
