ABOUT LOESS SOILS and the Natchez Bluff.
Distribution of Natchez (LOESS) soils by county-Lower Mississippi river. (Click to Enlarge)
LOESS soils are distributed nationally and exclusively in only three regions:
(1) The Palouse area, Washington State
(2) The Upper Missisippi (Illinois, Iowa area) and
(3) The Lower Mississippi. (see photo)
Layers 1 & 2:
Two Loess formations comprise the upper layer of silty soils, which is 50+ feet deep. Windblown, very light and calcium-bonded.
Natchez Formation consists of layers of interbedded clay, sand and silt changing to dense or gravelly sand with increasing depth.
Hattiesburg Formation is hard clay shale.
Failure types along the Natchez Bluffs.
Five kinds of failures of the Natchez Bluffs documented by the U.S. Army Corps of Engineers in its 1985 Natchez Bluff Study include:
1. Rotational Slump.
A Rotational Slump is the downward movement of a soil mass along a circular failure surface. Slumps can begin with a small crack even more than 50 feet from the edge of the bluff that runs parallel to the bluff line. An example of a large-scale slump at Natchez is the 1951 Clifton Avenue slump, which began with a crack on Oak Street. Water infiltration into the surface cracks by rainfall and leaky water lines, swimming pools, storm and sanitary sewer lines weaken the water and calcium bonds of the loess and increase the weight of the soil mass. The additional weight and the weakened loess eventually results in the cascading movement of a block of soil.
Small Slump: The top 50-feet of the bluff (the two loess layers) cascades downward in mass.
Medium Slump: The 50-foot loess layers PLUS the third layer of clay, silt and gravelly sand cascades in mass.
Large Slump: All four layers, including the hard shale clay layer at the bottom cascade in mass.
The likelihood of a slump is increased by the removal of previous slide debris from the toe of the slope by artificial excavation; and surface wash/erosion into the Mississippi River; and increased porewater pressure at the interface of the sands and the hard clay shale.
2. Soilfall and soil slide.
Soilfalls and soil slides are common in loess due to the steep slopes and the vertical jointing. They are triggered by water infiltration, desiccation (extreme dryness), freezing and thawing, undermining of the loess, removal of slide debris at the toe of the slope, and steep slopes caused by cascading effect of rotational slumps.
Mudflow occurs at the BASE of the bluff where the saturated slide debris has accumulated. Triggered by rainfall infiltration, groundwater seepage at the sand-clay shale interface and water from leaking utilities.
4. Soil creep.
Soil creep is a very slow, continuous, imperceptible movement of the soil. It is caused by rainfall, plant roots, freezing and thawing, burrowing of animals, and swaying of trees.
5. Surface wash.
Another continuous process of erosion by flowing water, mostly in un-vegetated areas, caused by predominantly by rainfall, and also seeping groundwater and leaky utilities.
Source: U.S. Army Corps of Engineers, Natchez Bluff Study, 1985