Landslides

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Downslope movements of rock and soil, collectively called landslides, cause many millions of dollar of damage yearly in Ohio. These gravity driven movements occur in many areas of the state where slopes and failure-prone materials exist. Although such movements occur naturally, many landslides are triggered by human activities.

Slow, downslope movements of unconsolidated surface materials called creep, is widespread as can be noted by tilting fence posts, cemetery monuments, telephone poles, and other objects that through time have slowly moved from their original vertical position. Creep is partly a result of alternate freeze-thaw cycles pushing grains apart.

Rotational slumps involve large blocks of rock or sediment that move as a unit along a glide plane. The bottom portion, or toe, of the slump moves outward and downward whereas the top portion of the slump block moves downward but tilts backward. Trees on top of the tilted block lean upslope. Rotational slumps commonly occur in red-colored shales of Pennsylvanian age in southeastern Ohio.

Earthflows occur in unconsolidated materials that are saturated with water and lose the bearing strength of grain-to-grain contact. The sediment moves down slope in a series of lobate flows.

All of these movements are relatively slow and, although seriously damaging to property and structures, generally do not pose an immediate threat to human life. However, rockfall is a category that can be of considerable danger because it involves a large amount of rock that suddenly moves downslope with little or no warning. Generally, rockfalls occur in thick, massive sandstones that are cut by vertical joints. Undercutting of the less-resistant rock beneath the sandstone, either by natural processes or human activities, removes support for the overlying sandstone. Water that penetrates a vertical joint tends to pry the block loose from the outcrop by freezing. Ohio’s only landslide fatality occurred in 1986 along U. S. Route 52 near Ironton when a rockfall crushed a passing car on the highway below.

The most significant landslide damage in the state, in terms of financial loss, occurs in the Cincinnati area. Steep slopes in this densely populated region are prone to landslides where an Ordovician-age unit, the Kope Formation, is present. This shale and limestone unit is prone to slumps and earthflows when vegetation is removed and the slope is undercut by excavations. Lake clays of Pleistocene age are also prone to landslides in this area.

Bluffs of till deposited by the glaciers of the Ice Age along the Lake Erie shore from Huron eastward to Ashtabula are particularly prone to landslides as waves undermine the bluffs, leading to collapse. Significant retreat of the shoreline has resulted in loss of homes, roads, and other structures.

Silts and clays deposited in former glacial lakes in the valley of the Cuyahoga River between Akron and Cleveland are susceptible to landslides, particularly when these sediments become saturated with water. This has been a problem for homes and roads in the area.

In southern Ohio, the Bedford Shale of Late Devonian age is prone to slumping in the valley of the Scioto River between Portsmouth and Circleville. Lake clays deposited in association with Lake Tight, and ice-dammed lake formed in the Early Pleistocene, are prone to landslides in some areas in southern Ohio.

In southeastern Ohio, red-colored shales of Late Pennsylvanian or Early Permian age tend to lose their bearing strength when they become saturated with water, leading to slope failure in the form of slumps and earthflows. Although the area is not densely populated, these landslides have been particularly troublesome along roads vital to transportation.

Most landslides in any category usually occur when the slope is undercut by natural erosion or by human construction activities, when vegetation, the roots of which bind earth materials, is removed, or the rock or when sediment becomes oversaturated with water. Care exercised in construction can commonly prevent future landslides if these factors are taken into consideration. In addition, avoiding construction on or through landslide-prone geologic units will avert the problem.

See Also