Geologic Hazards – An Overview

Geologic hazards of one variety or another occur over many parts of the United States, and represent a danger to both human welfare and public and private property. Generally, there are two categories of geologic hazards, including those that form on the order of less than 30 seconds to several days and tend to be more dramatic, to those that form gradually, taking decades to manifest themselves. Notably, not all geologic hazards are associated with erosional activity. Examples of instantaneous geologic hazards in the United States include volcanic eruptions, earthquakes, landslides, and sinkholes. Geologic hazards that form over the course of several months to several decades include ground settlement (such as the densification of loose surficial soils), aquifer compression, active (shrink-swell) soils, beach erosion / shoreline migration, and hill creep.

Winter Park Sinkhole
On May 8, 1981 the ground surface to the northwest of the intersection of Fairbanks Avenue and Denning Street in Orlando, Florida gave way, forming a sinkhole that ultimately measured about 350 feet wide and about 75 feet deep (left-hand photo). As the ground surface collapsed, a house fell into the sinkhole, along with several cars and part of a municipal swimming pool. In its present-day configuration (right-hand photo), the sinkhole is the centerpiece of a municipal park that includes “Lake Rose”, in honor of the homeowner who lost her house.

Active (i.e. “Shrink – Swell”) Soils
Certain varieties of clay minerals have an inherent property that expand or contract as water enters or leaves the molecular lattice much in the way that a dry sponge will expand as it absorbs water. In addition to natural changes in moisture content in the soil typically associated with seasonal variations in rainfall, the presence of large trees may also exaggerate the effects of “shrink – swell” soils. Recent literature reports that damage nationwide due to the effects of these soils is as high as $7 billion.

Hillside Creep
Although not common in Florida because of the generally flat terrain, hillside creep represents a long-term condition that may affect structures, roadways, and fencelines over several decades. This condition is caused by the long-term, gradual downhill movement of soil under the force of gravity. Conditions that greatly affect the presence and/or speed of “creep” include grain size (i.e. relative sand and clay content), vegetative cover, and moisture. Some sources do not classify “creep” as a hazard due to the long period of time associated with the condition. Even so, given enough time, creep has the potential to affect roadways and foundations.

Landslides / Mudslides
Landslides and mudslides are forms of “mass wasting”, a broad geologic term that is associated with instantaneous erosional events that also include avalanches and debris flows. Due to the relatively flat terrain in most of Florida, such events are quite unusual, although they may still occur when the slope of the ground surface is destabilized by rainfall. In the photograph below the ground surface gave way along the side of SR 50 in Clermont, Florida resulting in the soil from the embankment flowing into a duplex. No one was home at the time.