Abstract

Soil- and rock-related engineering geologic problems occur in a variety of geologic settings and are some of the most widespread and costly geologic hazards. Six types of problem soil and rock are present in southwestern Utah. The most extensive are expansive soil and rock. The majority of expansive soil problems are related to bentonitic marine shales near St. George, and Lake Bonneville or other deep-lake sediments in the Sevier Desert and basins west of Sevier Lake.

Subsidence of the ground surface due to collapsible soil has caused extensive damage in and around Cedar City. Collapsible soil is common in Holocene alluvial-fan and debris-flow deposits in southwestern Utah. Soil and rock containing gypsum are also susceptible to subsidence. Ground water and introduced waters from irrigation dissolve gypsum causing subsidence.

Limestone susceptible to dissolution and subsidence occurs throughout mountains west of Sevier Lake, west of Richfield, and south of St. George. No known damage to structures has occurred from ground collapse or subsidence related to limestone karst, but because karst ground-water systems have little filtering capacity, contamination of ground water is a major concern. Piping is a common problem in fine-grained Holocene alluvium incised by streams in much of southwestern Utah. Collapse of soil pipes and subsequent erosion has damaged roads and agricultural land.

Sand dunes occur in the Escalante Desert and west of Kanab. Migration of dunes across roads and burial of structures are common problems in areas where active dunes are present.