Thistle landslide: Was mitigation possible?
In April 1983, Spanish Fork Canyon, Utah, was engulfed by a massive landslide that dammed Spanish Fork Creek, creating a lake. The slide pushed and finally buried sections of the Denver & Rio Grande Western Railroad line and U.S. Highways 6, 50, and 89, which for decades had been located in Spanish Fork Canyon. When motion of this massive debris flow and/or landslide complex finally ceased, almost the entire 6,800-ft-long (2,040 m) and 800-1,100-ft-wide (240–330 m) mass, more than 200 ft (60 m) thick had moved. Spanish Fork Canyon, ~600 ft (180 m) wide at the location of the landslide, was filled to a depth of ~220 ft (66 m), causing a lake to form on the upstream side.
Data indicated that small displacements at the toe of the slide, adjacent to the Denver & Rio Grande Western Railroad, had been recorded since the early 1900s. Motion was correlative with years of above-average precipitation. Thus, the extraordi-narily high precipitation of 1983, which was only ~2.2% higher than the previous record high precipitation of 1875–1876, added sufficiently to the ground-water regime to cause slow motion to develop in early April, followed by rapid acceleration in mid-April. Borings placed in 1984 indicated the existence of an artesian condition, the pressure level ranging from 25 to 65 ft (7.5 to 19.5 m) above ground surface near the toe of the slide. Other borings drilled at a later date provided samples from which strength parameters were determined. Test data indicate strength as low as 17.5° angle of internal friction and 210 lb/ft2 cohesion.
Information obtained from all of these studies strongly suggests that had a surface-drainage system with horizontal (subsurface) drains been constructed prior to failure, landslide motion could have been prevented. Analyses indicate that a readily achievable lowering of the hydrostatic pressure on the slide planes would have produced a factor of safety greater than 1.0.
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Provides a variety of case histories, methodology to help identify, quantify, and mitigate landlsides, and legal cases affecting engineering geology. Part I provides basic information to aid in assessing geologic hazards related to compound landslides, surficial slope failures, and causes of distress to residential construction. Includes changes in the law relating to geologic investigations and disclosure of geotechnical information. Part II is a cross section dealing with recent significant landslides related to a single storm, intense rainfall, possible errors in the identification of and development on an existing or paleolandslide, and the use of pumping wells and horizontal drains to dewater slope failures. Also discusses how proper installation and use of drains prevent paleolandlsides from causing damage to modern facilities.