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Thistle landslide: Was mitigation possible?

By
James E. Slosson
James E. Slosson
Slosson and Associates, 15500 Erwin Street, Suite 1123, Van Nuys, California 91411
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Delmar D. Yoakum
Delmar D. Yoakum
6634 Valjean Avenue, Van Nuys, California 91406
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Gerard Shuirman
Gerard Shuirman
1426 Eastwind Circle, Westlake Village, California 91361
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Published:
January 01, 1992

Abstract

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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Contents

GSA Reviews in Engineering Geology

Landslides/Landslide Mitigation

James E. Slosson
James E. Slosson
Slosson and Associates, 15500 Erwin Street, Suite 1123, Van Nuys, California 91411
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Arthur G. Keene
Arthur G. Keene
2601 E. Victoria Street, #308, Rancho Dominguez, California 90220
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Jeffrey A. Johnson
Jeffrey A. Johnson
Jeffrey A. Johnson, Inc., 12818 Stebick Court, San Diego, California 92130
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Geological Society of America
Volume
9
ISBN electronic:
9780813758091
Publication date:
January 01, 1992

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