Missouri University of Science and Technology Scholars' Mine International Conference on Case Histories in (1988) - Second International Conference on Geotechnical Engineering Case Histories in Geotechnical Engineering 02 Jun 1988, 10:30 am - 3:00 pm A Finite Element Analysis of the Utah "Thistle" Failure Blaine D. Leonard LTR Associates, Salt Lake City, Utah Joseph M. Olsen University of South Alabama, Mobile, Alabama Follow this and additional works at: https://scholarsmine.mst.edu/icchge Part of the Geotechnical Engineering Commons Recommended Citation Leonard, Blaine D. and Olsen, Joseph M., "A Finite Element Analysis of the Utah "Thistle" Failure" (1988). International Conference on Case Histories in Geotechnical Engineering. 2. https://scholarsmine.mst.edu/icchge/2icchge/2icchge-session3/2 This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License. This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Conference on Case Histories in Geotechnical Engineering by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. Proceedings: Second International Conference on Case Histories In Geotechnlcel Engineering, June 1-5, 1988, St. Louis, Mo., Paper No. 3.48 A Finite Element Analysis of the Utah "Thistle" Failure Blaine D. Leonard Joseph M. Olsen President, LTR Associates, Salt Lake City, Utah Chairman, Civil Engineering Department, University of South Alabama, Mobile, Alabama SYNOPSIS: In the Spring of 1983, a large landslide occurred near the town of Thistle, Utah which blocked major transportation routes and impounded the Spanish Fork River, inundating the town with 200 feet of water. While much attention has been given to the slide and its impact, very little has been directed toward a quantitative understanding of its causes. An analysis was performed of the Thistle landslide using the SEEPSLOPE finite element system in order to evaluate the mechanisms, factors, and causes of the failure. An elastic, perfectly-plastic stress-strain curve was employed in the analysis to model the behavior of the overconsolidated clay soils. It is concluded that the landslide was a compound, progressive failure which initiated at the toe and progressed uphill. Seep­ age forces played a significant role in the failure. INTRODUCTION Early in April 1983, motorists driving through This study was undertaken to quantitatively Spanish Fork Canyon south of Provo, Utah, began address the geotechnical mechanisms involved in noticing cracks in the road near the small town the occurance of the Thistle Landslide. The of Thistle. During the next few days, the objective was to use a finite element stability cracks enlarged, the road began to heave, and analysis to draw conclusions about the physical the nearby railroad tracks began to be distorted. causes and mode of the failure which occurred By the 14th of April, the Thistle landslide, a there. Evidence is presented which demonstrates Quaternary earthflow deposit lying in a small that the slide was a progressive failure in over­ canyon essentially perpendicular to the road, consolidated clays which started at the toe of had moved sufficiently to lift the highway, the slide and progressed uphill. High seepage sever the railroad, and block the Spanish Fork forces played a significant role. in the failure. River. Within 30 days, this blockage had filled the canyon to a height of over 200 feet, butres­ BACKGROUND sing against a large sandstone formation known locally as Billies Mountain. The small town of The area of the slide is near the easternmost Thistle was buried with 62000 acre feet of water. edge of the Middle Rocky Mountain Province which is characterized by generally high mountain Property damage triggered a Presidential Disas­ ranges and plateaus transected by deeply incised ter declaration. Opening a passageway for the erosional valleys. The toe of the slide in ·the Spanish Fork River and rebuilding transportation bottom of the canyon was at an elevation of lines through the canyon subsequently cost local about 5030. The slide extended west approxi­ government and private entities in excess of mately perpendicular to Spanish Fork Canyon a 200 million dollars. The question of what to do horizontal distance of 1200 feet, and then with the slide mass is still unanswered. In southwest another 4500 feet, reaching an eleva­ addition, severe hardships were imposed on the tion of about 5900 at the top. A twenty-foot displaced residents of the small town who as of scarp at the top is noticable, and several re­ early 1987 had not been compensated for the loss lated slides adjacent to the main slide mass are of their property. also evident. Total slide widths vary from about 850 feet at the top to 1200 feet just This mass of overconsolidated clay, over a mile above the bend. Below the bend, widths are long and several hundred feet wide, has moved slightly less than 1000 feet (Duncan et al., several times previously in geologic time, and 1985). The volume of landslide material in the has plagued the railroad lines for most of this canyon has been estimated to be between 3 and century with occassional track movements due to 6.5 million cubic yards (FEMA, 1983; Dames and creep. Deformation had been noted by some Moore, 1985; Kaliser and Fleming, 1986). Total during the early Spring of 1983 which was attri­ volumes involved in the landslide are generally buted to the wet weather conditions. Precipi­ considered to be about 25 to 30 million cubic tation during the winter of 1982-1983 had yards. reached record levels, resulting in abnormally high antecedent moisture conditions. However, a Descriptions of the geology of the Thistle area movement of this magnitude had not been experi­ are given by Witkind and Page (1983), Duncan, et enced in recorded time. al. (1985), and Kaliser and Fleming (1986). According to these sources, three formations The geotechnical aspects of the Thistle land­ underlie or are present as outcrops adjacent to slide are complex and varied, and have been the landslide. The Triassic Ankareh Formation, treated only qualitatively in previous studies. which is a weak, reddish, shaly siltstone and Second International Conference on Case Histories in Geotechnical Engineering 593 Missouri University of Science and Technology http://ICCHGE1984-2013.mst.edu sandstone underlies the Triassic-Jurassic April 14, vertical deformation of the highway Nugget sandstone, a strong, light colored sand­ surface was so severe that the road was closed stone which in turn underlies the Tertiary to traffic, and an the following day, the road North Horn Formation, a weak, partly alluvial had displaced approximately 10 vertical feet partly lacustrine deposit consisting of mud­ (Dames and Moore, 1985). On the evening of stone, claystone, sandstone, conglomerate, and April 15, the last train used the tracks liNestone. The valley in which the Thistle (Kaliser and Fleming, 1986). landslide rests was cut from the Ankareh Forma­ tion which completely underlies the landslide From the first signs of movement considerable and is exposed an the north boundary of the effort was applied to prevent the Spanish Fork landslide. The. Nugget Formation farms the River from being dammed and to keep the canyon prominent ridge that delineates the southeast open. By April 17, it was clear that these flank of the landslide and underlies the land­ efforts were failing and the residents of slide in the canyon bottom. Duncan, et al. Thistle were evacuated. At this time efforts (1985) concluded that "all movement of the had already turned to unloading what was thought Thistle landslide apparently was above this to be the "head" of the slide, in the area imme­ bedrock unit." The North Horn Formation is diately upslope from the railroad cut. Attention exposed along ridges an both the southeast and also turned to preventing overtopping of the dam northwest borders of the landslide. The by the new Lake Thistle, and evaluating new majority of the landslide is composed of debris transportation routes. On April 22, Utah Gover­ and earthflaw material derived from the North nor Scott Matheson declared the area a state Horn Formation. disaster area, and on April 30, President Ron­ ald Reagan made the Thistle slide area Utah's The Thistle landslide is an early Holcene first National Disaster Area. (Kaliser and Fleming, 1986) landslide mass that has moved on several accassions in geologic Movement of the slide was measured by Railroad time (Schroder, 1971 and Duncan, et al., 1985). and county crews during the first crucial weeks However, Duncan, et al. (1985) argue that there of sliding. According to Duncan et al. (1985), was "no evidence that these alder, deep-seated the Railroad reported that the landslide was landslides, should they be present, were active moving at about 0.75 feet per hour on April 14. during 1983 or later." This average rate increased to a maximum of 2.5 to 2.8 feet per hour during the period of April In approximately 115 years of historic records, 17 to 19, and declined to 0.80 feet per hour by there is no indication of massive movement of April 25. Total horizontal displacement for the the Thistle slide, and no available written bottom of the slide during this period is esti­ accounts of small movement (Kaliser and Fleming, mated to be about 500 feet. Vertical displace­ 1986). However, the slide has caused repeated ments Of up to 1.5 feet per hour were noted problems to the rail lines located at. its toe (Dames and Moore, 1985). Peak sliding rates (FEMA, 1983; Sumsion, 1983). The most recent measured by Utah County were on the order of report of troublesome movements dates to just 6.6 feet per hour on April 19.