United States Department of Slope Stability Reference Guide Agriculture for National Forests Forest Service Engineerlng Staff in the United States Washington, DC Volume I August 1994 While reasonable efforts have been made to assure the accuracy of this publication, in no event will the authors, the editors, or the USDA Forest Service be liable for direct, indirect, incidental, or consequential damages resulting from any defect in, or the use or misuse of, this publications. Cover Photo Ca~tion: EYESEE DEBRIS SLIDE, Klamath National Forest, Region 5, Yreka, CA The photo shows the toe of a massive earth flow which is part of a large landslide complex that occupies about one square mile on the west side of the Klamath River, four air miles NNW of the community of Somes Bar, California. The active debris slide is a classic example of a natural slope failure occurring where an inner gorge cuts the toe of a large slumplearthflow complex. This photo point is located at milepost 9.63 on California State Highway 96. Photo by Gordon Keller, Plumas National Forest, Quincy, CA. The United States Department of Agriculture (USDA) prohibits discrimination in its programs on the basis of race, color, national origin, sex, religion, age, disability, political beliefs and marital or familial status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communication of program informa- tion (Braille, large print, audiotape, etc.) should contact the USDA Mice of Communications at 202-720-5881(voice) or 202-720-7808(TDD). To file a complaint, write the Secretary of Agriculture, U.S. Department of Agriculture. Washington, D.C. 20250 or call 202-720-7327(voice) or 202-720-1127(TDD). USDA is an equal employment opportunity employer. United States Department of Slope Stability Reference Guide Agriculture for National Forests Forest Service Engineering Staff in the United States Volume I August 1994 Coordinators: Rodney W. Prellwitz Thomas E. Koler John E. Steward Editors: David E. Hall Michael T. Long Michael D. Remboldt Dedication to Rod Prellwitz Courtrtey Cloyd, Engineering Geologist, Siuslaw National Forest The rugged terrain of America's national forests presents engineering geologists and geotechnical engineers with many challenges. Small budgets and staffs encourage innovative exploration techniques, while high design standards are necessary for compliance with Federal resource protection regulations. These sometimes conflicting conditions have fostered the development of skilled professionals, who often rely on the experiences of their colleagues for help with difficult problems. In this spirit of cooperation the Slope Stability Reference Guide was developed, a spirit that characterizes the career of the guide's instigator, Rod Prellwitz. In recognition of his years of guidance and support as a mentor and friend to geotechnical specialists in the Northwest, we dedicate these volumes, a summary of what we have learned together. Rod Prellwitz's career as a geotechnical engineer is the result of his love of the mountains and forests of the West and his love for the process of solving geotechnical problems. The symboIs of these two loves are his continuing enthusiasm for field work and the license plate on his four-wheel-drive pickup, GEOTECH. Rod's professional training began in 1957 arthe Montana Schoo) ornines (now Montana College of MineraI Sciences and Technology), where h'e &died geological engineering with a petroleum option. After graduating in 1961' he,worked for 3 years as a seismic geophysicist in Wyoming, Utah, and ~olor$o Gith Texaco, Inc. During that time he realized that career advancement in the petroleum industry meant spending long periods of time in such "God-forsaken places" as Houston, Tulsa, and Denver-a depressing prospect for a young man who loved the big, empty country of the West. In 1964, Rod heard that the USDA Forest Service was looking for a civil engineer on the Apache National Forest, and soon he was on his way to Springerville, Arizona, leaving the oil business behind forever. In 1965, he moved on to a position in preconstruction engineering in the regional office at Missoula, Montana, seizing a chance to return to the State he had grown to love. By 1966, he had moved on to Hamilton, Montana, and the Bitterroot National Forest, where he eventually became an assistant forest engineer. Rod later found that he was spending far too much time managing the motor pool and not enough time on engineering; so, in 1969 he transferred to the Northern Region Materials Investigation Unit back in Missoula as a geotechnical engineer. His role as a regional office geotechnical engineer gave him an opportunity to see much of Idaho and Montana and a full range of geotechnicd problems. Rod took graduate courses at the University of Idaho in the early 1970's, graduating with an Dedication iii MS. in civil engineering with a geotechnical option in 1975. In 1979, he became a research geotechnical engineer at the Intermountain Research Station in Missoula, continuing there until his retirement in early 1993. Slope stability analysis and ground water monitoring have been Rod's primary professional interests as a researcher. Since 1975, he has written more than a dozen professional papers, made numerous presentations before professional societies on geotechnical topics, and developed several practical guides and handbooks for use by Forest Service field specialists. Among his papers "SSMOS-Slope Stability Analysis by Three Methods of Slices with the HP-41 Programmable Calculator in 1985 and "SSIS and "SSCHKS-Preliminary Slope Stability Analysis with the HP-41 Programmable Calculator in 1988 were instantly popular with Forest Service geotechnical engineers and engineering geologists in the Northern and Pacific Northwest Regions. Through the development of those publications, the Pacific Southwest and Pacific Northwest Region geotechnical specialists learned what their Northern Region colleagues had known for some years: Rod was a research scientist who understood the field conditions they faced every day, and he was interested in helping them find the answers to their technical problems. He routinely provided technical support to national forest engineers, geologists, and soil scientists who were trying to apply textbook soil mechanics and ground water principles to steep forested terrain conditions and finding that the two were not always compatible. Rod is probably best known in the Forest Service for developing what is commonly called the three-level slope stability analysis concept. The three-level system grew out of his understanding that engineering geologists and geotechnical engineers could contribute their knowledge of soil mechanics and physical processes to forest planning and decision making. His 1983 presentation to the Transportation Research Board 62nd Annual Meeting, Landslide Analysis Concepts for Management of Forest Lands on Residual and Colluvial Soils (T.R. Howard and W.D. Wilson, co-authors), summarizes the three-level system and sets the stage for his later work on Level I Stability Analysis (LISA), Stability Analysis for Road Access (SARA, the level I1 program), and XSTABL (the level 111 program). LISA and XSTABL have become standard analysis tools for resource planning level and site-specific slope stability assessments, respectively, on national forests in the westem United States. As helpful as these tools have been, Rod's willingness to teach others what he has learned is his greatest contribution to the profession. He has always been more than generous with his time when people ask for help, prefening a field trip to look at the problem to talking about it on the phone. His informal style and ever-present sketch pad frequently turn simple questions into impromptu tutoring sessions, where gaining knowledge and understanding are as important as finding the answer to the problem. Over the years, Rod has been a frequent contributor to technical workshops held by the Forest Service, the Oregon and Washington Departments of Transportation, and the Federal Highway Administration. In the last 5 years, he has been a principal instructor at more than half a dozen Forest Service-sponsored workshops focused on the LISA computer model, demonstrating his belief that technical information is most effectively exchanged through discussion and shared experience. Even though we won't see the familiar Montana GEOTECH license plates as often in our office parking lots, Rod has left us a rich legacy of his knowledge and Dedication experience and a promise to be available occasionally for a look at an interesting problem. In thanks for his many gifts, we dedicate the Slope Stabiliry Reference Guide to Rod Prellwitz. Dedication Acknowledgments The authors, editors, and coordinators thank the following individuals for their review efforts and comments that helped to make this publication more user-friendly--quite often sparing the authors some embarrassment. We especially acknowledge Dr. Robert L. Schuster of the US. Geological Survey and Peter Jones of the Rogue River National Forest for their extremely detailed reviews. In addition to the internal review by the authors themselves, the following group of professionals made a tremendous contribution: Matt Brunengo, Washington Department of Natural Resources Tom DeRoo, Mt. Hood National Forest Carol Hammond, Shannon and Wilson Richard Kennedy, Bridger-Teton National Forest Allen King, Plumas National Forest David Knott, GAI