United States Department of Slope Stability Reference Guide Agriculture for National Forests Forest Service Engineering Staff in the United States Washington, DC Volume I11 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 publication. Cover Photo CAMP 5 SLIDE, Willamette National Forest, Region 6, Eugene, OR This photo shows the toe of a 250,000 cubic yard landslide that was initiated by a 15-foot horizontal road realignment. The mechanics of failure were largely controlled by excess pore-water pressures at the base of a clay soil having only residual shear strength. The slide was stabilized by installing 7,000 linear feet of horizontal drains which lowered the piezometric surface 14 feet. This photo point is located at milepost 0.1 on Forest Service Road 1926, approximately 13 miles northeast of Oakridge, Oregon. Photo by Michael Long, Willamette National Forest, Eugene, OR. 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 infona- tion (Braille, large print, audiotape, etc.) should contact the USDA Office of Communications at 202-720-5881 (voice) or 202-720-7808 (TDD). To file a complaint, write the Secretaly of Agriiulture, US. Depattment of Agriculture. Washington, D.C. 20250 or call 202-720-7327 (voice) or 202-720-1127 (TDD). USDA is an equal employment opporlunity employer. United States Department of Slope Stability Reference Guide Agriculture for National Forests Forest Sewice Engineering Staff in the United States Washington, DC EM-7170-1 3 Volume I11 August 1994 Coordinators: Rodney W. Prellwitz Thomas E. Koler John E. Steward Editors: David E. Hall Michael T. Long Michael D. Rernboldt SECTION 6 EVALUATION OF STABILIZATION ALTERNATIVES Principal contributors: Mike Burke, Geotechnical Engineer Mike Rembolt, Assistant Forest Engineer (Section Leader) USDA Forest Service USDA Forest Service Payette National Forest San Juan National Forest Supervisor's Office 701 Camino Del Rio P.O. Box 1026 Durango, CO 81301 McCall, ID 83638 Cliff Denning, Geotechnical Engineer Red Renteria, Geotechnical Engineer USDA Forest Service USDA Forest Semce Mt. Hood National Forest Regional Office Engineering 2955 NW Division Street 324 25th Street Gresham, OR 97030 Ogden, UT 84401 Ken Inouye, Geotechnical Engineer Ed Rose, Geotechnical Engineer USDA Forest Service USDA Forest Service Regional Office Engineering Klamath National Forest 2245 Morello Avenue 1312 Fairlane Road Pleasant Hill, CA 94523 Yreka, CA 96097 Gordon Keller, Geotechnical Engineer John Steward, Geotechnical Engineer USDA Forest Semce USDA Forest Service Plumas National Forest Washington Omce Engineering 159 Lawrence Street 201 14th Street, S.W. Quincy, CA 95971 Washington, DC 20250 Mike Long, Engineering Geologist Ted Stuart, Regional Geotechnical Engineer USDA Forest Service USDA Forest Service Willamette National Forest Regional Office Engineering P.O. Box 10607 2245 Morello Avenue Eugene, OR 97440 Pleasant Hill, CA 94523 Jim McKean, Engineering Geologist Richard VanDyke, Geotechnical Engineer USDA Forest Service USDA Forest Service Regional Office Engineering Siskiyou National Forest 2245 Morello Avenue Westside Engineering Zone Pleasant Hill, CA 94523 93976 Ocean Way Gold Beach. OR 97444 Rod Prellwitz, Geotechnical Engineer USDA Forest Service Intermountain Research Station 1221 S. Main Moscow, ID 83843 Page 6A Stabilization Considerations ............................................733 6A.I Introduction .................................................. 733 6A.2 Prestabilization Analysis Considerations ..............................733 6A.3 Selection of Feasible Stabilization Techniques ..........................734 6A.4 Analysis of Stabilization Alternatives ................................734 6A.5 When Does "Safe" Become "Unsafe"? ...............................736 6A.5.1 Litigation .............................................736 6A.5.2 Review of Designs by Others ...............................736 6B Modification of Geometry .............................................737 6B.1 General Information ............................................737 6B.2 Necessary Redesign Information ...................................737 6B.3 The Stability of Design ..........................................738 6B.3.1 Modification of Road Cross-Section ...........................738 6B.3.1.1 Flattened Slopes .................................738 6B.3.1.2 Steepened Slopes .................................739 6B.3.1.3 Lightweight Embankments ..........................740 6B.3.2 Shift in Horizontal Alignment ...............................741 6B.3.3 Modification of Vertical Alignment ...........................741 6B.3.3.1 Sag Vertical Curve ................................742 6B.3.3.2 Crest Vertical Curve ...............................742 6B.4 Construction and Preconstruction Considerations ........................ 743 6B.5 Relative Costs ................................................744 6B.6 Sample Problems ..............................................744 6C Surface and Subsurface Drainage ........................................767 6C.1 General Information ............................................767 6C.2 Surface Drainage ..............................................767 6C.3 Subsurface Drainage ............................................768 6C.3.1 Cutoff Trenches .........................................768 6C.3.2 Horizontal Drains ........................................781 6D Horizontal Drains ...................................................783 6D.1 General Information ............................................783 6D.2 Predesign Information-Investigation Techniques ........................784 6D.2.1 Area Reconnaissance .....................................784 6D.2.2 Ground Control Survey ....................................784 6D.2.3 Subsurface Interpretation ...................................785 6D.2.4 Drive Probe Exploration ...................................785 6D.2.5 Electrical Resistivity Profiling ............................... 785 6D.2.6 Drilling Exploration ......................................785 6D.2.7 Permeability Testing ......................................786 6D.2.8 Ground Water Tracing ....................................786 6D.2.9 Water Surface Contours ...................................786 6D.2.10 Test Drain Installation .................................... 787 6D.3 Drainage System Design .........................................788 6D.3.1 Ground Water Recharge Capacity ............................788 6D.3.2 Number of Drains Needed .................................789 6D.3.3 Slot Width and Spacing ...................................790 Table of Contents Page 6D.3.4 Effective End Spacing and Drawdown ..........................790 6D.3.5 Collector System .........................................797 6D.4 Construction Considerations .......................................797 6D.4.1 Suggested Construction Practices .............................797 6D.4.2 Inspector Duties .........................................798 6D.4.3 Alternative Construction Method ..............................799 6D.5 Post-Construction Monitoring ......................................799 6D.6 Case History Summaries ..........................................800 6D.7 Design Example-< amp Five Slide ..................................802 6E Buttresses .........................................................803 6E.1 General Information .............................................803 6E.2 Rock Buttresses ................................................803 6E.2.1 General ...............................................803 6E.2.2 Foundation Bearing Capacity ................................804 6E.2.3 External Stability ........................................804 6E.2.4 Drainage ..............................................805 6E.2.5 Construction ............................................805 6E.3 Earth Buttresses ................................................805 6E.3.1 General ...............................................805 6E.3.2 Internal Stability .........................................806 6E.3.3 Drainage ..............................................806 6E.3.4 Construction ............................................806 6E.4 Retaining Walls ................................................807 6E.4.1 General ...............................................807 6E.4.2 Foundation Bearing Capacity ................................807 6E.4.3 External Stability ........................................807 6E.4.4 Internal Stability .........................................807 6E.4.5 Drainage ..............................................807 6E.4.6 Construction ............................................807 6E.5 Buttress Sample Problem .........................................808 6E.5.1 Description of Area .......................................808 6E.5.2 Background ............................................808 6E.5.3 Field Survey ............................................809 6E.5.4 Field Investigation ........................................810