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NCHRP LRD 82 3 POTENTIAL LIABILITY ASSOCIATED WITH UNSTABLE SLOPE MANAGEMENT PROGRAMS Timothy R. Wyatt, GeoLawTM, Conner Gwyn Schenck pllc, Greensboro, NC I. INTRODUCTION Surface transportation is naturally tied to the topology of the earth, and it is unavoidable that highways will be constructed on or adjacent to earthen slopes, especially those highways provid- ing access to hilly or mountainous areas.1 In hilly or mountain- ous areas, cut-and-fill operations designed to produce a con- stant highway grade can result in slopes and cut faces adjacent to the highway that are even steeper than the natural topology.2 Even in relatively flat terrain, man-made slopes and embank- ments adjacent to highways (e.g., grade crossings) are common.3 All earthen slopes are at some risk of failure. The prominent types of slope failure include: â¢ Slides, or landslides. This is where a mass of earth on the surface of the slope overcomes the cohesive or frictional forces holding it in place, and slides or translates down the slope along a failure surface between the sliding mass and the stationary earth. This is most common with soil slopes.4 â¢ Falls, or rockfalls. This is where a mass breaks apart from the existing slope, often along an existing crack, and falls or tumbles down the slope, not necessarily in 1 See, e.g., Lawrence A. Pierson, Rockfall Hazard Rating System, 1343 Transp. Res. Rec.: J. Transp. Res. Bd. 6 (1992) (âIn mountainous states such as Oregon, many miles of roadway pass through steep ter- rain where rock slopes adjacent to the highway are common. Some of these man-made slopes are over 100 ft high and many are situated near the base of rugged natural slopes extending hundreds of feet further upslope, creating an inherent rockfall potential.â). 2 Id. (âUntil recently, it was standard construction practice to use overly aggressive blasting and ripping techniques to construct rock slopes. This construction practice facilitated excavation and frequently resulted in slopes prone to rockfall.â). 3 Paul D. Thompson, Geotechnical Asset Management Plan, Alaska Department of Transportation and Public Facilities (DOT&PF) Rep. No. STP000S (802) (B), 8 (2017) (â[T]here are reasons why the Department builds and maintains its slopes, embankments, retaining walls, and material sites.â). 4 See Gray v. State, 159 A.D.3d 1166, 1168, 72 N.Y.S.3d 208, 211 (N.Y. App. Div. 2018) (â[A] rock fall and a mudslide were two distinct geologic events caused by separate and distinct triggering mecha- nismsâa mudslide being caused by soil becoming oversaturated and ultimately separating from the underlying slope and a rock fall occur- ring when a piece or section of rock detaches from a rock face.â); Wash- ington State Department of Transportation, WSDOTâs Unsta- ble Slope Management Program (Jan.Â 2018), available at https:// www.wsdot.wa.gov/sites/default/files/2011/03/11/Unstable-Slope- Management-Program-Folio.pdf [hereinafter Washington State DOTÂ USMP]. contact with the stationary earth. This is most common with rock slopes and faces.5 Even well-constructed slopes are subject to degradation and failure over time, often as a result of weathering, predominantly by stormwater.6 For example, where a cut slope contains layers of different types of rock with different levels of susceptibility to erosion, the slope can become steeper over time as softer under- lying layers erode away, undercutting the support for harder rock layers at higher elevations, which ultimately results in rock- falls.7 Stormwater that infiltrates small cracks in rock slopes is subject to freeze-thaw cycles, causing the cracks to expand and ultimately causing rock fragments to break away from the face and fall.8 In soil slopes, stormwater can infiltrate voids in the soil, and the additional weight of the saturated soil can over- come the frictional forces holding the slope in place, resulting in slides.9 Slope failures such as landslides and rockfalls pose risks for highway owners such as State Departments of Transportation (DOTs) such as: 5 Id. 6 See, e.g., Darren Beckstrand, brent Black, Paul Thompson, Aine Mines, Benjamin George & David Stanley, Rockfall Haz- ard Process Assessment, Rep. No.Â FHWA/MT-17-008/8239-001, AppendixÂ A (Oct. 2015), taken from MDT Rockfall Hazard Pro- cess Assessment, State of Montana Project No. 15-3059V Task 1 Report, 30 (Oct. 2017) (âSlope failures are typically triggered by natural events, such as earthquakes, floods, ground saturation, groundwater movement, freeze/thaw, or general instability. These events are inher- ently uncontrollable.â); Martin J. Woodard, Development of a Rockfall Hazard Rating Matrix for the Ohio DOT, 11 (May 2004) (unpublished Ph.D. dissertation, Kent State University) (listing factors causing land- slides, including precipitation, erosion, freeze-and-thaw watering, shrink-and-swell weathering, and stormwater drain leakage). 7 See, e.g., Woodard, supra note 6, at 119 (âThe primary cause of rockfalls in the state of Ohio is the differential weathering of alternating series of durable and nondurable rocks.â). 8 See, e.g., Amended Memorandum of Points and Authorities in Support of Plaintiffsâ Opposition to Defendantsâ Motion for Summary Judgment, Terbush v. United States, No.Â 02-CV-05509, 5 (E.D.Â Cal. Dec.Â 8, 2009) (âEvery expert who has testified in this case .Â .Â . agree[s] that water getting into joints in the cliff face are the most common cause of rockfalls in the Yosemite Valley.â). 9 See, e.g., Gray v. State, 159 A.D.3d 1166, 1168, 72 N.Y.S.3d 208 (N.Y. App. Div. 2018) (stating that a slide is âcaused by soil becoming oversaturated and ultimately separating from the underlying slopeâ); Dan Pratt & Paul Santi, A Landslide Hazard Rating System for Colorado Highways, Proceedings of ASCE Rocky Mountain Geo- Conference, Lakewood, Colo., 127â28 (2014) (describing how soil saturation increases the âdriving forcesâ contributing to landslides).