ANALYSIS OF HAVEN ACRES LANDSLIDE, CLATSKANIE, OREGON

Along the Columbia River in the Oregon Coast Range, Columbia River Basalts overly weak Eocene marine sediments which creates unstable slopes in the river valleys. In Clatskanie, Oregon early Miocene Columbia River Basalts act as a cap rock over the weak Pittsburg Bluff marine sediments. The marine sediments presumably were undercut by the Clatskanie River; therefore, the slope lost its toe and failed rotationally. The area of the landslide is 2.235 million square yards with a volume of 248.627 million cubic yards characterizing the landslide as a huge slump. Over the past fifty years 176 homes and 7.3 miles of road have been built on the landslide with even more land being developed. Highway 30, the main highway to Astoria from Portland cuts through the landslide along with Northwest Natural's natural gas transmission pipeline running along the toe. Using the plasticity index tests and available well log data a factor of safety of 1.27 was calculated using Slope-W slope stabilizing program with the Morgenstern-Price method. Using the slices from the Morgenstern-Price method a mean thickness of 106 yards was calculated for the huge slump. With the high area of interest, the preliminary calculations suggest that there should be funds put into a detailed slope stability analysis of the landslide to assure the safety of the Clatskanie area. Piezometers must be installed to better identify the piezometric levels and the stratigraphy of the landslide. Significant cuts on the toe and loading at the head should be avoided.