Isolated surface slope failures plagued a previously constructed slope at the Westborough High School site. The 40± foot high slope is located along a paved parking area and football field. Northeast Geotechnical was engaged to assess the cause(s) of the slope failures and provide geotechnical engineering recommendations to stabilize/reconstruct the slope.
Northeast Geotechnical coordinated and observed a subsurface exploration program consisting of ten test pits performed throughout the slope area. The subsurface profile along the slope was assessed to consist of topsoil fill underlain by relatively loose sand fill and then dense, natural glacial till soils.
Engineering studies included a review of existing design and construction drawings, test pit information, and the results of the soil laboratory tests, which included sieve and hydrometer analyses, modified Proctor compaction tests, and an interface shear test. Northeast Geotechnical performed slope stability analyses using this test pit and laboratory information and developed an opinion there were insufficient factors of safety against sliding failures along the sand fill and natural glacial till interface, particularly when saturated.
Engineering recommendations included: excavation of existing fill materials down to the underlying natural glacial till soils; inspection and repair of the existing slope underdrain system; and construction of a new slope surface section that consisted of loam fill underlain by filter fabric and then crushed stone. Recommendations for installing an erosion control mat were also provided. Northeast Geotechnical developed a slope detail that was incorporated into the project plans.
