25 10 28 Slope Stability Review

Chairman and Members of the West Orange Planning Board Application No. PB24-01, West Essex Highlands, LLC, Warner Road, Block 179, Lot 32 Review of Applicant’s Engineer’s Slope Stability Analysis Reports, the Objector’s Engineer’s Response Reports; and the Applicant’s Engineer’s Rebuttal Report October 28, 2025 Page 15 of 21 surfaces passing through the rock. Langan employed a GSI of 65 in their initial analysis and considered a GSI of 50 in their supplementary analysis for their Rebuttal Memorandum. Princeton Hydro recommended a low value of the index of 20. Sandstone Bedrock (Feltsville Formation): The geologic mapping indicates that the sandstone contact with the Basalt Sill is likely located near the elevation 500 contour and that the sandstone dips below the basalt on a westerly inclination on the order of 10 degrees from the horizontal. Langan recognized in their Rebuttal Report (Ref.1C) that Borings LB-2 and LB-3 were likely to have been drilled into decomposed Sandstone bedrock and terminated at elevations 472 and 503 feet, respectively, which is below and slightly above elevation 500, which is the approximate contact indicated on the Geologic Maps. The analysis sections do not include the decomposed Sandstone bedrock in the failure surface penetrating the rock. We anticipate that strength indicated for the Decomposed Rock would be appropriate for the decomposed sandstone (cohesion = 0) since the failure surfaces in the overlying soil are likely to be the determinant factor for assessing the stability of the slope. Groundwater Conditions In our experience, establishment of a consistent groundwater surface in jointed bedrock in the area of a slope with relatively high topographic relief is very difficult to achieve. In addition, the intact rock has a very low permeability. The rock formation, however, has a variable secondary permeability which is created by the complex network of intersecting fractures. Depending on the width of the fractures, the filling and surface roughness of cracks surcharged by water levels high up on the slope can develop localized high hydrostatic pressures which could affect the local stability of the soil overburden. The persistent wetland delineated on the site plan in the stream valley about 350 feet west of the ridge is an indication of a possible groundwater level. The plan indicates a persistent stream that may give

sistent wetland delineated on the site plan in the stream valley about 350 feet west of the ridge is an indication of a possible groundwater level. The plan indicates a persistent stream that may give an indication of the groundwater levels behind the ridge. The nearest stream to the west of the ridge is shown to emerge near elevation 592. The wetlands markers to the west of the building indicate potential water levels as high as elevation 606 feet to the north of the ridge. The Langan explorations encountered seepage in Test Pit TP-1 at eleven feet below the ground surface at approximate elevation 595 at a point that is about 45 feet west of the crest of the ridge. Sor reported water seepage elevations ranging from elevation 584 at TP-E1 to elevation 581 at TP-E2, located in proposed infiltration Basin 11. They reported seepage at 10 feet below grade in Test Pit TP-D1 at elevation 571 feet which is 10 feet below that observed at TP-E2, which is only 45 feet away. The seepage level noted at Test Pit TP-D2 at elevation 572 is consistent with the level observed at TP-D1 which is 205 feet away.