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 19 of 21 2. 3. 4. 5. 6. 7. bedrock formation which appears to begin near Borings LB-2 and LB-4. Nevertheless, we do not anticipate that this will significantly modify the range of critical safety factors. The results of the Langan analyses that included a presumed groundwater level at the surface of the decomposed rock were reported, but the computer output plots were not included. These output plots should be submitted for the record. The results of the stability analyses by both the Objector’s and the Applicant’s Engineers exceed a safety factor of 1.50. On this basis, we may conclude that conventional analysis does not indicate instability for static and seismic stability. The results of the analyses do not discount the concerns for the risk of landslide indicated by the Hazus mapping which underestimated the effective angle of the slope. In our experience, landslides frequently occur in tandem with heavy rainfall. Neither of the Engineers has adequately considered the response of the slopes to heavy rainfall, and groundwater interflow. In addition, the Stormwater Management Plans propose introduction of substantial infiltration basins close to the top of the slope. The Applicant’s Engineers should propose and perform a stability evaluation using a conservative piezometric surface that would reflect the response of groundwater to heavy storm-induced rainfall. The Applicant’s Engineers should, in our opinion, review the mounding influence of the proposed infiltration Basins Nos. 10 and 11 and consider mitigation by either providing a cutoff (possibly using the building foundation walls) preventing infiltration flow to the east, or relocation of the basins further down the backslope. Recommended Requirements to be Considered by the Board The stability of the slope should not be considered a fixed or permanent condition. We recommend that the Planning Board and Municipal Agencies having jurisdiction establish and enforce the following requirements. 1. Clearing Site: The limits of earthwork and disturbance should be strictly adhered

hat the Planning Board and Municipal Agencies having jurisdiction establish and enforce the following requirements. 1. Clearing Site: The limits of earthwork and disturbance should be strictly adhered to maintain the stability of the slope. 2. Long-term measures should be required to maintain the health of the forestry on the slope. The health of the root systems provides reinforcement to the shallow topsoil layer on the slope. 3. The provisions of the Soil Erosion Plan should provide details for protection of the ridge and slope. Discharge of water from dewatering during construction should not be permitted. 4. Surcharges resulting from the stockpiling of materials and heavy equipment should not be permitted on the ridge immediately above the slope to avoid a reduction of the stability of the slope. 5. The requirements for construction should include an excavation and dewatering work plan addressing the stability of the slope during the construction. This work plan should be