O21 West Essex Highlands Geotech Slope Stability Model Memo 20250812pdf

Dr. Robert Bagoff, Chair August 12, 2025 West Orange Township Planning Board West Orange Township 66 Main Street West Orange, NJ 07052 RE: Further Evaluation of Langan Slope Stability Analysis, January 14, 2025, revised June 18, 2025 PB-24-01 West Essex Highlands, Inc Block: 179 Lot: 32 Zone: IH-1 Warner Road Project 2151.001 Dear Chair Bagoff, As was previously testified, the above-referenced executive summary and letter report, and the basis of analysis prepared by Langan, dated January 14, 2025, last revised June 18, 2025, present a flawed slope stability analysis, particularly in how they reflect the actual conditions on the eastern slope of the site. At the Planning Board meeting, it was asked why I had not prepared our own slope stability analysis. I had assumed that there would have been some rebuttal to my testimony, but since there was not, I have taken upon myself, with the approval of my client, to provide our approach to the modeling. I have recreated their stability model in the northeastern part of the site, between the location of proposed Building “D” and the existing residential properties along Howell Drive, to perform a sensitivity analysis of the bedrock parameters and the resulting Factors of Safety (FOS). The focus is on Cross Sections C & D, as identified by Langan. Recreation of Slope Stability Model Cross Sections C and D To further evaluate the slope stability at the site, Cross Sections C and D were recreated in the slope stability software Slide by RocScience using a combination of the above-mentioned Langan reporting, West Orange Township topography provided by We Care from the Spring 2010, and subsurface interpolation. Cross Sections C and D were evaluated as they are most likely to threaten the down-slope homes on Howell Drive, the homes immediately adjacent to the steep slope in this part of the site. For the surface topography, we used the Township- wide aerial topographic survey provided by our client, We Care, as we do not have access to the applicants’ survey. However, there is no significant difference between the two surveys. Subsurface layers were inferred from the boring logs supplied in the Langan report and applied to the approximate corresponding location on the cross-section. Interpolation of subsurface conditions in locations without specific borings also differs slightly. As the model geometry assumptions were

the approximate corresponding location on the cross-section. Interpolation of subsurface conditions in locations without specific borings also differs slightly. As the model geometry assumptions were not noted in the Langan report, at locations outside of the subsurface exploration and around rock outcrops, Princeton Hydro interpolated thicker layers of medium sand as opposed to assuming the presence of dense material, which is a more realistic representation of what is on site. Princeton Hydro also extended the cross-section on either end to capture more of the slope. We used the GLE/Morgenstern- Price method of slope stability analysis to recreate Langan’s results. Figures 1 and 2 below provide a visual comparison between the Langan model and the Princeton Hydro model. We were able to recreate their results with their modeled conditions. We then used this base model to input the actual conditions obtained from the boring logs into Cross Sections C and D.