O17 West_Essex_Highlands_Geotech_Slope_Stability_Review_20250716

Princeton Hydro, LLC Page | 3 2. The model sections provided assumed the highest RQD values for the entire slope section in every scenario tested, instead of using the actual RQDs obtained from the vicinity of each boring. For example, at modeled Section C-C that contains one of the steepest sections of the slope, and immediately above 15 Howell Drive, the RQD values in boring NLB-1, were 0% and 23”, not including the fragmented rock above the start of the cores, that indicate a very weak rock mass, significantly weathered and susceptible to slides. And for the borings at LB-2 and LB-4 obtained at the middle of the slope, the average RQDs for this portion of the would be less than 23%3 which would lower the GSI value from the 67 chosen by Langan to 15 to 20, reflecting poor and very poor joint conditions, common in heavily fractured rock. 3. Of particular concern is the use of a relatively high Geological Strength Index (GSI) in the slope stability model. While the model uses a Disturbance Factor (DF) of 1.0, which indicates a site that is disturbed by construction activity, it is the combination with an appropriate GSI factor that will truly dictate the overall stability of the slope. As illustrated in borings NLB-1, NLB-3, LB-2, and LB-4, the RQDs average 4. While Langan stated they used a uniform loading of 1,000 psf for the model (page 8 of 13 of the Langan report), they dismissed proposed Building D’s loading on the top of the slope, as they considered it “far enough from the top of the slope” to have surcharge impact (page 3 of 5 of the report), however the proposed building sits at the top of the slope, indicating that there would be an impact to loading on the slope. Langan did not discuss or model the impact of rock excavation to achieve the below-building garage (up to 25 feet of excavation, requiring rock excavation or blasting) subgrades on the stability of the slope. 3 due to the highly decomposed nature of the rock in borings LB-2 and LB-4, these borings should use a RQD of 0%.