ASCE 7-22 and IBC 2021 require detailed slope stability checks for any cut or fill steeper than 3H:1V within the Lehigh Valley. Here in Allentown, where the South Mountain escarpment meets the Lehigh River floodplain, we see a stark contrast in soil types — from residual clay on the slopes to alluvial silts near the creek beds. Our team has analyzed over a dozen failures along Hamilton Street and the Cedar Crest corridor since 2020. For deep-seated failures we combine limit equilibrium with finite element analysis to capture the actual stress-strain behavior of the colluvial soils. The city’s freeze-thaw cycles also reduce shear strength seasonally, so we monitor pore pressure through winter months.
Typical failure mode in Allentown is a shallow translational slide along shale bedding planes, triggered by perched water after heavy rain.