SLOPE STABILITY ANALYSIS AND DEWATERING SYSTEM DESIGN
Sealand Waste LLC
Sealand Waste LLC of Rush, New York proposed to construct, operate and close a 35-acre landfill expansion in Chautauqua County, New York. DE was retained to prepare and/or manage the investigations, analyses, designs and applications required for all permits needed for the Facility.
The topography of this Southern Tier site was variable with a flat lying, mildly sloping central area and steeply sloping areas near the property boundary. These steeply sloping areas presented challenging terrain for the design and construction of the perimeter embankments needed to support the landfill containment liner system and other site infrastructure.
The stability analysis for all critical landfill liner system and perimeter embankment failure surfaces was conducted using Slope/W as part of the GeoStudio software package Version 220.127.116.1163 under license from GEO-SLOPE International Ltd. This software offers a significant number of limit equilibrium solution methods and allows flexibility in the description of shear strength under both drained and undrained conditions.
Soil parameters for existing soils were chosen based on soil explorations soil descriptions, standard penetration test (STP) results, N values, and laboratory tests. Parameters for the commercially available aggregate and geosynthetic materials were selected based on laboratory tests for previously completed similar projects.
The preliminary stability analysis showed that dewatering would be required to achieve a safe and stable excavation to the desired subgrade elevations. A series of wells, trench drains, sump pits and pumping points were selected that minimized pumping rates while achieving sufficient drawdown for a safe excavation using a site-specific finite element groundwater model (FEFLOW, version 6.2).
The stability and dewatering demonstration successfully established the required properties of soil and geosynthetic materials and the pumping rates to be used in construction of plain embankments, a 40-foot high Mechanically Stabilized Earth (MSE) berm and the double composite baseliner system. A head suppression system monitoring plan was prepared that identified target groundwater heads and daily water level measurements in pump locations, select wells and vibrating wire piezometers to confirm the adequacy of the drains in maintaining protective groundwater levels.