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Interstate-99/ERPA – Engineered Rock Placement Area

In February 2004, during the construction of Interstate-99, construction crews unearthed vast quantities of acid rock at Skytop Mountain. A large portion of the acid rock was removed from the site to a remote location where it could be properly treated. The remainder of the area needed to be shielded from oxygen and water to prevent further oxidation of the pyrite and thus, additional contamination of the ground and surface waters. Methods similar to landfill principals and conventional lining practices were utilized for this purpose. A remediation practice termed an Engineered Rock Placement Area (ERPA) was recommended for this project and CMTs expertise was employed to develop a stability analysis for the ERPA.

CMT recommended that 4 vibrating wire piezometers be placed within the sediment of the sloped area during construction to monitor the pore water pressures. The pore pressures were monitored on a daily basis so that real world data could be incorporated and back calculated into the slope stability analysis in order to verify assumptions. Slope stability analysis and graphic slope stability results determined using the SLIDE computer program were utilized for our recommendations. The Bishop Simplified, Corps of Engineers #1 and #2, GLE Morganstern-Price, Janbu Simplified and Corrected, Lowe-Karafiath and Spencer methods were utilized to check the stability of the ERPA under different loading scenarios.

CMT Laboratories, Inc.
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