Slope failures in New Madrid Power Plant levee SRT System Stabilizes Levee at Power Plant A coal-fired power plant in New Madrid, Mo., required repair of shallow slope failures on its levees. The power plant had a 23 acre (93,000 sq m) raw water pond to store water for use in plant operations. Slope failures occurred over several years along a 1,000 ft (305 m) stretch of the levee as a result of rapid draw-down of the pond. The levee was approximately 20 ft (6.1 m) high with a slope inclination of 3H:1V. The depth of the slope failures varied from 5 to 7.5 ft (1.5 to 2.3 m). The levee was originally construc- ted of about 20 ft (6.1 m) of silty clay fill, underlain by native soft to very stiff silty clay. Repair of such slope failures can be costly because the existing levee must be functional during the construction operations. The levee failures at the power plant were repaired three times using earthwork regrading operations, but these fixes were limited by the groundwater associated with the adjacent pond. In the summer of 2012, after the fourth episode of slope failure, a more robust and permanent stabilization method was sought by the client. Initially, the project geotechnical engineer proposed installing a sheet pile wall at the toe of the levee in conjunction with excavation and recompaction. After several discussions between the geotechnical engineer, the owner and the engineers at Slope Repair Technology (now Geopier SRT), the patented SRT Plate Pile method was selected to provide a long- term repair. The SRT method required minimal earthwork and provided an overall cost savings of about $1,000,000 for this project. Geopier SRT System The Geopier SRT™ slope stabilization method consists of driving steel reinforcing elements called Plate Piles™ into and through a slide mass or a potentially unstable soil layer. Plate pile elements are installed in margin- ally stable slopes to increase the factor of safety against a shallow slope failure. They are also used in active or dormant landslides to restore the slope configuration and raise the factor of safety to acceptable levels. Plate piles consist of steel sections to which rectangular plates are welded. The pile shaft typically comprises a steel angle or S-shape section, and the plate is typically 12 in (305 mm) wide, with varying lengths based on the depth to failure surface. Plate piles are typically installed using small, tracked excavators with a hydraulic hammer attachment. Installation is a fast, clean, dry process that can occur even in AUTHOR Miriam Smith, Ph.D., P.E., Geopier Foundation Company DEEP FOUNDATIONS • MAR/APR 2015 • 61 bad weather. Following installation of the plate pile reinforcement, a vegetative erosion protection blanket may be placed over the reinforced slope area. The plate piles are driven through the unstable layer to penetrate the underlying stable materials, and are installed in a staggered grid pattern. Plate piles mobilize the strength of the soil through arching and transmit slide forces to the underlying stiffer soil. The downslope force on each plate pile is resisted by the shear and bending strength of the shaft in combination with the passive resistance of the soil behind the plate. The Geopier SRT Plate Pile technology is best suited for real or predicted slope failures up to 10 to 15 ft deep (3 to 4.5 m). It may be used on slopes with inclinations up to 45 degrees (1H:1V) and in all soil types (with the exception of very loose to loose sand) overlying a competent layer into which the plate piles penetrate. They may be installed into soft rock (e.g., siltstone, claystone, mudstone, weathered shale, etc.). The SRT system is not suited to stabilize deep-seated (i.e., greater than 15 ft [4.6 m]) failures, and cannot be installed into hard rock or soil with large boulders or other obstructions. FEATURE ARTICLE
