RAPID DRAWDOWN STABILITY OF DAM ON SILT/CLAY TAILINGS

摘要

This paper describes the design of an earth dam built on recently sedimented tailingsrnconsisting of low strength, low plasticity silt/clay and variably silty sand. The tailings werernencountered within the construction limits of Cell 7 at the 1000-acre Loxahatchee Reservoir inrnwestern Palm Beach County, Florida. The Loxahatchee Reservoir, also termed L-8 Reservoir,rnis part of the Comprehensive Everglades Restoration Plan (CERP), and consists of 7rninterconnected cells with a water storage capacity of approximately 46,000 acre-feet. Cell 7rnoriginally served as a process water/tailings disposal pond for limestone quarry operations,rnwhich preceded reservoir construction at the site.rnLaboratory shear strength tests using both Triaxial and Direct Simple Shear (DSS)rnequipment were performed on the tailings materials to analyze critical loading conditionsrnincluding “Steady-State Seepage” and “Rapid Drawdown” cases to determine the minimumrncalculated factor of safety (FS).rnThe minimum calculated factor of safety against rapid drawdown failure of thernapproximately 40-foot high upstream slope was 1.4 to 1.6 using triaxial shear strength testrnresults. This satisfied US Army Corps of Engineers (USACE) criteria, which requires arnminimum calculated Factor of Safety of 1.1 to 1.3 depending on pool level prior to drawdown.rnThe USACE calculation procedure involves use of a composite bilinear Effective Stress-TotalrnStress failure envelope.rnHowever, rapid drawdown slope stability analyses performed using the undrained strengthrnresults from DSS testing of the silt/clay tailings indicated minimum factor of safety values ofrnapproximately 1.0. In order to increase the factor of safety against a rapid drawdown failure, arn20 to 30-foot wide area of tailings material was removed beneath the upstream toe area, and arn20-foot wide toe buttress was constructed from the mine bottom at –14 feet NAVD up to anrnelevation of -2.0 feet. The calculated factor of safety against an undrained rapid drawdownrnfailure increased from approximately 1.0 to 2.8 as a result of removing the tailings andrnbuttressing the upstream slope.