Accelerated Testing Procedures for Evaluating Separation Layer Performance in Open-Graded Base Courses

An accelerated testing procedure was developed to evaluate pumping and deformation performance of separation layers used between open-graded bases and lime-treated and untreated subgrade soils. A pneumatically driven, electronically controlled repeated loading laboratory apparatus was developed. A low plasticity clay and a silty clay till soil were tested. A nonwoven geotextile and a dense-graded base aggregate were evaluated as separation layers. Tests were conducted at various loading levels and durations to develop the testing procedure. Comparisons of specimen pumping and deflection performance for stabilized, separated specimens were made relative to nonseparated and nonstabilized control specimens. Ultimately, loading at levels upwards of five times that expected in the field, for short durations (20,000 cycles or less), was performed to evaluate separation layer performance. Separation layer performance under accelerated loading correlated well to long-term field loading level performance. The testing method was sensitive to the use of geotextiles for separation between soils and open-graded aggregates, though some pumping of fines still occurred. The dense-graded separation level showed a marked drop-off in performance as moisture contents increased above optimum. A distinct breakpoint in subgrade soil performance occurred at a cone index of 200 (California bearing ratio 4). Soils with strengths less than cone index of 200 performed poorly, whereas soils above that strength pumped and deformed minimally regardless of separation layer type. Based upon performance comparisons with nonseparated test cases, the use of a separation layer between lime-stabilized subgrades and open-graded aggregate bases is recommended.