Consolidation characteristics of wastewater sludge

Capping is a cost-effective remediation method for soft contaminated sludge. The Madison Metropolitan Sewerage District evaluated different remediation alternatives to treat its polychlorinated biphenyl (PCB) contaminated sludge and the U.S. EPA agreed to permit capping as the method of remediation. Design of the cap required knowledge of the consolidation behavior of the sludge. In order to analyze this behavior, a series of consolidation tests was performed. The laboratory testing program included three large-scale consolidation tests and four conventional oedometer tests. Consolidation settlement of the sludge was observed in two field test cells capped with a wood chip/soil mixture which was reinforced with a woven geotextile. Instrumentation of the test cells included settlement plates, surface survey markers, and piezometers at different depths in the sludge. Laboratory and field observations were compared with results from conventional consolidation theory and a nonlinear finite-strain (large strain) numerical model (CS2). Conventional theory exhibited some limitations in analyzing the results and underpredicted the time for completion of consolidation. However, the large strain model more accurately predicted the behavior observed in both the laboratory and the field.