Seismic Response of a Composite Landfill Cover

The Olympic View Sanitary Landfill (OVSL) near Port Orchard, Washington, is a modern solid waste sanitary landfill covered, in part, by a composite cover system. The site was subjected to a free-field peak horizontal ground acceleration on the order of 0.16 g during the 28 February 2001 Moment Magnitude 6.8 Nisqually earthquake. To the knowledge of the writers, this is the first documented case history of a composite landfill cover shaken by strong ground motions. Postearthquake reconnaissance did not find any signs of earthquake-induced permanent displacement of the composite cover system. Accelerograms recorded within 1 km of the facility, the results of site-specific shear wave velocity measurements and laboratory interface shear testing, and these postearthquake observations provide a unique opportunity for a posteriori numerical analysis of this important case history. The seismic performance of the OVSL composite cover system was evaluated using four commonly used methods for seismic design of landfill cover systems. These methods include two simple screening procedures, a more rigorous screening procedure, and a decoupled equivalent-linear site response/Newmark-type permanent deformation analysis using the accelerograms recorded at the nearby strong motion station. The yield accelerations of the composite cover system, required for all four methods, were calculated using the results of construction quality assurance interface shear strength conformance testing. All four methods produce results consistent with the observed performance. However, the two simple screening procedures were significantly more conservative than the other two more rigorous methods.