Design Principles and Concepts for Enhancing Long-Term Cap Performance and Confidence

Abstract

The siting of new waste containment systems is becoming increasing difficult as the public and stakeholders want more confidence that contaminant barrier systems will perform effectively for very long durations and owners want to store more wastes in the least area while knowing and limiting their long-term liabilities. These developments motivate reexamination of long-term performance issues and their implications for barrier designs. Accordingly, design principles are herein considered from the standpoint of long-term performance and management, including the ability to monitor and repair barriers. Then, some design concepts that may be implemented on the basis of these principles, especially evapotranspiration (ET) caps are discussed. Five design principles are recommended on the basis of considerations of infrastructure implementation experience and facility management experiences in other fields. The principles are: establishment of a clear and defendable design basis; design for ease of monitoring and repair; analysis of barriers as dynamic systems; working with nature and not against it; and recognition that increased complexity can reduce, not enhance, net performance. The ET caps are an excellent embodiment of these design principles. The principles are applied to ET caps, as well as design variants such as erosion armor, capillary breaks, biointrusion layers, and low permeability material layers.