The use of spray applied waterproofing for tunnels in the Unites States is relatively new. Historically, PVC sheet membrane has been utilized with varying degrees of success for waterproofing tunnels. Sound Transit deliberately chose to offer an alternative to PVC sheet membrane and included a specification for spray applied waterproofing in its most recent project, the Downtown Bellevue Tunnel, which is a large-diameter SEM tunnel for light rail to the eastside of the Seattle region. This paper discusses the decision to allow the contractor the option of utilizing either spray applied waterproofing or sheet membrane, the testing of the product, required specification revisions, field testing, application, and performance of the waterproofing to date. Lessons learned are offered to the reader throughout the paper.
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