July 2016: Senior managers describe the process of completing the hyperbaric interventions into the pressurised excavation chamber for replacement of worn tools and cutterhead maintenance of the 17.4m diameter Bertha EPBM working on the SR99 elevated highway replacement tunnel in Seattle. The fourth hyperbaric maintenance intervention period has just ended and two more are scheduled on the remaining 1,878m of the total 2,825m long drive.