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  Performance of Double Shield TBM in Uma Oya Project, Sri Lanka, Lessons Learned

  Uma Oya Multipurpose Development Project was recently completed in Sri Lanka. This project involves 2 Rolled Concrete Core (RCC) Dams connected through a tunnel by the Drill and Blast method. The water is transferred from the 2nd regulatory dam through a 15.4 km Headrace Tunnel to the top of a 618 m deep drop shaft that feeds the high-pressure water to an underground powerhouse and turbine chamber for the generation of 120 MW of electricity. The 15.4 Km Headrace Tunnel (HRT) of the Uma Oya Project was excavated by two hard rock double shield TBMs with a diameter of 4.3m from both ends. These machines completed their work and broke through at the designated location on 2nd of October 2019. Many challenges were encountered during the mining of HRT tunnel including extremely hard rock, fault zones with pressurized mud, and water inflow reaching 4000 l/sec. This paper will offer an overview of the performance of double shield TBMs in the Uma Oya project and discuss operational parameters, rate of penetration, utilization, downtimes, and site-specific challenges in mining these tunnels. The summary of lessons learned in this project could be very beneficial to upcoming projects with similar ground conditions.

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