The 15km headrace tunnel for the Uma Oya project in Sri Lanka was excavated by double shield TBM and intersected large quantities of water. The study of this project with regards to better understanding the effects of the tunnel on the hydrogeology of the area, identifying relevant in-tunnel investigation techniques, and developing better pre-excavation and post excavation grouting systems is relevant to other long hard rock TBM projects.

The presentation will address why we need to limit the water ingress into tunnels under construction with regards to the potential effect on the environment with dewatering of surface and near surface aquifers, the effect on the construction and its schedule, and the methods available for prediction and control of ground water during tunnel excavation.

The paper will also present examples from other projects around the world – past completed projects, projects under construction and future planned projects, detailing how such hazards have been or will be treated.

About the Speaker:

David Lees has over 35 years experience in the design and construction of underground works, both in mining and civil tunnelling, with particular expertise in grouting and ground treatment. He has worked in over 14 countries around the world as both consultant and contractor. From August 2015 to August 2018 David was Chief Site Supervisor on a 15km TBM tunnel in Sri Lanka. The project had enormous projects with water ingress – up to 1000l/s from the face – causing heavy delays to the project and widespread social and environmental problems. David joined the project halfway through and took the opportunity whilst on the project to prepare a thesis for a PhD through the local university of Peradeniya. The presentation is based on his PhD thesis.