The Design and Construction of Grouting against Water Ingress in the Headrace Tunnel for the Uma Oya Project, Sri Lanka
The 15.2km long headrace tunnel (HRT) is a key component of the Uma Oya Multipurpose Development Project (UOMDP). The HRT takes water from Dyraaba Dam to the pressure shaft that feeds the power house. The excavation of the major 4.30m diameter headrace tunnel is done by using a double shield TBM. This is the first use of a TBM in Sri Lanka. High water ingress into the tunnel has been seen to impact on both the deep rock aquifer and the shallow near surface aquifer impacting domestic wells and agricultural land on the surface above. Both preexcavation grouting and post grouting has been carried out to overcome this. This paper will describe the key elements of the design and the issues faced during construction and how these were overcome.