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  Understanding Geotechnical Condition s in the Headrace Tunnel ofUma Oya Project, Sri Lanka

  The breakthrough of the 15.4 km Headrace Tunnel (HRT) of Uma Oya Multipurpose Development Project, which was excavated with two hard rock double shield TBMs of 4.3 m diameter, was achieved on 2nd of October 2019 after water ingress was challenging the excavation operations. One of the TBMs excavated upwards from HRT outlet and the other one excavated downwards from HRT Intake. HRT faced geotechnical unforeseeable conditions during the mechanised excavation. Then, after getting advices from international panel and experts, the methodology of the excavation had been changed. The new methodology was based on using some techniques like continuously using TSP test technology and pre-excavation drilling and grouting. The geological conditions ahead of a working face are predicted by analysing the geological data available from surface boreholes and tunnel face and using theories of geomorphology, stratigraphy, and structural geology from regional conditions to determine trends. However, whilst this can easily be implemented for normal conditions, it does not allow for prediction of abnormal conditions which can be catastrophic to tunnel excavation work. This paper discusses the case study of the methodology and process of understanding the geological and geotechnical conditions in the HRT of the Uma Oya Project.

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