Delineation and Treatment of Mega Shear Zone in the Main Dam Foundation: A Case Study of Punatsangchhu HEP-II, Bhutan

Pre-assessment of important geological features and their characteristics, while constructing any hydropower project in the Himalayan terrain, is always a challenging task. Most of the projects encounter one or more unforeseen adversely oriented geological features, either during excavation of dam abutments and foundation or tunnelling and back slope stripping of power house and pot head yard.

These features may be in the form of fault zone, shear or crushed zone with flowing condition, buried channel, heavy ingress of water, squeezing zone etc. which makes the task, of construction Engineer and Geologist as most difficult, leading to the over runs in respect of time and cost aspects. This paper brings out various aspects in respect of investigations and treatments of thick shear zone (maxi. thickness 17.0m) encountered during excavation of dam foundation for execution of Punatsangchhu-II HEP (PHEP-II), under construction on Punatsangchhu river along Wangdue-Tsirang National Highway in Wangdue Phodrang Dzonkhang of Bhutan. The shear zone was suspected during excavation of left abutment while drilling exploratory holes for demarcating abutment rock slope profile. Subsequently, in order to establish its nature and thickness, four additional exploratory drill holes were carried out. The shear zone was unique in the sense of its dimension, frequent changes in thickness, behavior and attitude with the depth. These characteristics of zone could only be established after excavating a 5.0m deep trench along its strike and dip. A detailed geotechnical assessment and geological mapping was carried out to demarcate shear zone thickness, continuity and infilling material. The shear zone was traversing from heal to toe of the dam, cutting across the dam foundation of block nos. 4, 5, 6, and 7. The treatment of the shear zone was designed by Central Water Commission (CWC) in the form of excavation of 125.0mm long, 5.0–35.0m wide and 13.0–15.0m deep trench (from u/s CH 22.0m to d/s CH 103.0m), consolidation grouting, providing rock anchors on the side walls and finally the trench filled with PCC and RCC. This paper exclusively deals with the detailed investigations that have been carried out at dam site and their inference and the methodologies adopted for delineating the shear zone in the dam foundation and its treatment. Besides, the present studies also briefly describe the evaluation of dam foundation and foundation grade rock mass characteristics.