Neelum Jhelum Hydroelectric Project: Pre-Excavation Grouting in Twin Headrace Tunnels

The Neelum-Jhelum Hydroelectric Project, located in Azad Jammu Kashmir, northeast Pakistan, involves a 56-m-high dam on the Neelum River and extensive underground works including a 969-MW underground power station and more than 52 km of large diameter tunneling. Construction has faced extremely challenging conditions including high rock cover up to 1900 m, highly deformed and weak sedimentary units, squeezing ground, high earthquake hazard, and active fault crossings. Pre-excavation grouting (PEG) was required in a critical reach of the headrace under the Jhelum River. A systematic program was developed and applied wherever probe-hole drilling indicated high pressure groundwater hazards (back pressures 15-20 bars). Perimeter and face grouting involved nearly 2.2 million liters of grout, at pressures up to 50 bars. Four major collapse incidents took place during excavation under the Jhelum River, each compromising stability of the pre-grouted tunnel periphery. Collapse remediation involved temporary plugs and staged re-grouting at up to 50 bars. The pre-excavation grouting program was very effective and enabled successful excavation hole-through of both tunnels in January-February 2016.