The forebay is 30ft wide x 35ft high x 165ft long, and is excavated with the same equipment as the intake tunnel. The excavation sequence comprised a top bench constructed with 15ft wide drift rounds, followed by slash rounds to take the ribs out to the full 30ft width. Rock support comprised a combination pattern of 10ft and 15ft grouted CT rockbolts, followed by 4in. of fibre reinforced shotcrete. The well shaft casings had been installed and grouted in before excavation, so the bottoms of the casings were exposed as the top bench was taken out.
After the bench was complete, the casings were cut to a uniform level below the arch, and shotcrete was applied to a 2ft radius around each one. The lower bench of the Forebay was then excavated in the same sequence as the upper bench.
22 well shafts were built to house pumps for the IPS-2 pumping station, arranged in two rows of 11 each, spaced 15ft on centre, and extending 331ft down to the Forebay chamber. The shafts were constructed in two phases: shaft drilling and casing installation.
After test holes revealed high water flow in the fractured rock below 100ft, blind boring took place downwards from the working slab. This method is unaffected by underground aquifers because shafts can be excavated and a watertight steel casing installed while the hole is full of water.
All 22 holes were drilled within the target cylinder. The drill rig’s cutters were changed to disc-type tunnelling cutters that produced fewer fines. The 6ft diameter cutterhead, with a 2ft stinger to follow the pilot hole, was set in a 7ft deep starter hole. Ten ton thrust collars were stacked on top of the bit. As the hole was bored, additional collars were added, giving a maximum thrust to the cutterhead of 160 tons.
When boring was completed on a hole, equipment was mobilised over the hole to install casings. Five sections of 66ft long, 50in. i.d. pipe were welded together to form the final string of casing in each shaft. Each section was aligned and welded to the next as it was supported over the shaft, then lowered in. Sand was poured through the inside of the casing to lock the string into place. A grout plug was pumped 10ft above the bottom lip of the casing and the annulus around the casing grouted in three to five lifts.
