The 2.5 km long tunnel was constructed by boring through mainly granite. The tunnel boring machine began the drilling from Stilling Basin near Butterfly Valley and it took 11 months to break though at the intake at Chak On Road South at upstream.
The 1.2 km tunnel was constructed by boring through mainly mix-ground and soft ground. After completing the drilling of the branch tunnel, the tunnel boring machine was delivered back to Stilling Basin for reassembly and began the drilling works at 40m below the urban area. It took 7 months to break through at the outfall.