Approach, Ventilation, Head Race and Emergency Tunnels and Surge were the principle underground work in Koyna Hydroproject stage I and III.
Approach tunnel was 915m. long with cross section 39 sq.m. to serve approach to underground Power House. Ventilation tunnel was 335m. long with cross-section 6 sq.m, These tunnels were driven through Deccan trap and were unlined. Head Race Tunnel was 3750m. long, circular with finished dia, 6.4m. An R.C,C. Intake tower 56m. high was constructed at the entrance. Surge shaft 12m. dia. and 91m. deep was constructed at
downstream. Four take offs 3m. dia. 76m./107m. long from surge met emergency valve tunnel.
For stage III works, three contracts were awarded to the Company. One for the Head Race Tunnel and two for the Tail Race Tunnel - 305m, and 228m. Stage three in a nutshell, can be described as a utilisation of Tail Water of stage I and II, taking the advantage of further drop of 130m. above sea level. From the reservoir, water was taken to the power house through 4 pressure shafts and discharge through Tail Surge and Tail Race Tunnel

HRT free flow tunnel 7.92m. dia, x 122m, long fully lined.
Pressure tunnel 7.47m. dia. x 4320m. long fully lined.
Tail race tunnel 10.36m. dia. x 1748m. long fully lined.
Shaft 4m. x 10m, 59m. deep fully lined.