Unlined Tunnels of the Snowy Mountains Hydro-Electric Authority, Australia

The design and construction of the most recently completed unlined tunnel is used to illustrate the practice adopted by the Snowy Mountains Hydro-Electric Authority, Australia. The economic advantage of an unlined tunnel over a hydraulic equivalent concrete-lined tunnel is dependent on the quantity of concrete lining required. The use of pneumatically applied mortar and grouted rock bolts reduces the amount of concrete lining. A summary is given of all unlined tunnels in the Snowy Mountains Scheme and also the behavior of four tunnels after several years service. Hydraulic data together with rock measurements are described. Tunnels currently (1964) under construction are generally larger than those completed earlier and operational experience is examined in assessing modification to design criteria and construction methods. A Schedule of Rates form of contract is generally used, and some details of special provisions are given.