1 - Alaska Energy Data Inventory

Since the air volume required in the tank for stability will increasewith tunnel diameter, and since it is unreasonable and may be costly torestrict the contractor to a narrow tolerance of excavated tunnel diameter,it was decided to relate average excavated power tunnel diameter (withinthe range of 11.0 to 12.2 ft) to required air volume in the air chamber. Asafety factor of 1.5 will be maintained for all tunnel sizes up to 12.2 ftin diameter. The safety factor is defined as the ratio of design airvolume to the air volume required for incipient stability.Minimum hydraulic losses were used throughout this stability analysisfor conservatism. This ensures a 1.5 safety factor even if the tunnelexcavation is smoother than expected.Another major design requirement was that a minimum depth of 3 f~ ofwater be maintained over the air chamber invert for the most severedrawdown condition.Whereas the Svee equation predicted the critical conditions forstabil ity to occur at a net head of 875 ft the "MSURGP program indicatesthat the critical condition will occur at 800 ft net head (equivalent topool elevation of 832.2 ft) and this condition was used for design.Stability runs at minimum pool (820 ft), as well as pool elevations of884.5 ft and 909.6 ft were performed for the 11 ft tunnel and were found toyield incipient stability volumes less critical (i.e., smaller) than thatat a pool elevation of 832.2 ft. The design air volumes and total tankvolumes for tunnels between 11.0 and 12.2 ft in diameter appear below:Tunnel Diameter (ft) 11.0 11.25 11.50 11 .75 12.0 12.2Design Air Volumefor Stability (ft 3 ) 51,000 54,150 57,750 61,500 65,625 69,000Total Tank DesignVolume (ft 3 ) 65,508 69,554 74,178 78,995 84,293 88,628Safety Factor 1.5 1.5 1.5 1.5 1.5 1.584-13