Discussion. Furnas Hydro-Electric Scheme, Brazil: Closure of Diversion Tunnels.

Abstract

President, International Commission on Large Dams) said that it was generally agreed that engineers could learn more from one detailed paper dealing with unforeseen difficulties than from many papers which only referred to successful completion of major engineering schemes. From that aspect, therefore, the Paper was particularly helpful and instructive. The courage of the coritractors and their workmen deserved the highest praise.65. Mr Guthrie Brown wished to emphasize the extreme importance of providing adequate and ample safeguards for deaIing with the unexpected in controlling water at high pressures.66. The safeguards which he took in somewhat similar circumstances in providing adequate precautions would be of interest to members. One section of the Conon Basin hydro-electric project in the north of Scotland required an intake to Loch Fannich nearly 100 ft below the surface of the Loch for the tunnel to Grudie Bridge Power Station. Fig. 20 showed the arrangement whereby the tunnel was worked cautiously forward until a rock plug or barrier of only 15 ft remained between the end of the intake to the tunnel and the bed of the Loch.67. If, after this barrier had been blown, the safety precautions had not been rigidly observed, the water would have discharged at high velocity down the tunnel into the uncompleted power station and thence into the adjacent valley-a major disaster.68. About one ton of explosives was necessary to remove the 400 tons of the rock barrier which remained. As the momentary pressure exerted by the explosion on the concrete bulkhead in the tunnel was estimated to amount to 10 OOO tons, it would be appreciated, therefore, that very careful safety precautions had to be made. For that purpose, one concrete bulkhead was made 15 ft thick as the first line of defence. Should that fail, then a second concrete bulkhead 6 ft 6 in. thick was available. The third and final safeguard was a temporary steel bulkhead which was lowered down the screen shaft.69. Careful study was given to the relative advantages of exploding the rock plug in the dry or with the tunnel full of water up to the concrete bulkhead; also what negative head for the water level of the gate shaft should be provided in relation to the surface of the Loch so as to encourage the shattered rock to fall back into the sump. All the provisions for firing the expIosives were in duplicate. The explosion was a complete success, the junction with the Loch being as designed, and the broken material falling into the debris sump provided for the purpose.70. If he were asked to carry out the work again, Mr Guthrie Brown said he would adopt exactly the same procedure. While not, in any way, criticizing the arrangements provided at Furnas, he felt sure that both Authors, in the light of their experience, if they had the work to do again, would make several modifications in the design for closing the diversion tunnels.

Tunnelling

“…Provided the potential risks are understood, control of minor inflow of methane should follow coal mining practice by reliance on assured dilution. The remarkable explosion in the diversion tunnel at Furnas, Brazil (Lyra and MacGregor 1967) was caused by methane derived from rotting vegetation, a more readily controlled source, once the potential hazard was identified.…”