Maraetai Tunnel Progress

Chronicle (Levin), 20 January 1948, Page 8

Maraetai Tunnel Progress

Nearly half the exeavation of the 1700ft. diversion tunnel for the Maraetai Jjydro-electrie dam is compieted an.l the eoncreting of the tunnel 's walls ifollowlng elose behind the exeavation. The work of driving the 25ft diameter tunnel is proceeding fairly well, but the wet and unstable nature of -tho countrv makes it anything but straight-forward job. From the outlot end, which has been driven in m,ore than 500ft, water is still being pumped at the rate of 7000, gallons a minute. The speed with which the tunnel can be finished.is the key to the completion of the whoie 180,000-Uilowatt schemo. In the last few months the number of men working in the tunnel has been increased from 130 to 150. These men are working three eight-hour shifts for five days a week. Mueh of the ground has had to be pinned with pipes an-1, - eoncrete. From the intake end the men have driven about 200ft. The lower half of the gate shaft which joins this end of the tunnel 180ft in is now being concreted. The whole of the. to'p of the inlet end of the tunnel has had to be lined with eoncrete as it was driven be- * eause of the heavv, unstable nature of the ground. At the outlet end, 360ft. of the tunnel 's walls have been lined witn eoncrete, some of the work kaving to be done under water. Unlike diversion tunnels on other schemes which are used onl.v during the building of the dam and power housn, the hlaraetai diversion tunnel will have ti permanent purpose. The power house will stretch right aeross the river below the dam, leaving no room for the orthodox type of spillway for surpltis head. The tunnel will be permanentlv used as a spillway. While the long, eurved tunnel is br.ing driven, as much outside work as possible is being done. This ineludes the power house and dam abutment excavations above water level. A great deal of exeavation "work will have to wait until the water is diverted through the tunnel away from the dam site. To stabilise the surrounding eountry, the engineers are putting in grouted core walls for some distanc.e on either side of the dam. This work is proceeding on one side" of the dam at present and consists of pumping cement under pressure into holes bored 500ft to 700f • deep to seal leaks in the co'untry.