Tongariro tunnel out of action

Waimarino Bulletin, Volume 6, Issue 273, 7 February 1989, Page 1

Tongariro tunnel out of action

Work is continuing on the repair of the Tongariro power scheme's Tawhitikuri tunnel and Electricorp staff were reluctant to estimate when the job would be completed at the time the Bulletin went to press.

Two holes have been worn through the tunnel invert (floor) by rock and gravel carried by the diverted water in the tunnel. One of the holes is 0.3 metres across and the other a

metre long by half a metre wide. The holes were discovered during a routine inspection of the tunnel on January 12. Because the diversion water is not flowing through the

tunnel, the reduced pressure is allowing water from both holes to enter the tunnel from the surrounding ground, carrying with it soil and silt through the larger hole. Tukaanu Area Manager

Len Malcolm told the Bulletin that it was very difficult to estimate how long the repair work would take because of the nature of the problem. Because manpower costs would make up a large part of

the repair costs he said it was "crystal ball stuff" to guess what the final cost of the job will be. Workers must first seal off the holes in the floor to stop the water Turnpage2

Tongariro tunnel

From page 1 entering the tunnel, then drill holes through the lining and pump grout into the cavity beneath the tunnel left by the lost soil and silt. The amount of grout needed has been estimated at 60 to 100 cubic metres. If the cavity is not filled the tunnel could become distorted because of the lack of support. Mr Malcolm said the problem is not totally unexpected, but that normally such holes would be repaired before they broke through the tunnel lining. He said the rocks and gravel that are carried by the diverted water scour the tunnel lining, creating pot-holes in some places. These potholes are normally filled during routine maintenance shut downs. 'Tm not sure if we've ever had a hole right through the lining before," said Mr Malcolm.

The tunnel has been in operation since 1973 and would have a life expectancy of at least 40 and up to 80 years, said Mr Malcolm. However, he said the lining of the tunnel is now getting more and more worn and may need major repairs along its whole 22 kilometre length soon. The rocks and gravel carried by water wears a groove in the tunnel floor, said Mr Malcolm, and rocks form eddies which wear larger and larger pot-holes, making the lining thinner. Repair of the wear on the lining would be a major exercise and very expensive, said Mr Malcolm. He said the decision when to carry out this work would take into account technical and economic considerations. "It could be in two years' time," said Mr Malcolm. High lake levels Though no water has been able to be diverted

from the Wanganui headwaters because of the Tawhitikuri tunnel problems, Lake Taupo's high level has meant there would have been little diverted through January. Mr Malcolm said that when Taupo's level reaches a certain point no foreign water, that is Tongariro Power Scheme water, can be put into the lake. He said there were only 13 days in January when water could have been diverted through the Wcstern Diversion.