Tunnel repairs

Ruapehu Bulletin, Volume 6, Issue 286, 16 May 1989, Page 2

Tunnel repairs

believe that the giant culvert is normally almost completely filled with a raging water flow. Room is tight, however, and entrances and exits from the vehicle have to be carried out somewhat gracelessly from the back door as the side doors are too close to the tunnel sides to open fully. Further explorations can be carried out after squeezing along the sides of the vehicle. Leakage Despite water from all the intakes into the tunnel being locked off and diverted elsewhere, the tunnel is by no means dry. Leakage, in the form of small sprays exists all the way along the sides, from minute holes between the section joins. Although not a danger in any way, they do create problems in that the tunnel floor is covered in a foot or more of water. And how are staff supposed to repair holes in the tunnel floor if it's always covered in water? "That," says Murray McEwen with a wry smile, "is the problem." Valves are in place to relieve the water pressure around the repair site. Working a kilometre into the dark one of the shifts of seven men can be observed at work. Len Malcolm was confident the tunnel would be back in action by May 12 or 14. Although the total cost of the major repair job was in the region of $830,000, Mr Malcolm says that the water lost on the Western Diversion while the tunnel was out of service had a value of nearly three times this amount. He added that the Western Diversion water loss has contributed to the present low level of Lake Taupo, and has meant less water has been stored in the lake for operating the Waikato River power stations during the coming winter. The 5.5 kilometre section of tunnel from Tawhitikuri to the Te Whaiau Canal differs from other sections in that the tunnel shape is completely circular.

Other sections have a built up floor that is flat giving the tunnel a "D" shape. The D shape is considered superior as gravel washed into the tunnel is spread evenly across the floor. In the circular tunnel section the gravel tends to travel on the lowest point causing greater abrasion. It's understood plans are afoot to try and get the D shape for the section implemented in next year's budget. The independent consulting engineers

"Works Consultancy Services" who have inspected the whole tunnel and are assisting with the present repair works have reported the tunnel to be now in good condition, said Len Malcolm. One of the less obvious problems of working down the tunnel is only noticeable when it's time to leave. If you drive down frontwards there's no alternative but to go out backwards. Turning bays are not a feature.

Reprinted from the Turangi Chronicle

Tunnel repairs a massive job

Peering through water by torch light at the floor of the Tawhitikuri tunnel, it's hard enough to see what was once the hole that caused so much trouble in the Tongariro Power Scheme, let alone understand how such a thing could cost some $830,000 to repair.

The air is cold and our only light the hand held torches and the headlights from the diesel powered vehicle we travel in. Petrol fuelled vehicles are not allowed into the tunnel because of the poisonous fumes they create. Surprisingly, although the tunnel eventually descends some 80 metres below ground level it is amazingly noisy. A constant roaring sound assails our ears. "With nowhere else to go the sound down here just bounces

around off the walls," explains Maintenance manager for the Tokaanu area, Murray McEwen. The hole in the tunnel was discovered four months ago on January 12. Electricorp staff identified it as being an old sump drain that had been left considerably less well reinforced than the rest of the tunnel along with the water from the Whakapapa Stream took its toll. Abrasion led to the wearing of a 40 centimetre square hole. Water pressure Urgent repair became

necessary as water pressure outside the tunnel, which Tokaanu Area Manager Len Malcolm estimates to be some 50-60 pounds per square inch, caused large quantities of the easily eroded volcanic material surrounding the tunnel to wash from outside, into the tunnel. Contractors estimated that in all some 800 cubic metres of gravel had been removed from around the tunnel and been washed inside. Early repair attempts had failed because of the groundwater pressure. However, when engineers and tunnel experts brought in a heavy steel liner with circular ribs to hold it in place, the water flow was stopped enabling concrete to be forced into the hole.

With the hole sealed the problem of having so much material missing from around the outer of the tunnel remained. Contractors were employed and three holes were drilled from ground level down through which gravel, and then a runny concrete mix, was poured. Although this seems to have been effective to a point, it was noted that only 120 cubic metres of gravel had been put in to replace an estimated 800 cubic metres of missing soil. The problem stemmed from the shelf-like hole created which extended away from the tunnel and the tendency of material put in from above to form cone shaped piles. Voids filled The final stage was to remove the steel liner from the tunnel, drill a pattern of holes through the tunnel wall, and pump concrete grout into the ground outside to ensure it was properly consolidated with all voids filled. Because of the repairs, the tunnel has been out of service for a total of 62 days. During this time repair works have been mounted on a continuous 24 hour basis using three shifts in an attempt to keep outage time and Western Diversion water loss to a minimum. Driving along the tunnel towards where the workmen are completing repairs on a secorid much smaller hole, it's difficult to Turn page 2