120 FEET DOWN

Waikato Times, Volume 127, Issue 21248, 19 October 1940, Page 6

120 FEET DOWN

INTO EARTH’S BOWELS ROCK-BOUND CHAMBER DRIVING DIVERSION TUNNEL KARAPIRO POWER SCHEME Deep down in the terrain of solid rock, at a point about 20 feet below the normal water level of the Waikato River, men are working day and night piercing the diversion tunnel which is the first major undertaking in the construction of the new hydroelectric works at the Crow’s Nest, Karapiro, near Cambridge. Preliminary to the tunnelling, two shafts of a depth of about 120 feet and 180 feet respectively were sunk. This was necessary to give the tunnellers access to the level of the diversion tunnel. With that stage completed, a start has just been made on the construction of the actual tunnel through which the river will be diverted while the dam is built across the river bed.

Great changes in the land formation at the site as a result of surface excavations are apparent to the visitor, but it is far down in the eolid rock that the scheme is progressing on its most important stage. -There, however, visitors are not permitted. Yesterday a Waikato Times representative descended the shaft and saw the tunnellers at work. To them the task is merely a routine of their normal lives, but to the layman the scream of drills, the changed atmosphere, and the gloom would be unbearable for many hours. Waiting For Gear Some delay has oeen experienced in securing the special machinery required for the tunnelling and progress is therefore restricted, though the handicap has not yet become serious. It is proposed to equip the shafts leading to the tunnel faces with lifts, but until they are shipped to New Zealand the tunnellers descend to their work by perpendicular ladders. The driving of the tunnel heads has been commenced on the No. 1 shaft and it will commence on the No. 2 shaft shortly. It was the No. 1 shaft which was inspected. The perpendicular ladder descends the face of the 9ft. square shaft for 1201 t. At the base, the shaft widens into a chamber about 30ft. square from where the drives |on four separate points have been I commenced. Crystal clear water seeps through the greywacke rock and pumps are operating continuously to prevent flooding. Tunnelling System The method of driving the tunnel is interesting. From each shaft tunnellers will pierce inward until they meet, while other men will drive outward from both shafts to meet the river at points selected for the entrance and exit of the diversion The drives will be made so that the tunnel is pierced in sections. The first step of driving wall headings has been commenced, a ninefoot section on either side of the base of the proposed tunnel being driven right through first. When that is completed the outer walls will be concreted to a thickness of two feet.

Similar wall headings on the upper section of the proposed tunnel will then be driven and the outer walls on this section concreted. With the completion of that section of the work a solid block of rock will remain, running in a longitudinal direction in the centre of the tunnel, it having been left in order to provide additional support. It will be removed in the final step of piercing the huge tunnel which, when the walls have been completed, will have an interior section of about 25ft. in a semi-oval shape. Charges of Gelignite With huge hardwood props supporting the rock in the central chamber of the shaft, and temporary larch wood supports in the four drives, the tunnel is similar to a mine*. Hydraulic hand drills are being used until the more elaborate drilling machinery arrives. To a depth of four or five feet about a dozen holes are drilled into the rock face in each drive and plugged with gelignite. The charge is fired and the rock tumbles as the drive extends farther and farther toward the objective. Then the debris has to be hoisted to the surface. At the present time a bucket is hauled up by a crane, but efficient hoisting gear will be installed when it arrives to facilitate the clearing. The greywacke rock will be valuable for the mixing of concrete for the dam. If the drilling and hoisting gear arrives shortly and the tunnelling can thus be speeded-up, it is expected that the construction of the diversion will occupy about three months. Surveyors yesterday put in the centre pegs and levels in the chamber of the No. 1 shaft to guide the tunnellers along the correct lines for the drives, which, when they meet on the inward cut, will be exactly on the same level. Smoke-choked Shaft When the gelignite is fired the workmen ascend the shaft, and often as long as two hours elapse before the tunnel is sufficiently cleared of smoke and the tunnellers may return to their task. Later a fan system will be installed to hasten the clearing after a blast, and also to provide fresh air for the men as the drives become longer. The actual cutting device of the drills consists of a chisel bit about five feet in length. Water will stream through them to prevent the dust which at present rises in the tunnellers’ faces as the drills grind out a high-pitched whirr. Up to 50 of these chisel bits may be rejected as worn out in one day and later, when the machine chills arrive, as many as 200 bits may be discarded. When the new gear is installed three men will drill in each head. Six-hour shifts have been worked, but a start has now been made with eight-hour shifts, so that the work proceeds day and night. At present live tunnellers work every shift with a foreman.