TAWA TUNNELS

Dominion, Volume 23, Issue 32, 1 November 1929, Page 13

TAWA TUNNELS

HOW BIG DEVIATION WORK IS PROGRESSING DIFFICULTIES OVERCOME TAPPING AN UNDERGROUND SPRING AU the detailed spade work has now been completed in connection with the Tawa Tunnel operations, and the scheme has settled down to a three-year attack on some three and half miles of treacherous rain-soaked greywacke dividing the outskirts of Wellington from the lowlands of Tawa and Porirua. The scheme, Tawa Tunnels, consists of two tunnels connected with one another by a bridge over the Ngahauranga Gorge Road, and 300 yards of very heavy formation work. . The tunnel that terminates at the sea is the shorter, being some three-quarters of a mile long. The inland tunnel that starts in Ngahauranga Gorge and terminates a mile or two beyond Johnsonville is two and three-quarter miles long. Hill Already Pierced. In both cases preliminary small tunnels some 10 feet by 8 feet square are driven through along the centre of the line that the permanent tunnel will occupy. In the case of the short tunnel to the sea, this preliminary “heading” has now been completed. It is thus possible to walk through the mountains from the bridge across the Hutt Road to the gorge, and possibly within a year right through to Tawa Flat. The final tunnel will be circular in shape and just over 20 feet in diameter. The work of enlarging to full size is proceeding from both ends, and at present just over 600 yards of full-size tunnel has been completed in this section. The connecting bridge across the gorge and the bridge across the Hutt Road are now completed. In fact, the gorge bridge is not only ready down to the last coat of paint, but is even laid with its final permanent way of 1001 b. rails. No Let-up on Work. Work ou the large tunnel, the hardest and most tedious nut to crack, continues night and day without a pause. Tunnelling gangs are busily at work not only at both ends but also in a shaft 130 feet deep sunk ou the line of the tunnel. Gangs in this shaft are working in one case towards Ngahauranga and in the other case towards the Tawa portal. The preliminary tunnel, in fact, is now completed between this shaft and the Tawa end, and about 200 yards of full-size tunnel is already' completed in this length. In the other direction, a mile gap of solid mountain still separates the workers at the faces. This mile section, indeed, is possibly the most difficult tunnelling operation of the whole scheme. Although at first there was little water to delay operations, scores of underground springs have been tapped recently. The matter is further complicated by the fact that at the shaft end every drop of water that seeps into the working tends to run down to the working-face, where it collects and has to be pumped away to soak pits beside the shaft. Thence it is pumped 130 feet to the surface. A few weeks ago work had to I . suspended entirely in this section owing' to the tapping of an underground lake. The workings were completely flooded in a matter of a few hours and nearly a week was spent pumping out the flood with every available pump. Although the spot from which this lake gushed is now hidden by enormous struts and steel rails, a cavity of considerable height still remains above the tunnel. Incidentally, when this lake was tapped,' a farmer up above in Johnsonville was somewhat upset to discover that his creek and the spring that fed it had run completely dry. Engineers some hundreds of feet below his farm are busily pumping out his errant creek from their cherished tunnel. Ten Feet a Day. Preliminary tunnel work advances at the rate of about ten feet a day. It is thus obvious that the mile gap in the longer tunnel cannot be pierced for at least another year. Enlarging to full size is proceeding hand in band with the preliminary work, and an advance of about five feet a day is made at the several spots from which these operations are proceeding. Enlarging of the heading is by no mean: a simple operation. Holes are drilled radially to full size in the smaller tunnel and the intervening material dislodged. The arched heights of the full size tunnel are l hen supported with wooden struts each cut to special shape and interlocking one with another. A steel profile of the completed tunnel, sixteen feet wide, is run into position on special rails. The two foot gap between it and the bare rock of the tunnel is filled with concrete by means of huge guns, not unlike the homely grease gun, but worked by compressed air. After being left to set for

a week the concrete is sufficiently strong to resist the strains of the hundreds of feet of rock above.

All the tunnels are laid with temporary rails complete with overhead trolley wires for the use of electric locomotives. These locomotives are used to haul away the rubble. Internal combustion engines, for obvious reasons, are strictly prohibited. In addition a side tunnel about half a mile long is being drilled from the gorge towards Tawa. It is about 66 feet away and parallel to the proposed site of the main tunnel. By means of interconnecting alleyways, spoil and rubble will >e removed much quicker and tunnelling operations greatly accelerated. Incidental Work. Apart from the tunnelling itself a host of outside work is rapidly reaching completion in connection with creek diversion and bridging. In addition to the t..q bridges already mentioned, theiv. besides another five bridges pro?' \ I for in the scheme. Three of tlic.-e are, now completed, including a 120 foot bridge that carries the Takapau road over the railway. All the necessary plant that has been eagerly awaited from overseas for months is now installed. A whole battery of powerful electrically driven air compressors can now provide mon? than enough air power to supply the hundred and one compressed air tools on the workings. These pneumatic tools range from a light pick to huge air-driven shovels weighing several tons, complete with their own crane and means of propulsion. Altogether over 400 men are employed on the job. They are accommodated in two large camps at Khandallah and Johnsonville. Both married and single quarters of a most up-to-date type are available. On the average those employed on the tunnelling operations earn, on the con’.ract system, well over a pound a day. Food at the cookhouse costs about 255. a week, and quarters, of course, are provided free.