MOUNTAIN WATER
A TRIP TO ORONGORONGO
INTAKE DAM ALMOST COMPLETED
TUNNEL GANGS TO MEET NEXT YEAR
A visit paid to the new waterworks at Orongorongo yesterday by a Dominion representative, in company with Mr J. M. Morice, water and drainage engineer to the City Council, gave a new vision of the manner in which a water supply for the Greater Wellington of the future is to be assured under the scheme planned and perfected by the city engineer (Mr W. H. Morton) some four or five years ago.
Twenty years ago it was realised by some people in Wellington that the Wainui Stream, however faithful, had its limitations as a source of water supply—limitations which in the mid-summers of the last five or six years have made themselves alarmingly evident. Four or five years ago almost a crisis arose. The then Mayor (now Sir John Luke) visited Wainui to find the big dam almost empty. It was so low that the state of things had to be kept from the public, and only a serious warning was issued as to the need for most careful use of water.
That was a warning also to the authorities, and was acted upon as speedily as possible. It was seen that the Wainui supply must be augmented from the Orongorongo, on the other side of the range. Plans were prepared, and over three years ago arrangements were made with co-operative tunnellers, led by Mr Robert Semple, to do the biggest work—the task of boring through a range of mountains 2500 feet in height.
Wellington’s Biggest Job
It is pretty well known how that work has proceeded, and how it stands today, but what the public have the poorest conception of is the magnitude of the task involved in tapping the Orongorongo and bringing its full-hearted flow right through to the upper dam at Karori, to a distance of about 24 miles. It was to obtain a grasp of three great works that the visit was paid yesterday.
The party left town at 9 a.m. in a motorcar, and after a brief rest near the Wainui tunnel end, mounted horses to travel over an exceedingly rough track to the Orongorongo camp. The weather was fair but overcast on leaving the base, but as the horses panted up the slippery, boulder-strewn track, which often developed a grade of one in three, rain began to fall, and at about the 1500 ft. level this changed to a damp, clinging mist. The riders were really in the clouds.
The track was not one to inspire confidence, and not even the glory of the primeval forest—tremendous beeches, giant rimus, lordly ratas, and magnificent clumps of putaputaweta ferns twenty-five feet in height—could altogether banish the fear that an accident might easily happen. For the Orongorongo range, softened in the distance by its mantle of splendid bush, is fairly precipitous in places.
There are a dozen places on this track where there is the bank on one side and almost a sheer fall of a couple of hundred feet on the other; and rounding a bluff yesterday in the mountain mist, one could see only space and an impenetrable grey veil. “Leave it to the horses,” was the injunction, and as there was really nothing else to do the order was obeyed.
The journey to the summit (recognised by a word of fervently expressed thanks nailed to a mountain birch) occupied 65 minutes, and the descent on the other side, about a quarter of an hour less.
Over the Range
When at length the Orongorongo camp was reached, the riders were wet and rain was still falling gently and purposefully. The camp, near the tunnel mouth, is set on the western side of a very beautiful deep-cut forest-mantled gorge, at the bottom of which rushes the stream which is to provide the Wellington of the future with a good proportion of its water.
It is this stream which has to be harnessed, and the doing of it is a task of absorbing interest. Modern tunnel-driving methods require power to supply the compressed air for the rock drills. This has been available at Orongorongo since last October. As the water has to be got under control to ensure a steady stream for the 21-inch pipe which is to bring it to town, it was decided early last year to push on hard with that part of the work, so that the water might in the meantime be utilised for power for the tunnellers.
So a long flume and a concrete catchment basin were built, and from there the 21-inch spiral steel pipes were conducted half a mile downstream to a power-house. In that well-arranged generating station an 80 horsepower turbine was installed, and impelled by a noble rush of water, this turbine, working 24 hours a day, from 8 a.m. on Monday until 4 p.m. on Saturday, supplies not only all the compressed air needed for the rock drills, but electric light and power for the entire camp.
The camp proper is a little to the south of the power-house and smithy (which are within a few yards of the tunnel mouth), and consists of two rows of iron shacks and fly-tents. The cook-houses of Semple’s men and the corporation staff are pleasant places, with their big open fireplaces and blazing fires of rata logs.
There are 20 men and two women cooks attached to the co-operative party, and sixteen corporation employees on the Orongorongo side. On the Wainui side Mr Semple has 17 men and a married couple (as cooks), and the corporation has 10 men. The corporation men include those concerned in the management of the two power-houses. Mr Donald Scott, of the city engineer’s staff, who is engaged in the work of taking levels from the upper dam at Karori to the intake at Orongorongo (for the purposes of the pipe connection), is also located at Orongorongo at present.
At the Intake
Perhaps the most interesting work is that now proceeding at the intake. Here an ideal site has been found for a dam, with solid rock on both sides and extending under the bed of the stream at a place only about 50 feet in width. It is stated that this serves all purposes most admirably.
Between the two rock abutments is being constructed a solid concrete dam, 12 feet broad at its base, and tapering to 2 ft. at the top. The dam wall will be 45 feet across on the creek bed, and 75 feet across the top. To enable this work to proceed, the greater part of the stream’s flow has been flumed off, and the water from the flume is discharged on to a concrete deck, which is walled off from the bed of the stream.
In the wall are several grated windows on a level with the deck, and when the dam is filled the water will flow through those apertures, and a proportion of it will fall through perforations into a concrete chamber below. But as it is possible that sand and gravel may be carried through the perforations into that chamber, a second chamber will be fed from the first by inlets at the top of the dividing wall, so that by no means can any gravel or sand reach it—it must sink to the bottom of the first chamber.
It is in this second chamber that the 21-inch valve to supply the pipe is located—this is the actual intake. There was a gang of half a dozen men working at the dam yesterday, and given fine weather, it should be completed next week. The work already done is regarded as perfectly satisfactory, and is built to endure. There is no likelihood of there being anything wanting at that end when the moment comes for the diversion of the Orongorongo stream into Wellington’s water reticulation system.
Progress at the Tunnel
The Orongorongo tunnel will be 10,494 feet in length. Of that distance, 6138 feet had been bored on April 30—4042 feet from Wainui and 2096 feet from the Orongorongo side. In the four months ended April 30, 1037 feet were bored (in 93 working days).
Mr Semple’s men at Orongorongo beat their own record for a month’s tunnelling by piercing 281 feet of mountain rock, which is a foot better than their previous best. Mr Semple, in conversation with the writer yesterday, said that he was confident his men would do 300 feet this month.
Date of Completion
Neither Mr K. M. Barrance (the corporation’s supervising engineer) nor Mr Semple now think that the tunnel will be bored by Christmas, as was at one time estimated. Both consider that the two gangs will meet about halfway between New Year and Easter of next year.
“It must not be overlooked,” said Mr Semple, “that we are laying the pipeline as we go, and that twenty-four hours after the tunnel is through the pipes will be joined up, as far as we are concerned. They are already laid from the intake to the power-house, and also in the tunnel. That is a big consideration, as they take some handling.”
The 21-inch pipes that are being used are spiral steel pipes, made in Wanganui. The actual thickness of the steel is only one-eighth of an inch, but they are bound in bitumenised canvas. These pipes cost 22s. 6d. per foot at the Petone Station, and the cartage all round is being done for £8 per ton.
All big weights have to be carted over the Waiwetu Hill, down the Wainui Valley, and up the roadless Orongorongo Valley to the tunnel mouth. Provisions and lighter material are sent out from there, packed over the hill on horses.
The tunnel will not be exactly level. Its highest point will be in the centre. On the Orongorongo side there will be only a slight grade of 1 in 500, whilst on the Wainui side the grade will be 1 in 160. This variation from the lateral is in order to drain the tunnel.
