Lookout Point Tunnel
On Thursday, the 26th Sept. , about three o'clock, the piercing of the Look-Out Point Tunnel was effected by the passing of a drill between the headings on each side, and at seven yesterday morning the whole of the heading was down, and a passage opened right through from Caversham to the Kaikorai Valley. The miners at each end met literally to an inch in line and levels, and, so far as can be at present ascertained, the distance is equally correct. As this is virtually the completion of the work, we give the following particulars regarding it :-: - The tunnel enters the range near Mr Thomson's property, Caversham, and passing almost under the Industrial School, comes out in the Kaikorai Valley near Mr Lawson's flax-mill. It is 43 chains or 946 yards long, 15 feet high, and 12 feet wide. It is all through solid sandstone, and there will be no lining except half a chain at each end, where the stone is subject to the action of frosts. The roof is a Gothic arch, that form being usually adopted in tunnels that are not lined.
From the north end, the tannel rises with a gradient of 1 in 66 for 26 chains, but beyond that it is dead level. This arrangement is made to facilitate drainage and working, and to get the easiest gradient for the portion of the line between Dunedin and the Look-out Point Range. The summit level is 160 feet above mean sea level, and there is a depth of 266 feet at the top of the range. The tunnel is included in one of Mr A. Smyth's Sbritracts. The total cost, including lining and ashlar fronts, is about £9500, or an average of a little over £10 per running yard, which is less than a third of the usual cost of single line tunnels.
Mr Smyth sub-let the work to Messrs Duncan and John M'Kenzie, who have carried it oiit in the most satisfactory manner. A commencement was made at the south end on the 21st September last, but in consequence of the length of open cutting at the north end, work was not begun there till the 13th March, at which date an advance of lGh chains had been made from the south. During the next two months nothing was done at this end. In consequence of the falling gradient it was impossible to go beyond a certain point without pumping, so nothing could be gained by driving further at that time.
With the exception of about ten days' delay, on account of a slip in the cutting at the mouth of the tunnel, the work has gone steadily on night and day, no stoppage being made except on Sunday. The method adopted in securing the correct alignment of the tunnel was as follows. Stone blocks were fixed into the ground at the summit, and at each end, exactly over the axis of the tunnel, their positions being verified by repeated observations. Those at the ends were placed at a level, to admit of a sight being taken into the tunnel. Now, by placing the instrument over these blocks, and looking to the one on the summit, its line of sight was made parallel with the axis of the tunnel, so by simply lowering the telescope and directing it to the heading, a point in the line could be given with great precision. It was, however, impossible to see when there was the least smoke or haze in the tunnel, and for some time back no direct sights could be taken to the heading at the south end, so the chance of error was considerably increased. The gradients were run by levelling from bench marks on the surface of the ground. To ensure accuracy both in line and level, it is necessary to know the exact distance between the ends. As this could not be done by chaining over the hill, a base line was measured in the Kaikorai Valley, and a system of triangulations carried over the range.
The practical work of setting out the tunnel was entrusted to Mr Adam Johnston, the Assistant Government Engineer, who has had charge of it throughout. The result of the meeting proves that more than ordinary care and skill were brought to bear on it.
The Messrs M'Kenzie deserve great credit for the manner in which the works have been carried out. Their arrangements were of the most complete description; there has not been a single hitch of any kind from first to last. And, although so much blasting and other dangerous work was in progress, no accident has occurred. This too, notwithstanding that the work was begun and ended on a Friday - that day of ill luck. Mr John M'Kenzie had special charge of carrying on the line and levels between the points given by the instrument, and he did the work with almost mathematical accuracy ; and so anxious was he about the meeting, that he has scarcely left the tunnel "for three days. Mr David Marchbanks has been the Government Inspector at the tunnel, and he also has given the work his best attention night and day. Taking advantage of the opportunity afforded on Saturday last by the men employed on the tunnel having knocked off work for the day, I determined to walk out and go through, the bore. The day was anything but fine, a strong gusty sou-wester blowing, with frequent cold showers of sleet and hail, which had the effect of softening the roads and making the walking unpleasant. I had not proceeded far, when I overtook Mr Blair, the Engineer of the works, and another gentleman, bound on a similar errand, and the three of us manfully breasted the storm, which seemed to blow harder the farther out we went. Indeed, over Cargill's Hill it blew a gale, sending the hail into our faces with a smartness that made them tingle, and assume the colour of a boiled lobster. Down in the hollow, and on through the village, it was not quite so bad, and the rain had almost stopped when we reached the beginning of the cutting which leads to the mouth of the tunnel. Here we found a small blacksmith's shop, and provided ourselves with candles. A truck was to have been in waiting to convey visitors in to the heading ; but neither man nor horse was to be seen. The former had probably gone to enjoy himself with his mates, in the belief that the stormy afternoon would keep away visitors. However, after visiting the little hut where the instrument was worked which kept the levels, and where we found a lighted lantern suspended in a peculiar way, intimating that the surveyor was in the tunnel all the time, taking observations, the bore being quite free from smoke, as we could see a light twinkling a long way in, we got down on the line, and proceeded through the cutting to the mouth of the tunnel. The first object that attracted notice here was a long wooden chimney that had been erected in connection with a system of " bratticing," in order to provide for the ventilation. By dividing the area of the shaft into two inequal parts - the square, or lower part, from, the arched, or upper part - a double current of air was induced, the cold fresh air coming in along the lower gallery, while the warm vitiated air found its way outward to the long wooden chimney at the mouth of the tunnel.
Hitherto, we had found the line very dirty - a thin bluish-grey mud covering everything to about ankle- depth - compelling us to walk as well as we could on the metals. This was not at all difficult in the daylight, but as we got into the tunnel, our lights were found of very little use, and hardly gave light enough to see the dimensions of the vault we were traversing, so that slipping from the rail into the mud was frequent enough. My two companions tried to steady themselves by grasping each other's arms, and walking on parallel rails, but that did not prevent slipping either. In short, it was walking by faith, and lot by sight. About half-way in we met a man coming out with a lantern : this we changed for one of our naked lights with some advantage, for it distributed better. A little further on we met the surveyor returning. In answer to some queries, we were informed that he found the lining admirable, and that they were not out a hair'sbreadth. This was very satisfactory, indeed. We soon arrived at the face, and examined as well as we could the point of meeting. But it was little we could see, and on scrambling up to the heading, found we could see less, for the strong wind coming up from the southern end at once put our lights out, lantern and all ; nor, though we tried to re-light them, could we succeed. So we trudged on through thick and thin, till the opposite face was reached, and then we got down on the line again. This end is level, the other part being up an incline of 1 in 50. We found the footing comparatively dry, only a wet bit here and there, and with the light becoming gradually stronger, we soon reached the other end, and emerged into the light of day - all three very much in need of an immediate application of Day and Martin [shoe polish]. This we ' tried to obviate by walking through some long wet grass, which took the rough off. From the mouth at the Kaikorai we proceeded up the hill to the summit, where a large stone was fixed, from which both of the outer observing points could be seen. Mr Blair explained how the thing was done, and very simple it seemed. A light was placed in the middle of the cutting in the tunnel, so that it could be seen from the observing point outside. The instrument was then brought to bear on the white rag at the summit, and then lowered till it met the light in the tunnel, when of course, if there was any error, it became at once apparent. In this way everything was kept straight, with the result of a meeting in the middle of the tunnel as true as if the whole thing had been worked from an open cutting. From this point we proceeded down the hill to t line again, to the point where we started in from, and found a man harnessing a horse to one of the trucks to fetch us out. But we saved him the trouble, by coming over the hill. From the blacksmith's shop we proceeded along the line, inspecting the various cuttings and formations till we got back to Hillside again, and so to town. I Pakbha. September 30, 1872.
From Otago Witness, Issue 1088, 5 October 1872, Page 3, Papers Past, National Library of New Zealand