HOW SEEPAGE OCCURS

New Zealand Herald, Volume LXIX, Issue 21251, 3 August 1932, Page 10

HOW SEEPAGE OCCURS

JOINTS IN THE ROCK HEADRACE LINED FOR 1500 FEET EXPENSE OF FURTHER TILING The tiled lining at Arapuni, which was laid down after the first case of seepage occurred in June, 1930, does not extend the complete length of the headrace. The actual distance which is thus lined is. section 1500 ft. in length irom the £ P " way The conclusion to be drawn from the information available is that seepage w occurring in unexpected volume from .he sides or bed of the unlined portion ol t headrace and that the flow is being collected by the drainage system jn down under the tiles last year specially th l P ;£wa S expressed by a number of people when the remedial measures were being undertaken in conformity with the Hornell report that the whole of the headrace should have been lined instead of appro?:imately half of it. It is nown that a section of engineering opinion advanced that view. On the indications then available, as a result of extremely thorough inspection of the geological character of the rock-bed, it was agreed by the Government that the tremendous added expense of paving the whole of the headrace would not be justified. Debate Among Experts t The question of extending the lining the whole way was debated for weeks between Professor Hornell, the Swedish expert the Public Works Department's engineers and the New Zealand geologists, who were called in to report on the geological formation of the country. These experts were unanimous that the source of the trouble was, in the first instance, the columnar vitric tuff, the joints between which were not as tight as had been presumed, thus allowing the water to get down and, after the course of two years or more, to find its way into cleavages such as exist in all rocks but which, in the case of the breccia at Arapuni, are continuous and run more or less parallel to the course of the river. By cleavages it is not suggested that open fissures exist in the rock. Natural divisions occur through which water in a general way cannot be said to flow, but into which, by ~a long process of soakage, it is possible for water to find its way. Professor Hornell therefore proposed that

all the columnar rock should be covered by» a lining. Reason lor Extending Lining. The columnar rock extended for just under 1000 ft. from the spillway and Professor Hornell proposed that the lining should be carried a distance of 1350 ft., no doubt on the principle that the lining should continue some distance beyond the limits of the crack, which coincided with the columnar rock. When executing the work, however, the department s engineers found that a better place to finish the lining was at a point 1500 ft. from the spillway, as at this place a considerable saving in the head-wall could be made which would almost counterbalance the cost of the extended tiling. The lining was therefore carried to a distance of

1500 ft. But the tiling of this portion of the headrace was by no means the only remedial measure adopted. At the intake end, where the headrace leaves the gorge, the bed and sides were covered with gunite to above water-level. This substance is a kind of mechanically-constructed plaster. In addition, all crevices and partings in the rock were scraped ,out, made Y-shaped approximately lft. deep, filled with bitumen to within a few inches of the ordinary surface, and then capped with concrete. This bitumen and concrete capping was carried the whole distance from the intake to the beginning of the Hornell lining. Anything that looked like a crack was thus dealt with, even though it showed no signs of ever having carried, or ever being likely to carry, water. To that extent, therefore, it is incorrect to say that the headrace is lined only along a limited portion. THE "TILED BATH" UNDERGROUND. TUNNELS MEASUREMENTS OF SEEPAGE Familiar to engineers as the "tiled bath," the lining of the lower portion of tbo Arapuni headrace is an elaborate tiled pavement absolutely impervious to waterThe first operation in building it was to lay down a filter bed, consisting of a layer of rubble and then layers of shingle and sand. A coating of bitumen and sand followed; and upon that was placed a layer of steel sheets coated with bitumen on each side. Finally, there was laid a surface of concrete paving blocks cast in a shape to enable angles to fit into one another. The joints so created were filled with bitumen. Beneath this impenetrable lining an extensive series of drainago tunnels was

bored. Actually two miles of these Innnels run underground at depths of 140 ft., 100 ft. and 60ft. from the bod of the headrace. One central underground drain extends along the length of the headrace to collect any seepage that might flow in from the sides. Other tunnels connect this central drain with the gorge. At tho upstream end of the "tiled bath" a cut-off wall was built extending downward to prevent, water from coming up under the tiles. Any water collecting above this wall is carried away by drains specially provided for the purpose. Two great shafts, 150 ft. deep, at each side of tho headrace at this point, are connected by an elaborate system of inspection galleries. Tho slightest seepage into any of these drains can be measured with great accuracy and the source traced, a watch being kept at regular intervals for this purpose. POWER FROM ARAPUNI FIRST RECEIVED IN 1929 Arapuni first came into operation in May, 1929, and on June 2 power from tho station was used in Auckland. It was delivered uninterruptedly to Auckland and the various power boards throughout the province for over a year. The fracture in the country embracing the penstock tunnels occurred at 1.30 a.m. on June 7, 1950, and on .Tune 11 the works were closed. They were reopened, after tho remedial measures had been carried out, on April 10 last.