ROCK FRACTURE

Hokitika Guardian, 21 July 1930, Page 2

ROCK FRACTURE

•**#> - ARAI’UNI SPILLWAY. GEOLOGICAL REPORT. WELLINGTON, July 17. A geological report on the fracture of the rock at the, Arapuni spillway w-a-s presented in the House of Representatives to-day by the Minister for Public Works (the Hon. W. B. Taverner), who .staletJ he also wished to say that, apart from the suggestions made in this report, an investigation was actively in progress on the water content and elastic content of the various materials in the neighbourhood of the dam and power-house, Already there had been obtained information which indicated that the material had about 1-lOtli the elasticity of concrete, which agreed with the conjecture that after the initial crack in the f'orebay had been formed the block between the forebay and the gorge had bent over as a result of liydrosatic force. The present investigations, said Mr Taverner, were aimed at an estimation of the behaviour of pumice breccia and rhyolite under different degrees of moisture content. Otie possibility suggesting it’-elf was that water seeping into tlie rhyolite and brec-ia caMed a volume of alterations which nrfcsibly resulted hi the original track. Tins anti Other liivestigiiticlis Were being actively pursued with a view to ttsclertainiiig more definitely the original cause of the disturbance. He had sumbitted the geological report to the Departmental engineers and had asked them to supply a report based on the geological position indicating what steps they considered should be taken from an engineering point of view to deal with the- trouble that had occurred. This engineering report would be submitted to Professor Hornell, the .expert from orQrsen>, who has been appointed by the Government to investigate the Arapuni scheme in order that lie might advise on the steps suggested by the Department. POWER-HOUSE DISPLACED. The geological report is a. lengthy document. After outlining the geological formations hear the power-house, it describes the indications of deformation, enumerating the cracks, leaks, and tilts. The report continues: “by survey the power-house had been found to he slightly displaced, but the precise amount and direction of the movement had not been closely considered. There were several known stresses and other possible causes that would tend to open the cracks and overturn the mass of rock between, the headrace and the gorge. These might he divided into .stresses and weaknesses created by the engineering works undertaken, geological factors, and earth tremors. . Under the heading of “Failure of Basal Rocks,” tlie report states:— “The ttiffaceouß indurated clays, banded sandstones, and breccias on which the power-house is built arc undoubt’ edly the weakest rocks of the area, and thoir position at the toe of a deep excavation places on them the maximum crushing and gravity streS’ ses due to the weight of the moved mass between the headrace and the gorge. The crushing strength ol those weak determined bj experiment in not much more than sufficient to sustain the weight of the superincumbent mass, This critiea area was carefully examined, but nc sign of failure was observed. Possibly the failure occurred below the rives level, but against this the block aftei its maximum deformation has moved back within a few days more thar half-way to its original position, a fad suggesting that the elasticity of rod is not destroyed as it would be ii failure and crushing had occurred Had sand and grit not entered the crack in considerable amount tlie fis sure would probaby have closed entirely again. Available data on tin correlation of strengths of rocks ii i small blocks and in the mass indicate dearly that as the area undei load is increasing the. load per uni 1 area may also be much increased with out crushing. It must, however, b< pointed out that most of the invest igations on strengths and elastic!tie; of rocks have described the propertie; of rocks much harder and denser than are the tuffs and breccias o Arapuni, and possibly tlie results o these studies are not altogether a.p plieable. When, however, it is alsc considered that the weak rocks at tin power-house have not failed during tlie hundreds of years the gorge ha: existed, it appears unlikely that de formation is due to rock failure anc crushing at this point.” i POSSIBILITY OF RECURRENCE. Dealing with the possibility of a re currence of the movement, the repori says: “Once the crack opened tin hydrostatic pressure of the water in I undoubtedly was the principal cause of the widening of the fracture anc the concurrent increase in the tilt ol the moved block. The evidence doe: not clearly show that purely geological factors were important in causing the crack. Possibly they were sufficient in combination with definitely known engineering stresses ol rupture the rock already weakened by erosion of the channel forming the headrace and by engineering excavations. The cause of the fracture must lie definitely ascertained, so that remedial measures may be taken. We suggest that the strata beneath the power-house be exp’ored to a depth of 100 feet or more by shafts or by bores of large enough diameter to yield a I continuous core, and that the differ- | ent rock layers he geologically exam- ! inod and tested for their strength and elastic properties.’’