Geologists Report On Arapnni Problem

Sun (Auckland), Volume IV, Issue 1027, 18 July 1930, Page 8

Geologists Report On Arapnni Problem

BASE OF POWER HOUSE NO ROCK FAILURE Pres* Association PARLIAMENT BLDGS, ThursdAr The geological report on the fractur. of the rock at the Arapuni spillwsr was presented in the House of Reprisentatives today by the Minister Public Works, the Hon. W. B. T»v erner. The Minister stated that he wished to say that apart from sbj. gestions made in this report an invet tigation was actively in 1 - ogress os the water content and elastic contest of the various materials iu the neighbourhood of the dam and power-house. Already there had been obtained is. formation which indicated that the material had about one-tenth the elasticity of concrete, a value which agreed with the conjecture that after the initial crack in the forebay had been formed the block between the forebav and gorge had bent oTer aj a result of hydrostatic force. The present investigations were aimed at an estimation of the behaviour of the pumice breccia and rhyolite under different degrees of moisture content. One possibility suggesting itself was that the water seeping into the rhyolite and breccia caused a volume of alterations which possibly resulted in the original crack. This and other investigations were being actively pursued with a view to ascertaining more definitely the original cause of the disturbance. He had submitted the geological report to departmental engineers and had asked them to supply a report, based on the geological position, indicating wha: 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 HorneH the expert from Sweden, who had been appointed by the Government to investigate the Arapuni scheme in order to advise on the steps suggested by the department. OPENING OF REPORT The geological report is a lengthy document and after outlining the geological formations near the power house it describes the indications of deformation, enumerating the cracks, leaks and tilts. The report continued that by a survey the power house had been found to be slightly displaced, but the precise amounts and directions 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 cf rock between the headrace and the gorge. These might be divided into stresses and weaknesses created by the engineering works undertaken and geological factors and earth tremors. Under the heading of failure d basal rocks, the report states: — "The tuffaceous indurated days, banded sandstones and breccias on which the power-house is built are undoubtedly the weakest rooks of the area and their position at the toe of a deep excavation places on them the maximum crushing and gravity stresses due to the weight of the moved mass between the headrace and the gorge. The crushing strength of these weak rocks as de termined by experiment is not much more than sufficient to sustain the weight of the superincumbent mass. This critical area was carefully examined. but no sign of failure was observed. Possibly failure occurred below the river level, but against this the block, after its maximum deformation, has moved back within a , few days more than half-way to its I original position, a fact suggesting that the elasticity of the rock is not destroyed, as it would be if failure and crushing had occurred. Had sand and grit not entered the crack in considerable amounts the fissure would probably have closed entirely again. The available data on the correlation of the strengths of rock* in small blocks and in mass indicate clearly that as the area under load is increr.ced. the load per unit o. area may also be much increased without crushing. It must, however, be pointed out that most of the investigations on the strengths and elasticities of rocks have described properties of rocks much harder and denser than are the tuffs and brec cias of Arapuni and possibly the results of these studies are not altogether applicable. When, however, it is also considered that the weak rocks at the power-house have not failed during the hundreds of vent* the gorge has existed, it appear* unlikely that the deformation is due to rock failure and crushing at Hu* point.” RECURRENCE OF MOVEMENT Dealing with the possibility of a recurrence of the movement the report states: —“Once the crack opened the hydrostatic pressure of the water in it undoubtedly was the principal cause of the widening of the rure and th concurrent increase/* the tilt oi the moved block. evidence does not clearly show th** purely geological factors were important in causing the crack. Possibly they were sufficient in combination with definitely known engineering stresses to ruptnre the rook *j* ready weakened by the eros *?, the channel forming the hca< and by the engineering excavationsThe cause of the fracture must _ definitely ascertained so that dial measures may be takensuggest that the strata beneath power-house be explored to a of 100 feet or more by shafts OT ++ bores of large enough diameter yield a continuous core and [Mt different rock layers be examined and tested tor strength and elastic properties. _