ARAPUNI BREAK

Timaru Herald, Volume CXXV, Issue 18622, 18 July 1930, Page 8

ARAPUNI BREAK

GEOLOGICAL REPORT RECEIVED. TECHNICAL BUT NOT ILLUMINATING. By Telegraph—Press Association WELLINGTON, July 17. A geological report on the fracture of rock at Arapuni spilling was presented in the House of Representatives to-day by the Minister of Public Works (Hon. W. B. Taverner). The Minister stated that he also wished to say that, apart from the suggestions made in this report, investigation was actively in progress on the water content and elastic content of various materials in the neighbourhood of the dam and the power house. Already there had been obtained information which indicated that the material had about l-10th the elasticity of concrete, and a value which agreed with the conjecture that after the initial crack in the forebay had been formed, the block between the forebay and the gorge had bent over as a result of hydro-static force. The present investigations were aimed at an estimation of the behaviour of pumice, breccia, and rhyolite under different degrees of moisture content. One possibility suggesting itself was that water seeping into rhyolite and breecia was causing volume 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 a geological report to the Departmental engineers, and had asked them to supply a report based on the geological position, indicating what steps they considefed 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 Homell, the expert from overseas who had been appointed by the Government to investigate the Arapuni scheme, in order that he might advise on the steps suggested by the Department. The geological report is a lengthy document, and after outlining geological formations near the power house, it describes indications of deformation, and 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 movement had not been closely considered. There were several known stresses and other possible causes that would tend to open cracks and overturn the mass of rock between the head race and the gorge. These might be divided into stresses and weaknesses created by the engineering works undertaken, geological factors, and earth tremors.

Under the heading of failure of basal rocks, the report states: “The tuffaceous indurated clays, banded sandstones, and breccias, on which the power house is built are undoubtedly the weakest rocks of the area, and their position at the toe of deep excavation places on them maximum crushing and gravity stresses, due to the weight of the moved mass between the head race and the gorge. The crushing strength of these weak rocks, as determined 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 the 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 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 a considerable amount, the fissure would probably have closed entirely again. Available data on correlation of strengths of rocks in small blocks and in mass indicate clearly that as the area under the load is increased, the load per unit area may also be much increased without crushing. It must, however, be pointed out that most of the investigations on strengths and elasticities of rocks have described the properties 9f rocks much harder and denser than are the tuffs and breccias 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 hundreds of years the gorge has existed, it appears unlikely that deformation is due to rock failure and crushing at this point.” Dealing with the possibility of a recurrence of movement the report states: “Once the crack opened, the hydrostatic pressure of water in it undoubtedly was the principal cause of the widening of the fracture, and concurrent increase in tilt of the moved block. The evidence does not clearly show that purely geological factors were important in causing the crack. Possibly they were sufficient in combination with definitely known engineering stresses, to rupture rock already weakened by erosion of the channel forming the head race and by engineering excavations. The cause of the fracture must be definitely ascertained, so that remedial measures may be taken. We suggested that the strata beneath the power house be explored to a depth of a hundred feet or more, by shafts, or by bores of large enough diameter to yield a continuous core, and that different rock layers be geologically examined, and tested for their strength and elastic properties.”