DISASTER AT ARAPUNI

Taranaki Daily News, 18 July 1930, Page 11

DISASTER AT ARAPUNI

REPORT FROM GEOLOGISTS RESULTS OF INVESTIGATIONS. CAUSES OF FRACTURE DISCUSSED. By Toksrayh.—Press. Aseo.clatlon, Wellington, Last Night. The- geological report on the fracture of rock at the Arapuni spillway was pre- ! rented in the House of Representatives : to-day by the Minister of Public Works, < the Hon. W. B. Taverner, who stated ■ that he wished to say that apart from i the suggestions made in this report the I investigation was actively in progress on the water content and elastic eon- ( tent of the various materials in the neighbourhood of the dam and powerhouse. Already there had been obtained information which indicated that the ma--terial was of about l-10th of 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 hydrostatic force. The present investigations were aimed at an estimation of the behaviour of the pumico breccia and rhyolite under different degrees of moisture content. One possibility suggesting itself was that water seeping into the rhyolite and breccia caused volume alterations which possibly resulted in the original crack. This and other investigations were being actively pursued with a view to ascertainingmore definitely the original cause of the disturbance. He had submitted the geological, report to the department’s 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. This engineering report would be submitted to Profesor Hornell, the Swedish expert who has been appointed by the Government to investigate the Arapuni scheme in order that he might advise on the steps ■ suggested by the department. FORMATION OF THE ROCKS. The geological report is a lengthy document. After outlining the geological formations near the power-house it describes the indications of deformation, enumerating cracks, leaks and tilts. By survey the power-house had been found, to be slightly displaced but the precise amounts and directions of the movements 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 headrace 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: “Tho tuffacious and indurated clays, banded sandstones and breccias on which the power-house is built are undoubtedly tho weakest rocks of the area and their position at tho too of the 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 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.

i “Possibly the failure occurred below i 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 the failure and crushing had occurred. Had sand and grit not entered the crack in considerable amount the fissure would probably have closed entirely. Again the available data on the correlation of strengths of the 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 tho strengths and elasticities of rocks have described tho properties of 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 tho power-house havo not failed during the hundreds of years the gorge has existed it appears unlikely that the deformation is due to rock failure and crushing at this point.” Dealing with the possibility of a recurrence of tho movement the report states: “Once tho crack opened the hydrostatic pressure of water in it undoubtedly was the principal cause of the widening of tho fracture and the concurrent increase in the tilt of the moved block. Tho evidence does not clearly show that purely geological factors were important in causing tho crack. Possibly they were sufficient in combination with definitely known engineering stresses to rupture the rock already weakened by erosion of tho channel forming the headrace and by tho engineering excavations. Tho cause of the fracture must be definitely ascertained so that remedial measures may bo taken. We suggest that the strata beneath the power-house be explored to a depth of 100 feet or more by shafts or by bores of large enough diameter to yield a continuous core, and that tho different rock layers be geologically examined and tested for their strength and elastic properties.”