THE ARAPUNI FRACTURE

Evening Star, Issue 20539, 18 July 1930, Page 3

THE ARAPUNI FRACTURE

GEOLOGICAL REPORT [l’-ek United Press Association.] WELLINGTON, July 17. Tho geological report ot the tracture of rock at tho Arapuni spillway was presented in tho House of Representatives to-day by the Minister of Public Works (Mr Taverner). The Minister stated that he also wished to say that, apart from tho suggestions made in this report, an investigation was actively in progress on the water content and the elastic content of tho 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 one-tenth, the elasticity of concrete and a value which agreed with the conjecture that after tho initial crack in the forebay had been formed the block between the forebay and the gorge had bent over as tho result of the hydrostatic force. The present investigations were aimed at an estimation of tho behaviour of the pumice breccia and the 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 ascertaining more definitely tho original cause of the disturbance. He had submitted the geological report to the departmental engineers, and had asked them to supply a report based on (he geological position, indicating what steps they considered should be taken from an engineering point of vew to deal with the trouble that had occurred. This engineering report would be submitted to Professor Hornell, an expert from overseas, who has been appointed by the Government to investigate tlje Arapuni scheme in order that he might advise on the stops suggested by the department. Tho 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 stated that by a survey the power house had been found to be slightly displaced, but tho precise amounts and directions of the movement had not been closely considered. There wore several known stresses and .other possible causes that would tend to open cracks and overturn the mass of rock between the headrace and tho gorge. These might bo divided into stresses and weaknesses created by the engineering works undertaken and tho geological factors and earth tremors under the heading of the failure of the basal rocks.

Tho report says: “Tuffaceous indurated clays, banded sandstones, and the breccias on which the power house is built are undoubtedly tho weakest rocks of the area, and' their position at the too of tho 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. Tho crushing strength of these_ weak rocks, as determined by experiment, is not much more than sufficient to sustain tho weight of the superincumbent mass. This critical area was carefully examined, hut ho sign of failure was observed. Possibly failure occurred below the river levels but against this tho block, after its maximum deformation, lias moved hack within a fev» days more than halfway to its original position, a fact suggesting that the elasticity of the rock is not destroyed, as it would be if failure and crushng had occurred. Had sand and grit not entered the crack in » considerable amount tho fissure would probably have closed entirely again. Tho available data on the correlation of tho strengths of the rocks in small blocks and in mass indicate clearly that as tho area under load is increased tho load per unit area may also ho much increased without crushing. It must, however, he pointed out that most of the investigations on the strengths and elasticities ot the rocks have described the properties of tho rocks as much harder and denser than are tho tuffs and breccias of Arapuni, and possibly the results of these studies aro not altogether applicable. When, however, it is also considered that the weak rocks at the power house have not failed dur ing 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 the movement, the report states: “Once tho crack opened the hydrostatic pressure of water in it undoubtedly was tho principal cause of tho widening of tho fracture and the concurrent increase in the tilt of tho moved block. Tho evidence docs not clearly show that purely geological factors were important in causing the crack. Possibly they were sufficient in combination with tho dolinitcly known engineering stresses .to rupture the rock already weakened by the erosion of tho' channel forming tho hehdrace and by the engineering excavations. Tho cause of the fracture must be definitely ascertained, so that remedial measures may bo taken. Wo suggest that the strata beneath the power house should be explored to a depth of 1001't or more by shafts or by bores of large enough diameter to yield a continuous core, and that different rock layers should b© geologically examined and tested for their strength and elastic properties.”