ARAPUNI HYDRO-ELECTRIC-POWER WORKS. GEOLOGICAL REPORT (TOGETHER WITH STATEMENT BY THE HON. W. B. TAVERNER, MINISTER OF PUBLIC WORKS).

Appendix to the Journals of the House of Representatives, 1930 Session I, D-01b

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1930. NEW ZEALAND.

ARAPUNI HYDRO-ELECTRIC-POWER WORKS. GEOLOGICAL REPORT (TOGETHER WITH STATEMENT BY THE HON. W. B. TAVERNER, MINISTER OF PUBLIC WORKS).

Laid on the Table of the House of Representatives by Leave.

T desire to lay on the table the geological report on the fracture of the rock at the Arapuni Spillway. T also wish to say that, apart from the suggestions made in this report, an investigation is 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 has been obtained information which indicates that the material has about one-tenth the elasticity of concrete, and a value which agrees with the conjecture that, after the initial crack in the forebay had been formed, the block between the forebay and the gorge bent over as a result of hydrostatic force. The present investigations are aimed at an estimation of the behaviour of the pumice breccia and rhyolite under different degrees of moisture content. One possibility suggesting itself is that water seeping into the rhyolite and breccia caused volume alterations, which, possibly, resulted in the original crack. This, and other investigations, are being actively pursued with a view to ascertaining more definitely the original cause of the disturbance. I have submitted the geological report to my departmental Engineers, and I have asked them to now supply me with a report, based on the geological position, indicating what steps they consider should be taken from an engineering point of view to deal with the trouble that has occurred. This engineering report will be submitted by me to Professor Hornell, the expert from overseas who has been appointed by the Government to investigate the Arapuni scheme, in order that lie may advise on the steps suggested by the Department. THE FRACTURE OF THE ROCK AT THE ARAPUNI SPILLWAY. We (Professor Bartrum, and Drs. Marshall and Henderson) arrived at Arapuni on the morning of the 23rd June, and left on the evening of the 25th June. We examined the Arapuni area generally, paying special attention to the area about the spillway and the power-house where cracks had recently formed and tilting occurred. Mr. Rabone, Engineer in Charge, and his staff, gave every facility for examining the works, plans, and records, and we take this opportunity of expressing our thanks and appreciation of the courtesy everywhere extended to us.

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Outline of Geology. There are four geological formations, near the power-house, which in downward order are — (1) Unconsolidated sands and. gravels, often 50ft. thick; (2) Much-jointed columnar rhyolite tuff, 50ft. thick: (3) Massive pumice breccia, 90 ft. to 100 ft. thick ; and (4) Tuffaceous clays, sands, and breccias, which extend to an unknown depth below the river. The first group of beds, the unconsolidated sands and gravels, here need no further consideration. The second and third formations are relatively strong rocks. They are porous rocks and when dry readily absorb water, but are not readily permeable, though the joints in them allow the passage of water. Weathering accentuates these joints, which are conspicuous on the faces of the cliffs. In the solid they are tightly appressed and often scarcely perceptible. Between the columnar rhyolitic tuff and the pumice breccia are a few feet of weak beds consisting of a poorly consolidated silty phase of the columnar tuff and thin beds of gravel, sand, and clay, the last mentioned being the soil of the gently undulating old land-surface of the pumice breccia on which the columnar rhyolite tuff was deposited as volcanic ash. These weak beds, especially the old soil, are decidedly impervious. The upper few feet of the fourth and lowest set of beds consists of indurated clays, probably the old soils of a former land-surface carved from weak tuffs and bedded sands. The power-house is built on these beds, of which only a small area is exposed. Indications of Deformation. Cracks. —The main crack ruptures the concrete at the corner where the spillway weir joins the penstock block and extends on the floor of the forebay nearly parallel with and close to the penstock block and along the headrace southward to a point 960 ft. from the north - east corner of the spillway. Northward, the small discontinuous fissures that appeared on the surface of the unconsolidated sands and gravels probably mark the extension of the crack in the rock below. These were traced for some distance, and, directly in line on the road from the low-level bridge to the transformer-house, several strong springs appeared at or near the base of the columnar rhyolite tuff. Cracks, more or less parallel with the chief break, cross the northern end of the penstock block. Another series of narrow discontinuous cracks in the covering beds extends at a slight angle from the main fracture, more or less parallel with the lower part of the overflow-channel, as far as the gorge eroded at its end. Strong leaks appeared at or near the base of the columnar tuff in this locality also. A table of measurements prepared by the engineering staff shows how the fissure in the concrete —the only place where it could be exactly measured —at first widened, and then, as the dam was emptied and the forebay drained, became, progressively narrower, until now it is less than half the width it was at the time of the maximum opening. The fissure in the forebay also showed progressive narrowing from the time it could be observed, and the fact that fragments of rock and pumice are pinched in it shows that decided closing occurred. Leaks. —The first indication that something untoward had happened was the flooding of the battery-room in the power-house. This vvas afterwards found to be due to the partial blocking of a drain which, on that account, could not carry the discharge from the leaks that appeared in the rock wall at the back of the power-house. Other leaks were discovered during the day. The leaks are in three localities : (a) Those near the power-house are from joint-planes in the pumice breccia and were at several points in the cliff some 30 ft. or 40 ft. above the main floor of the power-house ; another strong leak was through a joint-plane on the lower part of the No. 4 penstock tunnel. (b) Two decided leaks came into existence near the base of the columnar tuff on the road above the power-house and several smaller seepages at the same horizon in the cliff behind the power-house and in No. 4 penstock tunnel, (c) Five leaks were noted a little above water-level in the gorge excavated by the fall at the end of the overflow-channel ; these are at the same impervious horizon at the base of the columnar tuff. It should be noted that several of the leaks at first discharged milky water, but, in general, this discoloration disappeared within twenty-four hours. The lowering of the water in the forebay had also a decided effect on the amount of water yielded by all the leaks and seepages, as is well shown by the table prepared by the engineering staff. The leaks ceased altogether shortly after the water had drained from the forebay, though at that time the water still stood high in the dam. Tilts. —No. 1 generator had been levelled on the day preceding the formation of the crack, and, on relevelling, was found to have a slight tilt toward the river. The other generators, when the turbines stopped, were found to have tilts of similar amount, also toward the river. The 70 ft. tower at the west end of the suspension-bridge over the gorge was found to be 1| in. out of plumb, the inclination being toward the river. The ropes forming the handrails were noticeably slack, and, on measuring, the span was found to have been reduced by an inch or two. The tilts in the power-house and of the bridge tower steadily decreased as the forebay was emptied. Results of Resurvey. By survey, the power-house has been found to be slightly displaced ; but the precise amounts and directions of the movement have not been closely considered. The cracking and tilting above described could all be readily explained if the block of country between the spillway and power-house had tilted toward the gorge about one minute of arc, and indicate a maximum movement of the top of the block in that direction. If the rocks forming the block were absolutely rigid, the base of the block would be about 200 ft. below river-level.

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Possible Causes of the Overturning Movement. There are 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 may be divided into the stresses and weaknesses created by the engineering works undertaken, geological factors, and earthtremors. Engineering Factors. —The engineering factors require little more than mention. They include the hydrostatic head on the penstock block, the headrace, and the overflow-channel; the weight of water in the penstock tunnels ; the kinetic energy of the moving water ; the vibration of the generators ; and the weight of water in the headrace, and also the weight of the transformer-house. In addition, the blasting of the rock during the driving of the penstock tunnels, and the removal of the rock itself, no doubt weakened the mass. The excavation of over 100,000 yards of rock to provide room for the power-house and its extension is more important, as this mass formed a buttress supporting the moved block. Geological Factors. —" Any excavation in the earth's crust sets up stresses in the contiguous rocks, because of the unbalanced pressures created by the substitution of atmospheric pressure for the greater pressure of the material excavated." The horizontal components of these stresses exert a pressure toward the excavation, in this case, toward the gorge, and would tend to open cracks subparallel with the gorge and with maximum deformation near the surface. The floor of the old river-channel, followed by the headrace and the overflow channel, has now been largely stripped, by the moving water, of its cover of soil and forest debris that up till now almost entirely concealed the rock. A number of relatively straight cracks, some of them several chains long, obliquely cross the smooth gently hollowed rock floor of the old channel. The cracks are now sealed and as watertight as the somewhat altered rock they traverse. They are undoubtedly old and may have been formed in the manner suggested after the gorge was cut, a time which, as judged from the size of the trees growing in it, was at least several hundred years ago. One would expect cracks of this kind to gape more widely near the edge of the gorge than those farther back, and the cracks on the east side of the river above the dam may be cited. These are from 6 in. to 12 in. wide at the surface, but disappear at a depth of 40 ft. On the west or headrace side of the river the heavy cover of sands and gravels may conceal widely opened cracks, but there is no evidence of this. The crack that traverses the upper end of the headrace nowhere reaches an inch in width. The fracture that opened in the forebay on the 7th June closely resembles the cracks described above, and may have been produced in the same way. The well-developed vertical joints of the great bulk of the rocks greatly reduces the tensile strength of the mass, and it is not improbable that the tensional stresses developed when the gorge was cut were not wholly dissipated during the many years it has existed. The residuum of these tensional stresses combined with similar stresses due to the power-house excavations may have been sufficient to rupture the rock. Shortly after the crack formed, bubbles of gas were observed rising discontinuously at several points in the forebay. Later gas emanations were found on the floor of the forebay and headrace, and at one point gas was feebly rising to the 23rd June. The gas generally escaped from joint-planes near the crack, but some of the points of issue were some distance from it. The Dominion Analyst examined a sample and found it to consist chiefly of nitrogen, no oxygen being present. This gas may be interpreted as air so long imprisoned in the joint-planes, and pores of the rock mass, as to be completely deoxygenated. Its expulsion may be explained as being due to the slight contraction of the rock-mass as soon as the tensional stresses were relieved by the cracking of the rock. Failure of Basal Rocks. —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 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 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 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 considerable amount the fissure would probably have closed entirely. Again the available data on the correlation of the strengths of rocks in small blocks, and in mass, indicate clearly that as the area under 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 the strengths and elasticities of rocks have described the 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 the power-house have 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. Hydrostatic Head of Water in Rocks. —The rocks of the headrace, forebay, and overflow-channel, though porous, do not readily permit the passage of water. Small pools on their surface remain till the water evaporates, and the penstock tunnels driven as much as two years after the headrace was filled, were dry. There is, of course, some slow percolation and the base of the columnar rhyolite tuff exposed in the cliff above the power-house became slightly damper after the headrace filled. Assuming that the partings in the columnar tuff, though too small to allow the flow of water, were yet large enough efficiently to transmit the pressure due to hydrostatic head, this pressure must have been fully operative for over two years. The same applies to the underlying pumice breccia which floors the whole of the headrace from a point a few chains above the spillway.

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The tables prepared by the engineering staff, showing the changes in the width of the crack, the amounts of the tilts, and the discharges from leaks, clearly indicate the effect of the alteration of the depth of water in the forebay. Undoubtedly once the crack was formed the hydrostatic head in it was the main factor in determining the amounts of these. But until this crack formed the small amount of water (if any) that percolated from the headrace into the rock did not, in our opinion, exert an effective hydrostatic pressure. Earthquake Tremors. —Earthquake tremors cause deforming movements in rock-masses, and produce landslides and fractures in solid rocks. Several people at Arapuni now say that they felt an earth-tremor sometime after midnight on the 7th June ;. but as this slight earthquake was not felt in the surrounding district it is probable that the opening of the crack caused the earthquake, and was not caused by it. Possibility of Recurrence of Movement. Once the crack opened the hydrostatic pressure of water in it undoubtedly was the principal cause of the widening of the fracture and the concurrent increase in tilt of the moved block. The evidence does not clearly show that the purely geological factors, as detailed above, were important in causing the crack. Possibly they were sufficient, in combination with the definitely known engineering stresses, to rupture the rock, already weakened by the erosion of the channel forming the headrace and by the engineering excavations. The cause of the fracture must be definitely ascertained so that remedial measures may be taken. We suggest that the strata beneath the powerhouse be explored to a depth of 100 ft. or more by shafts or by bores of large-enough diameter to yield a continuous core, and that the different rock-layers be geologically examined and tested for their strength and elastic properties. J. Henderson. P. Marshall. J. A. Bartrum.

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