SAVING THE ARAPUNI PLANT

Dominion, Volume 24, Issue 18, 16 October 1930, Page 7

SAVING THE ARAPUNI PLANT

Prevention of Future Water Soakage OPTIMISTIC REPORT BY SWEDISH EXPERT Necessary Cost Estimated at £560,000 Although for various reasons Professor P. G. Hornell, the Swedish engineering expert, whose report upon the breakdown at the Arapuni hydro-electric works was presented to Parliament yesterday, gave no indication of the cost of remedial measures, the Public Works Department engineers estimate that £560,000 will be needed to make the country safe and the plan operative. Professor Hornell finds no fault with the plant, the site selected, or the way the work was carried out. He considers that although the country would not have shifted had the bottom and sides of the headrace been provided with a watertight lining, the engineers responsible did not take an unjustifiable risk in not doing this in the first place. ’ Chief among the remedial measures proposed in the report is the lining of the headrace with waterproof material, as the primary cause of the rupture is traced back to the presence of water in the headrace, although many other possibilities are explored. Professor Hornell is convinced that no further ruptures will occur if the present cracks are effectively sealed and the headrace lined. The opinion is advanced that the future of the plant depends upon the prevention of soakage. Suggestions are made for a drainage system below the headrace, the reinforcement of ground near the penstock intakes, the placing of a waterproof layer on the rock below the spillway to the crest of the falls, the stoppage of leaks in the main dam foundations, and the strengthening of country adjacent to the dam. Much attention was devoted to the outdoor transformer station when the ruptures occurred, but Professor Hornell, considers that this station had no appreciable influence on the fracture of the hill. He also b'elieves that the present power house can be safely operated without any major measures being taken. The site was found to be secure, and there was no risk in proceeding with the extension of the station along the lines proposed before the trouble occurred on June 7. The value of the diversion tunnel is emphasised, and it is urged that it should be ready at all times for operation. In the main, the report makes it clear that Arapuni can be saved as was previously predicted, the only remaining question of importance being the cost, which Professor Hornell, not being conversant with local conditions and circumstances, is content to leave to the Government engineers. He is convinced that there would be no danger in bringing the power station into commercial operation before the final arrestation of the erosion in the spillway channel is completed, provided the remedial measures he proposes are carried out.

CAUSES OF TROUBLE Water in Headrace STRESSES RELIEVED "The primary cause of the rupture must in my opinion be traced back to the presence of water in the headrace,” Professor Hornell says, after discussing various factors which might have caused the trouble. “As- the water cannot readily flow by gravity through the pores of the rock or in the narrow channels between different columns, the chief mechanical agent causing water to enter the rock must be attributed to capillarity. “The columnar rock at Arapuni, being of a porous nature, is particularly subject to the effect of capillary forces. This may be shown by the simple experiment of introducing a dry piece of the rock into water, when it is {found that the liquid is avariciously absorbed. It is also easily conceivable that capillary action takes place in the joint planes between the different columns of the rock. Swelling of Kock. “One of the most important characteristics of the columnar rock is the swelling of a dry piece of the rock when being saturated. Whether this is a direct or an indirect consequence of the capillary action is insignificant. It has been shown experimentally that the expansion per unit length due to saturation of a dry specimen is very great, amounting in one particular test to about 1 part in 2000.

“Another aspect of the capillarity has reference to the presence of gas in the ground. There is every evidence that water has been brought by capillarity into the ground under the bottom of the headrace, thereby causing a compression of gases in the deeper strata. This aspect of capillarity may account for the eventual escape of remnants of air through

the bottom of the headrace. It has not been possible to ascertain whether gases have also escaped from the deeper parts of the ridge.. “It is probable, however, that gases have also existed in these places, and that the gases have been compressed by the downward-travelling capillary water. The compressed gases have eventually tended to lift the overlying strata, and thus contributed to the-cause of failure.” The opinion is expressed that the cracking of the ground started at or down stream between the intake and the spillway. “Once a crack had formed,” he says, “certain internal stresses were automatically relieved. Water immediately filled the crack, developing the full hydrostatic pressure and widening the fissure until a new state of equilibrium was established between ' the hydrostatic pressure on one side and the elastic effect of the combined cantilever and beam action of the ridge on the other side. “Other subsidiary cracks appearing on the surface may be attributed to temporary phenomena due to the shock produced by the formation of the main crack. Whether or not some connection with deeper fissures was created at the occasion is uncertain. Evidently the ridge between the headrace and the powerhouse has, during the accident, been subjected to a severe test load without failing as a. dam. This indicates that the ridge will stand safely in the future, if it is adequately protected from the influence of destroying agents. Effective Remedy. "I am convinced that no rupture would have taken place if water under an increased pressure had been prevented from entering the ground in the bottom and sides of the headrace. I am equally , convinced that no rupture will take place in the future if the cracks which have form-' ed be effectively sealed and 1 adequate measures be taken to prevent the headrace water from entering the ground. “The ’fact that the crack started to close up when the water was lowered in the headrace clearly demonstrates the great elasticity of the rock. It also gives

th« reassuring evidence that no internal failure due to crushing of the rock or washout has taken place in the ridge or in the underlying strata. The fact that the crack has not completely closed is to a great extent due to the obstruction produced by materiar washed into it from the headrace. What remains of the crack indicates that there is still some compression in the columnar rock of the ridge, which will remain as an Initial stress in the future, tending to restore the ridge to its original position. “Obviously, the ground under the power-house, and the structure itself, have also been subjected to very great stresses due to the movement of the ridge. The fact that the building nearly regained its old position shows that the foundation is uncrushed and well capable of taking its normal load. Also, it may be said that the various concrete structural elements have very well withstood the severe condition temporarily imposed upon them through the rock movement.” REMEDIES PROPOSED Stopping Water Soakage LINING THE HEADRACE Basing his recommendation that any remedial measures taken must be focused on the necessity of preventing further cracks in the country. Professor Hornell says the absorption of water needed most attention. To prevent this he suggests sealing the existing cracks with concrete to a certain extent, and then covering a portion of the headrace in which thq troublesome rocks occur, with an impermeable covering. “It may safely be assumed,” he says, ‘‘that the block of country in which the crack lias occurred was in stable equilibrium before the river was diverted into the headrace, so that any remedial measures should be directed to maintaining the hydrological conditions in the state in which they originally existed. The question arises as to whether the presence of the existing crack has increased or lessened the possibility of future earth movement. At the present time there, is about the same amount of water draining into the headrace as would correspond with the hydrological conditions existing before the plant was built. Country Divided. “The conditions of equilibrium of the block of country are, however, not the same. The crack has separated the country into two parts, removing whatever support existed between them prior to its development. Even if the crack is filled with concrete the conditions will still not be the same. Due to the elastic properties of the rock, tending to bring the ridge back to its original position, the ridge will be exerting a greater pressure against the country to the west than was formerly the case. “This residual stress in the ridge will tend, of itself, to prevent the formation of similar cracks in the future. It might thus be said that the damage that occurred at Arapuni on June 7 provided the means of giving the ground an additional stability to that existing when the works were first begun. “I have come to the conclusion that the hydrostatic pressure of the water on the bottom and sides of the headrace will not, in itself, cause a dangerous earth-movement, provided such remedial measures as are proposed below are carried out. As regards the future, it is the forces brought into play due to the soakage of water into the ground that need the most careful consideration.”

To some it might appear illogical to attempt to safeguard the stability of the ground in future by means of preventing as far as possible the absorption of water. There was reason to believe, however, that such measures, if carried out too extensively, might lead to serious' trouble. “I feel convinced that the provision of an impermeable covering over the ridge between the headrace and the river gorge, which would allow the underlying rock to dry out, with a consequent contraction and production of deep, though possibly small cracks, would create a new danger rather than act as a safeguard,” Professor Hornell says. Drainage System. The measures to be adopted should in the first place consist of sealing the existing cracks with concrete to a certain extent, and then covering a porof the headrace in which the columnar rock, occurs with an impermeable covering, Professor Hornell recommends. He provides under this covering a porous drainage system in the form of a filter which will discharge into a trench excavated underneath the lining along the length of the headrace and discharging into pne of' the tunnels which have been excavated, for observation purposes between the power-house road and the headrace. All loose pieces of concrete in the penstock intake structure should be effectively grouted, a reinforcement of the ground itself should be arranged along the intake, tnd after this treatment the whole

of tfae water surface of the intake structure should be plastered with a gunite 1 facing. In certain cases where a earing in cost may be effected by so d°‘ n B> he provides that a concrete wall should bo built along one side of the headrace instead of extending the water-tight covering to the floor into positions where the headrace has very considerable width. Professor Hornell does not consider it necessary to provide this concrete lining for the whole length of the headrace, but he is of opinion that it will be sufficient if it extends from the spillway to a distance of about 350 feet upstream from the last point at which the columnar rhyolite appears in the floor of the headrace. He also recommends that a further waterproof layer be laid on the rock below the spillway as far down as the crest of the falls, and that measures be taken to drain all cracks that may exist in the deeper parts of the ridge to the left of the river gorge. “No matter what measures are taken in order to safeguard the stability of the ground,” he states, “the country, as a whole will retain properties relating to plasticity, and, in my mind a plastic lining of the bottom and the sides of the. headrace is more in conformity with actual conditions. I am convinced that such a lining can be arranged so as. to prevent effectively water from entering the ground, even if, contrary to my expectations, cracks should tend to occur in the ground. Strengthening the Spillway.

“As to the action of these remedial measures, a few words may be added. The aim of the porous layer under the impermeable lining of the headrace is twofold : it serves as a means of preserving the natural moisture of the ground, and at the same time to give warning should a rupture in this lining occur. As, in my mind, a rupture is possible only in case of an earthquake or a tectonic movement due to other causes, the first sign of it should immediately call for emptying the lake through the diversion tunnel. I would, therefore, in order to prevent any future disaster, propose that a device be installed, signalling instantly any exceptional increase of water in the drainage tunnel north of the power-house. “The more importance I give to this porous layer, the more strongly I recommend that it be arranged with proper care and with the use of proper materials. There is a theoretical possibility that it might partly clog on account of weathering of the underlying rhyolite rock. I feel, nevertheless. convinced that it will serve its aims if carried out properly. “I finally draw attention to one circumstance—the lining will greatly add to the stability of the spillway. For reasons partly akin to the reasons calling for a lining above the spillway, I. recommend that a watertight concrete lining be provided immediately on top of the rock below the spillway extension.” NO UNDUE RISK Engineers Held Blameless NO FAULT IN PLANT “In general it may be said that the works as executed have been designed in accordance with sound principles of engineering,” Professor Hornell says in replying to the order of reference handed to him when he arrived. He considers the best possible use has been made of the topography of the site, that the main dam is in its correct position in regard to ultimate power output, and that as far as public safety is concerned its stability under full head complied with the main rules now conceded as governing the design and construction of a solid gravity dam. In addition, the opinion is expressed that constructional work has been well carried out and that although no rupture would have taken place had the sides of the headrace been provided with watertight lining, the engineers did not undertake an unjustifiable risk in not lining the headrace originally. Professor Hornell expresses the view that the general surroundings, and the power-development possibilities at Arapuni are such that the locality is very suitable for the economic development of hydro-electric power. Concerning the class of country, he says that it by no means excludes the possibility of creating a successful water-power station on the site. He has no apprehension as to damage likely to occure from water leaking from the lake by way of Acacia Gully. Dam in Right Place.

The main dam is correct in its position in regard to the ultimate proposed power output, says the professor, but as there is evidence that the dam is leaking at some places in the foundation, even at the present lowered waterlevel with the river at its original state, it is essential that the origin of any appreciable leakage be located, and that the leaks be carefully grouted under pressure, so as to stop effectively the

water paasiag under ttw dam or through the rock foundation. Professor Hornell is also of opinion that the opportunity should be taken to further strengthen the country adjacent to the main dam, particularly on the western abutment, by reinforcing grouting and waterproofing the sides of the headrace between the headrace bridge and the junction of the headrace with the lake. In reply to questions relative to the development of the scheme, it is stated that the works are sited in such a way as to ensure the maximum power available consistent with reasonable expenditure. “In general,” Professor Hornell remarks, “it may be said that the works as executed have been designed in accordance with sound principles of engineering. . . From the civil engineering point of view I have no adverse criticism to pass upon the general location of the different parts of the plant in relation to each other. On the electrical side of the problem I can make no comment. . . As far as can be seen from a detail visual inspection on the site, the constructional work has been well carried out.” “Justifiable Risk.”

Discussing the question whether the crack which took place on June 7 might have been foreseen, Professor Hornell expresses the opinion that although the original plans and specifications indicate that the Public Works Department was not unmindful of the possible necessity for lining portions of the race, it took a justifiable risk in not carrying out this lining when the work was constructed. He is of opinion that features such as the driving and operation of the penstock tunnels and the weight of the outdoor transformer station had very little effect in causing the rupture. The view is also held that the fact that the crack has occurred will have no effect in preventing the continued operation of the existing power station, and that there is no need tor apprehension in extending the power station in its present site to the full capacity anticipated. No reason can be seen for abandoning the present headrace or power station if the remedial measures suggested by him were carried out. With regard to the definite consolidation of the falls, Professor Hornell states that, in view of the magnitude of the works involved, he would require further time to make more detailed examination before giving a finally considered opinion. As far as his investigations have enabled him to go, he is of opinion that the effect of the water entering the columnar rhyolite rock between the spillway and the head of the falls may have had a considerable effect in causing the rupture at the falls, and for this reason he has suggested waterproofing the channel. Diversion Tunnel Needed. In making this suggestion, Professor Hornell stresses the point that as the power station is further developed the amount of water which will be passing over the falls for considerable portions of the year will be very considerably reduced. In regard to the diversion tunnel, he is of opinion that this should not be completely closed up with concrete, but rather that a second gate should be provided immediately above or below the present one, so that repairs can be executed to the gates if necessity arises in after years. He considers that a considerable amount of grouting should be done in and around the diversion tunnel and immediately up stream of the main dam, and he further suggests that the diversion tunnel should be plastered with a gunite surface. Finally, Professor Hornell draws attention to the fact that throughout the report and his replies to the order of reference he has done very little by way of estimating the cost of the various sections of the work. “I feel that without considerably more knowledge of local conditions and prices I could not do this with any degree of accuracy,” he says. "In view of the fact that I have described the works proposed in some detail, I have now no doubt but that the engineers of the Public Works Department could provide estimates with a fair degree of accuracy. Should It be possible for them to prepare preliminary estimates of the work proposed before it is necessary for me to leave New Zealand, I should be very pleased to confer with them to see that the measures I have proposed have been all provided for.” STOPPING EROSION Power Station Safe OPERATION PRACTICABLE In a further statement upon the breakdown, the Minister of Public Works stated that after consideration of the expert's report he had deemed it advisable to obtain further information in relation to the consolidation of the erosion below the spillway, commonly called the falls. The

locality had considerable bearing upon the subject. In a letter to Professor Hornell read by hint the Minister asked that a more definite recommendation should be made in regard to what should be done at the falls, and he asked particularly whether it were considered there was any risk of further extensive erosion should the plant be operated before complete protective work at the falls had been carried out.

In his reply, Professor Hornell said he was of opinion that if the remedial measures for hampering the speed of destruction in the spillway were executed as outlined in his report, there would be no risk in bringing the power station into commercial operation before the final arrestation of the erosive processes in the spillway Channel, wherever this should be done. The remedial measures proposed in his report might even be sufficient to make any consolidation of the falls unnecessary, especially if the power station were soon fully developed to its ultimate proposed capacity. “Therefore, I consider it inadvisable from a technical economical viewpoint to appjy any scheme of consolidation of the falls without a very careful investigation of different possibilities,” Professor Hornell said. “In the event, however, that in the course of time a final arrestation of the erosion should be deemed necessary, a definite scheme should be worked out beforehand, and as soon as possible, ready to be applied when the need arises. “Preliminary considerations of different possible solutions have made me arrive at the conclusion that the arrangements required could be executed at a much lower outlay than authorised. Finally, I would call attention to the important point that the work for a final arrestation of the erosion in the spillway channel can be executed without affecting the operation of the power station.”

Cost of the Work. “Members will perceive from a perusal of the report that the professor, for obvious reasons, has been unable to give an estimate of the cost of his suggested remedial measures,” the Minister said. “An estimate of cost has, however, been compiled by the Public Works Department at £360,000 for the work recommended by the professor. To this, of course, must be added the cost of the consolidation, of the falls, estimated at £200,000, making a total of £560,000.”