ARAPUNI POWER HOUSE SAFE, SAYS PROFESSOR P. G. HORNELL

Star (Christchurch), Issue 19201, 15 October 1930, Page 9

ARAPUNI POWER HOUSE SAFE, SAYS PROFESSOR P. G. HORNELL

STATION MAY CONTINUE TO GENERATE ELECTRICITY

Plant Can Be Operated Without Major Measures Being Taken.

(SjJecial to the “ Star.”) WELLINGTON, October 15. T'HE FACT that a crack occurred at the Arapuni hydroelectric plant does not render the continued operation of the power generating station impossible provided remedial measures are carried out, according to Professor P. G. Hornell, whose report to the Government was made available to-day. “ I believe,” he adds, “ that the present power house can be safely operated without any major measures being taken. I consider that the position of the present power house is quite secure, and that there is no risk in proceeding to extend the station on the lines followed prior to the occurrence on June 7. I therefore believe that it will be practicable and safe to run the power house up to its present and proposed installed capacity.”

POSITION NOT AS BAD AS FEARED.

PROFESSOR HORNELL DISCUSSES REMEDIES.

In the earlier portions of the report Professor Hornell discusses various causes which may have led up to the fracture of the rock which caused the trouble at Arapuni on June 7. The various causes which have already been discussed are considered in detail, but he comes to the conclusion that the prime cause of the rupture can be traced to water entering the rocks provoked by capillary action, possibly augmented by the existence of open cracks in the deeper strata, in each case combined with the unusual, elastic properties of the rock. Movement of Crack.

He considers that the crack occurred first at or below the junction of the penstock intake structure with the spillway, and that it gradually extended from there up into the headrace and down towards the falls, the tendency to movement being increased by hydrostatic pressure as the crack opened. The Professor further explains that this same capillary action offers an explanation for the escape of gas through the bottom of the headrace. He is convinced that the rupture would not have taken place if water under an increased pressure had not been allowed to enter the headrace, and is equally convinced that a rupture will not occur in future if water is from now on effectively prevented from entering certain parts of the ground.

Discussing the ground at the powerhouse, the Professor is convinced that the foundation is in no way crushed, and is still quite capable of taking its normal load.

The report then goes on to discuss remedial measures, and deals with the various suggestions that have been made. Professor Hornell is of opinion that 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 with impermeable material a portion of the headrace in which the columnar rock occurs. 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 o* the headrace and discharging into one of the tunnels which have been excavated for observation purposes between the powerhouse road and the headrace. Headrace Dining. The Professor provides that all loose pieces of concrete in the penstock intake structure should be effectively grouted, that a reinforcement of the ground itself should be arranged along the intake, and that, after this treatment, the whole of the water surface of the intake structure should be plastered with a gunite facing. He provides that in certain cases where a saving in cost may be effected by so doing, a concrete wall should be built along one side of the headrace instead of extending the ■watertight covering to the floor into positions where the headrace has very considerable width. The Professor does not consider it jiecessary to provide this concrete lining for the whole length of the headrace, but considers 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. Construction, of Works. Professor Hornell is of opinion that the works have been designed and constructed in conformity with the ordinary considerations of hydro-electric engineering. He has no apprehension as to damage likely to occur from water leaking from the lake by way of Acacia Gully. In discussing the main dam he is of opinion that the opportunity should be taken to further strengthen the country adjacent to this, 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. Justifiable Risk. Discussing the question as to whether tae crack which took place on June 7 might have been foreseen, Professor Hornell is of opinion that, although the original plans and specifications indicate that the 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 the opinion that features such as the driving and operation of tee penstock tunnels and the weight of the outdoor transformer station had very little effect in causing the rupture. Professor Hornell is of the opinion that the fact that the crack has occurred will have no effect in preventing the continued operation of the existing power station, and is of opinion that there need be no apprehension in extending the power station in -its present site to the full capacity anticipated.

He can see no reason for abandonin; the present headrace or powsr statioi if the remedial measures suggested b' him are carried out. Consolidation of Falls.

\Yith regard to the definite consoli dation of the falls, the report is not conclusive in that the Professor 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. So far as his investigations had 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 quite a considerable effect in causing the rupture at these falls, and for this reason he has suggested waterproofing this channel. In making this suggestion, he 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. Diversion Tunnel. Discussing 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 could be effected to these gates should necessity arise in after years. He considers that a considerable amount of grouting should be done in and around the diversion tunnel and immediately upstream of the main dam, and further suggests that the diversion tunnel should be plastered with a gunite surface.

With regard to estimates, he is of opinion that these could better be carried out by the engineers of the Department, though he offers to cooperate with them to see that the ideas embodied in his report are adequately covered.

PROBABLE CAUSES OF THE RUPTURE.

SATURATION OF ROCK RESULTED IN SWELLING.

(Special to the “Star.”) WELLINGTON, October 15.

In the course of his report Professor Hornell discusses, in an interesting and largely non-technical manner the probable causes of the rupture that appeared at Arapuni on June 7. The expansion of the rock causing stress and movement was caused, he says, by saturation, probably due to capillary action.

** In view of the capillary characteristics of the columnar rock/* he says, *' the rupture may be interpreted in the following manner:—- “ When the water was impounded in the headrace, a constant source for feeding into the capillary interstices of the columnar rock was created over a large area, and the level of this source was lifted thirty feet to forty feet higher than the water-level of the swamp that had previously served as a supply of. water to the rock. Hydrostatic Pressure.

“It need hardly be explained that, under the new conditions, a certain amount of hydrostatic pressure developed in well-defined joint-planes of the columnar rock, where before there had been no such pressure, and where there had previously been a tendency to contraction, due to evaporation taking place from the ridge. In this way loosening of the joints would be effected.

“ The lower part of the columnar rock swelled to an extent corresponding to the difference between complete saturation and the saturation that had prevailed previous to the headrace being filled. The upper part and the overlying strata underwent very little change. Consequently, there was created a stress between the lower and the upper strata, tending to cause a break in the country.

" Compression and bending stresses in the very flexible columnar rock behind the intake, and tensile stresses in the joint-planes in front of it. added a factor of instability to the ground In this connection it should be noted that the tensile strength of the rock across the joint-planes is probably very small “ Any forcing-in of air into the deeper strata, which may have been brought aboilt by the capillary action of water

soaking into the ground, would also contribute a stress tending to cause a break in the country. Where Trouble Started.

“ I believe that the splitting of the ground started at, or down-stream of, the sharp corner between the intake and the spillway. “ Once a crack had formed, 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 cn 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. “ 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 formed be effectively sealed and adequate measures, as proposed below, be taken to prevent the headrace-water from entering the ground. Reassuring Evidence. “ 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 the 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 material 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 uiovement 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.”