THE DAMAGED DAM.

Rangitikei Advocate and Manawatu Argus, Volume XXXVII, Issue 10550, 14 January 1913, Page 5

THE DAMAGED DAM.

THE ENGINEERS’ REPORTS ON MARTON RESERVOIR.

The following is Mr Munro Wilson’s report:—

After a close inspection of the clam. I have come to the conclusion their ir Is better for the Oouuoii to face the expenditure of a drastic change in the method of construction, and thus prevent any risk of a recurrence of .a burst. Your engineer followed a method of construction recognised in India and recognised by eminent authorities. Had he carried out some freak design of his own, ho could not he ahsolvetTfrom blame. There is evidence of negligence in the construction, for throughout the interior of the embankment, black vegetable soil seems to have been freely intermixed with the clay. The latter was supposed to have been exclusively used. Tim fracture toot place where the last piece of earth work was completed. This frequently happens. There should be continuity in the embankment, one horizontal layer after the other right across —well puddled and chopped up 1 with spades and thoroughly consolidated. The existing slopes of the banks are 1% to 1. This is the usual slope given to an embankment where not. submerged or saturated with water. It is \ generally found that less than B to 1 ! is inadequate where water lias to be j retained. The existing reinforced concrete casiugYanuct be relied“upon as being impervious. It, has settled with the natural subsidence of the bank, and fractures have occurred most noticeable at the too wall, where the greatest deflection takes place. In the reconstruction, this concrete facing, even though not watertight will form a protection to the bank, and can remain. It is usual to pitch the up stream face with large stones to prevent the erosion of bank. The existing concrete facing is a very good substitute. The permanent ,-restoration willjuvolve a trench being cut right across gully well down to a solid stratum. This will be filled up with selected clay in layers of not less than Sin. —well chopped about with spades and well watered. The present embankment to be widened on the down stream side 10 feet with a batter of say 2}5 to 1. The old slope to he stepped out to form a union with the now work, and the latter deposited in layers and well consolidated. The concrete casing can be joined across break, but in addition supported at back with concretcyroiuforced struts and - properly drained. The concrete valve control tower can he taken down and the valves worked from the down stream side. This, however is only a detail. The tower can be repaired, I believe, at a very small cost. The foundations are not good enough for a concrete dam without a very heavy expenditure. From the appearance, of the country no solid rock foundation is procurable, and in this respect .your engineer took the only reasonable course in constructing an earthen dam. I estimate the cost of reconstruction with the additional safeguard of puddled core wall and increased width and slope of embankment, at £4500. G. LAING -MEASON ’ S REPORT. As authorised by your Council I obtained the services of Mr Muuro "Wilson, O. E., of Auckland, for the purpose of conferring with me, inspecting and reporting ou|the“hrokeu dam. I met Mr Wilson twice at Martou and we spent several hours on each occasion in making a close and exhaustive investigation. We more particularly examined the break itself; also the foundation below the-toe-wall where the trouble originated; and I had several pits from 0 to 8 feet deep sunk in the dam embankment, in order that Mr Wilson might see for himself and compare the nature of the material used in Its formation with the original stuff exposed by the break. Mr Wilson will submit his report to you in the course of a few days; and I have the honour now to put in writing the opinions I have myself formed. Immediately after the break I proceeded to Martou, and I made a thorough examination. As you are aware I interviewed your Council at a special meeting ; and I reported verbally the opinion I had formed in regard to the cause of the failure. The three subsequent inspections I made have not altered those views in any way; and I think Mr Wilson is in accord with me in this respect. Unfortunately in other respects' Mr Wilson’s and my opinions do not agree. Mr Wilson belongs to the school of engineers who favour one method of construction for earthen 'dams; whilst I am a believer in a more modern practice which is adopted by Indian engineers. In the latter country, where probably more earthen dams have been constructed than in the whole of the rest of the world, the old conservative practice as advocated in England has been greatly modified and changed. The chief points of difference between the English and Indian practices are two. In England—at any rate until recent years—it was the practice to construct earthen dams with core walls of puddled clay. These core walls were built through the length of the dam at the centre, and were carried up to the top. In the United States of America the use of clay puddle is condemned, and core walls where used, are of masonry or concrete or concrete and steei, but in many cases they are not used at all. In India uponßhe other hand such core walls are not considered to be necessary; and reliance is placed exclusively upon the material of which the dam is constructed. In these cases practically the whole embankment is a core wall. In both systems of course . a trench filled with masonry, concrete or puddle is carried downward below the surface to an impervious stratum. This puddle trench, as it is called in contradistinction to a puddle wall, is not carried upward above the natural snrface of the ground. Some engineers prefer to placo it at or near the front face of the embankment, whilst others put it at the centre line of the dam. The other point of difference between the English and Indian practice is tJie degree of slope given to the face of the embankment. In England it was the custom to make flat slopes; Bto 1 on the water side and 3 to 1 on the down stream side. In India much steeper slopes have been adopted, and in the Madras Presidency especially, these were never made flatter than 1 %, to 1 on either side of the embankment. The Eng:l4oV» 1 f.hnn

means all—now consider that these two principles are essential in earthen dam "construction; but on the other hand Indian engineers say that th.o3 r are not necessary, that j their dams are sufficiently stable j and that they are far more economical. Such are briefly, without entering into technicalities, the views held by English and Indian engineers on this subject; and ' they account for the difference of opinion between Mr Wilson and myself. Before designing the Martou dam, I took into consideration all the arguments for and against advanced by each side; and after mature thought I decided upon adopting the Indian practice, and for the following reasons ; —l. The vast experience of the , engineers in India and their un- j doubted success in this branch of engineering. 2. Tlie perfect suitability of the ground, and of the available excellent material on the site for the adoption of the Indian practice. 3. Economy of construction, which amounted in this case to at least 50 per cent. I considered, and still consider, that in the interests' of your Council and the ratepayers, I was fully justified in adopting this principle. In minor particulars only did I make any variation from the Indian practice. The first was in the placing of the toewall—or as it is usually called the puddle trench —in the front instead of the centre of the dam. Tins can hardly be termed a variation, because, as I have said, this position is preferred by many engineers. The second is in the manner of facing the water slope. This, neither in India nor anywhere else, is claimed or in ended to exclude the water, bat is only used for the purpose of breaking the wave action set up by the wind. The usual custom is to cover tne water face with dry stone pitching every joint of which is free to admit water. My method of using a “blanket” of reinforced concrete is much better, because although I do not claim that it will keep out the water entirely, yet it will certainly do so to a large extent. Even if it cracks and splits, as Mr Wilson seemed to think it would, it will still be much superior to stone pitching because there will be infinitely fewer open joints. Your old dam is faced with 3 inches of plain concrete as against -1 inches of reinforced concrete on the new dam. The disaster was due without any doubt, whatever, to bad workmanship at the “closure” of the dam. I paid visits of inspection to the work at frequent intervals, and on every occasion I saw that good clay was being put in, and that it was being properly laid and consolidated. I gave particular and explicit directions that only the best of material should be put in at the close which only occupied a few clays to complete. The break reveals that this was not done, and that much black soil was allowed to be put in; and notwithstanding even this, the dam stood full for over seven months with practically no leakage. No doubt, and reasonably enough, the failure has tended to raise doubts in the minds of your Council and of the public as to the design of the clam; and this distrust will not be allayed by the adverse opinions expressed by Mr Wilson. Nevertheless, I can only say with" certainty that this doubt is groundless, and that the dam can be restored to perfect efficiency provided, of course, that the broken part is made up faithfully as it should have been in the first instance. There is not one tittle of scientific evidence to show that a core wall is necessary or even aclvis- ■ able, or that the slopes need be any flatter considering the material of which the embankment is made. The break shows that this material will stand at from % to 1, to ver tical. Nor will frictional properties of this clay he reduced even if the whole cross section of the embankment were almost brought to ! saturation point. In proof of this I again point out that no endeavour is made in the Indian practice to exclude water from the embankment except by the selection of the material of which it is constructed; and it is found that water, to any harmful extent, will not find its way into the dam if the material is of the proper character. As I before said, the water face is covered with stone pitching with open joints; and the water which finds its way through them is absorbed or flows back again into the reservior by the same orifices when the water is lowered. In my case I have a facing of reinforced concrete which at -.its worst will not admit of a tittle of the water which would pass between the joints of stone pitching; under the very worst possible conditions. I am fully aware that after failure it is most difficult to re-establish confidence. I have given my reasons fully for the statement that there are no real grounds for loss of confidence in the and efficiency of the structure, and it must rest at that. My advice to the Council is as follows:—In order to insure the command of the water level during reconstruction, and also to keep that level down for some time after ■ wards, I advocate the making of another overflow channel, the bottom of which would bo four or five feet below the bottom of the existing overflow. This, with the aid of the tunnel and scour pipe, Jwill allow the water under any flood conditions to be kept down to any desired level, say 10 feet above floor of data. When chis is done the gap could be refilled with clay, well watered and rammed, and thoroughly consolidated. From the level of the toe-wall for a height of say 10 feet a thin layer of plain concrete (similar to that on your old dam) could be placed, almost at once, on the water slope to stop the wave action; the water level being, of course, kept below this by meansabove suggested. In a few months, according to circumstances, the reinforced concrete facing could bo laid on top of the thin layer of plain concrete, and the latter could be carried up another 10 feet and subsequently this could bo covered by the reinforced facing as before. The new by-wash or overflow would bo fitted with double concrete pillars in the centre to receive timber barrage board, which would enable the water level to be raised, in the future, during dry weather, to the designed level; [and so maintain a good storage for the town at such seasons. ■ The total cost of instalment in this manner would not exceed £650 to £7OO. As an alternative I have prepared a design, which I showed to Mr Wilson, for a reinforced concrete closure -,dam, which would close up the gap, and at the same time provide a spill-way at level that the height of the water Would be controlled exactly as above. This would be a very efficient and good way to reclose the dam, but the_cQgLaiaEflßi^il&

greater—-:sS’ ; ldoo’ to' £I2OO. in con elusion I yrish to reiterate emphatic ally that ths' design of the dam is in strict agreement with the well known and highly successful practice of engineers who in thi&p&v ticular class of work hayo had-*<prt> rbably more experience . others and certainly ver'J -\mti6.V more than either Mr Wilson Rolf. In respect to examination has without any doubt,' that exception of the part which was obviously bad, of the structures and good. I therefore say.tdw Council should slot throw this sound I suggestions carried out JaVmtentative I have incncatelv will p'ice t jBH ratepayers in tlit? possession work whieh will be eminently factory i'm a decade or more; by tbaif time probably storage capacity will bo and the present work can be used a basis fox enlargement to any tent that may then he found ti-. necessary. , iuHi