Correspondence. Concrete and Masonry Dam

Proceedings.] CORRESPONDENCE OS DAM-CONSTRUCTION. 49

the dam must, however, be looked upon as a portion of a cylinder ; m. Darley. and with a cylinder of equal thickness surrounded by water it was hard to see how any shearing stress could come in, the dam being in compression allround. With regard to t,he use of plums, they saved a large quantity of concrete, and good stonehad a higher crushing resistance than concrete. Where cement was costly, as in New South Wales, and cartage into places difficult of access had to be considered, it was important to save as much cement as possible, and therefore stone had to be used very largely in the walls.

*** The Author’sreply will befound atthe end of the Correspondence.

Correspondence. RP Mr. H. BELLET,of Lyons, remarked that the formula T = T, >lr,E&&. given by the Author forcalculating the thickness of an arched dam, was the stmdard formula ; it had the advantage of being extremely simple, but, on the other hand, it could only give the magnitude of the normal compression-stresses to which a curved wall was subject, and even that onlyaway from the immediate neighbourhood of the base. First, this formula supposed thatthe deformed wall maintained a circularcurvature, or thatin the same horizontal plane the various elements of the arch underwent the same radial displacement,like a cylinder which contracteduniformly. It supposed also that the compression to which the arch was subjected was distributed uniformly. But this was not exactly true, for the ends could not slide on the abutments, and therefore after deformation the wall took up a form approaching to an ellipse. The result was a bending-moment which introducedbending andshearing stresses, varying at different points in the length and thickness of the arch, and of these the standard formula took no account, How- ever,these stresses were small, so long as T was small compared with R. Again, the formula supposed that the wall was very thin, which was only approximately correct just at the top of the dam. The deeper the sections taken,the less exactdid this formula become. The pressure was always greater on the down-stream face [THE IRST. C.E. VOL. CLXXVIII.] E Downloaded by [ UNIVERSITY OF EXETER] on [24/09/16]. Copyright © ICE Publishing, all rights reserved. 50 COKI',%SPOSDEXCE OS DAM-COSSTRUCTIOX. [Minutes Uf

Itfr. Eellet. than was indicated by the standard formula, which only gave the mean pressure. The formula for cylinders with thick m-ails was :-

in which ?U was theexternal (up-stream) radiusand rd was the internal (down-stream)radius. Thisformula, however, was still insufficient to solve the problem effectually. Indeed, thevertical pressure pl, due to the weight of the material,had the effect of producing a transverse expansionp,/E m (E being the coefficient of elasticityand m the coefficient of transversedilatation), which tended to compensate for the contraction of the arch due to water- pressure, and which modified the distribution of pressure within the mall. For the Author's type No. 2 (wall battered on the up-stream face, with a vertical down-stream face) the vertical pressurep1 was a maximum onthe down-stream face, whichhncl the effect of increasing S' by the quantity

K being the density of the masonry.' For this reason such a type of profile was not to be recommended. For type No. 3 (wall battered on the down-stream face, with vertical up-stream face) the vertical pressure p1 was a maximum on the up-stream face and a minimum, ornothing, on the down-stream face,which had the effect of decreasing S' by the quantity

This typeof profile thus possessed distinct advantages over the others, and should be used in preference. For typeNo. 1 (wall having similar profiles on both its up-stream and down-stream faces) the pressure p, \%*asnot uniformly distributed, andbecause of the verticalcomponent of the water-pressure on theup-stream face, it was greater there than on the down-stream face. Although the pressure S' was less than for type No. 2, nevertheless, it was greater than thatprofile in No. 3, which still remainedthe bestform. With Lithgow No. 2 dam at 60 feet down , equation (1) gave in comparison with the standardformula an increase of pressure amounting to 10 per cent. But equation (2) showed that reallythis increase was no more than about 5.5 per cent., if no consideration were taken of the bending stresses mentioned at the ~-

1 For the proof of this formula see H. Bellet, " Barrages en Maponnerie et Murs de Ke'servoirs," Grenoble, 190'7, p. 310.

beginning of theseremarks. In the neighbourhood of the base, Mr. Bellet. however, the standard formula failed entirely, and was nolonger applicable. Underthe effect of the longitudinal compression, thearch diminished inlength, but the supporting surfaces at theends remained fixed, withresulting displacement of various otherpoints of the wall in adown-stream direction. But, at the base itself, no part of the wallcould slip, for it washeld to the foundation by the aclherence of the cement, and by friction due tothe weight of the structure. So long, therefore, as the actual stresses did not exceed these tworeactions, it would be found that, for the security of the dam, the followingconditions must hold: namely,no displacement waspossible atthe base itself, andthe longitudinal contractionmust bezero. That was to say, at the base the wall resisted not as an arch but almost exclusively by its weight, in the same way as a straight dam. If plm was the mean vertical pressure and pamwas the mean horizontal radial pressure, and8 the mean unit contraction of the arch, then at thebase.

KH For a mall of type No. (3), p1 = -, and pain = H, H being the 2 height of the wall, which for simplicity was taken to be also that of the water. Further, if h represented such height of water, which the wall at its base could resist by arch action, there was obtained for the approximate value of h

Now, it followed from thetests carried out by theFrench Commission onReinforced Concrete, that forpressures of 0 to 13.6 kilograms per square centimetre, the coeficient l/nh was equal toabout 0.16 when the force was perpendicular tothe largest dimension of the concrete), and about 0.22 when at right angles to this clirection.1 Taking the mean value of 5 for nt and of 2.2 for K, it would be found that m I h = 0.42- H R

1 The composition of the concrete was : 800 kilograms (660 lbs.) of Portland cement, 400 litres (14 cubic feet) of sand, 800 litres of gravel; gauged with 8.2 per cent. of water (by weight) to the dry mixture. The concrete was tested at 104 months ('l Rapport de la Commission clu Ciment Arme'." Paris, 1908). E2

arr. Bellet. The remaining head of water 11' (= H - h) had to be resistecl 1>~' gravity action, as in a straigllt dam.

For Lithgow No. 2 clam 1~ = 0.10 H ; It' = 0.90 H ,, Medlow dam 1) = 0.06 H ; 1)' = 0.94 H ,, Tamworthdam h = 0.04H; h' = 0.96 H ,, Parranlattadam 11 = 0.04 H ; h' = 0.96 H In proportion as the top was approached, the deflection of the wd1 could increase, so thatthe contraction S of the arch mouldalso increase, and the wall would resist less and less by its weight and more and more as an arch. But what law this progression followed, it appeared almost impossible to say, eren approximately. All that could be stated for certain was that the dam resisted as an arch in its upper part and by its weight in the lower part. Now, it was known that the best section for a dam of the gravity type was a triangular form with the up-stream face vertical. For this reason, type (3) should be employed in preference to the others. Tllus, from theoreticalconsiderations of resistance it was foundadvisable to employ this same section, which from practicalreasons of construction hadalready been preferredby theAuthor. Again, owing tothe uncertaintyas to the value of thestress dereloped inthe dam where it resistedonly by its weight, it wouldbe well notto adopt too great a pressure in the calculations, and not to exceed the value of 10 tonsper square foot, unless the concrete was strengthened and interlaced by some conveniently-arranged metallic reinforcement. That method of construction, also, would enable the top of the wall to help in the resistance of the remainder of the work, and to exert its fullpressure, the top for practical reasons of construction being always thickerthan was theoretically needed. With reference to vertical cracks, it was quite evident that if the intense cold occurred when the reservoir was empty, curvature of the dam would notprevent them, and they were as likely to be produced as in a straight dam. These cracks took placeespeciallg- at the top, because at the base displacement was impossible, for the reasons previously given, whereas contraction WRS free to take place at the top. By reinforcing theupper part sufficientlywith ironrods it wouldbe possible nodoubt to obviatethese cracks. The Author's fears that thiswould cause horizontal cracks, however, were groundless, especially if themetal arnlouring went down suEciently low ; but there was some fear that the abutmentsurfaces of the wall against the sides of the valley would then constitute surfaces of least resistance. The cracks would be transferred to that position on account of the shrinkage, and would produce a separation

of the wall from its abutments. In order to remedythat the metallic ~r.Bellet. :lrmouringembedded inthe concrete could beattached to large cables secured to headings in the abutments, thusgiving horizontally the effect that was obtained vertically in the suspension-bridge. In this wayshrinkage could onlydiminish the ‘‘ rise ” of the arch. It was perhaps,however, as simple to arrange for joints of least resistance in the body of the dam and to cause cracks to take place at points which could be fixed beforehand, as the Author suggested.

Such a method had been employed for the ‘I screen ” wall on the dam of La Bouillouse, situated in the Pyrenees at a height of 2,000 metresabove sea-level. Thisscreen-wall was a secondwall built against the up-stream face of the dam, and provided with wells or shafts communicating with drains, so that the water-pressure did not act directly on the clam itself. The wall was straight on plan and was 14 metres (46 feet) high and 363 metres (1,190 feet) long. At every 30 metres (98 feet) were arranged vertical joints of weak concrete. A sheet of softcopper 3 millimetres (0.12 inch)thick, the edges of which were bedded inthe masonry,carried any longitudinal contraction or expansion, and covered this joint for its entireheight. But the devicehad notyet been tried sufficiently long in practice for any very great value to be attached to it. Mr. H. A. BLOMFIELDobserved that on the concrete dam built for &fr.Blomfield. theJunee water-supply, New South Wales,a great saving was effected by the use of largeplums ; the width at the base being more than 30 feet,blocks 4 tons in weight couldbe used. The grey granite available was easily quarried in a roughly rect- angularform and the plumswere set in the wetconcrete itself; this was mixed with plenty of water, care being taken tosee that the wet, fine stuff was not allowed to run down the sides of the wall and be lost. The blocks were well wriggled and carefullybedded, and as faras he was aware no cracks hadbeen found in thewall, which was straight.With regard to facing the damwith moulded concrete blocks and setting them as individual stones in mortar, probably a great difficulty would be experienced in making sure that these were well bedded, and that they did not enclose air and so lead water to penetrate behind. With thesystem adopted at Junee, afacing-block, ns it were, 18 inches thick, 30 inches high, and about 50 feet long, was made in situ, and there was no joint whatever between it and the heartingof concrete and plums, because both were mixed and put in place at the sametime, making one setting of the mass. This would have a better chance tokeep out water than any system of masonry setin mortar. Anotheradvantage was thatthe better impervious class of concrete was put on the upper side and

mr. Blomfield. the hearting material was carried to the outeredge, on the principle that if water did get into the structure it was better to let it out than to have a stronger skin of concrete on the down-stream side impounding it within thewall. Mr. Brucc. Mr. A. FAIRLIEBRUCE remarked thatthe Author appeared to havedemonstrated very successfully the advantages of segmentnl damswhen circumstances admitted of theradius beillg macle sufficiently small, and reliable abutments couldhe secured. The Author seemed to havearrived, by dint of experiment, atthe usually-accepted profile with all the batter on the down-stream face. Although the curved form of dam was the most suitable where a narrow gorge was available, there were many sites’ where it wa,s necessary toadopt a straightdam dependenton gravityfor its stability. He would be glad to know if the Author had considered the advisability of adopting any modification of the hollow dam, which had met with somesuccess in America. Therisk of the reinforcement in the slab forming the up-stream face rusting away might be obviated by connecting the cross walls with flat arches, so that all the work would be in compression. Mr. Bruce hac1 recently worked out the calculations for a dam of this type, which showed a saving of about 50 percent. as colnpared withthe quantitiesfor a solid dam of correspondingheight. Some data regarding the tests passed by the cement employed in the works described in the Paperwould seem to be desirable, as, notwithstanding the excellent quality of the sand and stoneused, and therichness of the mixtures, the results obtained in the crushing-tests on the concrete were very disappointing. The round figures given for that used in the smaller works only worked out at 780 to 1,560 lbs, per square inch, and even the strong mixture of the KO.1 concrete at theCataract dam only withstood 1,805 lbs. persquare inch. On the Bahia Blonca waterworks, recently carried out underMr. Bruce’a charge, the best materials obtainable in the district mere pit sand, containing a verylarge percentage of fine particles, and Tosca, a kind of poor limestone, of which a bed about 4 feet thick was found immediately below the surface. Theproportions were 1 : 24 : 4. One sample cut from the work crushed at 2,034 h.per square inch at between 1 and 2 months old, and another sample, cut into three 6-inch cubes withstood 1,955, 2,185 and 2,411 lbs. per square inch respectively. The second sample was cut from thetop of the division-wall in the service-reservoir, so placed that it could not be kept moist, and wasexposed to thefull heat of theArgentine summer for about 4 months. The agewhen tested was nearly 6 months. It appeared,therefore, that had the Author used good

Englishcement he might have secured a concrete at least 50 IIr. Drum?. per cent. stronger,with a proportionate economy inthe section of hisdams, at least in those of the non-gravitytype. It was generally found safer to cut out and refill doubtful joints in the rockfoundations of a dam, but t,here were cases in which such joints contained loose but clean materials, when they could safely be made goodby forcing ingrout under pressure. Mr. Bruce had alwaysfound what was usuallyknown as a “wet” mixture (though he would hardly bedisposed to describe it as “sloppy ”) much the most satisfactory in every respect ; it required less labour, W~Smore dense and consequently watertight, and was quite equal in strengthto ‘‘ dry ” concrete.Unless such an excess of water was used as to drown the cement,which of courseshould not be done, he could not see that a wet mixture was specially liable to the vertical cracks referred to by the Author ; all concrete was more or less subject to these, and he would seem to have generalized on the somewhat insufficient basis of the Mudgee dam. These cracksseemed to be quite as often due to contraction in setting as tothe expansion and contraction caused by changes of atmospheric temperature ; a considerable amount of heat was given off from the former cause, which the concrete often did not lose for a very long period. The latter muse of contractionmight be counteracted to some extent by carrying on the work on awall at two or more points alternately, if circumstances admitted of this being done, and so allowing each section to contract to some extent before the next was added. Light reinforcement WRS useful, but it should be of small section and well distributed near the face. The remaining alternative was to provide expansion-joints as proposed by the Author. Mr. Bruce had found it a good plan to zigzag them and fill them with asphalt. Rendering was rarely a successful expedient to produce a watertight face, as it generally peeled off. A much better and more satisfactory method was to form an inner skinof a rich mixture, say, in the proportions 1 : 2 : 4, which amalgamatedwith the body of the concrete ; it must be remembered,however, that the larger the proportion of cement the greater was the liability to expansion and contraction under changes of temperature. The Author appeared to have used plums of n more or less rectangular shape ; there was less danger of voids if they were of a conical form, the flat side being well bedded with the point upwards ; wet concrete would then close round them without difficulty. Referring to the Cataract dam, whichwas so important a work as to be well worthy of a distinct Paper, it would almost appear that the drains placed in the body of the work were an unnecessary

Nr. Dluce refinement, as when once the stone was built in position it was unlikely to change materhllyin condition. It would bevery interesting to know if experience hadiindicated that they servecl :I useful purpose. Drainage of the foundation was, of course, a very different matter, and should be provided in all large gravity dams. The weight stated for the concrete-l40 lbs. per cubic foot-seemed rather light ; Mr. Bruce had usually fold concrete of that class to weigh about 145 lbs., in workcarried outby him. The ontltxt arrangements struck himas being rather unusual, and it would be of interest if theAuthor would explain why the common [type of upstand tower, with sluices at different levels, was departed from. With reference to this branch of the subject, the Author might give his experience as to the durability of the draw-off pipes used in the case of the smaller dams. It would appear at first sight that there would be a great liability to failure in thehinge, owing to corrosion. Regarding the execution of the work, seeing that so much was done departmentally, would it nothave been betterto have dispensed with contractors altogether i! The principal advantage in employing a contractor consisted in the fact that much of the plant required might be used by him in a succession of works, whereas a corpora- tion or company had no further use for it after their works were done. Otherwise it seemed rather wasteful to employ a duplicate staff, one to do the work and the other-to see that it was done. Purely departmental work also allowed of greater flexibility, per- mitting of modifications beingintroduced during the construction of the works, without incurring claims for extras. The only other point to which Mr. Bruce wished to referwas the cost of the concrete ; this appeared to work out at an average of XG 10s. per cubic yard, whichseemed veryhigh. If theAuthor would give information on the points here referred to, Mr. Bruce ventured to think that it would enhance the value of the Paper. Mr. Cardca. Mr. J. HAYDONCARDEW remarked that the subject of the Paper was such a large one, that the Author had hardly been able to c10 justice to the description of the Cataract dam, the first of a number of large dams which must eventually be built in New South Wales for the conservation of water, and the forerunnerof the greater dnnl now beingbuilt at Ba.rren Jack on the River Murrumbidgee. As the pioneer characterof the Cataractdam wa.s of considerable interest to engineers engaged on wat'er-conservation in this State, some information as to the design of the structure would have been very acceptable, and it was to be regretted that the Authorhad omitted 311 reference to such an importantmatter. Referring first to curvecl dams for country towns' water-supply, it must be admitted th;tt tllc

ecomonial limit for such dams was generally speaking a restricted Mr. Cardew. one and that theywere most suited for narrow valleys ; but as there were many such valleys in this State suitable for the storageof water, the use of curved dams was likely to be extensive in future; therefore the Paper was of considerable practical and educational value. His opinion differed from the Author’s as to the adaptabilityof a curved m:tll to meet contraction, as he thought it presented decided advantages: the moment of inertia for the cross section of a curved wall would be greater than thatof a straight wall, and therefore the stress mouldbe less;and further, the liability to tension at the inner toe would be diminished. The extraexpense entailed by a curved wall mould be justified if the effect was to prevent the cracks referred to, which in some of the dams mentionedwere of considerable magnitude. The Author’s statement that the dangers arising from these cracks were more apparent than real, in dam walls which relied upon their curvature for stability, appeared tobe a contradiction of the statement made previously that a curved dam presented no adaptabilityto meet contraction. Mr. U. W. Darley, M. Inst. C.E., lateEngineer-in- Chief for Public Works, had stated ina Paper on “ Curved Concrete Walls for Storage Reservoirs,” read before the Engineering Section of theRoyal Society of New South Wales,’ that“curved dams have a decided advantage over straight gravity-dams, for when thelatter form of wallcracks, which all long walls of concrete areliable to do when subjected to a change of temperatureand stand dry for any length of time, it does not so readily close up again.” He agreed withthe Author, that the wallwas far more liable to crack if constructed of sloppy concrete, than if constructed of dry, especially if the thinner pwt of the wall were built during hotweather, when the evaporative agencies were most active, or if a dry period followed the completion of the dam andthe reservoir was not filled for some time ; and his practice had always been to put the concrete in as dry as possible. But in spite of all precautions he thought it inevitable that cracks would occur in any concrete dam unless some metal reinforcement was used, especially in theupper and thinner pzrt of the wall. In the design and construction of the Burraga dam inNew South Wales, described by him in a Paper,2 a reinforcement of three 70-lb. rails was provided in the crest of the dam, where the cracks usuallyoriginated, and the results had been very satisfactory. The dam referred to was curved

. -

1 Journal and Proceedings of the Royal Society of N. S.W., vol. xvxiv (1900), p. dig. 2 Minutes of Proceedings Inst. C.E., vol. clii, p. 237.

Mr. CardcF. to a radius of 539 feet, and although subject to great fluctuation of the mater-level in the reservoir, no cracks, longitudinal or otherwise, hnd occnrred ; and considering the similarity of the coefficients of expansion of the two materials, namely, 0.0000076 for concrete per degree Fahrenheit, and 0.0000067 for wrought iron, it was dificult to seehow anylongitudinal cracks couldoccur asfeared by the Author. It was veryundesirable to havecracks in a dam, as they would admitwater into the structure, thus endangering itsshbility ; he would not provide partingjoints, therefore, to allom of the wall opening radially instead of cracking, as advocattd in the Paper, but he would strengthen the skin by the insertion of expandedmetal just under the surface, to enable it to resist the tensional stresses due to temperature-changes. With regard tothe Cataractdam, having been appointed, by the Royal Commission of Inquiryinto its construction, to make certain investiga.tions as tothe mode of constructionand other matters affecting thestability of the dam, andto advise and report thereon, he had had the opportunity of making very close observations of the work during the course of erection. The body of the dam was described by the Author as being of ‘‘ cyclopean ” rubble masonry, a term not to be found in an engineer’s vocabulary, and the dictionary meaning of which was in one sense ‘‘ gigantic,” and in another, “ pertaining to a prehistoric style of masonry with immense stones of irregular form ” ; in familiar terms it was random or un- coursedrubble with mortar beds and concretejoints. In this class of work great difficulty was experienced, when bedding large stones on green mortar, in excluding the air from under the seat of the stone being set, and no precautions could altogether prevent the formation of air-cavities ; the large vertical joints of concrete were also open to objection on account of the difticulty of ramming the concrete solid, especially if the sides of the stones were at all uneven. The work, however, had been done under the closest and strictest supervision, and was undoubtedly of superior quality. The Author drew attentionto the liability of the Hawkesburysand- stone to expand and contract with the absorption or loss of water ; fhe former action might cause internal stresses of unknown magnitude, and the latter might cause the large stones to draw away from the rigid concrete joints, thus making cleavage-planes for water to pass through. Specialprecautions, however, hadbeen taken to protect the heart of the dam from intrusions of water, by providing an impervious skin on the up-stream face, of moulded basalt concrete blocks set in cement mortar of special quality.The im- permeability of thestructure dependedupon the facing, in view

of theporosity of the sandstone blocks ; and inspection of the M’.Cardew. manufacture of the moulded blocks showed that the mode adopted bythe Author of rammingand working concrete into moulds secured the best results in its consolidation and in the reduction of voids. The weakness in block-facingwas found inthe joints,as the mechanical operation of forming a mortar joint so as to completely fill the space between the blocks with mortar was a very difficultone. As faras the bedswere concerned the difficulty was not serious, but even there, voids were apt to occur bythe block imprisoning air when descending on the mortar. It might be concluded, however, that the efficiency of this class of work was higher thanthat of mass concrete, which, as a whole,was not so compact and dense as the mouldedblocks, and which wasalso liable to skin cracks. The method of construction of mass-concrete dams could be greatly improved upon, and it was a question worthy of consideration whether the concrete could not be moulded in con- fined sections of limited size, each section breaking joint, vertically and horizontally, and having numerous offsets dovetailing the blocks together, every precaution being takenin the joiningof the successive portions together in order to secure an absolute bond and complete monolithic construction throughout,in thesame way as theSan Mateo dam in California was built.’ This system of moulding in sections mould secure high density, and if the exposed faces of the concrete were reinforced with expanded metal and the surfaceof the up-stream face were made impervious by brushing neat cement into thepores of the skin, asdescribed by the Author and asadopted at Burraga, then n dam of economical construction, immune from cracking and impervious to water, would be obtained. From the Author’s figures it appeared that thework cost 21 9s. 3d. per cubic yard, butby the use of an inferior concrete-though suficiently good if built in the manner indicated-a considerable saving might be effected in future works of a similar character. During the deep excavation for foundations across the bed of the gorge, an inferior bed of water-bearing sandstone was struck in the trial shafts, andsome controversy arose as to the necessity of excavating it, or of putting in a gullet under the base of the dam and filling it with good concrete, to prevent percolation from the reservoir and so removing all danger of uplift. As, however, the foundations were sunk about 35 feet in the trench over half the bottom width,the dam was held firmly in ZL jaw of very strong rock, so that no fears need be entertained AS to the stability of the

1 Vnited States Geological Surrey, Kightecnth AIII~U~~Report, 1896-97. Pnrt iv, Hydrography, p. 688.

Mr. Cardew. structure. Taken altogether, the dam might be considered a credit to theengineer who designed it and to theengineer who carried it out. Mr. De Burgh. Mr. E. M. DE BURGH(until recentlyPrincipal Assistant Engineer, nowChief Engineer,for Harbours and Water-Supply, Department of PublicWorks, New SouthWales) had been associated with the Author in the construction of the more recent of the curved dams referred to, namely, Katoomba, Lithgow No. 2, and Medlom. The use of these curved damshad rendered it possible to effect a great saving in the cost of a class of work which was of primaryimportance in such a country as Australia.The success of the thirteen curved dams dealt with in the Paper might be accepted, he thought, as justifying the use of the simple formula used in calculating the section, but as stated by the Author, the complex nature of the stress which might occur in such structures had to be neglected. It was evident that complex stresses must occur in those portions of the dams where a junction wasformed with the solid rock at the base, especially near the centre; while on the other hand, those due to expansionand contraction in the upper portionswere probably well dealtwith by the smallradius of curvature admitting an alteration in the versed sine without undue stress in the material. Atthe same time(and evenkeeping in mind the fact that the Bear Valley dam in America stood) he did not think it was necessary to use such stresses as the 25 tons per square foot allowed in the Cootamundra dam. The actual quantity of material used was not a direct measure of the total cost when the cost of opening up the work, access, framing, plant, etc., was considered : and the saving in quantities due to the adoption of the curved plan and the formula mentioned, appeared to him sufficient to satisfy the engineer without forcing the ultimate stress so high. Judging by practice, 15 tons per square foot was a high stress in a gravity dam, and there appeared to be no good reason for exceeding it in these curved structures. In the case of those in which he had noted on the Author’sbehalf, he had kept the stressdown to 15 tons, or taking the material intoconsideration, less, as at Lithgow No. 2. The precaution of making vertical radial joints in the walls so as to allow of cracks developing radially was, in his opinion,very necessary; it appeared as if the great danger in these thin curved clams was the occurrence of cracks in vertical planes the “ reverse ” of radial, if he might use the expression, so that the pressure would tendto force out a piece of the wall (Figs. 18). And while the introduction of verticalradial joints was a reasonable precaution against such a fa.ilnre, there did not to his mind a,ppexr to be much objection to the introduction of some horizontal iron or steel rails

between such vertical joints, or if the dam were short, the introdnc-Mr. De Burgh. tion of the bondingiron andthe omission of the joints. In connection with the supply of water for domestic purposes to the workmen employed on the construction of the great dam at Barren Jack, he had erected a curved concrete dam on Barren Jack Creek 38 feetin height, 80 feet in radius, 2 feetthick atthe top, 5feet atthe bottom ; the compression stressas calculated by the formulagiven by theAuthor was 17 tonsper square foot, slightlymore thanthe 15 tons Mr. DeBurgh usually adopted. While there was no necessity to introduce reinforcement, he, having a quantity of 20-lb. rails on hand,had placed them in the dam with a view to observe the effect. The concrete used in the work was mixed inthe proportions of 375 lbs. of cement, 12 cubic feet of sand and 20 cubicfeet of stone broken to 23-inch gauge. Thesteel rails were placed vertically, 10 feet apart,within the Figs. IS.

0' a (T dam, and 1 footfrom the up-stream face, their lowerends being let into the solid rock. Railswere alsoplaced 10 feetapart, but alternatingwith these 1 foot fromthe down-stream face. These latter, however, were only carried up until they intersected the line of the up-stream rail. Such rails as were not long enoughto reach to the top of the dam were lengthened by having othersfished to them. Other similar rails curved to the radiusof the dam were then placed horizontally and secured on the up-stream side of the vertical rails; a line of these horizontal rails was put in atevery 2 feet 6 inches in the height of the dam, the up-stream and down-stream rows alter- nating-(Fig. 19). The dam n-as completed in December, 1908, when it was given one coat of cement wash, no rendering being used. At that time the Creek had stopped running and there was no water in

Mr. De Burgh.

Scale. I Inch = 8 Feet FEET 5 0 on I,,,,, I SECTIONOF DANFOR DOMESTIC-SUPPLYRESERVOIR AT BARRENJACK, SAOWINQ JIETHOD ADOPTED FOR >IEASURINQ ~IOVEJIENTS.

the reservoir. In order to ascertain what movements took place in Mr. De Burgh. the dam under variations of pressure and temperature, metal plugs were fixed centraIIy in the concrete of the down-stream face at each 5-feet level, measured from the top. A steel wire, No. 16 S.W.G., was securely fixed on the left bank and passed over pulleys grooved to receive it on the right bank, where it was strained by a weight of 2 cwt. In plan this wire was 20 feet from the down-stream face of the dam, and to it was fixed (by means of a suitable clamp) a No. 31 steel wire, strained and securely fixed to the rock vertically under thetop connection.The horizontal distances between the plugs in the dam and the vertical wire were then measured, and any variation in thesemeasurements would indicate the movement of the wall. The method of measuring was the following :-A staging (Fig. 19) was erected between the dam and the vertical wire, with n platform on a level with each metal plug. The measurements were taken with cedar measuring-rods slightly shorter than the distance between the metal plugs and the vertical wire ; these rods were shod with knife-edges of metal. One knife-edge was placed against the metal plug in theconcrete and therod was supported horizontally on the platform with the other end in line with the verticalwire. The verticalwire was connectedwith one pole of a batterythrough an electric bell, theother pole beingconnected with themetal shoe atthe end of the measuring-rod, andthe distance was then measuredwith a micrometer, placed againstthe end of themetal shoe. Whenthe micrometertouched the vertical wire the circuit was completed andthe bell rang. By this means the movements of the dam couldbe measuredwith a fair degree of accuracy. Measurements were taken first withthe dam empty onthe 24th December, 1908, temperature 87" F.; 26th December, 1908, empty, temperature 57" F. ; 31st December, 1908, empty, temperature 100" F. ; 2nd March, 1909, water 15 feet from top, temperature 80" F. ; 14th April, 1909, water 12 feet from top, temperature 50" F. Themovements thus obtainedwere shown to a distorted scale in Fig. 20. It wouldbe seen that the maximummovement up-stream with change of temperature was 0.115 inch, and down- streamwith decreased temperature 0.076 inch, whilewith a decrease of 7" and water up to 15 feet from the top of the dam the maximummovement was 0.152 inch. This was increased to 0,272 inch, withthe temperature at 50" andwith water up to 12 feet from the top. Since then the dam had been filled, and the water had been running over the by-wash to a depth of 3 to 15 inches. The deflections readwith the damfull were also plotted in the Figure.The fact that the leftabutment of this dam ran

Mr. De Burgh into the solid rock up to the level of thecrest, while thestress

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9 b S 0 5 of the right abutmentfor the upper 10 feet was distributed into the rock by means of a concrete abutment and a wing-wall, might have a

certain effect upon the deflections in the upper 10 feet. There was, Mr. De Burgh. however, nothing to show that any yield had taken place, and until the dam was again empty it could not bestated how much, if any, of the deflection shown was permanent. It might be mentioned, however, that up to August, 1909, there was noleakage through the dam,and only a slight uniform appearance of moistureon the face of the dam upto water-level. The dam was behaving satisfactorily in every way. The method of measuring the deflection was admittedly a primitiveone, but it was,not easy to devise a better means by which any veryexact measurement might be taken without incurring heavyexpense ; and the figures here recordedmight be of interestto those who had occasion to designcurved dams. The Author referred to the fact that the Picton dam was designed with a view to future extension. Mr. De Burgh wasnow carrying out that work and adding 8 feet to the height of the dam.The original structurehaving beendesigned with aview of raising it, and being accordinglybuilt to the full thickness required at thebase, there appeared to be no objection to doing this, though, of course, much care was necessary in order to bond the old and the new structure together. A proposal had been brought before him recently to increase the height of the Wollongongdam, a section of whichwas given in Fig. 9, Plate 1. In this case no provision had originally been made for raising the structure, and in order todo so it would be necessary to increase the thickness of the dam as well as the height, unless it were contemplated to carry up the top of the wall with its thickness of 3 feet 6 inches, and toincrease the pressure onthe material, already assumed at 20 tons per square foot. It was suggested that the wall should be thickened by building on to the down-stream side, a space being left between the new and the old work as was being done at Assuan, such space to be grouted in when the new work had set : the building of the increased height to be then proceeded with on the top of the combined structure. He had declined to approve of this proposal. If it were possible to empty the reservoir and keep it empty until the work had been completed and the new concrete had set for a considerable length of time, the new and the old work might perhaps be consideredas acting together and the stress as being uniformly distributed, but to carry out the work, as would be necessary, with the reservoir full appearedto him very objectionable. Theaddition of concrete would notin his opinion relieve the existing stress, so that when the height of the wall was increased the stresses in the old work would be increased above the 20 tons, while the stress could only be communicated tothe newwork [THE INST. C.E. VOL. CLXXVIII.~ F

Mr. DCBurgh. against it to an extent proportionate to the distortion produced inthe old workby theadditional pressure.As stated by the Author, Mr. DeBurgh had acted as supervising engineer on the construction of theCataract dam, andthere were severalpoints of interest with regard to that structure to which he might 81-w special attention. It would be noticed that the full section of the dam was carried to a depth of 35 feet below the bed of the river, and though the removal of this large quantity of rock was due to the previousexistence of awater-worn hole inthe strata, partly unsound, the sides of the excavation, when completed, presented a sheer wall of solid rock with only fine bedding joints. Thedam, where its height was greatest, was therefore encased to a depth of 35 feet in solid rock, which he thought would have a considerable effecton thedistribution of the stresses inthe lower parts.TO suppose that such a structure could slide was, of course, absurd ; sliding could only take place by a shearing of the material in the dam itself at the surface level, andtension in the up-streamtoe if such, in view of recentinvestigations, occurred, must be dis- tributedin the formation, The Author did not mention thefact thatduring the progress of the worka Royal Commissionwas appointed to inquire into the question of its cost, and during that inquiry attacks hadbeen made on the Author’s conductof the work, its design,execution, and indeed everything connected therewith. The inquiry ended in the complete justification of the Author, and was only of interest on account of some of the points raised. The Paper referred to the occurrence of springs in the sandstone underlying the foundation and their treatment: it was urged at the inquiry that theexistence of these springs constituted amenace to the safety of the dam. Mr. DeBurgh had dealt with the remarkablysmall quantity of water met with in the foundations in the usual way, and he had not the slightest doubt that if other shafts were sunk throughout the horizontal layers of sandstone underlying the dam, water would befound whichwould rise in suchshafts, and this water might have for its source rainfall on the country surrounding the dam, at a higher level than the top of the structure. It was inevitable that in these great horizontal beds of sandstone in which some layers exceeded others in porosity, water should be met ; but to assume that such water controlled by the enormous friction as it percolated throughthe rock coulddevelop adangerous uplifting pressure over any large area of the base of the dam was absurd. The Author pointed out that the springs dealt with in the foundation of the damhad not responded tothe level of thewater impounded in the reservoir, and Mr. DeBurgh agreed with him

thatthey had their source inthe surrounding hills. If these Mr. De Burgh. springs werecapable of exertinga great upwardpressure over a large area they should have burst up the huge layer of sandstone, 15 feetin thickness,on which the dam was founded afterthe excavations were complete, and before the weight of the dam was placed onthem. The pressure which thiswater might exert was neutralizedby friction, and in any case the layers of sandstone extending under the dam could only be forced upwards by the up- heaval of the adjoining hills, under whichthey extended continuously. It was certain that the strongest springencountered could onlyexert a pressure equal to 4 feet of water, which gradually was reduced until the spring ceased to flow. It had been urged that instead of constructing the dam of sandstone blocks ranging from 2 to 4 tons and over, packed with concrete of harder stone, the sandstone should havebeen broken upand the damconstructed of concretewith sandstoneplums, such as couldbe placed byhand. The stone, however, lent itself admirablyto cheap quarrying in rectangular blocks, which being of all sizes, admitted of assemblage into a wall free from the great defect of extensive horizontal beds, a defect so difficult to avoid in concrete construction when only small displacing stones were used. The Author gave the crushing-resistance of the sandstone at 276 tons per square foot, and to break this up with a view to cementing it together again as concrete would have been to substitute poor concrete for magnificent stone, and fortunately for the work t,he engineer in chargedid not do so. Theadhesion between these sandstone blocks and the mortar of the concrete and bedding was so good that whenever he had occasion to liftor remove a block which had set in place, a layer of stone was left on the mortar. In conclusion, he thought that it would be found impossible to construct a wall of cyclopean concrete in which the proportion of large stone (65 per cent.) reached in this case was exceeded. It was impossible to place the stones closer together and still use concrete betweenthem. Workwith a greater percentage of large stone would be masonry, and the stonewould have to be dressed ; it would cost more but would make no better work than that atCataract. Mr. GEO.L. DILLMANconsidered that the Paper was a valuable Mr. Dillmm. addition to the literature of dams, dealing, as it did, with actual structures.The formula givenfor curved dams was undoubtedly safe. As the radius of curvature increased, the thickness increased, until at some point a section equal to the usual gravity type was reached. After this was reached, anyadditional thickness (apparently demandedby the cylinderformula) wouldbe absurd.The logical inference was, therefore, that gravity had some effect before F2 Downloaded by [ UNIVERSITY OF EXETER] on [24/09/16]. Copyright © ICE Publishing, all rights reserved. 68 CORRESPONDENCE OX DAM-CONSTRUCTION. [JIinutes of RP 31r. Dillmsn. such a point was reached, and a dam of thickness based on T = - S had an increasing factorof safety as the radiusincreased. The economy dueto curvature decreased asthe radius of curvature increased, until, at some point such as that mentioned above, it ceased. Thus, for dams demanding a radius of curvature of, say, 500 feet or over (varying with requirements and hypotheses), no benefit was derived by adopting the curved plan, unless it were an increase in safety of unknownamount. The Author had hit the keynote of the situa- tion in his conclusion that, where it waseconomical to do so, it was admissible to use tangential lengths of gravity wall in order toobtain a curve of sharpradius. The “tangential lengths of gravity wsll” were simply buttresses for the arches between them. Properly designed buttresses connected by short arches were always economical in dam-construction. They were more : they were safer, eliminating the unknown upward pressure of water and the effect of unknown but probable percolation. The usual uniforn-sectioned gravitytype was to-day obsolescent. Improved cement, together with reinforced-concrete construction, rendered possible the greater economies and increased safety of the buttress and short-radius-arch type. A buttressed-walltype of damcomputed with 7 2 tons per square foot asa maximum stress (100 lbs. per square inch) compared with the Cataract dam described at the end of the Paper, showed the following economies :-At a height of 30 feet, 31 per cent. ; at a height of 50 feet, 28 per cent. ; at a height of 100 feet, 23 per cent. ; at a height of 150 feet, 12 per cent, ; at a height of 190 feet, 9 percent. These economies were entirelyunimportant if safety was to be decreased, but safety was actually increased in the buttressed-wall type. He submittedthat a uniform-sectioned dam of the usual gravity type bore the same relation to a dam built with buttresses and arches that a plate girder did to a truss in bridge- construction. There was in eachcase a great deal of material not working. Economy of material,and cert.ainly of transmission of strain, lay with the truss in the one case and with the buttressed type in the other. afr. Max am Mr. MAXAM ENDE observed that the Author stated on p. 2 that Ende. he had calculated the walls described in the Paper as sections of cylinders subject to exterior water-pressure and had disregarded the assistance afforded by their weight. He would suggest thatthe case was the following: If the wall acted according to the theory as stated, its stability would be very doubtful and reliance must be had on conditions which had been regarded as too complicated toenter into the calculation, namely, thatthe wall as a whole

was cantilevera of circular section, in additiontowhat it was am according to the Author’s theory. According to this theory every. horizontallayer was anarch in plan. In the upperlayers the middle third or core of the arch was only 12 inches thick. A deviation in the form and position of the pressure-line of the load of only 6 inches from the centre of the core would be sufficient to produce tension probably resulting in cracks and collapse. Whoever had calculated a masonry arch must know that much greater devia- tions than this were possible in a length of some hundreds of feet. The fact that the arches still stood he therefore regarded as a proof of the importance of the disregarded elements of stability, and the construction of these dams was as an experiment on a large scale based upon an unsatisfactory theory. Meanwhile the differences of opinion as to the stresses damsin of the ordinary simple construction were still unsettled. The Cataract dam, Fig. 15, Plate 1, was one of these, and it appeared that the middle third or core was measured on the horizontal sections as was often done. In the discussion on a recent Paper read before The Institution,’ he had pointed outthat it was not right to apply the bending theory of beams or cantilevers to horizontal sections unlessthey were normal sections. Taking, first, a prfsmatic cantilever and through it two normal sections near together, from observation of the bending process it was found that eachsection turnedround an axis, thatthe deformation of the material between the two planes took place mainly in a longitudinal direction, and that it increased in proportion to the distance from the fulcrum. It was therefore possible to speak of fibres and of a neutral fibre of the material. The fibres were originally of equal length, and according to the law of elasticity, “ut tensio sic vis,’’ it was to be concluded that the forces also increased with the distance. Conversely, assuming suchforces, it was found by the same law that the planes remained planes while they turned, and the observation confirmed the theory. The conditions here described, especially the moments of the forces, couldbe expressed in simplealgebraical form, and the summation of the expressions resulted in the usual bendingtheory. Theoretical investigation of the effect of the shearing forces suggested that owing tothem the normal planes did not remain perfect planes, but the imperfection was generally tooslight to be noticeable. In wedge-shapeda cantilever, however, such as was shown in Fig. 15, the conditions were rather different.The fibres betweentwo normal planes were not of equallength, the forces were not parallel, but converged to one

1 Minutes of Proceedings Inst. C.E., vol. clxxii, p. 164.

:;.pam point, and they must be assumed different from the &st case SO as to agree withthe probabledeformation of the fibres. The symmetrical a,rrangementof the latter, however, rendered it possible to set up algebraic expressions which could be integrated. (A slight inaccuracy muststill be allowed to pass, but itwas unimportant if the angle of the wedge was not great; to do away with it, cylindrical sections might be suggested in place of planes, butthen other difficulties arose which had not yet been overcome.) Taking now a wedge-shaped cantilever and two parallel sections which were not normal, it was observed that the fibres were all of different length ; their arrangement was not symmetrical and, moreover, the planes became curved as all except normal planes did, even in prismatic cantilevers. A theory in these circumstances was impracticable, and thereforeonly normal planes were open to direct investigation. Stresses in horizontal or vertical planes could, however, be derived from those in normal planes. Using these in Fig. 15, it would be found that the core of the cantilever was not so thick as shown, and that the supposed margin of safety, namely the margin outside the pressure-line, had almost disappeared. Mr. Qaudard. Mr. J. GAUDARD,of Lausanne, observed that, a long homogeneous wall being subject to oracks under the effects of large temperature- variations, the Author was right in advising the adoption of parting joints placed systematically at intervals, thus affording some transverse filtration without danger to the stability. It would be possible forthis danger to arise in astraight dam, if the crackingwere allowed to act asit pleased, following oblique or zigzag lines, such as C D (Figs. 21); for then the down-stream part of the section A A would receivefrom the fissure an interior hydraulic pressure,re- placing that of the reservoir, and this piece of reduced thickness would probably be incapable of resisting such pressure. Or rather, from the plan it would be seen that the part, E D F could not give way untilthe masonryhad sheared along EF. Thus thepart E D F constituted a cantilever loaded by the thrust of the water actingon E D. Should it give way the up-streamportion would finditself suddenlyunsupported, and would fail in its turn; at least, it could onlyresist as a cantilever. Dam-wallsconstructed on plan so as to resist as arches under the thrust of the water were of advantage, in that the pressuretended toprevent or to close cracks caused bycontraction under cold. However, to consider exactly what happened, the formula for,T, with which the Author was satisfied, was only an approximation, andstrictly speaking appliedonly to a very thin arch. In reality, the arch followed more complicated laws, as in a bridge ; that was to say, the curveB

of pressurewere not concentricwith eitherthe intrados or Mr. Gaudard. the extrados of the arch, the planes of failurebeing constrained to fixed positions, but varyingwith the conditions. Under contraction, whether due to compression of the curve or to drying of the mortar, or againto lowering of thetemperature, the arch tended to flatten, and the curve of pressure to rise, as indicated in Fig. 22 (a). If, on the contrary,the loadwas diminished, or an expansion from the absorption of water or from heat occurred, the arch would rise andthe curve of pressure would flattenas in Fig. 22 (6). But it should be noted that, with an empty reservoir, I=AFigs. 21.

SECTION A A.

PLAN

the arch did not undergo these movements if its axis was vertical. Thus, in this state it might happen that, under intense cold, the wch-stones would part completely, as shown in Fig. 22 (c), and thatthis crack would only closeslowly duringthe subsequent filling of the reservoir. In order to prevent loss of water, and thus the inconvenient and sudden closing of the voussoirs, Mr. Gaudard thought that it would be convenient to construct the arch with such inclination that, even with an empty reservoir, it would be under the partial action of gravity (Fig. 23). Thus the voussoirs would remain always in contact, and the danger of filtration and of sudden closingwould be diminished.Arched dams, then, shouldhave a pronounced curvature, since according to the formula their thickness increased with the increase of radius, In order that the system might be applied to long lengths of dam, it became necessary to use a series of arches supported by gravity buttresses ; but, as indicated in Fig. 22 (a),shallow cracks still tended to half-open on the water

Mr. Qaudard. side at the joints of the springers, or in case of expansion by heat at the joints of the keystone (Fig.22 (b)). To prevent the water from entering and weakening the masonry, it became necessary to apply, especially in the neighbourhood of these parts, a coating which was not only impermeable, but was also elastic enough to expand slightly without cracking. Fortunately the Authoronly observed very slight leakages from the cracks that were produced in some of his dams, and Mr. Gaudard supposed that there was no question of the cracks extendingthe entire thickness of the mall, fromone side to the other.Again, it might .be pro- Fig. 23. posed to constrnct arches divided bythree joints, interposing metallic “ hinges ” to concentrate the pressuretowards the middle of thekey-joints and springer$ ; but that would be a rather deli- cate procedure, owing tothe thickness of the arch increasing with the depth, and because the great pressure exercised by the blocks of masonryon the base wouldoppose __ any such moTTe- ment. Besides, temperature-variations would not be felt equally onthe external and the internal beds of masonry. Mr. Gaudard thoughtthen that it was necessary to fall back onthe Author’s conclusions concerning the unimportance of the leakagewith the mortars chosen, and to remember also that in a dam the loading came on slowly and gradually and not by sudden andviolent shocks, as in a bridge. &.Hanna. Mr. W. J. HANNA,Under Secretary for Public Works, Sydney, remarked that the formula upon which these curved dams had been designed, though corresponding with that given in most text-books, could be taken as only a crude approximation at thebest, and it TWS gratifying to note that nofailures had occurred, nor had any serious leakage resulted, in spite of the thin walls adopted. No figures were given as to thecost, but it might be taken tha,t the cost per million gallons of water stored had been much less than would hare been possible with dams of gravity section, and this economy in cost was a strong reason for the use of the curved type in situations such as those referred to in the Paper, where in most instances the valleys to be dammed were narrowand with good rockfoundations and abutments. With regard to the proposed use of parting joints, he had some doubt as to whether these would prove entirely effective

in preventing cracking, unless spaced at short distances apart ; in IIr. Hanna. which case there might be some leakage, even if the joints were re- bated or otherwise constructed to minimize the escape of water. At the same time, it would be of interest to ascertain theeffect of these joints, and attention would probably be given to the matter in one of the curved structures to be erected in the future. With regard to the use of sloppy concrete where plums were included, this was, in hisopinion, a wise precaution, asthe exact bedding of these stones would otherwise be a difficult matter. This had been found to be the case in connectionwith thelarge stones used inthe caisson of PyrmontBridge, constructed someyears ago. Similar concrete should also beused where steel was embedded, especially if the members of the reinforcementwere spacGd close together. With regard to the Cataract dam, he had had many opportunities of viewing the work during construction, and had noticed the great care exercised in choosing and dealing with the materials used, and in arranging the hearting blocks so as to avoid through joints in anydirection. The dam had, indeed, proved a verysatisfactory structure, and, though never yet filled to the top water-level, had been of great service in supplementing the water-supply of the city during several dry periods that had occurred since water was first stored in August, 1906. Mr. ALLEN HAZEN,of New York, considered that the Author Mr. Hazen. was to be heartily congratulated upon having taken advantage of the favourable natural conditions tothe fullest extent, and on having utilized them in a manner to effect a great increase in the water-supply of Sydney at a reasonable cost. He believed that thedrains built into the masonry of the dam toprevent the possibility of internal pressures added greatly to its strength and safety. TheAuthor’s statements in regard tothe archeddams whichhe had designed andbuilt were the clearest and most definite that Mr. Hazen had seen ; and it was gratifying to find the basis of the designs stated with such frankness and in so concise and simple a manner, It might be that thesmaller range of temperature in New South Wales, as compared with American conditions, had made the entire neglect of temperature-strains a saferprocedure than it wouldbe in America. If thestability and safety of archeddams within reasonable limits couldbe made clear to all who made and controlled the design of such dams, it was certain that a great saving might be effected by their use in many narrow rocky valleys, TheBear Valley dam in California was one of the * earliest and best-known examplesof a dam of this type; and adam at Ithaca, New York, designed and built by Mr. Gardner S. Williams,

MI. HaZen. M. Am. Soc. C.E., was one of the best-known of recent American examples. Mr. Hazen had recently designed a curved overflow dam extending 50 feet above river-level, now being built, in which the masonry sect,ionwas sufficient toresist by its weightalone the pressure of water up to the level of the crest, and in which the arch action (180 feet radius) was depended upon to carry the additional pressure resulting fromsurcharge during floods. It would also carry any additional thrust from ice upon the dam, which might be very severe at times, in a cold northern climate and in a narrow rocky valley. He believed that underthese conditions, an arched dam of moderate radius and comparatively light sectionwas stronger andsafer than amuch heavier straight dam. Thefoundation of this dam was mica schist, which afforded excellent support for the masonry, but was not considered hard enough to be safe under the effect of erosion from the 50-foot fall of water over the top. For this reason the lower toe of the dam was curvedoutwards and extended to break the force of the falling water, and to keepit from eroding the rock immediately down-stream from the dam. Some of the dams described by the Author restedupon granite or other rock, so hard that there seemed to be no danger of erosion from falling water. He would like to ask what had been the actual effect of the overflow in producing erosion in the various cases, and especially in those cases where the rock was not particularly hard ; and he would beinterestedto know what consideration had beengiven to this aspect of the matter in the Author'sdesigns. afr. Hill. Mr. JOHNW. HILL,of Cincinnati, found several points of special interest in the Paper. The first and most important from a politico- economic point of view, was the question of raising money for municipal works, by issuing Government bondsor by the Government guaranteeing the fixed charges on bonds issued by the municipality. Applying this policy to works in the United States, it wouldbe found that thecost could be reduced by several percent., which would be of great benefit to many small corporations. While the general Government could borrow money at a rate of interest ranging from 2 to 23 per cent., he could recall no city, not even New York, that had ever sold its bonds at a. lower rate than 39 per cent., and this at a time when the Government was selling its bonds at 2 per cent. interest. The difference of 14 per cent. would be so much saved to municipal corporations like Boston, Kew York, or Philadelphia, if it were legally possible to conduct the finances of public works in theUnited States in the manner described inthe Paper. To small municipalities the saving effected in fixed charges wouldbe greater, few of these having sold bonds at any time bearing a lower

rate of interest than 4,$per cent. At the present time the smaller nr. Hill. cities and towns in the eastern part of the United States had to pay as much as 5 per cent., while the general Government could readily float a public loan at 24 per cent. Loans for small waterworks and other “public utilities” in the United Stateswere usually issuedto run for a term of 20 years, and the difference in interest-charges on each $1,000 of capital for this timewould amount to $500, or one half the principal ; while the saving in interest-charges to the small municipality if it were possible to adopt theplan of financing the construction of public works in vogue in New South Wales, would enable a community to increase its water or other “utility ” by 50 per cent. during a term of 20 years. They could thus keep pace with the improvements in pumping-machinery, filtration-works, etc., with no greater cost to the tax-payers or rate-payers than now occurred for interest-charges alone, no part of which was returned to the people. Further, while the smaller cities and towns east of the Mississippi River could place a loan now at 5 per cent. interest, those west of the Mississippi Riverand in the southwere forcedto pay higher rates of interest, and to them the saving to the taxpayers by the New SouthWales plan wouldbe evengreater. In the United States, however, there was an insuperable bar to this method. of financing any except Government works ; that was to say, works which were controlled by the FederalGovernment, such as the improvement of rivers, the construction of harbours, and the reclamation of arid territorial lands.About 30 yearsago the bonds of theUnion Pacific RailroadCompany, the first transcontinentalline to the Pacific Ocean, were guaranteed by the Federal Government, but so much scandal grew out of the transaction, that there had been a manifest unwillingness since on the part of Congress to guarantee the bonds of any semi-publicenterprise. This was one of the obstacles now raisedto a subsidy of steamship-linesto South American ports, however much it might in time inure to the com- mercialadvantage of the country.The National Government, by reason of the reserved rights of the States, could not step in as the Author said was done in New South Wales, and aid the State of Ohio, or any other State, in an enterprise that was only State- wide ; even though it might be legal for the Government to construct or enlargecanals for transportationthrough States in the interests of national commerce. As a matter of fact, however, the expenso of enlarging the Erie Canal afew years ago, amounting to $104,000,000, was borne entirely by the State of New York. Concerning the pressures allowed on good concretemasonry, the limits fixed in the New South Wales dams seemed to him to be

Bfr. Hill. rather low, although the magnitude and importance of structures would always have a modifying influence on the load to be imposed. In constructing a large waterworks pumping-station a few years ago, in which concrete was freely used in all work below the ground-line, he specified the following proportions and tests:-The concrete was to be made in the proportions of l05i lbs. of Portland cement, 14 cubic foot of sand, 13 cubic foot of limestone screenings, and 5 cubic feet of ballast. Six-inch cubes of concrete taken from the batches used in the work were to show not less than the following crushing-resistances :-

At 30 days ...... 1,500 lhs. persquare in cl^. ,, 60 days ...... 1,550 ,, ,, ,, ,, ,, 90 days ...... 2,200 ,, ,,,, ,,

Cubes were made regularly each day that concrete was being mixed and placed,were registered asto date, weather, and location in the work, and in duetime mere forwarded tothe general laboratory, there to be tested by assistants who had no connection with or interest in the constructional works from which the cubes wereobtained. Considering thenature of thestructures in which the concrete was used, building-foundations,chimney- foundations,engine-foundations, pump-wells, valve-chambers, etc., the working(static) loads could withsafety be madeone-fifth to one-fourth of the knowncrushing-resistance of the concrete. This was rarely as low as 2,200 lbs. per square inch, and theworking- stress could be takenat 28 * 5 to 36 tonsper square foot. It would probably not be advisable toput so great a load on concrete in a high dam, but on a good foundation and with rigid supervision of the material,mixing, and placing of concrete, maximum loads of 23 to 27 tons per square foot were not to be regarded as excessive, even in suchstructures. The method described by the Author for rectifying natural defects in the foundation, by drilling into the rock and pumping in grout under pressure, seemed to be the onlyremedy for a defect of this kind. Almost the same condition had been discovered in the foundation of a masonry dam now being built across the valley of the east branch of the Cahaba River, in the State of Alabama, a work for which Mr. Hill at the present time was acting as consulting engineer. The site of this dam had been selected from surface conditions, with no borings to show the condition of the rock foundation, and after a large amount of money had been expended on thework and the foundation had been found defective, he was placed in advisory charge to assist in rendering the work safe. In thisinstance the stratifiedlimestone forming the

foundation for the dam was in layers 6 to 8 feet thick with clay Mr. Hill. seams, voids and pockets filled with gravel up to 2 feet in thickness occurring between adjacent layers. The rock lay at aninclination of 45 degrees to the horizontal, or at this angle to the base of the dam. Holes 34 inches in diameter were being drilled with a diamond drill to a depth of 65 to 70 feet below the base of the dam.These were thenfurnished with short pieces of wrought-iron gas- pipe, threadedon their upperends anddriven tightly into the bore-hole.Connection was madefrom this piece of pipe tothe discharge of asteam-driven grout-pump, which was proportioned to pump groutinto the hole nndera pressure of 150 lbs. per square inch, with a view to forcing it into the smallest and most remote cavities within reach of the bore-hole. As a precaution before pumping the grout, each hole was pumped with clear water, and notes were taken of the rate at which water was lost under a given pressure ; from these data a fair inference could be drawn as to the amount of grout that would be required, the size and extent of crevices and voids which the bore-holeintersected, and generally the condition of the foundationwith respect to voids between adjacent layers of rock. Precautions of this kind might not always be necessary to ensure security to the foundation and to place it in condition to carry a heavy unit load ; but apart from the strength of underlying rock, there was often the question of watertightness of the foundation after the dam had been completed and the reservoir had been filled with water. It was scarcely probable in the majority of cases that water thus lost by passing under the base of the dam would have a dangerous effect in balancing a portion of the weight of the masonry, and thus diminishing the stability of the structure. Such, however, might be the case where the rock from side to side of the gorge or valley was everywhere of open structureas at Cahaba, and containedvoids filled with clay,which in duetime might bewashed outand formchannels under the dam.Here, unless unusual precautions were taken to fill all voidssolid with grout, the manysmall streams of waterflowing through the foundation under pressure might exert sufficient upward pressure to counterbalance, in a dangerous degree, the weight of the masonry. Of course ea.ch situationmust beworked out carefully upon its own natural conditions, and it was to be regretted, if the drilling and grouting of the foundation under the Cataract dam constituted a matter of concern to the constructors, that the size, spacing and depth of holes drilled, the condition of the rock passed through by drills, with respect to crevices and voids, and finally the precise manner in which these holeswere afterwards pumped with grout

Mr. Ilill. were not described in the Paper. The amount of grout SO expended, the voidsfilled, andthe pressureunder which the grouting was performed,should have been noted. Doubtless theAuthor, in replying, would supply this information. The experiment made by the Author to determine the changes in dimensions of. the block of sandstone,wet anddry, was very interesting, and the provision of drains in the body of the Cataract damto provide constantand uniform saturation was novelto Mr. Hill. While it was not unusual to provide drains or weepers in largemasonry dams, to conductaway anywater that might find its way from the back of the dam intothe interior of the masonry, the proposition toconstruct drains that would provide a uniformsaturation of the masonry inthe dam was a novel proposition. Of course, it was clear from theAuthor’s statement that the use of the Hawkesbury sandstone, which showed remark- able contraction and expansion between a thoroughly dried and a saturated condition,required some precaution of this kind.The extreme change in length of a block 20 feet long was shown to be inch, sufficient to destroy the bond between adjacent stones and to provide innumerable thin channels for the escape of water from the reservoir. Whilethe changes in dimension of sedimentary rock, due to temperature, were well known, this was the first case within Mr. Hill’s knowledge where an attempt had been made to provide in the construction of the work for changes of dimension due to the temperature of the water. It was a question whether, with a thorough bonding of stones in both directions and perfect mortar beds and joints, the expansion andcontraction of the stonesactually built into the wall would be any material percentage of that shown by the test of the Hawkesbury stone at thequarry. Xr. Iieele. Mr. T. W. KEELEhad been disappointed to find so little information givenwith reference tothe cost of the works, which, in the case of the Cataractdam, he considered, had been excessive. Verylittle information was givenas tothe height of the dam, the foundations, the catchment-areaand its physiography and capabilities in regard to rainfall and run-off, and judging by the interest taken in these subjects when discussing similar Papers read before The Institution, the omission would no doubt be a disappoint- mentto members. He felt some diffidence in taking part in the discussion, in view of the positior, he had found it necessary to take up with regard to the work from its inception to its completion. It wouldbe impossible to explainsatisfactorily the reasons forthe heavy cost of the work, without making some reference to itshistory. He would therefore state, briefly, that in 1902 the question of the

water-supply of Sydney was referred to a Royal Commission. One &fr. Keele. of their recommendations was that a concrete dam, 120 feet high to top water-level, and capable of impounding 7,000 million gallons of water, should be constructed on the Cataract River at the site on which thepresent structure stood. Thedesign for this dam, together with one to impound 18,000 million gallons at an ultimate top water-level of 150 feet, had been prepared by Mr. Keele in his capacity of Principal Engineer for Harbours and Rivers, thebranch previously charged with such work;and he was also one of the members of the Royal Commission, but signed the minority report in favour of the larger dam. The Royal Commissioners’ recommendation was submitted by Parliament to its Public Works Standing Committee for inquiry and report, and after considering the merits of the two schemesreferred to, and one other prepared by theAuthor, to impound 14,000 million gallons, the Committee recommended the construction of the 18,000-million gallon dam 150 feet high as designed by Mr. Keele, at a cost notexceeding 2,217,500. Parliament authorized the construction of this work in October, 1902, the Act providing that the cost should not exceed 10 per cent. more than the sum stated, so that the limit of the authorized cost was 2,239,250. The work wascommenced immediatelyby day labour, under the PublicWorks Department as the Constructing Authority. The design andestimates of the authorizedwork were based on the condition that it wouldbe built of concretewith displacers or plums of roughquarried stones to beembedded in the matrix. This class of work had been adopted with very satisfactory results at theHelena dam at Mundaring inWestern Australia for the Coolgardiewater-supply, and also atthe Barossa dam inSouth Abtralia for the Gawler water-supply. It was considered that such work would be preferable to masonry, for reasons which would be givenpresently, and wouldbe less costly, asno skilled labour or expensive plant would be necessary.The Constructing Authority, however, decided to build the dam in the manner described by the Author, namely, of cyclopean rubblemasonry for the hearting, with a facing of moulded concrete blocks set in cement mortar. The total estimated cost of the plant required for the authorized work was2,17,000 ; the actual cost of theplant used inthe work as carried out was &40,237. While the work was proceeding, it soon became evident thatthe ConstructingAuthority was making a departure from the authorized work, the intention being to lower the height of the damfrom 150feet to 145 feetat top water-level, and the rock forming the by-wash was therefore cut down to suit the dam at the lower level. The Board of Water-Supply and Sewerage,

Mr. Iieele. for whom the dam was being built, objected to this very strongly, and urged that the authorized height, which it had been ascertained by morecareful survey would impound 21,400 million gallons, instead of the 18,000 million gallons as previously contemplated, should be adheredto. As this would entailadditional expense, owing to the cutting-down of the by-wash, and as the Constructing Authority considered thatthe Act was complied with by the impounding of 18,000 milliongallons at the 145-feet level the request was refused, andthe work proceeded. Thedisputes, however, concerning the departures from the Act were ultimately referred to two separate Royal Commissions : one, “ to inquire into the discrepancy between the estimatessubmitted tothe Public Works Committee and placed before Parliament for the building of the ‘dam as authorized, and the amount it was anticipated it would cost to complete the structure, namely, 2350,000 ” ; and the other Royal Commission, “to inquire into the height to which the dam should be built.” The result of the first inquiry was to show that the work done, and the probable cost to complete it, would amount to 2342,050, and the amountof excess cost,for which it was shown the designer of the authorized work was in no way responsible, would amount to2124,550 ; but adding 10 per cent.as allowed by the Actto the original estimabe, and allowing creditof 50 per cent. onthe prime cost of the plant, reduced the estimated ultimate cost to %325,496,and the amount of excess cost to 286,246. The total cost as given bythe Author was 2329,136, and the actual excess was therefore $89,886. Theresult of the second Royal Commission of inquiryinto the height to which the dam should be constructed was a recommendation that it should be builtto the originalheight as designed and authorized,namely, to 150feet. Directions were thereforegiyen by the Government tothe Constructing Authority to carry the recommendation into effect. Thecutting down of the by-wash to suit the lower level necessitated the construction of a masonry wall 715 feet long to hold up the water to the authorized height, andto act as a spillway weir in connection withthe by-wash. The extra expense entailed by this procedure could not have been less than 218,000. The cost of clearing and burningoff the timberon the reservoir-area amounted to $22,908 for 2,456 acres, or, at the rate of $9 6s. 6d. per acre. This workwas doneby day labour. The heavy timber mentioned by the Author existed over an area of only 272 acres, and the greater part of it was felled into the creek bed and was never removed. Two experienced railway contractors stated, in evidence, that the whole work of clearing the 2,456 acres could have been done for 26 39. 8d. to 27 per acre. If it had been

let by contract there couldbe no doubt that the expense would have Mr. Keele beenconsiderably less. Theestimate for rockexcavation inthe foundations was 30,300 cubic yards ; but owing to the necessity for going deeper, as described by the Author, this estimatewas exceeded by60,898 cubic yards. Notwithstanding this necessaryadditional expensebeyond what was contemplated, aninquiry madeunder Royal Commissionshowed on the evidence of threeengineering experts thatthe whole work could havebeen carried outin concrete with displacers, instead of the expensive cyclopean rubble masonry and concretemoulded-block facing, at a cost within the amount allowedby the Act.The total saving,therefore, could havebeen atthe least X90,OOO. TheHawkesbury sandstone formation, on which the dam had been built, was composed of beds of varyingthickness, horizontally disposed, with occasional false bedding and seams of shale, through which watermight readily pass under comparatively little pressure. This was exemplified when buildinga weir a few miles below the present dam site in 1883. When excavating in the bed of the river for the foundation, a crack or fissure betweentwo sound beds of rock was metwith, and followed for a considerable distance, until it ran out quite abruptly at a depth of about 19 feet below the bed of the river.A large portion of the stream then running found itsway through the crack into theexcavation, causinggreat trouble and expensein unwatering. Notwithstandingthis experience, and with the knowledge that a water-carrying bed of sandstone was ascertainedby bores and shaftsto underlie the bottom of the excavation, as shown in Fig. 15, Plate 1, the Constructing Authority, after the fullest consideration, deemed it unnecessary to make any provision to cut off possible leakage under the base of the dam, other than the shallow trench referred to by the Author. A glance at this Figure would show that it would have been wise to cut a narrow gullet, tobe filled with concrete,in therock under the up-stream toeof the dam, and to mrry it well down through the water-carryingbed referred to. This work was necessary in such an uncertain and treacherous formation as the Hawkesbury sandstone, and the extra expense entailed would have been amply justified as an insurance against one of the most seriousrisks to which structures of thisnature were subject. It was true that no signs of leakage were apparent under the foundations with the head due to the height to which the water in the reservoir had been raised up to the present time, namely, 107 feet; but it was impossible to say what might happen with the additional head of 43 feet when the reservoir was full, It would be observed from Fig. 15 that provision for drainage of the water which might [THE INST. C.E. VOL. CLXXVIII.] Q

Mr. Keele. leak into the heartof the dam had not been made lowerthan 65 feet below the top, therefore there was nothing to prevent water from accumulating slowly up to that level ; so the fact that there was no leakage at present below the dam was no proof that it was not actuallygoing on inthe mannersuggested. With respect tothe cyclopean masonry which formed the hearting of the dam, the work carried outhad demonstrated the impossibility of avoiding horizontal beds over considerable areas, and also vertical and horizontal joints in line overseveral courses. Although every care had been taken to comply with the terms of the specification, which strictly prohibitedsuch work, the photographs of the progress of the structure fromtime totime showed very clearly these defects. The Author claimed that the stones had been bedded on the mortar without any imprisonment of air underthem. It was a fact that every endeavour was made to avoid this ; but Mr. Keele contended that nothing could prevent this very serious defect from occurring in even the most carefully-built ashlarmasonry, let alone in the roughly hewn stone used in this dam. An object-lesson in this connection was given atthe failure of the weirpreviously referred to, at Broughton’s Pass, about 5 miles below the present structure. In February, 1897, a flood displaced several of the top courses and swept them away, the stones being scattered along the bed of the river below the weir. The underside of every one of these stones had nota particle of mortaradhering to it; but the full bed of mortar was found to be adhering to theupper surface of each block, from which it could not be detached without carrying away portions of the stonewith it. The strength of the cementmortar used in the work, and the fact thatwhere there was no imprisonment of air there was completeadhesion, wasproved by some of the stones, which, after being displaced and rolled over the rocky bed of the stream to100 feet away from theweir, were found strongly unitedat the side joints. The weir was built of masonry in 18-inch courses, eachstone havicg an area not less than 15 squarefeet, the bed joints being chisel-drafted 14 inch and boasted and tooled fair and full to the square between the draft. The vertical joints were axed fair with a 14-inch margin draft worked all round the arrises. The masonry was setfull in cement mortar composed of 2 parts of cement to 3 of sand.For purposes of grouting, grooves were cutin the sides of eachstone, as well ason the bottom, and a hole was drilled throughthe centre of everystone toadmit of the escape of air. Each stone waswell wettedbefore being laid, and was dropped on its bed of mortar twice andraised for inspection, andthen struck severaltimes in place witha heavy

wooden rammer. Notwithstanding every precaution thus taken to Mr. Eeele. ensure the best work, air must have been imprisoned, and spread in a thin film over the whole bottomsurface of eachstone, thus effectually preventingany proper adhesion to the mortar. Verylittle leakage was noticeable afterthe weir was completed, and it stood intact for 15 years, beingsubjected tothe passage of frequent floodsover its top, and to the impact of heavy drift timber.There couldbe no doubt that, having thus been severely shaken, the water foundits way under the beds, and there being no adhesion, in the manner described, there was nothing to prevent the stones from being lifted and carried away. If air was imprisoned underthe beds of masonry of this description, surely it was reasonable toanticipate that a similardefect wouldbe foundto exist in the cyclopean rubblemasonry used inthe Cataract dam, which was composed of blocks hewn to a rough rectangular shape, the lower beds being roughly picked only. The comparatively large cavities undersuch beds from the pick-holes alone, without con- sideringthe other cavities that could not beavoided in such roughly quarried stones, must contain a large amount of air, and no wriggling of the stones with levers as described by the Author would entirely expel this air. The procedure would be more .likely to force the mortar into the cavities, thus driving out the air, and spreading it all over the underside of the stone in a thin sheet, so preventing adhesion to the mortar in exactly the same manner as at the weir just described, The fact of a structure of cyclopean rubble masonry being watertight did not prove that the beds were impervious to water, for the thick mortar joints in a large mass of random uncoursed masonry would bond well with the mortar in the beds, thus preventing leakage from one bed to another. A structure withonly partial adhesion of the stones tothe mortar on the beds, if leftquite undisturbed, might last for an indefinite time.Unfortunately, however,no country inthe world was exempt from earthquakes, and it was easy to conceive what would certainlyhappen if adam of this descriptionwere subjected to shocks, and the rocking about which it might some day be called upon to sustain. There couldbe little doubt that whatever slight adhesion theremight be between the lowerbeds of the stones and the mortar wouldbe gone,under such conditions as existed recently at Messina or at San Francisco.The lesson to be learnt therefore was to avoid masonry of every description for dam-construction, and to adopt concrete with displacers as provided for in the authorized design for the Cataract dam. In addition it should be partially reinforced with steel bars a5 suggested by Sir Benjamin Q2 Downloaded by [ UNIVERSITY OF EXETER] on [24/09/16]. Copyright © ICE Publishing, all rights reserved. 84 CORRESPONDENCEDAM-CONSTRUCTION.ON [Minutes Of

m.Keele. Baker, in the discussion on the Coolgardie water-supply.' The fact that, almost everywherethroughout the areacovered by the Hawkes- bury sandstone, basalt was found in the shape of dikes from 1 foot up to manyfeet in thickness, should suggest caution in mining under reservoirs with so little cover as 800 feet, especially when it was considered that these dikes were vertical cracks or fissures in the earth's crust, through which the once liquid magma had been forced from below. The heat frequently altered the structure of the rocks through which the intrusive materialhad cut its way, and sometimes converted the sandstones into ahardened prismaticrock, with jointsso open as topass water under pressure quiteeasily. In many instances where the dikes occurred in the Hawkesbury sandstone formation, the basalt itself had been altered by decomposition into an earthy substance, so soft as to be crumbled by mere pressure of the fingers. Further, at least onedike of original basalthad been discovered within the area tobe covered bythe impounded waterin thereservoir, and asit was well known that occasionally the magma hadbeen found to have cooled before reachingthe surface, it was possible there might be others ; it would appear therefore that a very considerable risk had been taken by the decision to ignore the protests of the Water Board in allowing the reservoir to be undermined. Mr.McKinney Mr. H. G. MCKINNEYobserved that whilehe had no doubt that all ordinary care was exercised in the construction of the concrete dams referred to in the Paper, and that the work was quite up to the usual standard, it hadalways appeared to him that the consolidation of concrete in New South Wales was never so well carried out as he had seen it done on the canals in India and particularlyon the Lower Gangescanal. The systematic rammingof every layer, however, and the breaking upof the surface to make the consolidated layer bind with the next above it, as practised in India, would be a very expensive item under Australian conditions. To meet this ditficulty it was the general practice in Australia to use a larger proportion of water in mixing the concrete, so as to form a plastic mass and thus to fill all interstices withthe minimum expenditure of labour. Port- land cement as manufactured for a considerable timepast, and on an extensive scale, in New South Wales was a material of a much higher class than the kunkurcement used in Upper India. This superiority of the cement, and the greater proportion of water used, served in some measure to balance the drawbacks arising from the smaller amount of labour in consolidation. He hadnot, however, seen in New South Wales any concrete which when broken up bore such

Minutes of Proceedings Inst. C.E., vol. clxii, p. 126.

resemblance to compact conglomerate rock, as concrete made under Mr.McKinney. Indian conditions. He observed thatthe Author stated that the practice now adopted was to construct the walls of concrete mixed with a minimum quantity of water and well rammed. The value of ramming was certainly more appreciated now than formerly, but it would always be difficult, under Australian conditions, to have this well attended to. Mr. A. MALLOCKhad no doubt that temperature-variations were Mr. Mallook. the chief cause of the vertical cracks which had developed, and he would like to know if any observations of temperature hadbeen made within the substance of the dams. For the thin part of the walls the annual range of temperature must havebeen considerable throughouttheir thickness.A statementas to whether any observations on this point were made, and also as to the daily and annual range of temperature of the air and of the surface of the masonry would, he thought, add to thevalue of the Paper. Mr. E. MATTERN,of Herne, Westphalia, observed that in the Mr. Mattern. continued efforts of engineers in all countries to reduce the high cost of dams built with sections of the gravity type, due to their large masonry contents, the idea of utilizing the arch principle had always been conspicuously in view, and this series of arched dams afforded further proof of the practicability of the idea. The Paper described the testing of the archprinciple on an extensive scale, forming a worthy addition to the earlier examples carried out in France,America, and elsewhere. He agreedgenerally with the Authorin the view thatan archeddam afforded a safe and economical means of storing water; but he wouldadd-provided the site allowed of the use of a dam of small radius, and provided the storage-capacity of the reservoirwas moderate. The latter condition was imposed by theinadequate existing knowledge of curveddams. It was necessary to be cautious in accepting the demonstratedutility of archeddams, in individual cases and under particular conditions, as proving their general applicability underother conditions. The reservoirs in New SouthWales were of comparativelysmall capacity, andthe heights of the wallswere moderate;further, the climaticconditions of the country werefavourable. It might also beconcluded thatthe damswere situatedin valleys which were butlittle settled and where costly damage from any failure was not to be feared. These simpleconditions being given, it was quitenatural that the designersshould have contented themselves with approximate methods of calculation into whichconsideration of the elasticity of the materialdid not enter. He believed, however, thatan

Mr. *attern. engineer would alwayshesitate to buildcurved dams, 150 feet ormore in heightand storing many hundred million gallons of water, inthe mostthickly-populated mountain-valleys of Europe, because in addition to the statical conditions there were manyother factors to be takeninto account.The climatic conditions of CentralEurope precluded the building of dams of such slight thickness as in Australia, even if the dimensions corre- sponded with the requirements of theory ; the German dams had in general a minimum thickness of 13 feet at thetop. To diminish this thickness,even if thearching actionpermitted it, wouldbe inadvisable ; in thickwalls frost-cracks woulddevelop on the surface only, in thin walls they would penetrate the core. He had observed this when superintending the construction of a dam with concrete hearting built to a radius of 165 feet, and in which the concrete core, rather more than 3 feet thick at the top, was of itself able towithstand the whole of the water-pressure.Such cracks mustinvolve danger, on account of thedetrimental influence of weathering on the durability of the masonry. Everything possible should be done to collect experience of works already constructbd, and it was desirable that thewalls already built Newin South Wales should be watched as regarded their behaviour under the influence of water-pressure and changes of temperature, and that the elastic movement of the walls should be measured. In this way valuable data wouldbe obtainedfor the calculation of archeddams. TheAuthor was of the opinion that dams of gravity cross sectionshould be builtstraight in plan ; andthat the curved form would not offer such advantagesas to justify the extraexpense due to the longer length of wall. On this point Mr. Mattern would observe that nearly all German dams were curved in plan notwith- standingtheir gravity section. Thearched form waschosen in order to give play for the movements of dam walls which occurred under the influence of changes of water-pressure and temperature, particularlyunder the effect of the sun’s rays, It afforded room forthe expansionwhich occurred when thetemperature ros0 above the meantemperature of the mass during construction. It did notprevent the formation of cracksdue to contraction when the temperature fell considerably below this mean ; but in that event the water-pressure when the reservoir was full -as was generally the case in winter-tended to close the cracks. Rankine hadpointed outthat in straight walls thematerial on the down-stream side must be subjected to tension, inasmuch as withfulla reservoir the damcurved slightly under the pressure of the water.Further, the archedform had a better

effect in regardto adjustment of theinternal strains and Mr. Mattern. shearing stresses, dueto the difference inthe head of water at the middle and at the ends of the dam.These reasons, as well as thegreater safety and the reservearch action, whichwould come into play inthe event of thestability of the wall as a gravity sectionbeing called into questionfrom any cause, had determined the preference in Germany for the curved form of dam. The advantages gained wereheld to be fully worth the extraexpense entailed. He agreed with the Author that drying of concrete which had been mixed very wet entailed the risk of its not proving watertight,the loss of watercausing the formation of cavities and crevices. But in fixing the quantity of water regard should be had tothe season of the year.Further, the drier the concrete was mixed, the more carefully must the ramming be done. In practice, concrete was usually not absolutely watertight, even when it ought theoretically to be so because it had been mixed so that the quantity of mortar correspondedwith the interstices inthe aggregate. It was difficult, if not impossible, to make a concrete wall really watertight. In view of this fact, almost all German dams were given on the water-facealayer of cementabout 1 inchthick, and to protect this layer against the weather and from damage the upper portion of the wallwas faced with stone, while the lower portion was protected with a bank of earth. This device had always been successful, the dams being watertight. The above-mentioned movements of wallsunder the influences of temperatureand water- pressurepointed tothe desirability of using an elastic mortar. Thorough investigations had shownthat under equal loading greater elastic movements took place in trass mortar than incement mortar, and thus the tendency to the formationof cracks was counteracted. On this account a mortar consisting of 1 part by volume of unslaked lime, l&part of trass, and 13 part of sand was generally used in Germany in place of cement mortar. Trass was a natural pozzolana, and possessed the property of forming, with the slaked lime,a mortar which hardened under water. The tuff which was crushed to form trass occurred in largedeposits along theRhine. The special properties of trassmortar, besides itselasticity, were its density and its slow loss of binding-power.Sudden interruptions of work,such as occurred often inmountain regionsfrom heavy rain, entailed no pecuniary loss, because the mixed mortar remained usablewithout any ill effect forseveral days. Its plastic yield- ing after setting had begun permitted the laying of tramways and the transportof materials of construction over green masonry without harm. Where trass was obtainable locally, trass mortar was cheaper

Mr. Mattern. than cement mortar, andwhere the cost of using trass alone would be too heavy, owing to the expense of transport, it was mixed with cement mortar. It combined with the excess of lime in the cement and prevented the efflorescence of cement mortar, besides increasing its strength and density. The stresses in the materials of the arched walls-up to 25 tons per square foot-were high, while the stress of 83 tons persquare foot in the Cataractdam followed the usual practice. Two things caused him to regard the stress of 25 tons persquare foot as high; one was the approximatemethod of calculating the arched dams, the other was the fact that the mortar in an actual work never possessed the same strength as in the test briquettes. He had found it to have only one-half to two-thirds of thatstrength. Experiments with concrete blocks upto cubes of S& inches edge showed a decrease in strength with increase in size, and pointed to the fact that large masses of masonry could not withstand as high unitstresses as small blocks. Again, he considered a careful examination of the rock foundation in regard to its soundness and its suitability for supporting a high dam, as well as the testing of the watertightness of the site of the reservoir, to be one of the most important conditions for the construction of a solid and stabledam. For this purposewide experience was necessary, and the engineer and the geologist must work together, the engineer however, having the controlling hand. A central power-station and the use of electric current wherepossible had also proved to be advantageous in Germanworks. The measures for discharging floods during the construotion of the Cataract dam, however,seemed to have been insufficient, as the floods rose over the wall. The execution of the works in New South Wales through contractors, but with the condition that the cement was found by the ConstructingAuthority, was the methodusually adopted by the Prussian State, and proved generally satisfactory. The construction of dams for German municipalities was frequently carried out by means of general contracts, under which the supply of the whole of the materials, the building of the necessaryconstruction-railways and roads, the quarryingof stone, the provision of plant for prepara- tion of the concrete and the requiredlabour, together withthe other arrangements for the conduct of the work,were all undertaken by the maincontractors. But it was impossible todispute the correctness of the Author’s view, that the subdivision of the work into smallercontracts widened the circle of tenderersand rendered possible the participation of the smaller contractors. In this way the plan had a good socio-political effect, as had also the method of providing the whole of the necessary plant and lending

it to the contractors. The total cost of the Cataract dam for the Mr. Mattern. water-supply of Sydney must be considered to be comparatively low. The special conditions of the Cataract damin regard to the underlying coal-deposits were of much interest to him, because he was almostdaily confronted with problems arising out of similar conditions. At the presenttime the Prussian State was building theRhine-Herne ship-canal, andhe was in charge of the constructional work Cbnnected withthis undertaking. Branching off fromthe Rhine, the canal would extendfor about 24 miles across the colliery-districts of Rhinelandand Westphalia. The depth of water was 11 -5 feet,and thebreadth at water-level was 115 feet. There wereseven double locks 600 feetlong and 33 feet wide, withafall of 16 to 20 feet.Several harbours would be constructed, and the canal would be navigable for ships of 1,000 tonsburden. In the line of this canal the coal-beds were overlaidby a bed of marl 525 to 650 feetthick. Already subsidences of several yards had taken place in the surface as the result of mining, but experience showed that these occurred without the rupture of the strata extending to the surface, where the marl was at least 150 feetthick, and elastic enough toprevent the formation of cracksthere. Streams had keptto their channels,

eventhough their levels hadbeen altered. ' Fromthese facts it was concluded that no danger of an irruption of water into the mines need be feared; but the question was how to deal with the subsidences. It was not desired to preventthem entirely, but to reduce them and render them as gradual as possible, and with this object all spaces left by mining beneath a strip of land extending for 1,000 feet on each side of the canal had to be filled again by hand. In this way the subsidence was reduced to about one-half. Further, allthe structural works, locks, harbours, bridges, etc., were so constructed as to admit of subsidence to the extent of about 15 feet. According toFig. 14, Plate 1, the coal-beds under the Cataract reservoir were of sandstone and basalt, and from their thickness it could be assumed that uniform subsidence would leave the bed of the reservoir intact and not entail any risk of water bursting into the workings. But the pillar to be left under the dam itself would create a special state of affairs. The experience of coal-mining in Westphalia had shown that when pillars were left in this way, the effect of subsidences was most injurious at the boundaries of the pillars. On this account it was consideredpreferable notto have such pillars. But it must be admitted that the damcould not in any case be shielded from the risk of subsidence, because it was not easy to take such measures in its construction as would protect a

Mr. Mattern. structure of suchheight against fracture and damage, as was practicable with bridges and locks. The boundary line of the pillars underthe Cataract dam,therefore, should be carefully tested in respect to the brittlenessof the strata there and thepossibility of the irruption of water. m. Matthews. Mr. E. R. MATTHEWSremarked that his experience, which extended to reservoir-walls only, confirmed the Author’s conclusions in regard to securing watertightness in concrete dam walls. He did not agreewith theAuthor, however, in hispreference for “dry ” concrete. He preferred, in reservoir-walls, to adopt the middle course, namely, to use concrete which was moderately wet, and to deposit this inlayers, well ramming each layer, A solid mass was thereby formed, whichwas certainlymore watertight than whena dry concrete wasused, eventhough the compressive strength of thelatter might be greater. This- increased strength with a drymixture had beenascertained in experiments carried out by Mr. Geo. W. Rafter, M. Am.Soc. C.E., some littletime ago, forthe State Engineers’ Office, New York.There the strengths on compression were :- Strength on Compression. Dry mixture . . 156 blocks . . 2,470 Ibs. per square inch. Plastic mixture,. . 144 ,, . . 2,294 ,, ,, ,, Wet mixture . . 148 ,, . . 2,180 ,, ,, ,, The blocks tested were 12-inchcubes, 18to 24 months old. Mr. Matthews’smethod of securing the watertightness of reservoir-walls was the following: theproportion of parts inthe concrete was usually 1 partPortland cement, 1 part of sharpsand, and 3 parts of l-inch brokenstone or gravel (the former preferable). Immediatelyafter the formshad been removedhe had the concreteface pointed with neat cement, andthe whole area twice washedover withneat cement. This method of securing the watertightness of walls was usually adopted in the United States. He did not consider that the introduction of plums, asin the damsreferred toin this Paper,tended to increase the watertightness of the wall, but rather the reverse. Wherever a plumoccurred a cavityexisted around it, which was increased by thecontraction of the concretewhen setting, and leakage was frequentlyfound to occur at these points. Except in the foundations of areservoir-wall or damhe did not approve of the insertion of plums. He believed that the gravity type damsreferred to would havebeen constructed more econo- mically and efficiently of reinforcedconcrete than of ordinary concrete.

Mr. REGINALDE. MIDDLETONadmired the pluck of the engineers Mr. Middleton. who designed the several reservoir-dams illustrated in the Paper, but he doubted if the information afforded was of much value in England. It was, in hisopinion, unfortunatethat plans of the several dams werenot shown,' nor was any comparison drawn between the cost of the curved dam and thatof a gravitywall capable of doing the samework. It was admittedthat some of the concretehad cracked both vertically and horizontally, and that thecracks opened and closed with variations of pressure due either to the height of the water in the reservoir, to changes of temperature, or to both combined. H0 would fear that the length of the cracks would tend to increasewith time. The engineers had been very fortunate in securingfoundations of fairreliability with little excavation : he was doubtful if with deep trenches the dams would have given such good results ; there might have been, he thought, a tendency to crackhorizontally at or nearground-level. Concrete cracked irregularly, which might result in a section of the dam being with- outsupport. With an elastic structurethe whole of the coal might be worked from under thereservoir without danger. A pillar of coal under the masonrydam, while it would preserve that structure, would not by any means necessarily protect the floor of the reservoir from fracture and leakage, but, on the contrary,would almost certainly result in fracture, if not in leakage. Mr. STUARTMURRAY remarked thatthe principleadopted in Mr. Murray. the design of curved dams, asstated in the Paper, commended itself tohis judgment asentirely satisfactory. Theformula employed to determine the conditionsdemanded to give effect tothis principle seemed equallyadequate ; while its simplicity RP rendered it specially easy of application. The statement T = - S needed no demonstration ; but the conditions were to be taken as subject to certain practical considerations--- involving certain assumptions,which must be warrantable if successwas to be assured. Of these considerations the chief were : first, the quality and consequent strength of the concrete, and especially its resistance to crushing and shearing, in other words, the value to be assigned to S in the equation ; and secondly, the factor of safety proper to be adopted,as representing the inherent relation betweenresistance to crushing at the moment of impact, and resistance to crushing throughan indefinite period of time.The proportion of cement

1 Plans of some of the dams have been furnished by the Author since the discussion took place (see p. 3).-sEC. INST. C.E.

Mr.Murrsy. tototal aggregate,mentioned inthe Paper,namely, 1 : 7.66, might be takenas generally sufficient, thoughnot necessarily always so ; the factor of safety, on the other hand, appeared IOW. Thestrength of concretedepended less on the proportion of its several constituentsthan on theirstrength, or rather on the strength of the weakest of them, and on the nature and closeness of their contacts. The strongest concrete was made of tough, hard stone, broken into theform of rough cubes, with water-worn shingle, also of hardstone, in sufficient quantity to fill the interstices of the metal ;of sharp siliceous sand ; and of cement, addedin thecorrect proportionsto thoroughly fill all interstices. Thesubstitution of shiversfor Tshingle in the worksdescribed in the Paper was probablyinevitable ; otherwise, the adoption of thismaterial seemed the most questionable feature of the concrete employed ; that was, if by shivers was meantminute 0at spawls,such as were produced bya mason inthe process of hewing atone. It was not always possible,however, to obtain ideal concrete ; first, because of the frequent difficulty of obtaining the materials ; and, secondly, because the excessive ramming generally required to bring about the assumed closeness of contact would entailthe risk of destroying the grip of the cement, of disturbing it, in fact, after the process of setting hadbegun, The factor of safety was referred to, in the body of the Paper, as 5, to be applied to the test values obtained by crushing 6-inch cubes, but it was assumed as equal to 74 for concrete in bulk. In the summary,however, the factor 5 was revertedto. It would certainly be more satisfactory to have the actual tests made on largerblocks, say, not less than 1 foot cube, if possible ; while, for so inelastic a material as concrete, in effect artificial stone, it might wellbe doubted whether so low a factor as 5 was at all admissible. The use of explosivesin rock that was to form the seat of a dam was always and everywhere open to grave objection. Not only did it result in minute cracks which weakened the adjacent rock, as the Author justly observed ; but it was apt to open thejoints following the cleavage-planes, andthus to give rise to dangerous leakage. Of the advantage of ‘‘ fairly dry ” over “ sloppy ” concrete there could be no question. Cracks in concrete, arising from the discharge of its contained water, were a familiar phenomenon;on theother hand, the difficulty of ramming stiff concrete so as to close all interstices in the mass, without breaking the bond, was avery real difficulty. Probablycracking was most effectively prevented by allowing the cement to be thoroughly air- slaked before use. The proposal to provide parting joints to allow of the cracks forming on radial lines had much to recommend it.

In this connection it might be permissible to call attention to the Mr. Murray. following statement in a publication on the Goulburn Weir :-

e “ The body of the weir is not a monolith ; it is a wall of large blocks of cement concrete,bonded afterthe fashion of ordinary coursedmasonry, bedded and jointed in cement mortar. An undoubted advantage of this mode of construction is that the cracks, that will in time inevitablybe developed in the mass, will follow the planes between the blocks, instead of, as often happens with monolithic weirs, forming a rent extending inone line from top to bottom of the work, and traversingit from front to rear.” No cracks, except as hereafter explained, had,so far, shown themselves in the down-stream face of the work ; nor was any leakage visible eitherin the bedding orin the upright joints of the steps. The exception was in the shallow part of the concrete wall near the right abutment, where the work was necessarily thin ; so thin, in fact, that the adoption of the block system was there impossible. The leakage was of trifling import, the emitted volume beingso small as to be of no material consequence, and the movement in no degree an element of danger.The six conclusions reached and clearly stated by the Author, might be accepted as safe working-rules for practical dam-building ; with the exception of the second and third. The factor of safety, 5, seemed too low. It should not be adopted for important structures, until its sufficiency had beenshown by protractedexperience, under test conditions.The introduction of radial parting jointswas deserving of the most careful consideration, with a view to adoption possibly with modifications in the direction indicated by the extract here given. Mr. MALCOLMPATERSON suggested that the Author’sfirst con- ~~.pat~rson. clusion (p. 11) might be extended so far as to embody a general engineeringcanon that in suitablerock formation, curved walls, relying for stability on their resistanceto both leakage and crushing, form the safest and most economical means of storing large volumes of water^; and would ask what objection, if any, the Author could bring against going to this length. As to the insertion of the large blocks of sandstone, etc., were these embedded in the concrete by bringing up the mortar in layers within the narrowspaces between thom and the freshly-made concrete layers ? He would also like to know whether any tests had been made to determine how far, as a maximum, the verticalcracks, which did notleak, had penetrated into the masonry, and if so, what was the ratioof this distance to the width at such cracks. For instance, how deep andhow open were the cracks in the Mudgee wall ? Did they leak ? and had the 3-inch crack in

“ The Goulburn Weir, etc. ; a Descriptive Memorandum,” p. 8. Melbourne, 1893.

Mr. Patergon. the Cootamundra wall leaked ? If so, what was done to close it perma- nently ? Also in whatmanner and of what materialwere the parting joints made, and were they carried from face to face right through the bank P Touching the proposed working of coal under the Cataract reservoir. He had seen much disturbance by coal-getting in connection with his own works, with varying thicknesses of bed and at varying depths, from a 20-inch seam at 65 yards to a 4h-foot seam at 200 yards and 540 yards. These were in the sandstones, limestones and shales of the coal-measures, and had caused settlement at the surface averaging two-thirds to three-fourthsof the thickness of the seam gotten, witha decrease in the settlement as the depthbelow the surface increased. In certain dist,ricts he had found most of the underground watersaffected by workings within, say, 200 yards of the surface ; and atshallower depths the streamswere largely reduced in volume, and even in some casesdried up, andsmall reservoirs were put out of use. Within the last monthor two he hadhad levels taken over a sewer disturbed at thesurface bya 4-foot seam 550 yards deep. How far this had tapped its contents, if at all, he had not determined, but there was no disturbance visible in the surface of the land, and it was probable that there was none, for at such a depth the surface would probably not be ruptured but only sunken. He would, however, not feel quite secure in working the 5-foot Bulli seam under the upper end of the Cataract reservoirat a depth of 267 yards. Yet if the water were let down, probably the sandstone strata would carry it off and the mine would escape flooding, as it was in some degree protected by the shales which in thecoal formation were intercalated with thesandstones ; but with a full head from a huge reservoir great scour in leaks might be setup. InBritish coal-districts, the surface above a worked coal-field, even where the coal had been long gotten, was avoided as a reservoir-site. In view of the great coal-interestsat stake under a reservoir so large, the risk might be taken, the dam being absolutely secure. The division of labour by relegating the foundation to day work, and the superstructure to the contractor, was admirable, and equally satisfactory was the supply of cement by the Government,with the check upon wasteful use. It was a pleasure to study the details of such a Paper, which showedhow much valuable experience, on a large scale and of a rather novel kind, was being gained by Australian engineers. Nr. Smith. Mr. CHARLESW. SMITEconsidered that the Paper, especially if amplified in some respects, would form a valuable addition to the Institution records. Though the title was " Dam-Construction " it could not be overlooked that some of the Paperwas occupied with the subject of " Dam-Design," at least in the portion devoted to curved

dams. The introductionof this class of dam was of no recent datein Mr. smith. Australia ; witness the work built at Parramatta by Mr. Moriarty, M. Inst. C.E., in 1858, and also the Lower Stoney Creek dam, Victoria.’ The latter structurehad a section similar to many of those described in the present Paper, though its section was much larger than it would have been had it been designed as that of ‘‘ a rigid cylinder subject to exterior water-pressure”; and Mr. Dobson in his description of the work admitted thatexcess of stability was given in view of the action of waves, breaking in stormy weather to a height of 4 feet above the coping. The Author stated that it had been found convenient to pass flood-waters over a portion of the crest in all the cases cited. But it would add to theinterest of his Paper were the maximum flood-levels marked over the cross sections, so admitting of the sections being scrutinized in relation to the additional pressures thus liable to act onwalls the ; otherwise thestatement following, that the maximum depth of 3 feet had been dealt with at Parkes, might, rightly or wrongly, be read as implying that such a depth was the maximum to be apprehended in all cases. Neither was it stated whether any provision had been made for protectingthe toe of the dams against scour byflood- overflows. Anotherquery suggested itself where theAuthor said that plums had been used in thewalls where the cross sections were of sufficient width to allow of it being done. It wouldbe sntis- factory to know towhat extent, if a.t all, they entered into the composition of such a slender wall as, for example, that shown in Fig. 12, Plate 1. In referring to verticalcracks in some of these curved dams, the Author stated thatsince they occurred naturally they did not affect the stabilityof the walls ;but thatconclusion, in the form in whioh it stood, was not cogent ; still less did it appear so when, in the next sentence, it was stated to be better not to allow the cracks to occur naturally. No doubt the Author meant that since, as he admitted,vertical cracks didnot as a rule go down in a vertical plane, but twisted in their course, it was better to control their development by providing parting joints that should compel such cracks to follow radiallines, and therefore to develop in vertical planes they would so act rather as voussoirs, whose backs would be disposed towards the water-pressure in the manner best calculated to assist stability. The Author had doubtless had special opportunities of observing whether the reservoirs behind such thin walls as those illustrated inFigs. 7, 9, 11, 12, and 13 of Plate 1 had been very low in hot spells, and what ill effect, if any, the sun’s heat

Minutes of ProceedingsInst. C.E., vol. lvi, p. 94.

Mr. Smith. had caused in them by expansion, perhaps followed by rapid con- tractiondue to considerable fall in temperature. Any definitely- ascertained facts in that connection would greatly enhance the value of a Paper devoted to design andconstruction. Conservation of water in Australia, where perennial streamswere few, wasso essential to the proper development of the country, that any advancement in knowledge touching the economic storage of water was sure to be welcomed ; and the Public Works Department, by instituting these cheaper forms of dams, had done much to make settlement possible in many outlying districts. It was regrettable that the actual cost of constructing the curved dams, compared with the estimated cost of dams of gravity sections at those same sites, had been omitted, since no one could be in so favourable a position as the Author for preparing and showing such a comparison. The record of the Cataract dam-construction might belooked upon as the chronicle of a work carried out under ideal local conditions as regarded site, foundations,building-stone, and sand, together with accessibility and comparative freedom fromtrouble with water. The reservoir had not been filled, but thehead of water stored hadso far proved the excellence of the construction. The reservoir was the first, but was not likely tobe the last, of a series of similar storagesreservoirs that must be provided within the catchment-area, if the wants of Sydney were to be adequately met. This was foretold in the Reportof a Royal Commission, of which Mr. Smith was a member, appointed in March, 1905, to determine the capacity of the Cataract reservoir, and other matters connected therewith. Mr.tStrsnw; Mr. W. L. STRANGEremarked thatin India, as in New South Wales, the Public WorksDepartment designed and constructed most of the municipal water-supply works, except those for the largest cities, which maintained their own engineering staffs. There was an obvious advantage both to Government and to the municipalities in this arrangement : the Government staff obtained wide]. experience, andthe municipalities gained the advantage of having the services of that staff atthe minimum cost to themselves. He agreed with the Author that there was little, if any, advantage in building curved instead of straight masonry dams of gravity section, while the alignment of the former introduced additional practical difficulties in construction. It seemed tohim that the formula determining the thickness of a complete ring was not applicable to the calculation of the section of a masonry dam which was to be built only as a segment of a curve, When B complete rigid cylinder was subjected toexternal pressure, its form remained circular, although its diameter was diminished, andthe distribution of

pressure was similar on all of its cross sections. When, however, Mr. Strange. only a segment of cylinder was underexternal pressure, and its ends were practically fixed at the abutments, the internal curve of pressure would not be parallel toits bounding surfaces, andthe distribution of pressure would be much more unequal on the cross sections nearer the abutments thanon those more remote from them. In this case the segment of the cylinder really became an arch with fixed abutments, and the determination of the stresses became so complex as to be almost incapable of solution. Fig. 24 (U) showed diagrammatically the effect of pressure on a complete cylinder, and Fig..24 (6) that on a segment with fixed abutments. The sections of the dams shown in Figs. 1 to 13, Plate 1, seemed very light. The use of sloppy concreteto obtain a skin next themould-boards seemed open to objection, as thereby the inner portionof the outer thickness

Figs. 24.

(U) COMPLETECYLINDER (b)SEGMENT OF/ CYLINDER UNSTRESSED SURFACES SHOWN THUS * STRESSER ,.. ,, 3. .. - -. - - of the concrete, which should properlybe most watertight, would be moreporous and less dense than the interior of the hearting. A betterarrangement wouldbe to form an imperviouscasing with concrete richer in cement than that of the hearting, by placing the twc? varieties of concrete on each side of a temporary parting board, which wouldbe subsequently removed to enable the two to be rammed into perfect union with each other. The bedding of plums onsloppy concrete must tend to make the whole structure non- homogeneous, and therefore liable to internal stresses. The better plan wouldbe toprepre in the rammedconcrete beds for the plums, and to cover these beds liberally with mortar for the recep- tion of the stones. Fig. 14 indicated that onlyone bore-hole had been driven to determine the geological conditions under the bed of the proposed Cataract reservoir. Looking to the largeness of the [THE INBT. C.E. VOL. CLXXVIII.] H Downloaded by [ UNIVERSITY OF EXETER] on [24/09/16]. Copyright © ICE Publishing, all rights reserved. 98 CORRESPONDENCEDAM-CONSTRUCTION.ON [Minutes of

Mr. Strange. interests involved, it would apparently have been better to test the watertightness of the basin by more bore-holes. Perhapsthe geologists considered that the upward tilt ofbhe strata away from the damrendered improbable anyextensive leakage of water. In the Cataract dam the use of hearting masonry of doubtful soundness seemed open to question, as the natural rock from which the stone was obtained showed signs of deterioration, although it was not exposed directly to external influences. The system of drainage adopted to prevent the infiltration of water intothe masonry hearting might possibly lead to decay, by allowing air and water to penetrate along definite lines. It would seem preferable to exclude everything by a watertight skin of cement concrete. Mr. Symonds. Mr. J. SYMONDSobserved that he had been associated with nearly all the concrete dams of any magnitude constructed in the State of New South Wales, and was familiar with all the cases cited in the Paper. In the construction of curved dams it had been found to expedite thework considerably if one face was vertical, as thereby any portion constructed was a guide to laterwork, and saved the frequent setting and resetting out of the curve, so often necessary if both faces were battered, and so awkward on a narrow wall where men were continually working. In placing concretehe wits of opinion that it should only be “well wetted,” and not sloppy, since in the latter case any working-up of concrete toobtain a face against timbering brought the cement to the top andsides, to the detrimentof the bulk d the concrete. After stripping timbering from concrete, washing or paintingthe work with cementgave a thin, almost impervious skin which prevented access of water to thepores incon- Crete ; but in order tobe of value it must be applied to theup-stream face of the work, and should be put on shortly after theconcrete had received a thorough soaking, otherwise the moisture would evaporate too quickly and thecement would fall off. If the percentages of sand, cement, shivers, and broken stone in concrete were carefully ad- justed, SO thatthe finer materialsleft no voids between the largermetal, the cement-washing of the water-face of the dam would render it watertight;but if the ingredients were not properly balanced, a more or less honeycombedwall resulted, the face of which it was very hard to coat properly. In placing plums in concrete they should only be damp ; excess of moisture over and above what they would absorb was likely to result in a film of water gathering on the underside of such stones and preventing perfect contactwith the concrete. From experiments made on concrete carefully dissected, it had been noted that excess of moisture on the underneath sides of hand-stones had resulted in a thin void

or voids, or the shrinking away of finer particles of concrete from Mr. Symonda the plums on account of the presence of too much moisture. Plums should always be set on edge, as it was mainly side adhesion that could be guaranteed, and weakness due to imperfect beds was thus reduced. The bedding of plums, say of 50 lbs. to 150 lbs. weight, in compo in small damsof light section, would so greatly increasethe cost by reason of extra labour and cementthat a wall of all concrete would prove cheaper; but when large stonescould be handled by mechanical means, the method adopted at Cataract of compo beds and concrete side-packing was expeditious, effective, and cheap. He had observed that the Wellington dam expanded radially 3 inch by midday in summer time (reservoir empty), also that the curve flattened (from the normal) by a similar amount when the reservoir filled. He was of opinion that cracks in concrete dams in this State were almost invariably due to temperature-changes. Dams built in the summer months developed cracks after the first severe frost, and any main cracks occurred just above where a decided rise or fall existed in the foundations. All cracks were naturally widest on the outside of the concrete, decreasing towardsthe centre of the dam away from the influence of the atmosphere, and in nocase had he noticed any material leakagefrom such cracks;further, the expansion of concrete in warmer weather entirely closed them for such time. Mr. Symonds was of opinion that above where any very decided change of level occurred in thefoundations of curved dams,aradial parting joint might with advantage be employed, say, for instance, a thin paper joint. Generally small curved dams in remote districts would not permit of much outlay in plant : they should preferably be constructed all of concrete. Strainsmight then be moreevenly distributed, and any cracks would probably be radial ; but in larger structures, such as the Cataract dam, with, say, electrically-driven plant, large plums and concrete would proveall that couldbe desired, Cableways were now so perfectly controllablethat concrete and stonework could beplaced almost entirely by such means. He had had the opportunity of employing various meansof concrete-mixing in New South Wales, and found that a " cube " mixer for consecutive charges gave the best results. Professor W. H. WARRENconsidered that the Author's remarks Prof. \vsrreD. on curved dams for country water-supply were of great interest, as they showed that, where the conditions were suitable, small concrete damsconstructed as horizontal arches, andhaving a xersed sine of about one-third of the span, might be economical and efficient structures. The method used in calculating the thickness at any level could only be regarded as a rough approximation, but the strength

Prof. Warren. andstability of these structures wereassured by theadditional thickness at the top, by the neglect of the weight of the dam itself in the calculations, and lastly, by the limiting pressure allowed on the materialbeing based onits known strength.The practice of using plums of not less than 20 cubic feet, of roughly rectnugular section, andthoroughly embedded in tine concrete or mortar, reduced the cost of large masses of concrete ; it was suggested by him and used for the hearting in the towers of the North Sydney suspension-bridge. In regard to the safe compressive stress allowed on concrete, Professor Warren’s tests on some thousand cubes and prisms of various proportions, crushed at ages of l month to 2 years, showed that such concrete as the No. 1 and No. 2 described in the Paper would have a crushing strength of 100 tons to 120 tons per squarefoot at 90 days, andmight giveas much as150 tonsto 180 tons per square foot after a year. Adopting 150 tons ultimate crushing strength, the safe stress of 20 tons per square foot gave a factor of safety of 74 ; so that in small dams designed on the arch principle 20 tons per square foot was a safe maximum pressure. In gravity dams,however, the maximumcompressive stress (not the verticalcomponent of the stresson horizontal sections) should certainlynot exceed 20 tons persquare foot on concrete. In the Cataract dam the vertical pressureallowed on horizontal sections did not exceed Sl, tons per square foot, and the lines of pressure were everywhere well within the middle third. The method adopted in the small arched dams for minimizingthe formation of cracks in the concrete, and for securing watertightness in the dam-wall, by the use of comparatively dry concrete faced on the up- and down-stream sides with a skin of neat cement, was worthy of note, with a view to its application where the climatic conditions were similar to those existing in New South Wales. In this and in all similar cases of long wallsof concrete, however, it was better toprovide parting joints to take up any contraction or expansion that might occur, whether causedby variations intemperature or moisture. The Author’s remarks on curved and straight gravity damswere open to question, more especially in view of the facts andprinciples recently established in connectionwith the stresses in dams,’ which showed that the practice of building dams convex to the up-stream side reduced the tensile stresses on the face near the inner toe, and provided a greater moment of resistance to bending stresses. Professor Warren considered the advantages of this practice far outweighed the mere economy of material inthe straight dam. In the case of the

1 Minutes of Proceedings Inst. C.E., vol. clxxii, p. 89 et sq.

Cataract dam, which was built straight across the valley, the wall Prof. Warren. was SO &mly attached to the rock sides in trenches excavatedin solid rock, that it must receive considerable support from its connection withthe slopes of the valley, andmust act as an arch. He had visited the site of the works at the Cataract dam during its construction,and hadbeen favourably impressed with the general organization of the methods employed in every detail of the work, which had resulted in a thoroughly satisfactory structure. Mr. WM.WATTS remarked that water escaping under the flanks Mr. Watts. of the wall, and finding an exit some distancedown-stream, was evidence that the central trench had not beencarried sufficiently deep to cut off the lines of continuity in the beds and joints of the horizontalstrata under the floor of the reservoir. Whenan imperviousfoundation for adam-wall was required, it was a mistake to use explosive substances in excavating the last few feet of the finished level. Therocky material should be quarriedout with pick and wedges, and no beds should be disturbed that were not removed. For a concrete floor on which to commence the wall- foundation, it could not be left toojagged and uneven,nor too smooth for puddle. Rendering the face of the walls, and trusting to a skin of cement to secure watertightness, was an unsafe method on which to rely, especially when the structure was under considerable hydrostatic pressure andwas exposedto changes of temperature asthe water rose and fell in the reservoir. The material in thewall should be relied upon as a whole to resist percolation, and one quality only should be used. To wash the walls with a preparationof cement grout gave them a neater appearance,but it only added a uniform colouring effect to thefinished work. With reference to the use of sloppy and stiff concrete, the former was certainly more reliable as animpervious core than the latter, andit was sa.fer to use too much sand and water than too little. Punning concrete in thinlayers was unnecessary ; the operation pressed the stony aggregate down and brought the " fat '' to thetop, which formed a line of weakness to forces acting laterally. For convenience of handling, and of securing a more homogeneous concrete,small plums were, in Mr. Watts's opinion,preferable to large ones tons in weight, and each plum should be carefully bedded in the matrix in the condition of a jelly without the use of mortar. It seemed risky to provide parting joints in a dam-wall exposed to a great head of water and to changes of atmospheric temperature. Under such circumstances the stability of the superstructure must be endangered by the provision of joints in the material toopen and close in obedience to physical laws. If thisdanger could not be mastered bythe engineer, it would militate against theuse of concrete

Mr. WattB. for dam-walls above the ground, which many engineers were pemuaded would never supersede earthen embankments. By the use of well- seasoned cement and clean stone, carefully mixed with a plentiful supply of sand and water, cracks should not appear in setting. He thought the cracks referred to by the Author were caused byfailure to secure proper homogeneity in mixing and placing the matrix in position. Horizontal and vertical cracks suggested that overheating

' or over-liming must be the cause, especially in the horizontal openings, which were due to a force acting at right angles to the vertical, a condition that should not exist in a carefully-prepared and homogeneous material. It seemed very objectionable to allow the flood- water from a large drainage-area to rise and overflow the dam-wall whilst it was being built, and also to expose the lower outlet-pipes tothe merciless action of thewater, carrying before it, and in suspension, various debris. Grates or screens placed over the outlets should be kept under observation during the time the works were in course of construction to Bee that they did not get choked, and it was unwise to fix valves to remainsubmerged. The overflow, or spillway, in one of the dams seemed much too small and should be widened to fullytwice its presentwidth to be safe in time of a maximum flood. To allow a depth of 4 feet 6 inches to flow over a weir-crest with a dam-wallno more than 7 feethigher than the normal top water-level was highly dangerous. With a full reservoir and a gale blowing, the waves had great power owing to the long stretch available in which to form. This danger was increased by the possibility of ice packing in front of the spillway, which human efforts were helpless to keep clear in stormy weather. The Author's designs had been carefully prepared mathematically, but some of the sections seemed toogracefully slender toresist the ravages of time,the force of waves, andthe packing up of ice in front. Stability and utility in all reservoir-dams should be aimed at, to present lines of beauty and repose and to afford peace of mind and safetyfor all time. Saving material in a structure should be a secondary consideration, when it was known where to place it in order to resist the forces to be overcome. xr.wegmann. Mr. E. WEGNAKNobserved thatthe Paper contained some veryinteresting points relating to the design of masonry dams. Until recentyears the generalpractice had been to give curveddams a gravity profile, andto consider the arch actionresulting from the curving of the plansimply as an additional factor of safety. According to the Author, however, the curved dams built across narrow valleys in New South Wales had been designed more logically, by considering them to be sections of

rigid cylinders subject to exterior water-pressure. That the stabilityMI-. Wegmann. of such dams could be safely left to arch action was proved by the well-known cases of the Zola dam, built in France about1843, and of the BearValley dam, built in California in 1884. Both of these dams would fail if their stability had to depend only on the resistance to overturning offeredby their weight.The latter dam had probably thethinnest profile everadopted for sucha structure.' The engineer in charge of the work, Mr. F. E. Brown, made this bold design, as the available financid means were very restricted, andas all the cement, tools and supplieshad to behauled for

Pig. 25.

Scale I Inch = 100 Feet FEET SO 25 0 100 FEET LLyLLyIILL___I PATHPINDERDAM. about 70 miles over rough mountain roads to the site of the dam. The Author was quite correct in recommending that the plans of dams should be curved only in cases of narrow valleys. Where the width of the valley was considerable, greater strains would result from arch action than would occur in straight dams, and although it had been claimed that with a curved plan the tendency of the pressure of the water would be to close the joints in the masonry and to prevent cracks, there was not enough advantage in this to compensate for the larger amount of material required in a curved

Air. J\7'egmann. dam. Under the direction of the United StatesReclamation Service some high, curved masonry dams were being built across narrow canyons inthe western States.The most important of these structures were:-TheRoosevelt dam in Arizona, which would have a maximum height of 284 feetfrom the foundation to the top of theparapet. Its length would be 210 feet at mean low water and 780 feet at the crest of the clam, and its width at the

Fig. 26.

! ! I

Scale I Inch = 100 Feet F€ETlOO 75 so 25 0 100 FEET L,,.,l.. ,/....I -1 SHOSHONEDAM.

base and at the roadway wouldbe, respectively, 170 feetand 16 feet. In plan the dam was curved to a radius of about 400 feet. The Pathfinder dam (Fig. 25),in Wyoming, would have a maximum height of about 210 feet above the foundation. The canyon across which it was built had a width of 60 feet at the bottom and of 160 feet at the level of the top of the dam; the plan of the dam was curved, the radius of the centre-lineof the profile being 150 feet. The Shoshone dam (Fig. 26), in Wyoming, was to have an extreme

height of 310 feetabove the foundation,The canyon in which Nr. Wegmann this dam was located was 70 feet wide at the bottom and 200 feet wide at the crest of the dam, and the radius was 150 feet as in the last-mentioned example. While the profiles of the curved dams mentioned inthe Paper weredesigned according tothe simple formula given, a ra,ther more complicated method had been used in designing the Pathfinder and the Shoshone dams.’ The second point of interest in the Paper was the adoption of considerably higher limitsof unit stress in themasonry than had been used hitherto. In the design of thefirst dam having a scientific profile (the Furensdam, built in France in1862-66), the pressures in the masonry and on the foundation were limited to 6 kilograms per squarecentimetre (5.5tons persquare foot). Professor Rankine recommended in his ‘‘ Report on the Design and Construction of Masonry Dams,”2that thelimitof vertical pressureshould be 15,625lbs. per square foot at the down-stream face of the dam, and 20,000 lbs. at the up-stream face. Since that Report was written, engineers had been gradually increasing the limits of safe pressure for masonry dams. When Mr. Wegmann was engaged, in 1884, in designing the profile of a dam 275 to 300 feet high, which was to be constructed across the Croton Valley, for the water-supply of New York, he was obliged to adopt higher limitsof safe pressure onthe masonry than had been used up till then, as with the customarylimits the base of the dam would have become abnormally wide, and the down-stream face would have been too sloping.Under thedirection of the chief engineer in charge of the work-the late Mr. Alphonse Fteley-he based the profile of the dam ona limit of pressure of 32,000 lbs. (14.3 tons) per square foot. Although this limit was almost double what had been allowed up to that time, it was known that the maximum pressures in the oldest masonry dam in existence-the Almanza dam, built in Spaintowards the end of the sixteenth century-amounted to 14 tons per square foot. The dam across the Croton River was built in 1892 to 1907, and was known as the New Croton dam. It had a maximum height of 297 feet above the foundation, and a maximum width at the base of 206 feet. No bad effect had thus far resulted from the pressure of 16 tons per square foot, which existed in the masonrynear its foundation. In the dams in New South %-ales even higher pressures (12 tons to 25 tons per square foot) appeared to have been permitted,and there was no reason whygood masonryshould not sustain safely suchpressures. With reference

~ __~ Engineering News, vol. liv, p. 141. a In Major H. Tulloch’s Report on “ The Water-Supplyof Bombay,” Appendix D, p. 226, London, 1872 : also The Engineer, vol. xxxiii (1872), p. 1.

Mr. Wegmann. to the kind of masonry which was adopted for many of the dams mentioned in the Paper, namely, concrete in which large stones or boulders were embedded, he would say that this class of masonry had been very successfully used in many of the dams built recently inthe United States, beingknown thereas cyclopean masonry. In all of these cases, however, the stones placed in the concrete were handled by derricks or cableways, and measured usually 3 cubic yard to 4 cubicyards. As the result of usingsuch large stones, only about one-half of the structure consisted of concrete. This kind of masonry couldbe laidvery rapidly, as the concrete was usually mixed very wet, and the large stones were dropped into it. Cracks caused by changes in temperature, Beem to be almostunavoid- able in long masonry dams, and the Author’s suggestion to provide parting joints forallowing the cracks to form on radial lineswas good. An attempt to prevent cracking by placing steel bars in the upper part of a dam was to be made in the Croton Falls dam, which was now being built to form an additional storage-reservoir for the City of New York. In connection with concrete dams he would like to draw attention to a typeof reinforced-concrete dam, which hadbeen used in many cases in the United States. This type, which had been developed and introduced by the Ambursen Hydraulic Construction Company, of Boston, Massachusetts, consisted of piers or buttresses placed 12 to 25 feet apart, and extending under whole the depth of the dam. The piers were braced laterallyby concrete beams, which served to support the scaffolding during the construction. The piers were usually vertical at the down-stream face of the dam and sloped at the up-stream face, where they supporteda deck composed of reinforcedconcrete, which extended over thecrest of the piers. Openingsleft inthe piersrendered it possible to pass through the interior of the hollow damwhile water wasflowing over its top, anda coveredpassage-way was thus providedfor crossing theriver. In some recent cases a power-househad been con- structedin the interior of thedam. About fifty dams of this kindhad thusfar beenconstructed inthe United States. Most of them were of moderateheight (20 to 60 feet),but a dam of thiskind 136 feet high wasnow beingconstructed at Douglas, Wyoming ; anda similar structure, 80 feethigh and 2,970 feet long, would soon be built across the Savannah River, for the War Department of theUnited States. Four other similar dams, 80 to 125 feet high,had been contracted for. In conclusion, he would like to say a few words on what seemed to him to be the best profile fora masonry dam. In the profile typesdesigned by the French engineers, de Sazilly and Delocre, who were the first

to propose rational profiles formasonry dams, the principle Mr. Wegmann. followedwas to have the samepressure inthe masonry, at boththe up-stream and the down-stream faces. They called the designs theythus obtained ‘‘ profiles of equal resistance,” and foundthat with the limits of pressure they adopted(about 5.5 tonsper square foot), their profilesoffered sufficient resistance tooverturning, to sliding, andto the crushing of the masonry. Professor Rankine followed, in his report on masonry dams mentioned above, the principles laid down by the French writers, with the exception that he claimed that higher pressures could be permitted in the masonry at the up-stream face of the dam than at the down-stream face. The effect of this moditicationintroduced by Rankine was to make the up-stream face of the logarithmic protile designed by him much steeper than the corresponding faces in the French profile-types fordams. Rankine also pointed out that the lines of resistance must be kept within the middle-third of the profile, if tension in the masonry was to be avoided. It seemed to Mr. Wegmann that theprinciple just mentioned was the main considera,- tion upon which the design of the profile of a masonry dam should bebased. With the highestwater-level at thecrest of the dam, the resulting profile would be a triangle having its up-stream face vertical. For a weight of masonry of about 145 lbs. per cubic foot, the base of the triangle would be about two-thirds of its height. In order to ensure sufficient strength against shocks from waves and floating objects, an inverted triangle would have to be added at the top of the protile so as to make the top-width for ordinary cases about one-tenth of the height of the dam. This simple profile could be safely adopted for a dam 200 feet high, without causing higher pressures in the masonry than 13 5 tons per square foot, and by applying the best mathematical formulas for dams, the area of this simple-profile type could not be reduced more than about 5 per cent. He knew of no theoretical orpractical objections against makingthe up-stream face of a dam vertical, until the limit of safe pressure on masonry was reached. Afterthis point was passed, both faces of the dam would have to be sloped sufficiently to keep the pressures in the masonrywithin the adopted limits. The static pressure of water was represented graphically by a right-angled triangle, and it wouldseem to be rationalthat the profile of a masonrydam to resist the water-pressureshould, also, be a triangle. He noted, with much interest, that the Author proposed for curved dams a

See E. Wegmann, “The Design and Construction of Dams,” etc. 5th ed. New York, 1907. Plate XIV. Downloaded by [ UNIVERSITY OF EXETER] on [24/09/16]. Copyright © ICE Publishing, all rights reserved. 108 CORRESPONDENCE ON DAM-CONSTRUCTION. [Minutes of

MY. Wegmsnn. triangular profile similar to the one recommendedabove. Such a profilewould thereforeappear to offer advantageswhether a dam were built straight or curved in plan. The Author. The AUTHOR,in reply to theDiscussion and Correspondence, stated that no horizontal cracks in any of the curved walls had been observed near thebase. In contemplating the possibility of shearing at or near the base of a curved wall, mentioned by Colonel Pennycuick, it must be borne in mind that thelower portion of t.he structure was wedged in between the sides at thenarrowest part of the gorge, and, owing to the curvatureof the wall, crushing must take place before shearing.The Author, whenmaking the statement to which Mr. Cardew and Mr. Mattern had drawn attention, namely, that gravity-section walls should be built straight in plan, referred more particularly to the cases of narrow gorges, and it was in connection with these that a comparisonbetween the economy of straight, gravity section and curved thin walls was being made. In such narrow gorges he considered that no benefit commensurate with the increased cost was derived from constructing a gravity-section wall curved in plan. Before finally deciding upon the ground plan of a gravity wall to be constructed in a narrow gorge, the question of the wedging in of the base should be takeninto accountwhen the relative costs of straight and of curved plans were being considered. In such situations crushing must take place before tension could be experienced in the up-stream heel of the structure. With regard to information asked. for by Mr. Burge on the curvature provided in theBarren Jack dam now under construction, thatstructure would be 240 feet in height above foundations, and although it had been curved to a radius of 1,200 feet, it was so wedged in between the walls of the gorge at its base that tension in the up-stream heel was not possible withoutcrushing of the material inthe lower part of the wall. The configuration of the sides of the gorge at the site, however, was such thatthe quantities in the flanks of the dam had not been increased by the curvaturegiven to the wall. It might here be mentioned that the Cataract dam, which was built straight inplan, was also wedged in at the base in a rather similarmanner. The Author considered thatan examination of the longitudinalsection of the bulk of the high dam-walls built across narrow valleys, would show that the same conditions held. The probable effect of deformation or flattening of the curvature of a wall as the result of pressure, referred to by Mr. Bellet, Mr. Strange,and Mr. Martin, woulddepend upon theextent of the deformation. Informationon that point was afforded in the Correspondence by Mr. De Burgh, in the diagram covering the

behaviour of a very thin reinforced wall built for domestic-supply The Author. purposes at the Barren Jack works. An inspection of the diagram on p. 64 would showthat thedeformation practically varied uniformly from top tobase, and that the amountat any point was probably no greater than might be due to a rough setting-out of the concrete work. The temperaturesrecorded on that diagram were air-tempern- tures, and probably did not represent accurately the temperature of the wall. Theactual temperature of the wallwould, inthe Author's opinion, be more nearly represented by the temperature of the water stored behind it, as there was always a slight soakage through such thin walls, which must keep them at practically the same temperature as the stored water. The down-stream movement of this wall with a full reservoir was probably due more to reduced temperaturethan to pressure. The diagram was interestingin that the movement between the crest and the base of the wall was represented practically by a straight line, although the lengthsof the arcs of the wall at different levels did not vary uniformly. If the deformation of the wall were due only to pressure, then the diagram would certainly show the greatest deflection at a point below the crest, as suggested by Mr. Martin.The greatest deflection wss however, at the crest, Temperature records had not been kept in connection with thewalls described ; the reason being that theworks passed out of the control of the Public Works Department to thelocal bodies immediately after completion, and implicit reliance could not be placed on the results of local observations. In reply to the various queries of Mr. Bruce, the Author did not consider that the hollow dams which had met with somesuccess in America would be a suitable substitute for thecurved walls described, these hollow damsbeing more suitablefor a wider class of valley with more extensive flat land in the bottom. In narrow gorges difficulty would probably be experienced at the flanks in providing the great width of base required for the former type of structure. In connection withthe crushing-resistances of the concrete used, thestandard cement-test of the Public Works Department of New South Wales provided for the following tests:-Tensile strength of neat cement not less than 585 lbs. per square inch at 7 days, and 715 lbs. per square inch at 28 days ; for cement gauged 1 to 3 parts with sand, 165 and 250 lbs. per square inchrespectively. Compressive strength neat: 5,000 lbs. per square inch ; gauged 1 to 3, 1,750 lbs. per square inch, both at 28 days. In addition there were tests for time of setting and fineness, with hot and cold tests for soundness. It would be seen that the standardwas a high one, and probably one that would not be reached by a greatmany brands of English and Conti-

The Author. nentalmanufactured cements. It mustnot be assumed that the crushing-resistancementioned in thePaper representedany- thing near the maximum results obtained, the samples of concrete being taken at various times from the work under construction ; but in round figures it represented what it would be fair to take as an average standard for the different classes of material. As regarded the operation of thedrains left in the dam-wall, the completion of the works at the Cataract dam had been followed by a succession of dry seasons, and the water stored behind the Tvalls had not yet risen to a sufficiently high level, or been maintained for a sufficiently lengthy period atits then highest level, to enable experience to be obtained in this matter. In designing the outlet- works, provision had to be made for the passage of large bodies of flood-water during construction. This necessitated the insertion of the four lines of &-foot pipes through the body of the dam. The outlet-works were then designed with a view of utilizing these pipes for supply purposes afterwards. No doubt the volumes required to be supplied afterconstruction might havebeen passed through one pipe, but as they were in existence it wasdeemed advisable to utilizetwo of them. Mr. Bruce was inerror in puttingthe average cost of concrete at %6 10s. Od. ; the average cost for the wholework at the Cataract dam was less than 33s. Od. per cubic ynrd. With respect to scour at the toe of the walls dueto the overfalling flood-waters as suggested by Mr. Hazen, such scour had been found to occur in the softer rocks, and concrete aprons and water-cushionshad been provided in ordertocounteract it. Additional pressures were suggested by Mr. Smith as liable to be set up in the walls owing to the maximum floods passing over them ; an inspection of the cross sections of the walls illustrated in Plate 1, however, would show that in all cases the maximum depth of flood-waters that might pass over the crest of the wall had been considered in designing these cross sections. Particularly was such the case at Picton, where an overflow of 10 feet in depth had been provided for in designing the cross section of the wall.

16 March, 1909. JAMES CHARLESINGLIS, President, in the Chair. The discussion upon Mr. L. A. B. Wade’sPaper, “Concrete and Masonry Dam Construction in New South Wales,” was continued andconcluded.