DAMS

until quite recent times the great ma- jority of dams were constructed for the purpose of storing water to ensure regular supplics to communities for domestic purposes. the storage required to give a regulated supply and to utilise the water resources of large areas for hydroelectric power supply and irrigation on a large scale, in countries having the requisite conditions and resources, is in many cases cnormously greater than that required for water supply to the largest towns. hence, the developments of large dam structures in recent years have been extensive. : earth dams.—farth dams are largely used on account of their permanence and cheapness and convenience of construction where suitable material is available at the site. recent develop- ments relate to the method of forming the watertight core. the puddle core has been largely abandoned, even where suitable material is procurable, on account of the long time required for its proper construction. a thin watertight core wall of rein- forced concrete finds more common use now as being practicable for most sites and quicker to construct than a puddle wall. the trend of development in amcrica for the construction of large earth dams is towards the attainment of watertightness by the formation of a wide hearting of fine material bound with fine clay and silt separated and washed into the centre by hy- draulic sluicing from the earth as dumpcd in banks along the upper and lower sides of the dam. fig. 1 shows a cross section of the david bridge dam in vermont,! 200 ft. high, with an carth- works volume of 1,900,000 cu. yd., which was constructed in less than two years by this method. the upstream and downstream banks were formed by side-tipping from standard tracks working towards the centre. between the banks there was formed, to the width of the hearting, a central pool of water, in which floating craft with high pressure pumps and monitor nozzles operated to. sluice material from the inner faces of the banks down into the pond. the outer and inner banks were raised in stages as required, ‘partly by the use of trestles and partly by the use of a dragline, which formed, from the material already tipped, a raised bank to carry the track for the tipping of the next stage. . rock-fill dams.—a rock-fill dam depends for its stability on an embankment of tipped rock material and for its watertightness on a skin of conercte or reinforced concrete laid on the hand-built wwaterface and carried down in the form of a vertical cut-off wall to an impervious stratum. lack of suitable carth material and excessive cost of concrete necessitate consideration of the rock- fill type of dam. the embankment is usually foraned with rock of all sizes as blasted from the quarry, the outer face being roughly built to a plain surface with large blocks and the inner face more carefully and solidly built to a uniform surface and formed with grooves or keyways to furnish a grip for the con- crete. for important dams, special drainage arrangements are made for catching up at the back of the skin any water which 1 engineering news record (feb. 27 1924). | 808 may percolate through it, and leading it by special drains through to the downstream side so as to obviate risk of scour under the dam. the water face is made steep (1:1 or steeper) to minimise the quantity of skin concrete, the outer face being gen- erally somewhat flatter, about 4:3. the largest dam of this type yet constructed is the dix river dam in kentucky, u.s.a., built 1923~5.1 this has a height of 1400 1300 nov. 1922 >. 1200¢ qian ey 0 ~ 200 400 275 ft. and a volume of rock-fill of 1,747,000 cu. yards. the upstream slope varies from 1:1 at the top to 1-2:1 at the bottom. downstream, the slopes are 1:1 at top and 1-4:1 at the bottom. a thick layer on the upstream face was hand-built with large stones, while the rest of the material was side-tipped from wagons. the watertight skin of reinforced concrete varies in thickness from 8 in. at the top to 18 in. at the bottom. gravity concrete dams.—the tendency in gravity dams is to use the simple triangular form for even the highest dam and to use concrete to the exclusion of masonry. the triangular form gives a uniform stability condition at any horizontal section, and the maximum stresses in the material are directly proportional to the depth below t.w.l. assuming a usual inclination of the waterface, say 1:20, and concrete of normal density, the neces- sary inclination of the outer face will depend on the amount of upward water pressure on the base. if by means of effective drainage arrangements upward pressure is eliminated, an outer slope of about o-65:1 will suffice. if the conditions require an allowance of upward water pressure on the base varying from the full head at the inner face to zero at the outer face, the outer slope will require to be about o-80:1. if, in addition, it is necessary to allow for heavy ice pressure at the top of the wall, still greater slope, up to 0-85:1, will be required. these dimensions ensure that the resultant will pass within the middle third, so that there will be no tension on the concrete, and the maximum pressures at inner and outer toes when the slope is o-80:1 will be slightly less than that produced by a column of concrete the same height as the wall. if, therefore, the com- pressive stress limit is put at the moderate value of 400 ib. per ‘sq. in. the practicable height of the triangular gravity dam will be over 4oo ft. (since 1 ft. depth of concrete produces a stress of about 1 lb. per square inch). - sound construction requires a rich concrete mixture next the waterface and on the bottom to ensure watertightness and also in the regions of maximum stress at the inner and outer edges near the base in the case of high dams. the proportion of ce- ment to mixed aggregate may vary from 4 cwt. per cu. yd. for the body of the dam to 5 cwt. or more for the places of maximum water pressure and stress. the barberine dam in switzerland, completed 1925, and shown in cross section in fig. 2, illustrates the application of the triangular form, together with excellent arrangements for sealing the base by cementation through drill holes, and for providing for expansion and shrinkage in the upper part. the dam is of concrete with mixtures varying from 250 to 300 kgm. of cement per cu. metre, is without drainage arrangements and is designed for water uplift on the base varying from full-head at the inside 1 engineering news record (april 2 1925). dams to zero at the outside. the slope of the waterface is 1:20 and of the outer face o-80:1. the cross section also indicates the masonry facing of gneiss applied on the outer slope, which has a southern exposure and is subject to severe temperature variations. horizontal arch dam.—the horizontal arch dam is suitable only for narrow gorges with sound rock at the sides to resist the great thrust from the abutments, the water pressure at any 600 feet fic. 1.—cross section of the davis bridge dam. 800 horizontal plane being constant, the appropriate arch form at that plane will be a segment of a cylinder of uniform thickness. if the radius of the extrados is constant at all depths the arch thickness required will be directly proportional to the depth of water. the vertical section will then be of triangular form, and if the waterface is vertical the angle of the outer slope will vary with the radius and the working compressive stress in accordance with the formula:— 0-43r tan a — 07431 c where r =radius of extrados in feet. c =average compressive stress in |b. per square inch. the value of c for actual dams is usually between 200 and 400 ib. per sq. in., and should depend on the quality of concrete used. it should be noted that with c=300 and r=450, tan a will be=0-65, which is the slope for the minimum gravity wall. as an arch dam is necessarily longer than a gravity dam for the same site, it follows that a radius of 400 ft. is near the upper limit of the field for installation of arch dams. the greatest economy of materia] can be shown to occur in a horizontal arch of given span when it subtends an angle of 133° at its centre. if the radius is constant and the gorge has flar- ing sides the arch angle will be variable and maximum economy of material will not be attained. jorgensen’s so-called constant angle 16877 | = a pe amare se jae filling of hs, slot deferred te yee | te: sit aati | -pd- 2 bp --on bane te a lela oes gia | si eernaes ramen expansion joint | cee eee ay rere. section xx 1 (end /92 958 vio sn ce ie be ener | ape 2 ere ae eee saccoee ix-paecibop ake x bo fetches hh end 1923" g oo ee ge ec masonry face ' as he “oe — sonar a \ n see ea es ates bee: n lise eee “ty see seer oe. tet oo pee be mene em | netesm: 22 ee ee | a fs ee ee - ny oy sod alate tara serr mae “on 1-00 | spee os ri one a ee of ee ees ee 2 taree ieee seamer eerare 10235) peeves a oo gad wey eee trb [gene are en" 100" 5 z 5 = oe ar welts cour pipe . a lr cg meme te ee om * aed . en em a a oe i a 4 man hoe i eel i yy, : - j 48/0 leben lap ror 2 mgt iy 5 0 52 1 « e “ a “d a wy: e . / * drill holes for palate re rs c7 + e *® cementation fic. 2.—cross section of barberine dam, switzerland. d a m s plate opo eee oo _ rae net. - + - pas py me wre rd " 7 ie “on 7 iaine . a \ 48444608 n ye etal ‘i ni r,. ay - lls \\1 peat fic. 1 sennar (or makwar) dam on the blue nile, sudan the photograph presents a view of the dam from the east. the dam, designed as the principal unit of the gezira irrigation scheme, 1s composed of rubble masonry in mortar, continued on each side by earth embankments with heavy concrete facing walls, the total length of the entire structure being 9,925 feet. the height from the lowest point in the foundation to the top of the parapet is 130 ft., and it contains about 556,000 cu. yd. of masonry. (photo by central press.) fic. 2. wilson dam on the tennessee river, u.s.a. ___ the photograph presents a view of the dam from the north bank. the dam is a monolithic concrete structure, 4,500 ft. in length, 95 ft. high from river bed to crest, 140 ft. high from foundation to the level of the operating bridge, and 105 ft. thick at the base. it is the largest masonry dam in the world, containing about 1,353,000 cu. vd. of masonry. dancing type of arch dam has been introduced in recent years to economise in concrete, and results in variable radius of arch and warped curved surfaces of less satisfactory appearance than the cylindrical and conical surfaces of the constant radius dam. the salmon creek dam in alaska is an example of the con- stant angle arch type. it has a height of 168 ft., a maximum ra- dius at the top of 332 ft. and a thickness varying irregularly from 6 ft. at the top, to 48 ft. at the bottom. an arch dam with a height of 444 ft. and a bottom thickness of 57 ft. is projected for the narrow gorge of the drac at sautet, france, by the societe des forces motrices bonne et drac. multiple buttress dams.—in multiple buttress dams the water load may be supported and transferred to the buttresses either by reinforced flat slabs or by arches. with flat slab construction h.w. 109 007 masonry buttresses reinforced concrete arches t fy 5” centres 157 span somes 9-75 | | | | | | | s t =— = . = ve) | { | | i | | 48-60 sh p son ee goers a ce le aa : fae sn an i a i ~ v fic. 3.—cross section of the tirso dam, sardinia. a spillway section can readily be formed by providing flat slabs also on the outer face, formed to a suitable spillway curve. the junction brook main dam of the newfoundland power and paper company is of this type and carries a single line railway. the buttresses, spaced at 18 ft. centres, are of plain concrete from 16 in. to 44 in. thick. the slabs vary from 16 in. to 47 in. in thickness and are heavily reinforced as simple beams to sup- port the water pressure. flat slabs form an uneconomical type of construction for support of heavy water pressures as com- pared with arches and entail a multiplicity of buttresses with close spacing, the practicable maximum being about 20 feet. in multiple arch dams the buttress spacing may be from rg to 6o ft., depending on the general height of the dam. the highest multiple arch dam yet constructed is the tirso dam in sar- dinia,' a cross section of which is shown in fig. 3. the buttresses, spaced at so ft., centres, are of masonry with courses sloped at the outer face to suit the direction of maximum pressure, and are designed to be independently stable and safe under the max- imum loadings transmitted to them by the arches. the arches are of concrete, nearly semi-circular, lightly reinforced for tem- perature and shortening stresses, and have a thickness varying from 20 in. at top to 53 ft. at bottom. the dam provides storage for the triple purposes of power production, irrigation and river regulation, the power house being worked into the spaces between the buttresses. except in italy, where masons are plentiful, the buttresses are gencrally formed in concrete or reinforced concrete, and the arches are made monolithic with the buttresses, so that the whole structure may be reckoned on for stability. the slope line of the arches usually makes an angle with the horizontal of between 45° and 60°, the outer face of the buttresses being considerably steeper. proportions are generally so arranged that the maximum resultant pressure cuts the base of the buttresses nearer the middle than the one-third point, so that increased stability is obtained as compared with a gravity dam. another satisfactory feature is the entire avoidance of water uplift on the bases. when properly designed, this type shows marked economy of material, the volume being only about one-fourth of that of a gravity dam and the possible saving in cost from 30 to 50 %. 1 engineering (nov. 7 1924). 809 river dams.—special circumstances arise in regard to dams on large rivers, where it becomes necessary to control the flood and backing-up height above the dam, and especially where the wet season flow of a river carrying much silt has to be stored to give regulated supply during the dry season. for such cases, movable openings are necessary on a scale commensurate with the flood conditions, a series of large rectangular steel gates of stoney roller type operated by machinery being most com- monly used. automatic gates operated by the rise of the water are also used to some extent, but for the extreme case where the barrage must not be allowed to become a trap for large volumes of silt brought down during high floods, a continuous series of gates is used, extending from bank to bank and having their sills at the level of the river bed. the gates are separated by piers, usually of concrete, which must be designed to support the maximum water load from a panel of the barrage, as well as the erections and machinery for handling the gates. the vaal river barrage? is an interesting example of this type of dam. (see hydroelectric engineering.) brptiograruy.—l. r. jorgensen, ‘‘ the constant angle arch dam,” trans. amer. soc. c. e., vol. 78; ‘ design of rock-fill dams,” engineering news (march 9 1916); w. p. creager, engineering for masonry dams (1917); e. wegmann, design and construction of dams, 7th ed. (1922); c. t. johnston, the design of hac ai wl