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HORIZONTAL PRESSURES ON VERTICAL JOINTS
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See also:HORIZONTAL PRESSURES ON See also:VERTICAL See also:JOINTS .
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where P is the horizontal pressure of the See also:earth against the See also:wall limes have often been used in the past. Any See also: Both the Neuadd and the See also:Fisher See also:Tarn dams arelargely dependent upon the support of earthen embankments with much See also:economy and with perfectly satisfactory results. In the construction of the Vyrnwy masonry dam See also:Portland See also:cement concrete was used in the joints. When more than six months old, 9 in. cubes of this material never failed under See also:compression below III tons per sq. ft. with an average of 167 tons; and the mean resistance of all the blocks tested between two and three years after moulding exceeded 215 tons per sq. ft., while blocks cut from the concrete of the dam gave from 181 to 32o tons per sq. ft. It has been shown that the best See also:hydraulic See also:lime, or volcanic puzzuolana and lime, if properly ground while slaking, and otherwise treated in the best-known manner, as well as some of the so-called natural (calcareous) cements, will yield results certainly not inferior to those obtained from Portland cement. The only objection that can in any case be urged against most of the natural products is that a longer time is required for induration; but in the case of masonry dams sufficient time necessarily passes before any load, beyond that of the very gradually increasing masonry, is brought upon the structure. The result of using properly treated natural limes is not to be judged from the careless manner in which such The actual construction of successful masonry dams has varied from the roughest See also:rubble masonry to ashlar work. It Materials. is probable, however, that, all things considered, See also:random rubble in which the flattest See also:side of • each See also:block of stone is dressed to a fairly See also:uniform surface, so that it may be bedded as it were in a See also:tray of See also:mortar, secures the nearest approach to uniform elasticity. Such stones may be of any See also:size subject to each of them covering only a small proportion of the width of the structure (in the Vyrnwy dam they reached 8 or 10 tons each), and the spaces between them, where large enough, must be similarly built in with smaller, but always the largest possible, stones; spaces too small for this treatment must be filled and rammed with concrete. All stones must be beaten down into their beds until the mortar squeezes up into the joints around them. The faces of the work may be of squared masonry, thoroughly tied into the hearting; but, in view of the expansion and contraction mentioned below, it is better that the See also:face masonry should not be coursed. Generally speaking, in the excavations for the See also:foundations springs are met with; these may be only sufficient to indicate a continuous dampness at certain beds or joints of the rock, but all such places should be connected by See also:relief drains carried to visible points at the back of the darn. It should be impossible, in See also:short, for any See also:part of the rock beneath the dam to become charged with water under pressure, either directly from the water in the reservoir or from higher places in the See also:mountain sides. For similar reasons care must be taken to ensure that the structure of the water face of the dam shall be the least permeable of any part. In the best examples this has been secured by bedding the stones near to the water face in somewhat finer mortar than the See also:rest, and sometimes also by placing pads to fill the joints for several inches from the water face, so that the mortar was kept away from the face and was well held up to its work. On the removal of the pads, or the cutting out of the face of the mortar where pads were not used, the vacant See also:joint was gradually filled with almost dry mortar, a See also:hammer and caulking See also:tool being used to consolidate it. By these means See also:practical impermeability was obtained. If the pores of the water face are thus rendered extremely See also:fine, the surface water, carrying more or less fine detritus and organic See also:matter, will soon See also:close them entirely and assist in making that face the least permeable portion of the structure.
But no care in construction can prevent the compression of the See also:mass as the superincumbent weight comes upon it. Any given yard of height measured during construction, or at any time after construction, will be less than a yard when additional weight has been placed upon it; hence the ends of such dams placed against rock surfaces must move with respect to those surfaces when the superincumbent load comes upon them. This See also:action is obviously much reduced where the rock sides of the valley rise slowly; but in cases where the rock is very steep, the safest course is to face the facts, and not to depend for water-tightness upon the cementing of the masonry to the rock, but rather to provide a vertical See also: The first masonry dam of importance constructed in Great See also:Britain was that upon the See also:river Vyrnwy, a tributary of the See also:Severn, in connexion with the See also:Liverpool water-See also:supply (See also:Plate I.). Its height, subject to water-pressure, is about 134 ft., and a See also:carriage-way is carried on See also:arches at an See also:elevation of about 18 ft. higher. As this dam is about 1180 ft. in length from rockito rock, it receives practic-ally no support from the sides of the valley. Its construction See also:drew much See also:attention to the subject of masonry dams in See also:England—where the earthwork dam, with a wall of puddled clay, had hitherto been almost universal—and since its completion nine more masonry dams of smaller size have been completed. In connexion with the Elan and Claerwen See also:works, in See also:Mid-See also:Wales, for the supply of See also:Birmingham, six masonry dams were projected, three of which are completed, including the Caban See also:Goch dam, 590 ft. See also:long at the water level, and subject to a water-pressure of 152 ft. above the rock foundations and of 122 ft. above the river. See also:bed, and the See also:Craig-yr-alit Goch dam, subject to a See also:head of •133 ft, The latter dam is curved in See also:plan, the See also:radius being 740 ft. and the chord of the arc 515 ft. In the See also:Derwent Valley See also:scheme, in connexion with the water supplies of See also:Derby, See also:Leicester, See also:Nottingham and See also:Sheffield, six more masonry dams have received See also:parliamentary See also:sanction. Of these the highest is the See also:Hag-See also:glee, on the Ashop, a tributary of the Derwent, which will impound water to about 136 ft. above the river bed, the length from rock to rock being 98o ft.. Two of these dams are now in course of construction, one of which, the Howden, will be io8o ft. in length and will impound water to a depth of 114 ft. above the river bed. In 1892 the excavation was begun for the foundations of a masonry dam across the Croton river, in -connexion with the supply of New See also:York. The length of this dam from rock to rock at the overflow level is about 1500 ft. The water face, over the maximum depth at which that face cuts the rock foundations, is subject to a water :pressure of about 26o ft., while the height of the dam above the river bed is 163 ft. The See also:section, shown in fig. 17, has been well considered. The hearting is of rubble masonry, and the faces are coursed ashlar. -t iv N. Iale s t I ttI I e. aS - Fin. 17.-Section of Croton Dam. So-called " natural cement " has been used, except during frosty weather, when Portland cement was substituted on See also:account of its more rapid setting. An important feature in connexion with this dam is the nature of the foundation upon which it stands. Part of the rock is schist, but the greater portion See also:limestone, similar in See also:physical qualities to the Carboniferous limestone of Great Britain. The lowest part of the surface of this rock was reached after excavating through alluvial deposits to a depth of about 70 ft.; but owing to its fissured and cavernous nature it became necessary to excavate to much greater depths, reaching in places more than 120 ft. below the See also:original bottom of the valley. Great pains appear to have been taken to ascertain that the cavernous portions of the rock had been cut out and built up before the See also:building was begun. The Furens dam, already referred to as the earliest type of a scientifically designed structure of the kind, is subject to a pressure of about 166 ft. of water; the valley it crosses is only about 300 ft. wide at the water level, and the dam is curved in plan to a radius of 828 ft. Much discussion has taken See also:place as to the utility of such curvature. The See also:recent investigations already referred to indicate the desirability of curving dams in plan in See also:order to reduce the possibility of tension and infiltration of water at the upstream face. In narrow rock See also:gorges extremely interesting and complex problems See also:relating to the combined action of horizontal and vertical stresses arise. and in some such cases it is evident that much may be done by means of horizontal curvature to reduce the quantity of masonry without reduction of strength. The Bear Valley dam, California, is the most daring example in existence of the employment of the See also:arch principle. years after this, and about fifteen years after the dam was first Its height from the rock bed is 64 ft., and it is subject during floods brought into use, it overturned on its See also:outer edge, at about the level to a head of water not much less. The length of the chord of the arc indicated by the dotted See also:line just above the See also:counterfort; and there is across the valley is about 250 ft. and the radius 335 ft. The dam was no See also:good See also:reason to attribute to the movement of 1884, or to the begun in 1883, with a See also:base 20 ft. thick, narrowing to 13 ft. at a height vertical cracks it caused, any See also:influence in the overturning of 1895. of 16 ft. The cost of this thickness being regarded as too great, it Someof the worst was abruptly reduced to 8 ft. 6 in., and for the remaining 48 ft. it cracks were, in-was tapered up to a final width of about 3 ft. The masonry is de- scribed by Mr See also:Schuyler as " a rough uncut granite ashlar, with a hearting of rough rubble all laid in cement mortar and See also:gravel." This dam has been in satisfactory use since 1885, and the slight filtration through the masonry which occurred at first is said to have almost entirely ceased. In New See also:South Wales thirteen thin concrete dams, dependent upon horizontal curvature for their resistance to water pressure, have been constructed in narrow gorges at comparatively small cost to impound water for the use of villages. The depth of water varies from 18 ft. to 76 ft. and five of them have cracked vertically, owing apparently to the impossibility of the base of the dam partaking of the changes of curvature induced by changes of temperature and of moisture in the upper parts. It is stated, however, that these cracks close up and become practically water-tight as the water rises. Something has been said of the failures of earthen dams. Many masonry dams have also failed, but, speaking generally, we know less of the causes which have led to such failures. The Failures. examination of one case, however, namely, the bursting in 1895 of the Bouzey dam, near Epinal, in See also:France, by which many lives were lost, has brought out several points of great See also:interest. It is probably the only instance in which a masonry dam has slipped upon its foundations, and also the only case in which a masonry dam has actually overturned, while curiously enough there is every See also:probability that the two circumstances had no connexion with each other. A short time after the occurrence of the See also:catastrophe the dam was visited by Dr W. C. Unwin, F.R.S., and the writer, and a very careful examination of the work was made by them. Some of the blocks of rubble masonry carried down the stream weighed several See also:hundred tons. The original section of the dam is shown by the continuous thick line in fig. 18, from which it appears that the work was subject to a pressure of only about 65 ft. of water. 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