146 



THE IRRIGATION AGE. 



quantity of gravel mixed with clay and extending 

 for a considerable distance upstream. This adds 

 to the strength of the dam by reason of its extra 

 weight on top of same and also by reason of its 

 imperviousness to seepage. This last is one of the 

 most important features in any type of dam con- 

 struction, for as soon as the water percolates under 

 the base of the dam it lessens the weight thereon 

 by just 62 l / 2 pounds per cubic foot, thus increasing 

 the force tending to overturn or slide the dam on 

 its base. 



The dam described above is used successfully 

 where it has not to extend for any great length nor 

 has to be very high. Other types of timber dams 

 where the height is an important factor to consider 

 are constructed either by cribbing after obtaining 

 a satisfactory foundation on rock and filling against 

 the upstream side of the dam as already described, 

 or by constructing a triangular structure. 



Where the dam assumes any considerable 

 height or length it is not conceded to be economy 

 to build this type inasmuch as it requires so much 

 necessary cross-bracing to insure complete rigidity 

 and it is so hard to obtain a satisfactory foundation 

 that unless the cost of labor and timber is very low 

 it is cheaper and more satisfactory to build a con- 

 crete or masonry dam. 



These dams are generally used only where 

 there is abundance of adjacent timber and can be 

 constructed in shallow water with labor at $1.50 

 per day, timber not exceeding $20 per thousand, 

 stone $1.00 per cubic yard, and gravel 50 cents per 

 cubic yard, iron for bolts 2 cents per pound com- 

 plete at from $3.25 to $3.50 per cubic yard of crib. 



There are innumerable types of concrete dams 

 which have proven to be entirely satisfactory. 

 Some of the various types are briefly as follows : 

 Buttress type, arch type, gravity type, the hollow 

 construction type, and revolving type. 



The gravity type of dam is one which resists 

 sliding and overturning by the action of its weight 

 alone, the center line in plan being straight and 

 perpendicular to the general direction of the flow 

 of the impounded waters. 



An arch dam is one that is built so that the cen- 

 ter line is curved with a concave surface upstream. 

 This type is securely mortised into the rock at each 

 side of the stream and is only to be used where the 

 sides of the stream are more or less perpendicular 

 and are composed of solid rock. It will be readily 

 seen that this dam can necessarily be of much 

 lighter construction than the gravity dam, inasmuch 

 as the force of the water against it brings into 

 action the same resisting forces that an arch has 

 and transmits same directly to the abutments at 

 each side. This of course is an assistance to gravity 

 in resisting the tendency to slide or overturn. It 

 has a much prettier appearance than the straight 

 gravity type and while the materials used in its 

 construction are less than those in the gravity type, 

 yet the difficulties attending this construction off- 

 set more or less the saving in the material. 



Naturally when the two types, the gravity and 

 arch, are combined, although the cost is increased, 

 yet the structure is much stronger than either one 

 of the above types and such insurance of security 

 is sometimes to be desired regardless of the increase 

 in cost. 



I will not attempt to discuss in detail either the 

 hollow or rolling type of dam inasmuch as they are 

 ordinarily only used on very large work, particu- 

 larly for hydro-electric development and where it 

 becomes necessary to maintain a more or less uni- 

 form water level. The features of importance in 

 either of these are similar to those already men- 

 tioned. 



The most common cause of failure of a dam is 

 where a defective foundation has allowed the water 

 to percolate under the dam. Thus it becomes abso- 

 lutely important to have the foundation on bed 

 rock and a core wall of sufficient size as to success- 

 fully intercept any seepage that might occur. On 

 very large dams, as, for example, the Ashokan dam 

 in the lower Catskill region, which was completed 

 in 1912, the height of which is 220 feet and the 

 width of base 190 feet with a top width of 26, there 

 has been placed in this dam two openings which 

 are called inspection galleries, one near the top of 

 the dam and the other close to the bottom. By a 

 system of drains any seepage which occurs can lie 

 observed at once and the proper steps taken to 

 handle it in the event of its becoming necessary to 

 'do so. 



Another feature in dam construction which is 

 important to observe is the construction of the 

 downstream side of the structure. On the concrete 

 types this side is usually constructed on a curve 

 which allows the water to flow over the surface of 

 the dam rather than falling sheer. This is for the 

 reason that it is desirable to prevent the water's 

 washing away the material in the river bed at the 

 heel of the dam, thus weakening the dam at that 

 point. 



This is easily remedied by designing the dam 

 so that the water either strikes the side of the dam 

 about half way down or follows it the entire way. 

 Another important consideration in the event that 

 the water falls from the top of the dam to the bot- 

 tom without touching the face is that there is a 

 partial vacuum formed in that space which very 

 materially increases the pressure on the back of the 

 dam. This of course is something that is not to be 

 desired as it decreases proportionately the factor of 

 safety considered in the design. 



Nothing has been said so far about rubble dams 

 or cyclopean masonry having ashlar facing. A 

 rubble dam is a masonry dam with very heavy ce- 

 ment joints ; cyclopean masonry consists of large 

 stone imbedded in concrete ; ashlar facing consists 

 of cut masonry laid with cement joints on both 

 sides of the dam. 



As before mentioned the costs of these types 

 vary with the locality and the size, difficulties at- 

 tending the foundation and construction, and also 

 the difficulties of handling the water during con- 

 struction. A fair average cost per cubic yard taken 

 from a great many dams situated all over the coun- 

 try is as follows: Ashlar facing, first class, $20: 

 second class and backing, $12 to $14; rubble, $8.50 

 to $10; concrete, $6.50 to $10. 



There are seldom two spillways constructed 

 along the same line, owing to the fact that local 

 conditions govern almost entirely the construction 

 of this part of the project. The essential features 

 to be considered in designing spillways are to keep 



