THE IKKIGATION AGE. 



339 



Economy in Embankment. 



We once more emphasize the fact that the hollow core 

 wall as a whole is absolutely impervious. Its cellular con- 

 struction and heavy reinforcement will defend it against 

 cracks due to settlement or uneven pressure but assuming 

 cracks which would permit the passage of water, it is still 

 impervious as a whole, since any water passing into the 

 interior is carried away by the bottom drains. The core wall 

 itself is, therefore, effectively the dam, and the earth em- 

 bankment being merely adjuncts we can deal intelligently 

 with each prism according to its functions. 



In the first place we move the core wall, as stated, to the 

 upper edge of the bank, thus getting more material in the 

 lower prism, where being perfectly dry it is most effective. 

 The upper prism, on the contrary, has no function ex- 

 cept to support the core wall in place when the pond is 

 empty. It can, therefore, be materially reduced in section. 

 To this end the top of the slope may be dropped from 6 to 

 10 feet below the top of the core wall, and the slope instead 

 of being made 2 to 1 or ZV 2 to 1 may be reduced to say l l /> 

 to 1, varying with the angle of repose of the saturated 

 material. Fig. 14 indicates the large saving in earth which 

 will result an amount generally more than sufficient to offset 

 the extra cost of the core wall. Fig. 5 shows the economy 

 even more strikingly by contrasting the relative areas covered 

 by the upper and lower prisms. (As these sketches are 

 merely illustrative we omit all details such as berms, etc.) 



Conduits and Gate Well. 



In building an earth dam provision must be made for 

 discharging the stream at various stages during construction. 

 Our method of doing this is illustrated in Figs. 15 and 16. 

 The first step is to construct one or more reinforced concrete 

 outlet conduits, in size and number adequate to carry the 

 flood flow of the stream and located in the lowest point of 

 the stream bed. This conduit is reinforced longitudinally as 

 well as in circumference so as to withstand both internal and 

 external pressure, and prevent cracks by displacement. The 

 conduit communicates with the hollow core wall, its con- 

 tinuity being broken at that point. During construction it is 

 left open for discharging the stream ; see Fig. 15. 



When the dam is completed a service conduit of cast iron 

 or steel is laid through the concrete tunnel and supported at 

 proper intervals; see Fig. 16. It is sealed into the tunnel 

 by a water-tight concrete block at the upper end, as shown 

 in Fig. 17. In nearly all states an emergency gate is re- 

 quired in addition to the service gate. Where there is no 

 large amount of sediment, the use of a gate tower can be 

 avoided by arranging the secondary gate to operate along 

 the slope of the embankment, thus avoiding the danger from 

 ice pressure, which is always a menace to the detached gate 

 house. 



We are now at work designing a service gate and a 

 practical means of operating the same from the crest of the 

 dam without a separate gate house. We had hoped to include 

 in this circular all these details. The accumulating de- 

 mand, however, for the hollow core circular will not permit 



If the gate house were built like a well in the body of 

 the ordinary earth fill dam, that portion of the conduit up- 

 stream from the gate would always be under pressure due to 

 the head in the pond. Hence, any settlement of the em- 

 bankment would be liable to crack the conduit or open its 

 joints. This would lead to over-saturation through the body 

 of the fill, and would establish underground channels and 

 ultimately create a washout as was the case in the Empire 

 failure Fig. 8. All the convenience of the interior gate well 

 is, therefore, attained with complete avoidance of its dangers. 



By the device here shown of a conduit within and in- 

 dependent of the tunnel, not only is the possibility of cracking 

 reduced to a negligible quantity, but assuming a crack to 



Fig. 6. Altoona Dam, Showing Incipient Failure From Wave Spill. 



us longer to delay its publication, and, on the other hand, we 

 prefer to show no details except such as are thoroughly 

 worked out. In Fig. 17, therefore, we have only conven- 

 tionally indicated an emergency or secondary gate. 



The main service gate is in the bottom of the hollow 

 core wall which therefore supersedes the usual costly gate 

 house, and leaves this item as a further credit to offset the 

 cost of the core wall. 



Fig. 5. Pplan View Illustrating a Single Trunk Drain with Laterals as 

 Applied in a Short Dam. In Long Dams the Trunk Drains Are 

 in a Multiple, Without Laterals. 



occur, it could affect only the outer tunnel or if we push 

 the assumption still further and admit an open joint or crack 

 in the service conduit itself, any discharge of water is never- 

 theless immediately drained away by the main tunnel assisted 

 by the various drains running from the hollow core wall. 

 The possibility of saturation from this source is, therefore, 

 entirely eliminated and a material economy in cost secured. 



Protection Against Ice. 



In cold climates embankments are often injured by the 

 thrust of ice either during formation or by the force of 

 current or wind pressure. The usual rip-rap with which the 

 face of the slope is paved is not calculated to afford the best 

 protection. Its surfaces are too rough and the unit masses too 

 small, hence, ice has a better chance to get a 

 mm^^gm hold on it. We prefer to pave our slopes with 

 concrete slabs in large masses adequately re- 

 inforced, having smooth surfaces and more 

 or less joined together. Saturation from the 

 pond is thereby reduced to a minimum and 

 danger from ice practically eliminated. 



At the moment of writing we have under 

 advisement an improvement in the paving of 

 earth slopes, which, if satisfactorily worked 

 out, will appear in our advertisements and in 

 a future edition of this circular. 

 Wind Protection. 



We have already shown in Fig. 6 one in- 

 stance of incipient destruction even on a very 

 small reservoir. Fig. 7 is a photograph of the 

 Lower Latham dam in Colorado near 

 Greeley. A severe wind storm in April, 1912, 

 acting on a reservoir surface of considerable extent set up 

 a wave action which very nearly swept away the entire 

 crest of the dam. Heroic work on the part of the com- 

 munity enabled them to save the dam from actual disaster 

 by the use of sand bags. 



Plate B illustrates the protection afforded by the hol- 

 low wall and pavement. 



(Concluded in August issue.) 



