338 



THE IRRIGATION AGE. 



a water barrier which will absolutely we say absolutely 

 prevent any degree of saturation either from the pond or up 

 through the foundations, thus maintaining the lower prism 

 in its integrity of cohesion and effectiveness of weight. Such 

 is the hollow core wall. 



The Hollow Core Wall. 



The hollow core wall is built of reinforced concrete, 

 using a system of slip forms which admit of rapid and 

 economical construction. Its preferred location is not in the 

 center of the dam, but substantially at the upstream edge of 

 the embankment : See Fig. 1. This throws more of the 



Structurally, the hollow core wall consists of two corn- 

 material into the lower Hry prism, where it will do the most 



Fig. 2. Composite Dam with Hollow Core Wall. 



good. Structurally, the hollow core wall consists of two com- 

 paratively thin parallel walls spaced by ribs which are mono- 

 lithic with the whole structure and capable of withstanding 

 the external collapsing pressure. The upstream wall is keyed 

 into the impervious sub-structure, whether of rock, clay, hard- 

 pan, etc., thereby effectually intercepting the "creep" of water 

 along the junction plane. The downstream wall need not be 

 keyed in at all or at most but slightly. If deep-lying, water- 

 bearing seams are suspected they may be reached and tapped 

 off by a series of drill holes along the center line of the 

 core wall. 



Drains of large capacity are led from the core wall at 

 frequent intervals out through the lower embankment, thereby 

 discharging all collected water away 

 from and below the dam. Small lateral 

 drains under the lower prism may lead 

 into the main drains, thus effectively re- 

 moving all possibility of saturation from 

 the under footing of the lower fill ; see 

 Fig. 3. As these drains are located 

 wholly below the sealing core wall of 

 the dam and afford a sure means of re- 

 lieving any leakage accumulating in the 

 core wall chamber, it is evident that 

 they entirely avoid the otherwise ob- 

 jection of forming a "lead" for water to 

 follow along their outer surfaces. 



It is obvious, therefore, that any water which may find 

 its way through the sub-material or through any joints in the 

 core wall or from any source whatever is absolutely inter- 

 cepted by the hollow wall and drained away. For the first 

 time, therefore, ir<? have secured the essential of an abso- 

 lutely dry earth prism for the downstream section of the 

 dam. 



Moreover, the hollow core wall penetrates the heart of 

 the whole mass. It may be made of any width between walls 



located and if possible closed or if not closed we at least 

 make certain that it is powerless for harm. In cost, the 

 hollow core wall as a unit compares favorably with the cost 

 of a solid core, but there are other features wherein the 

 hollow core wall operates to reduce the cost of the dam 

 as a ii-hole below that of a dam with a solid core wall or 

 even, with a clay puddle. This will appear later. 



Crest Protection. 



All earth fill dams should have a crest of indestructible 

 material to prevent wave wash or even the wash of spray 

 which can collect into a runnel and ultimately enlarge into 

 a wash out. Such a condition in its incipient stages is 

 shown in Fig. 6 which is from a photograph of the little 

 earth dam at the famous Horseshoe Bend on the Pennsyl- 

 vania Railroad near Altoona. This dam 

 is so located that it is subject to violent 

 wind gusts coming down through a 

 gorge in the mountains. A few years 

 ago such a gust pushed the water to 

 the lower end of the pond and actually 

 blew it over the crest of the dam. The 

 result was the beginning of a severe 

 wash as shown in the view and had the 

 pon-1 been larger there is no doubt that 

 the destruction of the dam would have 

 followed. 



Now in many locations the slope 

 of the land is such that the dams are 



necessarily in the end of the valleys opposite the prevailing 

 heavy winds. Hence, a wave run is set up which extends 

 its full force on the dam and is a serious menace. Fig. 7 

 is a striking photograph of the Lower Latham Reservoir 

 near Greeley. Colo., and will be referred to later. 



We effectually meet this condition, as illustrated in Plate 

 B, by carrying the downstream wall of the hollow core up 

 above the level of the embankment, thus forming a parapet 

 and effectually intercepting any wash either as spray or direct 

 wave spill. It will even take care within limits, of an acci- 

 dental surcharge on the dam due to extraordinary floods or 

 carelessness in handling the waste gates. Such a device as 

 the hollow core wall at Safety Crest would have saved many 



Fig. 4. Line of Saturation with Clay Puddle Wall. 



without material increase of cost. Its trussed or cellular 

 structure gives it great inherent stiffness. It is open to day- 

 light from above and is accessible by permanent ladderways 

 of iron rungs at frequent intervals. Openings through the 

 partition walls at various levels give access to every foot of 

 both surfaces. An unsuspected leak may, therefore, be 



Fig. 3. Illustrating Line of Saturation; No Core Wall. 



disasters which, like that at Johnstown, Pa., have gone down 

 into history. Moreover, such a wall is embedded in the 

 heart of the fill so that, unlike a pavement, water cannot 

 work around and under it. The extension of a concrete facing 

 vertically upward to form a parapet wall, while common 

 practice, makes the stability of the parapet dependent on 

 that of the facing, whereas when the parapet and core wall are 

 monolithic no such weakness exists. 



What such a protective device as the hollow core wall 

 means is best illustrated without com- 

 ment by object lessons. Figs. 8 to 13 

 inclusive are disasters to earth dams in 

 the state of Colorado alone and in a 

 period of only two years, namely, 1909 

 and 1910. Every one of these disasters 

 would have been absolutely prevented 

 with such a form of construction as we 

 specify. Any engineer interested in the 

 detail of these disasters can find same 

 fully described in the fifteenth biennial 

 report of the State Engineer of Col- 

 orado for these years. This portion of 



the report covers some 30 pages, and to quote freely from 

 the recommendation of the State Engineer as to how the 

 dams should have been built or how they should be repaired 

 would read almost like a specification of a hollow core wall 

 dam. 



