THE IERIGATION AGE. 



395 



thus avoiding standing water in the reservoir during con- 

 struction the drainage of the sluicing water from the upper 

 fill is greatly accelerated, due to the increased head towards 

 the upper toe. 



The next step is to begin the hollow core wall which is 

 carried down to and sealed into the foundations underlying 

 the dam. Sheet piling will be used below the core wall if 

 the supporting foundation is porous. All the remarks which 

 have heretofore been made with regard to the effect of the 

 hollow core wall in maintaining a dry lower prism in an 

 earth fill apply with equal force in hydraulic fill. 



The hollow core wall is built up with the rise of the fill 

 itself, being kept just above the level of the summit pond. 

 By this means temperature cracks are avoided as the core 

 wall is maintained by the fill at a substantially uniform 

 temperature. No expansion joints are therefore needed. 



The summit pool on the crest of the fill is retained by 

 hand-made levees on the outer slope and by the walls of the 

 core on the inside. The pool may be from 1 to 5 feet in 

 depth, depending upon the rapidity with which the sluicing 

 water drains away. It is generally found, even with sluiced 

 material containing a high percentage of clay, that in depths 

 exceeding 5 feet the deposited material will solidify and 

 assume a consistency of cheese from which the suspended 

 water will ooze and sweat until the material passes slowly 

 through a moist and semi-plastic condition into its final set. 



In practice the idea of the hydraulic fill has heretofore 

 been to create an impervious deposit of clay within the heart 

 of the fill, overlaid and supported on both slopes by a rela- 

 tively heavy and porous fill. The first function of a support- 

 ing porous fill is to assist the drainage from the summit pool, 

 and the second is to sustain the central body of the dam when 

 completed. Box drains of some form have 

 been frequently used to assist in the drainage, 

 but these have proven uncontrollable and are 

 a source of much annoyance. The hollow core 

 wall, on the other hand, gives wide range and 

 absolute certainty of control. 



The placing of material by hydraulic sluic- 

 ing involves what is known as "water sorting," 

 and this results in distinct lines of approxi- 

 mately horizontal stratification. When there 

 is no core wall it is necessary to break up the 

 continuity of these various strata, otherwise 

 they form lines of percolation through which 

 the water from the reservoir may pass with 

 more or less freedom, thus tending to keep 

 the embankment in a state of constant super- 

 saturation, even if not resulting in actual dam- 

 age. To prevent this it has been customary 

 to force boards edgewise into the soft ma- 

 terial, and then withdrawing same to allow the 

 sediment to fill into the voids, thus intercepting 

 the planes of stratification. 



The hollow core wall introduces altogether a different 

 condition. Its hollow chamber absolutely insulates the lower 

 prism from any percolation of reservoir water through the 

 upper prism. Hence, the planes of stratification which would 

 be a serious menace in the ordinary dam now become a dis- 

 tinct advantage, serving as a medium for draining both 

 towards the outer slope and towards the core wall ; see Fig. 

 39. So far then from attempting to prevent stratification it 

 may, n-iih certain classes of material, become an advantage 

 to produce stratification. 



Moreover, drainage from the upper prism should be 

 facilitated by laying a course of broken stone against the out- 

 side upstream face of the core wall as shown in Fig. 19 and 

 faintly in Plate C. This affords every facility for the drain- 

 age water to reach the various gates at various levels, and 

 the gates themselves are controlled at will from the interior 

 of the core wall. 



The gates are of the simplest possible design, since they 

 are to be used only during construction. They are preferably 

 a cast iron flanged thimble set in the concrete of the wall 

 with a covering plate in which is cast a small auxiliary 

 thimble with its independent cover. 



Another form is a smaller cast iron thimble with a coarse 

 thread on the inside into which is lightly screwed a hard 

 wood plug. 



Incidentally, the numerous points of drainage act upon 

 smaller bodies of material. The whole mass, therefore, much 

 more quickly reaches its final set than is the case when the 

 drainage of the mass as a whole cannot be completed until 

 the sluicing is discontinued. 



Fig. 12. Turkey Creek Break, July 15, 1910. 



Reduction of Cost Due to Hollow Core Wall. 



As in the earth fill dam the hollow core 

 wall pays for itself in the actual first cost of 

 construction. This arises from the fact that 

 the rapid drainage permits of steeper slopes 

 instead of slopes of i in 3 to 1 in 5, as is 

 necessary under other methods. The rapid 

 drainage permits of substantially the same 

 slope as in an ordinary earth fill dam, say 

 from 1 in V/2 to 1 in 2}4. The reduction in 

 the mass of material will, in most cases, 'offset 

 the cost of the hollow core wall. 



Send $1.00 for 

 The Irrigation Age 



one year and 

 The Primer of Irrigation 



Fig. 10. Telluride Power Co. Break, October 2, 1909. 



