394 



THE IRRIGATION AGE. 



HYDRAULIC FILL DAMS.* 



Hydraulic Sluicing Process. 



(Continued from July issue.) 



The development of hydraulic mining has demonstrated 

 the practicability and economy of loosening and transporting 

 large masses of earth or gravel by the hydraulic sluicing 

 process. This method has been adapted to the construction 

 of large earth dams where conditions have permitted. Where 

 materials of construction are at hand and water under natural 

 or even artificial head is abundant, there is no method of con- 

 struction which can compare in economy with the hydraulic 

 sluicing process, or which can be handled with so small a 

 working force. The principal disappointments in the hydraulic 

 method have arisen from the difficulty of securing adequate 

 drainage for the sluiced material, and from the treachery 

 of the sluicing water. 



Drainage of Hydraulic Fills. 



In a hydraulic fill dam the drainage of the sluicing 

 water is, therefore, of controlling importance. To begin 

 with, the sluiced material should be such that it will not 

 retain the sluicing water for an undue time. Unless the 

 water is drained with reasonable rapidity a marked settlement 



Fig. 8. Empire Reservoir Break, August 15, 1909. 



with consequent cracks is bound to ensue when 

 the fill ultimately dries out. 



The sluicing water on the fill is main- 

 tained in a summit pool by hand-made levees. 

 It is found by experience that in varying 

 depths down to 5 feet, the material in suspen- 

 sion becomes comparatively solid and will then 

 hold its shape and consistency. The sluicing 

 water, however, must necesarily be under con- 

 stant and well distributed drainage if rapid 

 construction and solid embankments are to be 

 realized. 



A hydraulic fill dam during construction 

 usually collects a body of water in the im- 

 pounding reservoir above it, which rises sub- 

 stantially with the rise of the dam and but a 

 little below its level, thereby reducing the 

 drainage head in that direction. Assuming 

 that there is no core wall, the sluicing water 

 is, therefore, forced to pass largely through 

 the down-stream fill in order to free itself un- 

 less artificial drains in some forms are pro- 

 vided. The passage of the drainage water 

 through such a mass of material is slow and 

 hence full advantage cannot be taken of this 

 otherwise rapid method of hydraulic construc- 

 tion. 



Again the material of the fill will not take 

 its final set until the fill is complete and sluic- 

 ing stopped. The fill is, therefore saturated 

 during construction, and saturated material is always of 

 greater bulk than dry material. This fact accounts in a 

 measure for the excessive settlement in hydraulic fills. 



Drainage Through a Hollow Core Wall. 



The most practical method to secure controllable, well- 

 distributed and adequate drainage in a hydraulic fill dam is 

 by building a hollow core wall through the center of the em- 

 bankment carrying it up with the work and providing it with 

 numerous inward drainage gates of simple construction. 

 Plate C, on page 15, illustrates this, but unfortunately the re- 

 duction of the plate is so strong as to make the drainage 

 gates to the interior of the core wall very small and faint. 

 1'ig. IS shows them more plainly. 



It is evident at a glance that we can thereby accomplish 

 two things : 



First. We have provided an effectual water barrier, 

 whereby when the lower prism of the dam is once drained it 

 is forever protected against re-saturation. 



Second. The problem of drainage is entirely under con- 

 trol and can be hastened or retarded at will. Drainage head 

 is secured in two directions instead of one, i. e.. towards the 

 core and towards the toe. Fig. IS illustrates this somewhat 

 crudely by the difference in shading of the sections. The 

 material more quickly receives its final set and unexpected 

 settlement is thereby avoided. The time of construction is 

 also greatly shortened. 



Once the fill is completed the drainage gates into the 

 hollow core wall from the lower prism are left permanently 

 open, thus insuring absolute and permanent 

 dryness of the lower prism a result never be- 

 fore attained. 



Materials of Construction. 



The materials best adapted for hydraulic 

 fills are those in which the percentage of clay 

 doss not exceed one-third of the total mass. 

 Too great a burden of clay prevents adequate 

 drainage of the sluicing waters, but a certain 

 percentage of clay is of distinct advantage, 

 both as to the lubrication of transported ma- 

 terials during the sluicing process, and as to 

 the future impermeability of the fill. The best 

 material is an admixture of sand, gravel, silt 

 or clay with boulders of varying sizes. The 

 peculiar action of "water-sorting" will classify 

 and stratify these materials into varying 

 gradss in horizontal layers and will admit of 

 slopes which may occasionally range as high 

 as 1 1 / 2 to 1, although usually much flatter, de- 

 pending almost wholly upon the rapidity with 

 which the material is drained of its water. 



Fig. 9. Trout Lake Break, October 10, 1909. 



Order of Construction. 



The preferred order of construction is first to build an 

 outlet conduit as described in connection with Fig. 15. By 



*Courtesy Ambursen Hydraulic Construction Co. 



