THE IKRIGATION AGE. 



337 



EARTH FILL DAMS 



The ideal earth fill dam is an immovable mass overlaid 

 with an impermeable face. If this ideal condition could be 

 realized the entire mass would remain free from saturation, 

 it would retain its effective dry weight, and its normal 

 cohesion would be unimpaired. Unfortunately, the ideal con- 

 dition cannot be even approximated. We must, therefore, 

 search for a practical solution of the difficulty. 



There are many high authorities on earth fill dams who 

 contend that the embankment itself is the dam, and that to 

 bisect it vertically by any form of core wall introduces a line 

 of cleavage at the vital point of the dam, to its detriment 

 rather than to its advantage. But there are two sides to this 

 quesion as to all others. 



Nor must we overlook the importance of an impenetrable 

 barrier of some sort to prevent the boring of muskrats, craw- 

 fish, etc. All engineers seem- to agree upon the value of this 

 function of a core wall if there were no other. 



PLATE A. 



Typical Section of Earth rm i^am with Hollow Core Wall. 



Now let us utterly divest our minds of all preconceived 

 ideas and try a fresh line of reasoning. 



The thesis is the core wall, not the embankment, is the 

 dam therefore it must be (1) initially water tight, (2) not 

 subject to accidental cracks, (3) indifferent to them if formed, 

 and (4) permanent in position. 



The conception of an earth fill dam, 

 therefore, might pass through three 

 stages. 



The first stage. Referring to Fig. 1, 

 if the care wall standing alone were the 

 dam, it would comply only with the 

 first and third conditions. That is, it is 

 initially water-tight, and by making it 

 hollow and providing exits for the water 

 it is indifferent to chance cracks but it 

 is not stable. 



The second stage. We will now 

 support the dam by an earth embank- 

 ment. A, below it, of sufficient mass 



and consistency to prevent any deflection or movement due 

 to hydrostatic pressure from the pond. The fourth condi- 



1 he third stage. Therefore, we back it up on the up- 

 stream side as well by an earth fill, B, so that it may be 

 stable during construction and whenever the pond is drawn 

 down. 



For convenience let us speak of B as the upper prism 

 and A as the lower prism. We now see clearly that the 

 functions of two prisms of the dam are wholly dissimilar. 



The lower prism must have mass, must be maintained 

 dry and must be free from settlement. 



The upper prism which is now only a subordinate part 

 of the dam need have mass only in proportion to its duty 

 as a backing for the core wall, and may be wholly indifferent 

 to saturation or permeability. In fact, there is no reason why 

 the upper prism of the dam should not now be built of 

 gravel, boulder or quarry stone without any attempt to 

 secure impermeability see Fig 2. This view of the case 

 becomes entirely practical when the amount of earth avail- 

 able is only sufficient for the lower prism. Since we cannot 

 prevent loss of weight by submergence anyway, whatever 

 the material, the question of its porosity becomes indifferent. 



Control of Saturation. 



Of prime importance in an earth fill dam is the control 

 of saturation within its mass. Lines of 

 saturation are heretofore inevitable in earth 

 fill dams, being caused by percolation of 

 water from the reservoir and by subsatura- 

 tion through the foundations. 



Fig. 3 illustrates the ordinary satura- 

 tion of an earth fill dam without a core 

 wall. The only part of the dam of full 

 efficiency as to weight is that shown in the 

 light shading, and it will be noted that 

 I even this rests upon the unstable saturated 

 mass below it. The line of saturation 

 -. varies considerably according to material, 

 but in some degree it is always present. 



If a clay puddle wall is used as in 

 v Fig. 4, the situation is still not materially 

 changed if there is a possibility of sub- 

 saturation through the base. This particular 

 sketch is from an actual example in which 

 the line of saturation in the lower prism 

 was determined by test. Saturation, there- 

 fore, not only deprives the fill of its full 

 effective weight, since all the material be- 

 low the line of saturation is virtually sub- 

 merged, but it introduces instability into 

 the base of the dam itself. In other words, 

 as to stability the ordinary earth fill dam 

 is upside down. 



Clay puddle wall or solid concrete core 

 walls, etc., have been and still are used 

 and unquestionably serve a certain useful, 

 purpose. But a clay puddle wall has no 

 inherent stability, and is not impervious to boring animals 

 or immune against shrinkage and the concrete wall if 

 massive is very costly, and if light is liable to destructive 

 distortion. 



The saturated material in the dam loses effective weight 

 in proportion to its buoyancy. The degree varies with the 



Fig. 1. Section of Earth Fill Dam with Hollow Core Wall. 



nature of the material, but some loss always exists. Thus, 

 if the material were open gravel, weighing when dry say 120 



tion, namely stability under pressure is now fulfilled and the pounds per cubic foot, it would weigh when submerged only 

 second condition partially so. Unfortunately, with the pond 4 pounds per cubic foot, owing to its displacement and 

 empty the dam would be unstable from the thrust of the allowing 40 per cent for voids and similarly with other 



earth embankment, and the core wall would tip over backward. 

 ~~^rtesy Ambursen Hydraulic Construction Company, Boston, Mass. 



material. 



Our thesis, therefore, makes it incumbent to interpose 



