Preliminary Study of Profile for Dam.
The preliminary calculations for designing a profile for an earth dam are simple and will here be illustrated by an example. Let us assume the following values:
a. Central height of dam, 100 ft.
b. Maximum depth of water, 90 ft., with surface 10 ft. below crest of dam.
c. Effective head, 90 ft.
d. Weight of water, 62.5 lbs. per cu. ft.
e. Weight of material, 125 lbs. per cu. ft.
f. Coefficient of friction, 1.00, or equal to the weight.
g. Factor of safety against sliding, 10.
The width corresponding to the vertical pressure of 1 ft. is,
| 62.5 × 10 | = 5 ft. |
| 125 |
FIG. 19.–GRAPHICAL INTERPRETATION OF STUDIES OF BOARD OF EXPERTS
ON THE ORIGINAL EARTH PORTION OF THE NEW CROTON DAM.
The hydrostatic pressure per square foot at 90 ft. depth is,
62.5 × 90 = 5,625 lbs.
The dam, having a factor of safety of 10, must present a resistance of,
5,625 × 10 = 56,250 lbs.,
or 28 tons per square foot.
The theoretical width of bank corresponding to 90 ft. head and a factor of 10 is shown by the dotted triangle (A-B-B) to be 450 ft., ([B, Fig. 19]) with slopes 2½ on 1.
To this must be added the width due to the height of crest above the water surface in the reservoir and the width of crest.
The former would be,
2 (2½ × 10) = 50 ft.,
and the latter by Trautwine’s rule,
| 2 + 2√ | 100 | = 22 ft., |
giving a total base width of 522 ft.
Let us now assume that the slope of saturation may be 35 ft. per 100 ft. We observe that this intersects the base 40 ft. within the outer toe of the bank slope. If the plane of saturation was 33 ft. per 100, it would just reach the outer toe. It would be advisable to enlarge this section by adding a 10-ft. berm at the 50-ft. level, having a slope not less than 3 on 1 for the up-stream face, and two 15-ft. berms on the down-stream face, having slopes 2½ on 1. The additional width of base due to these modifications in our profile amounts to 65 ft., giving a total base width of 587 ft., and increasing the factor of safety from 10 to 13. It should be remembered that if the bank becomes saturated this factor of safety may be reduced 50%, the coefficient of moist clay being 0.50.
The loss of head due to a core wall of masonry, as designed for the New Croton Dam, was assumed by the board of experts to be 21 ft., or 17% of the depth of water in full reservoir. It has been stated by several authorities that the primary object of a masonry core wall is to afford a water-tight cut-off to any water of percolation which may reach it through the upper half of the embankment. It appears that absolute water-tightness in the core wall is not obtained, although the core walls of the Croton dams are said to be “the very best quality of rubble masonry that can be made.”
Mr. W. W. Follett, who is reported to have had considerable experience in building earth dams, and who has made some valuable suggestions thereupon, is emphatic in saying,
that the junction of earth and masonry forms a weak point, that either a puddle or masonry core in an earthen dam is an element of weakness rather than strength.
He also thinks the usual manner of segregating and depositing materials different in density and weight, and thus subject to different amounts of settlement, as bad a form of construction as could be devised.
Core walls may prevent “free passage of water” and “excessive seepage,” but are nevertheless of doubtful expediency.