The third design, which was proposed by Mr. Morison, is shown by [Fig. 29]. The topography and configuration of this dam site is not unlike that of the San Leandro Dam, California, soon to be described, while the general design is similar, as has been remarked, to the North Dike of the Wachusett Reservoir.
This third design contemplates a compound structure, formed by two rock-fill dams situated about 2,120 ft. apart, with the intervening space filled with loose rock, earth and other available material. Immediately below the upper and higher rock-fill dam, it is proposed to place across the canyon a puddle wall 50 ft. in width, resting over two lines of sheet-piling 30 ft. apart. This piling would probably not reach farther than 50 ft. below tidewater, the solid rock floor being about 100 ft. deeper.
Mr. Morison made use of Mr. Hazen’s filtration formula for estimating the rate and quantity of seepage through the permeable strata below the dam. This formula is:
| h | t + 10° | ||
| V = | cd² | — | —— |
| l | 60 | ||
where
- V = rate of flow in meters per day through the whole section.
- c = constant varying from 450 to 1,200,
- according to cleanness of the sand.
- d = “effective size” of sand in mm.
- h = head in feet.
- l = length or distance water must pass.
- t = temperature of the water (Fahr.)
This formula should be used only when the effective sizes of sands are from 0.10 to 3.0 mm. and with uniformity coefficients below 5.0[5].
Mr. Morison used the following values:
- c = 1,000;
- d = 1.0 mm.;
- h = 90 ft.;
- l = 2,500 ft.;
- t = 90°;
for the solution of this problem, and obtained a velocity of 0.002 ft. per sec. The bed of sand and gravel was assumed to have a sectional area of 20,000 sq. ft. for 2,500 ft. in length. This gives a seepage of 40 cu. ft. per sec.