The observations indicated that pressures existed as follows: Section A-B on one side of the bottom of the foundation; full head at heel to one-half at toe; section C-D next in order, full head at heel to about one-fourth at toe; section E-F next, three-fourths of full head at heel to about one-fourth at toe; section G-H at the other side of the bottom of the foundation, from full head at heel to zero at toe.
Observations in 1910 showed in most cases a reduction in the pressures, although in one pipe there was a slight increase and in one (the downstream pipe of section G-H) a considerable increase to about one-fourth full reservoir head. Holes 19 ft. from heel showed in 1907, 85 per cent, of full head and in 1910, 71.2 per cent. In one of the crevices, which had been excavated about 8 ft. to 13 ft. lower than the general foundation, a spring (just upstream from the upstream pipe of section G-H) had been piped. This spring ran 19 gal. per hour with reservoir empty and 870 gal. per hour with reservoir full.
The foundation of the Neye dam was about 26 ft. below original surface in graywacke and shale which was on an inclination of 15 deg. to 35 deg. toward the downstream side, and likewise without cut-off trench. In February, 1910, after the reservoir had been full for some time, pressures were observed in nine pipes, water in the reservoir being 102 ft. above the general level of the foundation. The two lines agreed very closely, the pressure at 14 ft. inside the heel being about 57 per cent, of the total and decreasing to about 32 per cent, near the toe.
An erroneous conclusion was drawn from the Oester observations: "That deeper foundations conduce to decrease in pressure as more solid and impervious rock is reached".
Section G-H was deeper than section A-B and the other two sections came between, both for elevation and pressure. However, the effect of the tapped spring at the upper end of G-H would account for the difference in pressure. We also see a confusion of ideas in that pressure is considered dependent upon quantity" of leakage whereas there is no necessary relation between the two. From the observations on the two dams two conclusions were drawn:
"That full static pressure does not act over the entire base but the resistance, which the water encounters in its flow toward the downstream side, gradually uses up the head and makes the pressure at the toe less than at the heel." Precisely what was meant by this is not clear; probably something indicating an analogy to the gradient taken by water flowing through a homogeneous material. This analogy if it exists at all is entirely too precarious and accidental to deserve any weight whatever.
"Deep foundations are conducive to decreasing the uplift pressure." Here the deeper foundation and less pressure at the Neye was a coincidence, and is made the basis for an unwarranted conclusion in connection with the fictitious (from ignoring the effect of the spring) result at the Oester. The conclusion is unwarranted in any case but all the more so in this from the ignored fact that at Oester the stratification was vertical and that at Neye it approached the horizontal.
Deep foundations conduce to a decrease in quantity of leakage. Pressure, however, depends upon the extent to which any amount of leakage is confined.
To state the case again. The head at any point (A) under a dam subtracted from reservoir head gives a certain difference in head which is required to force a certain quantity of water from the reservoir to point A. The head at point A minus the head at the toe gives the difference in head required to force the same quantity of water from A to the toe. It is seen that the head at A does not depend upon the quantity of leakage but upon the relative freedom of the channels upstream and downstream from A.
If the difference between A head and reservoir head is small compared with that between A head and head at toe, the tightest position of the channel is downstream from A. Read large instead of small in the above sentence and the tightest part of the channel is upstream from A. The entire treatment of a foundation should have for its aim the location of point A as near the heel of the dam as practicable, and the making of the difference in head upstream from it as great as possible and the difference in head downstream from it as small as possible.
An observation at the Roosevelt dam must be prefaced by considerable explanation. The foundation was a hard fine-grained sandstone, the strata dipping upstream at an angle of 29 deg. and at right angles to the dam. Some seams carried water but the widest seams were filled with a material resembling clay (so fine as to betray no grit whatever) and were absolutely watertight. The issuing water generally was at the river temperature but at two or three points the water was considerably warmer (no deg. Fahr.). These warm springs were piped together and led to the basement of the power house at the toe of the dam; the other springs were grouted.
The rock was so sound and suitable that it was not necessary to excavate much. Over portions of the bottom practically none was excavated and the average for the entire area was less than 5 feet. Just inside the heel of the dam and extending across the canyon was a fault in the rock with a displacement of about 8 in. Part way up the sides of the canyon the dam curved away to the downstream side of it. Where the fault came inside the masonry lines advantage was taken of it to form a cut-off trench or V 6 ft. to 8 ft. deep. The rock on the upstream side, being solid, was left in place and the rock on the downstream side readily broke to the fault with a minimum shattering or disturbance of the foundation. The elevation of the bottom ranges from - 20, to - 35, with a lowest point of - 38, elevation 0.0 being the original low-water elevation in the river. The exact location of the warm springs was not made a matter of record, but a close enough approximation would be 50 ft. or 60 ft. from the heel of the dam. Previous to any work upon the dam, warm springs existed on the river bank a mile upstream from the dam. As these were covered by the slowly rising waters of the reservoir, warm springs appeared 6 miles upstream from the dam where none had been observed before. During the construction period the water in the reservoir was held for a year or more around elevation 100. During that time warm springs appeared between 400 ft. and 500 ft. downstream from the dam. Water of the same temperature (no deg. Fahr.) emerged from crevices in both canyon walls at elevations between zero at river level and 20 ft. above, and very likely also under the river. The streams, visible above river level, although scattered so as to be difficult to estimate, might have amounted to 2 sec. ft.