A description of this work will be of interest in showing the general
practice followed in California for carrying water across deep mountain
gorges. In order to augment its water supply, the North Bloomfield Gravel
Mining Company desired to conduct water from a stream known as Texas
Creek, in Nevada County, California, across the Big Cañon branch of the
South Yuba River into the main Bloomfield flume or aqueduct, which was
located on the side of Big Cañon Creek, at a vertical elevation of 620
feet above the bed of the latter stream. The quantity of water to be
carried was about 32 cubic feet a second (1,250 miner's inches), which
could be diverted from Texas Creek at a point 480 feet vertical above the
Bloomfield flume. An aqueduct about 4,000 feet long, partly of ditch and
partly of flume, was needed to bring the water from the catchment dam on
the creek to the brow of the gorge. The vertical head for the pipe could
therefore be from a maximum of 460 feet down to any lesser head; with a
head of 460 feet, the pipe would be 4,790 feet long; and with a head of
220 feet, the length would be 4,290 feet. Assuming a maximum tensile
strain upon the iron of 16,500 pounds per square inch, with the formula
for the greatest head of about
d = (.359 l/h)^{1/5}, [or, v = 68 (dh/l)^{1/2}, and Q = 32],
and a lower value of the coefficient in the last equation for the lesser
heads, it was found, by calculation, that the least cost could be obtained
with a head from 300 to 350 feet. The head fixed upon was 303.6 feet, with
a length of 4,438.7 feet. A profile of the pipe, with nearly the same
horizontal and vertical scales (horizontal scale, showing slope lengths),
is given in Fig. 14; details are given in Figs. 15 and 16. The pipe was of
double riveted sheet iron, made in lengths of about 20 feet, and of the
following thicknesses:
1,349 linear feet, 0.083 inch thick.
220 " 0.095 "
240 " 0.109 "
250 " 0.120 "
320 " 0.134 "
610 " 0.148 "
1,450 " 0.165 "
Some of the iron was of the very poorest quality; the pipe was made by
contract in San Francisco, without the supervision of an inspector, as the
contractors were a firm of good reputation; the bad quality of the iron
was not detected until too late to have it corrected. Since then, the
writer has always had such pipes--the mines of which he has been the
manager using large quantities--made directly on the ground where they are
to be used; the pipe makers, in the latter case, always reject such sheets
as are too much below in thickness the standard gauge, and those which
show in passing through the rolls the bad quality of iron; tests of each
joint by hydrostatic pressure would add too much to the cost.
[Illustration: FIG. 16.]
The maximum tensile strain upon each of the seven thicknesses of iron used
was intended to be 16,500 pounds per square inch. Some of the sheets were
below the standard gauge, so that, in reality, the tensile strain is
sometimes as high as 18,000 pounds. The mean diameter of the pipe was
1.416 feet. The entrance into the pen-stock was tapered, so that the
coefficient of contraction was about 0.92. For pressures not exceeding say
380 feet, the joints were put together stove-pipe fashion. For greater
pressures, the joints were made by an inner sleeve riveted on one end of
the joint, with an outer lap-welded band, as shown by Fig. 15; lead was
run into the space between the outer band and the pipe, and then tightly
driven up by calking-irons. The pipe was laid under the bed of the Big
Cañon Creek, a large stream when in freshet, where the head below the
hydraulic grade line was 760 feet. Some of the lead joints leaked slightly
at first, but this was soon remedied by more careful calking. No man-holes
or escape-gates were used. The pipe for the larger part of the year is not
filled at its upper end; when such is the case, the water at the inlet
carries down the pipe a great quantity of air, for which escapes must be
provided to prevent a jarring or throbbing, which would soon destroy the
pipe. The escape air-valves used are shown by Fig. 16. They consist simply
of a heavy flap valve of cast-iron, with recess for lead filling to give
greater weight set on top the pipe, seating on a vulcanized rubber
cushion, and swinging on a loose hinge. When the pipe is only partly
filled with water, the valves drop down by their own weight, allowing the
air to freely escape; when the water rises above the level of a valve, it
is tightly closed by the resulting pressure. There are fourteen of these
valves, those on the lower end being designed to allow air to freely enter
the pipe in case it should burst in the deeper portion, and thus prevent
any collapse from atmospheric pressure. The valves have answered the
desired purposes most effectually. The pipe was hauled over a road built
to the inlet end, and shot down the mountain side by means of a V-shaped
trough of wood. For the lower end, the joints were hauled up the cliff
side into place by a crab worked by horse-power. On steep inclinations,
the pipe was held firmly in place by wire ropes fastened to iron pins in
the solid rock, as shown by the sketch. The covering of earth and stone
was 1 foot to 2 feet in depth; with steep slopes, the earth was kept from
sliding by rough dry walls, or by cedar plank placed crosswise. The pipe
was laid in 1878; the first year it broke twice, owing to the wretched
quality of the iron; since then, it has given no trouble, and has required
practically no attention. The cost of this work--ditch and flume 4,000
feet, and pipe 4,440 feet--was $23,779.53.
A comparison of the relative values of n, in the formula v = n (r s)^{½},
for the foregoing ditch, flume, and pipe will be instructive. The ditch
has a width on the bottom of 3 feet, on the top of 6 feet, with a depth of
3 feet, and an inclination of 20 feet per mile; its sides are rough, being
cut in part through the rock and with sharp curves, although fairly
regular; with a flow of about 1,300 miner's inches (32.8 cubic feet per
second) the ditch runs about full.
Therefore:
6 + 3
a = ----- × 3 = 13.5 ;
2
[TEX: a = \frac{6+3}{2} \times 3 = 13.5;]
a
r = ------------- = 1.41 ;
3.3 + 3 + 3.3
[TEX: r = \frac{a}{3.3 + 3 + 3.3} = 1.41;]
20 1
s = ------ = ----- ;
5280 264
[TEX: s = \frac{20}{5280} = \frac{1}{264};]