Evening star. [volume] (Washington, D.C.) 1854-1972, 1893-07-12
UNDER THE CONDUIT ROAD,

From the north connection for a distance of
1853 feet aqueduct tunnel No. 4 is lined with
brick and for a further distance of thirty -one
feet it is lined with rubbie stone Of not very good
quality, but from here. from within few feet
of the point Of crossing below the aqueduct
tunnel OF the line Of the drainage tunnel, the
aqueduct tunnel is not lined. 1t is a great
misfortune that Gen. Meigs did not
completely line aqueduct tunnel No. 4,
for, although the rock is fairly good.
found when emptied the conduit between
Great Falls and the distributing reservoir and
inspected 1t in September, 1891, that there have
been several slides from the roof and sides OF
the tunnel. A serious one, il it ever occurs.
will block up the tunnel and cut off the supply
Of water to the city, but it is too late TO line ii
now. This can only be done after duplicate
conduit from Great Falls (Which will doubtless
some time, perhaps twenty years hence, be
deemed necessary for the safety of the supply
as well as for its increase) has been con-
structed.
DELICATE OPERATION.
The excavation Of the draining tunnel through
Dalecarlia Hill and under the aqueduct tunnel
will be delicate operation. The blasting out
OF the former in such u manner as not to pro-
duce shocks and jars the unsupported rock
Of the latter will have to be done with the great
est care and caution on the part Of all con-
cerned. Else we may not only have falls of
ing Of serious leaks through some Of the
numerous seams that exist everywhere in
the rock. Such leaks, if large, not only would
interfere with and add to the expense of the
excavation of the lower (the drainage) tunnel.
but they might be an irreparable damage to
the upper (the aqucducD tunnel through the
difficulty of finding (by lantern light and in
the short periods of time during which the sup
ply to the distributing reservoir can be inter
rupted without seriously impairins the head in
the city mains) the sources of the leaks in this
unlined tunnel. should mention that all parts
OF the aqueduct tunnel are under pressure as
its roof is several feet lower than the level of
the water in the distributing reservoir.
2. With q grade of .OO31 the velocity of
the water in the conduit when just full viii be
ten feet per second. This will be the condition
OF the conduit when the discharge from the
entire watershed will be 385 cubic feet per Sec
ond, and this will obtain 80 rarely that no
abrasion Of tho invert by sand and pebbles car
ried along by the water need be feared
am told that in Washington there has not
been found any abrasion Of inverts Of sewers
when made Of vitrifed brick from velocties
as high even as l6 feet per second.
When the we'll is supplied with less water
than enough to make the conduit Just full the
velocities will be less than 10 feet per second.
but at all times the velocities through the con-
duit will be sufficient to prevent the
iodgment OF any detritus that is likely
of water that in storms now produces a depth
OF six inches on the weir of the spillway (Which
Is, under existing conditions, the only escape
from the reservoir Of the discharge from the
watershed), the depth of water in the conduit
will be as represented in section 2. The veloc-
ity In this case will be 81 feet per Sec
ond Storms that produce this depth
OF water on the weir Of the apilway
are very fnfrequent and do not on an average
occur more than two or three times a wear.
There Is no doubt, then, as to the capacity Of
the conduit to carry the amount Of water dis
charged from the watershed in ordinary
storms, or Of the safety of the lining of the
drainage tunnel from abrasion, and the main
question is, will it carry the amount that will be
discharged in the greatest food and freshet
that is liable to occur and under the circum-
stances most favorable for producing the
highest percentage Of discharge? This GUAM
tity, as we have seen. il l.132 cubic feet per
second. Before the water could be damned up
in the valley Of Little Falls branch (and the
two other Valleys and above the well to a suf-
icient height to allow the water to pass over
the dams into the reservoir il would have to
have a head over the lower mouth OF the outlet
conduit Of 59.S feet
IN FAIRLY GOOD CONDITION
This portion Of the aqueduct is in fairly
good condition, but there is small amount OF
sediment in the bottom, say an average Of six
or eight inches in depth. It is under pres
sure OF about four feet at the crown of the arch
and the velocity is small, about .95 Of a foot
per second. find that l,Os3 cubic feet per
second is the amount that the outlet tunnel
would carry before the water behind the dam
would overrun the dam
do not consider, therefore. that seven feet
FOR the diameter OF the conduit would be ex-
cessive, and one much smaller could not be
constructed economically.
ESTIMATE OF COST.
A careful estimate Of the cost of the receiv-
ing shaft Of well, the drainage conduit and the
open cut at its mouth. allowing 10 per centum
for contingencies. gives $50. $50. The re-
m.inder OF the $50, 900 appropriated by the act
Of March 8, 1888, will be required for the pur-