Evening star., 1896-01-23
THE TUNNEL REPORT
Continued from First Page.)


rosal, and presents the following brief re-
sumo of it's conclusions, which, in all es-
sential features. are practically unanimous
Brief Description of Tunnel.
The plan, profile, sections and photo
graphs accompanying this report give an
idea of the Washington tunnel. But a
brief description may not be out of place.
The tunnel was designed to connect the
main distributing reservoir in the suburbs
of Georgetown, on the westerly side of the
city, with the new distributing reservoir
near the Soldiers Home, the northerly
side of the city. 1t is about four miles
long, constructed entirely in tunnel,
through rock and at a depth of about 150
feet below the surface 1t is well known
that before the tunnel was completed grave
defects were discovered in its construction,
and that in consequence the work was
abandoned in 1889.
The principal object for the construction
of the tunnel was to provide means for
carrying a large amount of water, with
but small loss of head, from the terrni-
nus of the Washington aqueduct to the
new reservoir, thus maintaining the pres
sure in l1rge part of the city situated at
a distance from the main distributing res
ervoir. The pressure in this portion of the
city is now drawn down in times of maxi
mum draft BY the very large consumption
of water and the lack of sufficient mains
to re-eEforce the supply.
There are other methods by which this
result can be attained, such as the laying
of additional mains, the construction of
tunnel nearer the surface of the ground, or
by a combination of these plans On ac-
count of the topography coed line for
surface aqueduct connecting the reservoirs
does not exist.
When the amount necessary to complete
the present tunnel is considered it is
opinion that no other means can provide
economical and efficient method for
conducting large quantities of water from
one Part of the District to another. pro-
vided, of course that it is practicable to
cOmDlpfp the tunnel in SIch a H.
ner as to insure safety in its operation
and maintenance.
With the slight differences OF level exist
Ing between the distributing reservoir and
the new reservoir, which is about two feet,
the capacity of the tunnel according to
our computations. is equal to that of nine
forty-eight-inch pipes or more than three
six-fcot pipes, of the same length. Con-
sidering the requirements of the city in
the near future, the addition to the dis
tributiHg system of structure of such
large carrying capacity must prove CT in
estimable advantage.
1t became apparent, therefore, early in
our investigation. that the portion of your
letter of instructions which referred to the
determination of The feasibility of COM
pleting the tunnel, was the first and most
important subject for study by the COM
mission.
Feasibility of Completing Tunnel
Conduit.
Careful examination of the nature and
condition of the rock through which the
tunnel is built has convinced us that it can
be relied upon to Withstand safely the pres
sure to which it will be submitted when
the tunnel is filled with water and sub
jected to the maximum hydrostatic pres
sure We found the rock. with but few
exceptions, to be sound, strong and hard.
The report of Messrs. Main and Sparrow,
already alluded to. confirmed our own
opinion as to the condition of the rock, in
asmuch as these gentlemen found it in al
.
it was in 1889. Although practically un
supported the rock has only fallen in two
or three places, and TO amount which
We consider trivial With the exception of
few schistose veins, the rock shows no
signs of disintegration. These veins
few in number, and even at these points
the rock appears, to a large extent, to be
free from percOlation and assures, and We
believe that when closed in by proper ma
soEry construction all disintegration will
cease In brief, We believe there is moth
ing in the nature of the rock to cause un-
easiness to call for the exercise of un-
usual engineering expedents
In 1SS9 it found that in some portions
of the tunnel the cement was soft and of
suspicious character 1t appears from
the examination of Messrs. Main and Spar
row that this cement has now become
hard, and it has evidently improved with
age. We personally tested the hardness
of the mortar throughout the tunnel, as far
possible, and found it to be in salts
factory condition.
One of the objections against the tunnel
that has been urged with much force is
the liability of leakage or percolation out
ward from the tunnel into the ground
when it is subjected to the pressure of the
water. This is a point to which we have
devoted much time and attention. 1t is
evident that should the leakage be suffi-
Ciently great, the dangers to be apprehend-
ed from this source might he considera BILE
IEeSe possible dangers, however. A.
terially lessehed by consideration of the
fact that from all the information have
been able to collect, it appears that for
considerable portion of the tunnel the
natural level of the water in the ground
is not far different from that of the hy-
draulic grade line of the water in the tun-
mel.
Since this tunnel was abandoned, the new
Croton aqueduct supplying the city of New
York with water has been completed. a
portion of this aqueduct, embracing a
length of about seven miles, is subjected
TO internal pressure with maximum
pressure due to a head of about 130 feet
In this case no serious percolation has been
observed. although the surface of the
ground is at several points much below the
hydraulic grade line. An experiment made
Tor the purpose of determining the amount
of this leakage outward from the aqueduct
showed that the total loss in the whole
seven miles from this cause did not ex-
ceed 22J,O00 gallons in twenty-four hours
With this practical example before the
commission, and guided by our judgment
and experience, believe that if the
spaces now existing between the brick
work and the rock thoroughly filled
with rubble masonary and grouted the
tunnel can be made sufficiently watertight
and that the leakage under head of 1JO
feet will not be large enough to prove an
obstacle in the way of the successful COM
pletion and operation of the tunnel. In this
connection We recommend that the interior
surface of the whole tunnel should receive
a double coating wash of pure Portland
cement This treatment will aid materially
In making the brick work more inperviouS
to the passage of water.
Objection has also been urged against
the tunnel on the ground of the danger of
contamination to the supply from the per-
Collation. of ground water into the tunnel
when it is empty. This condition can oc-
cur but seldom, perhaps but once or twice
a year, at the most when the tunnel is
pumped out for examination. The amount
OF this percolation is very slight even under
present conditions, with the lining prac-
,tically open. While our examinations of
itne tunnel were under way samples of wa
ter at various points were collected for
analysis, and not without some difficulty on
account of the small amount of wafer