Engineering Wonders of the World
Volume III
Forfatter: Archibald Williams
År: 1945
Serie: Engineering Wonders of the World
Forlag: Thomas Nelson and Sons
Sted: London, Edinburgh, Dublin and New York
Sider: 407
UDK: 600 eng- gl
With 424 Illustrations, Maps, and Diagrams
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110
ENGINEERING WONDERS OF THE WORLD.
On account of the enormous
hydrostatic pressure to which
it is subjected, a siphon tunnel
must be deep in
Exploratory fectI sound
Work. 1 , J
rock. Conse-
quently, wherever this type of
construction was found neces-
sary, very extensive explora-
tions had to be made by means
of wash or core borings before
the route of the aqueduct could
be definitely determined. The
magnitude of this preliminary-
work will be evident when it is
remembered that along the line
of the Ashokan Aqueduct the
surface—or, as the geologists
call it, glacial material—usually
covers the rock to a depth of
several hundred feet.
And here let it be noted, for
the benefit of the uninitiated,
that drilling in earth, espe-
cially at a great depth, is
usually more difficult than
boring through the hardest of
rock. If the rock is of uniform
quality, a progress of 10 to 30
feet a day can often be main-
tained ; whereas the presence of
a gravel bed or boulder in the surface material
may bring on troubles sufficient to cause a
delay of weeks in boring a few feet. Where
the rocks are very hard, the diamonds and
other cutting agents wear away rapidly ; but,
speaking generally, this trouble is of small
account compared with, that caused by the
occurrence of a soft spot, resulting in the
caving in of the walls of the hole.
Up to the present the total length of wash
and core borings in connection with the Asho-
kan Aqueduct exceeds 25 miles ! The borings
have ranged in depth from a few feet to
nearly 700 feet in the Bondout Valley and
STEEL MOULD FOR CUT-AND-COVER WORK, CATSKILL AQUEDUCT.
{Photo, by courtesy of the “Scientific American.'")
over 1,000 feet in the gorge of the Hudson
River, where, for reasons which will be ex-
plained later, the exploration difficulties have
been exceptionally great. West of the river,
in addition to three streams, three wide valleys
will require siphons, each from 3| to 4|- miles
long. On the other side, also, several tribu-
taries of the Hudson must be crossed by
similar means. The following are all the
siphons in this section of the aqueduct—from
Ashokan Reservoir to Croton Lake—arranged
in geographical order from north to south,
with the type of construction and approximate
length of each :—