The city of Chicago plans to construct a large number of dropshafts
which will connect various parts of the existing surface sewer system to
large underground storage tunnels. The St. Anthony Falls Hydraulic Laboratory
has made model studies of various dropshafts proposed for use in the
comprehensive plan. A particular dropshaft may be in use continuously (dry
weather flow dropshaft) or during storms only (storm water dropshaft), or
it may operate as a combination of the two. The mode of operation will have
a considerable influence on the design for each type. The dropshafts will
also vary in size depending on the discharge for the particular location.
The function of a dropshaft is to convey the flow from the surface to the
underground tunnels, which in some cases are over 200 ft beneath the surface.
At the bottom of the dropshaft is a sump which has to withstand the impact
forces of the falling water, separate the air from the water to prevent the
air from entering the tunnels and return it to the surface, and convey the
water to the tunnel at a reduced velocity. Various designs of dry weather
flow dropshafts have been developed for study. One interesting variation
was the incorporation of a helixin the dropshaft to convey the water downward
and thus reduce the impact pressures and eroding forces in the sump;
at the same time, for high tailwater elevations the head loss through the
system was expeoted to be large and hence detrimental to its use. Because
there are locations where this type of installation could be considered,
De Leuw, Cather and Company initiated a project to study the flow in the helix
in a dry weather flow dropshaft. Observations have been made on fluctuating
and static pressures, air removal in the sump, and overall hydraulic characteristics
of the structure.
Anderson, Alvin G.; Dahlin, Warren.
Model Studies of Dropshafts for the Chicago Underflow Plan Dry Weather Flow Dropshaft - Helix.
St. Anthony Falls Hydraulic Laboratory.
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