Model Studies of Dropshafts for the Chicago Underflow Plan Dry Weather Flow Dropshaft - Helix

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.