from that shown in the 1966 Chicago Underflow Report. The utilization of this low-level point of discharge for peak flows becomes more attractive. A project, already approved by Congress and awaiting funding, provides for the widening of the channel to 225 feet. The Chicago Drainage Plan now proposed by the Illinois Division of Waterways recommends the widening of this reach to 325 feet to accommodate barge tows which currently operate on the Illinois Waterway as far upstream as Brandon Road Pool and to increase flood conveyance capacity. The Division of Waterways Plan also recommends a 10-foot deepening of the Canal for a width of 150 feet to further increase conveyance capacity. The combined Underflow-Storage Plan herein recommended assumes a widening to 325 feet, without any deepening. Combining storage with conveyance The City of Chicago presented, in September, 1968, the Composite Drainage Plan, which considered the possibility of providing mined storage areas at four locations along the main tunnel, plus one in the Calumet Area and one along the Des Plaines River Tunnel. This was proposed in order to provide temporary detention storage closer to the various origins of spillage water and thereby reduce conveyance distances, and consequently the cost of conveyance tunnels. This concept of geographical spreading of the underground storage volume is further extended in the Underflow-Storage Plan. It is now suggested that the main tunnels be re-sized so as to serve both as conveyance tunnels and as continuous storage reservoirs. The total length of underflow-storage tunnels is about 106 miles. It is proposed that the principal tunnels be 26 feet wide and 50 feet high and have paved inverts plus sidewall lining (in their lower portions only). The principal or mainstream tunnels from Lockport to Lake Street would be Twin Tunnels. It is also proposed that the inverts slope to low points opposite each of the three major existing treatment works. Pumping stations are proposed at these points having a combined pumping capacity of about 2,000 cfs, which is about equal to % of the total ultimate dry weather average flow through the three major treatment works. The total storage volume of the tunnels shown for construction is equal to 18,000 acre-feet, or 1.12 inches of runoff from the entire 300 square miles of combined sewer area. It would accomplish, on a long term basis, an average of more than 98.5 percent entrapment of combined sewer B.O.D. spillages. IV-CHICAGO UNDERFLOW PLAN In 1966, the City of Chicago proposed a sewer project which would demonstrate the principles of the Underflow Plan but, of course, on a much smaller scale. The City of Chicago's Five Year Capital Improvement Program called for the construction of a new Auxiliary Outlet Sewer System to provide relief from basement and underpass flooding of an area bounded by the North Branch of the Chicago River, Irving Park Road, Oriole Avenue and Devon Avenue. Preliminary hydraulic studies indicated that a trunk sewer in the vicinity of Wilson Avenue from the North Branch of the Chicago River to Melvina Avenue with branches extending north and south to intercept and unload existing trunk sewers would provide the necessary flood relief for a direct drainage area of 3,620 acres. The proposed sewer system in that program was designated the EastwoodWilson Avenue Sewer System and varied in size from a 2 barrel 13-foot by 13foot section at the lower end near the river to a 7.5-foot circular section at its upper end. Consideration was given to lowering the profile of this sewer to increase the storage available during small storm periods and to cause it to flow full before discharging to the river. The storage thus generated would reduce the frequency of spillages from this combined sewer to the river. Lowering the profile would necessitate pumping of sewage to the existing sanitary intercepting sewer, increasing the overall cost. It would also require that more of the construction be performed by earth tunnel method. Recent development of earth mining machines has resulted in lower bid prices in earth tunnel contracts. However, preliminary soil investigations indicate that heavy primary steel lining and occasional rock sections would negate the savings from the use of such machines. Costs would greatly exceed that of the conventional open cut construction method. Recent improvements of the rock mining machines (Moles) have reduced the cost of tunneling in various kinds of rock materials for large irrigation and hydroelectric projects throughout the world. Preliminary cost estimates revealed that mining in rock may be competitive with open cut methods. Lowering the profile of the Eastwood-Wilson sewer over one hundred feet into bed rock and constructing it as an "Underflow Sewer" looked promising. Sanitary flow would not normally, in dry weather period, enter the tunnel and therefore would not be pumped on a continuous basis. Pumping would be required, however, for dewatering of the tunnel to the existing sanitary intercepting sewer in the post rainfall period. The Department of Public Works retained the Harza Engineering Company to study alternate methods of constructing the proposed Eastwood-Wilson Auxiliary Outlet Sewer System. The studies were to include a comparison of costs of constructing the sewer by open cut and tunnels, the maintenance and operating costs, and their recommendations on the best method to fit the City's needs. It was decided to construct a lined tunnel sewer in the Niagaran limestone formation approximately 250 feet under the surface of Lawrence Avenue and demonstrate the feasibility of the "Underflow" concept. The rock tunnel would be excavated by a tunnel boring machine. Lawrence Avenue, an arterial street, was selected as the route of the sewer because of the requirement of the mole to travel in nearly a straight line. Because the tunneling would be so far below the surface, traffic in that arterial street and commercial activities would not be interrupted, as would be the case with the conventional open cut construction. The tunnel would be 12,800 feet long at 12 feet in diameter and 9,300 feet long at 17 feet in diameter. A branch tunnel in Harding Avenue extending south from Lawrence Avenue to Berteau Avenue, a distance of 4,000 feet would also be 12 feet in diameter. Approximately 18,000 feet of new conventional branch sewers would relieve the overloaded existing sewers and convey the flow to the tunnel inlet shafts. Ten inlet shafts would be constructed to supply the tunnel and one 25-foot diameter outlet shaft for the discharge drainage. The total storage in the tunnels and shafts will be about 4,000,000 cubic feet or about 0.30 of an inch over the 3,620 acre drainage area. This storage would provide space for the runoff from rainfall accumulation up to about 0.9 inches without overflowing to the river. The construction of these Underflow Sewer Systems will demonstrate the feasibility of constructing, economically, a detention reservoir to greatly reduce the pollution caused by overflows from combined sewers, far below the surface in public right-of-way, while providing the conveyance capacity to reduce basement and underpass flooding. It will also demonstrate the practicability of constructing a much enlarged Underflow System beneath the waterways to serve the entire 300 square mile combined sewer area in the City of Chicago and the surrounding Metropolitan Area. When the enlarged Underflow System is completed, the Lawrence Avenue Underflow Sewer and the two being constructed by the Metropolitan Sanitary District will become branches to the trunk lines under the waterways. At that time the pumping stations serving these three initial Underflow Sewers will be abandoned. Project cost The largest single item of cost of the proposed project is the excavation, hauling and disposal of rock. To arrive at a conservative estimate for the rock excavation (including hauling and disposal) conferences have been continued with manufacturers of rock drilling, blasting and hauling equipment. These companies have had wide ex |