Flooding prompts city to repair sewer
The 1993 floods that inundated much of the Midwest wreaked havoc on Cedar Rapids, Iowa. In neighborhoods on the city’s west side, sanitary sewers overflowed and flooded residential basements. Because of the potential health effects, the city’s engineering staff reacted quickly to find a permanent solution; it commissioned a local consulting engineering firm to study the west side’s sewer system and design solutions that would allow the city to meet future demands on the system.
The area’s original, 42-inch sanitary sewer was built in 1956. Hundreds of homes built on the west side from 1950– 1980 have basement drainage tiles that feed into that system. During storms, infiltration and inflow (I/I) of surface water to sanitary sewer pipes through cracks and deteriorating joints contribute to the loading problem.
Additionally, because of industrial expansion in the area, portions of the 1956 sewer are so close to buildings that they no longer are accessible. Finally, large storm sewers interfered with the construction of a new sanitary interceptor sewer.
The city engineering office originally envisioned a $2.5 million sanitary sewer improvement project to serve the Time Check neighborhood, one of the hardest hit by sewer flooding. The consultant’s study, however, convinced the city that installing a larger sewer to serve the entire west side would be more cost-effective than building a small sewer immediately and a parallel sewer later.
With $1.1 million in Federal Emergency Management Agency grants, Cedar Rapids began to redesign and reconstruct the west side sanitary sewer system. The project was aimed at accommodating future growth, as well as reducing short-term sewer flooding.
Final project design began in August 1995 and was completed in October. The project’s prime contractor, Barbarossa and Sons, Osseo, Minn., completed installation in one season, working from March to December 1996.
The project’s magnitude presented the biggest challenge. Ninety percent of the 12,000-foot, 54-inch, 60-inch and 66-inch trunk line was routed through developed residential and industrial sections of the city. In those sections, other underground utilities, including 72-inch and 84-inch diameter storm sewers; a 96-inch-by-72-inch rectangular storm sewer; 8-, 10-, and 12-inch water mains; and gas, telephone and fiber-optic cable already were in place and had to be accommodated.
The existing large storm sewers presented another formidable challenge. Buried 18 to 20 feet underground, the storm sewers had the “right of way,” meaning that the sanitary sewer had to be placed at depths of 20 to 30 feet. Because of the depth and because of slope requirements for construction trenches, installing the large sewer pipes necessitated the demolition and removal of entire widths of city streets. Disruption of residential neighborhoods, therefore, was a potential problem.
During deep trenching, the boring met bedrock, which had to be blasted and excavated. The contractor exercised extreme caution during the process to avoid disruption of the existing sanitary sewer in the vicinity. Additionally, part of the project ran parallel to the Cedar River, so trenching required dewatering.
Because of soil conditions, the contractor had to open-cut the sewer trench across First Avenue, a major downtown street. Traffic control routing allowed for one lane to be open at all times.
Cooperation of local residents was a critical element in the project’s success, as were utility coordination and construction crew coordination and management. Four construction crews worked at one time, all supporting the project’s mission.
This article was written by David Elgin, city engineer for Cedar Rapids; Charles Saxton, resident engineer for the city; and Danny Wall, project manager for Howard R. Green Co., Cedar Rapids.