English
Des Moines sewer project underscores commitment to infrastructure, flood control
Winter 2013
The downtown business district of Iowa's vibrant capital city is dissected from east to west by the Des Moines River, a 525-mile-long tributary of the great Mississippi, and the longest river flowing through the state. Named for Iowa's largest city, the Des Moines River has long beckoned tourists to the downtown area for a variety of festivals, celebrations and entertainment venues. Now, a recent infrastructure improvement project promises to further enhance the downtown experience, affording visitors the opportunity to traverse both banks of the river for walking, jogging, biking and blading, or dining at one of several planned riverfront restaurants.
The Principal Riverwalk is a multi-year, downtown improvement initiative jointly funded by the City of Des Moines, the State of Iowa, U.S. Army Corps of Engineers, Federal Highway Administration and Principal Financial Group, one of the city's largest employers. When complete, the Principal Riverwalk will feature a 1.2-mile recreational trail connecting the east and west sides of downtown via two pedestrian bridges, and a 12-foot-wide multi-use trail along the balustrade at river level for joggers and bikers. Night-time lighting and security will turn the river loop into a 24/7 attraction as downtown Des Moines prepares to take on a new and exciting recreational role.
The expansion-infrastructure equation
Planning officials agree that while city expansion projects — especially those designed to attract tourists — are exciting and beneficial for municipalities, new development can also place additional strains on existing infrastructures, especially storm water and sanitary sewer services. The Des Moines Riverwalk project is no exception. Yet advance planning by city engineers and cooperative efforts among all parties afforded officials the opportunity to incorporate a much-needed upgrade to aging sewers while addressing an ongoing threat posed by the river nearly every spring — flooding.
After the historic flood of 1993, earthen berms and levees — many of which had failed to protect the downtown area — were raised and reinforced. And while the levy enhancements proved effective in containing the rising water within the river's banks during a subsequent flood in 2008, the aged, inadequate storm water sewer system was unable to prevent flood waters from backing up into the downtown area again, surrounding many businesses. So city officials focused on developing a solution.
Coined the Court Avenue and Water Street Sewer Separation Project (no coincidence that Water Street runs parallel to and within a few feet of the Des Moines River), city officials unanimously approved nearly $10 million in funding to address the inadequacies of the existing sewer system. The project includes the construction of storm sewer along Court Avenue from Water Street west to 2nd Avenue; then north three blocks to Grand Avenue, and west again to 3rd Street — a distance stretching approximately six blocks.
The larger storm sewer was specified as RCPP; a reinforced concrete pipe with a gasketed joint. The deeper sewers are below the river level and will always have infiltration into the pipe. Also, during high river levels, the pipe will become pressurized. Therefore, the gasketed pipe will reduce the infiltration and material being conveyed into the pipe through the joints.
The first phase from Water Street along Court Avenue and north up to Grand Avenue was completed on schedule in fall 2011. H & W Contracting selected subcontractor Rognes Excavating, based in Ankeny, Iowa, to install the storm sewer portion. The open-cut method identified for the majority of the project was influenced mostly by cost. Yet there were also locations along the sewer alignment in which open-cutting the pipe was not feasible; hence five trenchless installations were completed along the project.
Doosan excavators onsite
Rognes Excavating worked with RTL Equipment to secure a Doosan DX225LC and DX235LCR crawler excavator to complete the trenching and installation of the 66- and 54-inch reinforced concrete pipe (RCPP) sewer. The RCPP along Court Avenue is all 66-inch diameter, while the pipe diameter reduces at 2nd Avenue to 54-inch for the extent of the upstream pipe.
"There were confined space and lane width considerations given that this is a high-traffic, often-congested downtown area," Rognes says. "RTL equipment recommended the Doosan DX235LCR excavator for the job mainly because of its near-zero clearance capabilities. This excavator has just 3 inches of overhang and has the ability to maneuver within inches of construction barriers. It fits well into tight spaces and has less tail swing radius.
"Aside from the compact frame and extended reach, it's also easy to perform daily maintenance on the DX235LCR," Rognes says. "All of the routine maintenance points are easily accessible, allowing us to complete daily maintenance more quickly, and keep the machine running most efficiently. The DX235LCR also has extended vertical reach that requires repositioning the excavator less often. The extended arm reach is also capable of loading spoil into high-frame truck boxes."
Subsurface exploration was critical. As a result, there were no delays due to utility conflicts. Communication was also an important component to the project's success. Contractors, businesses and media were kept informed continuously, not just about specifics of the project, but also of progress and delays.
"Aside from some unavoidable traffic backups and congestion during peak times and a few minor, unsightly inconveniences to the thousands of Downtown Farmers Market goers, the project went really well," says David Kamp, an engineer with the city and instrumental in the design of the project. "Contractors, workers and the city tried to minimize inconvenience and mitigate the occasional frustration of drivers and downtown pedestrian traffic, but some of the challenges were simply unavoidable. The vast majority of Des Moines residents are unaware of just how significant this project really is, or what has taken place underground that now makes the downtown area safer from flooding. It really is an out-of-sight, out-of-mind sort of thing."
