Iowa River Sanitary Sewer Interceptor

Marshalltown, IA

In 2008, the Iowa Department of Natural Resources issued a consent order to the city of Marshalltown requiring elimination of sanitary sewer overflows and basement backups for at least the 5-year storm event. As a result of this consent order, the city initiated a program to reduce infiltration/inflow (I&I) in the sanitary sewer system which would provide collection system improvements for conveyance of up to a 10-year storm event.

In 2011, the city asked FOX Engineering to assist in developing a plan for the city to review their interceptor and pump station capacities.  The system evaluation indicated that significant improvements were necessary to convey the 10-year event to the Water Pollution Control Plant (WPCP). 

Several projects were initiated as a result of this study; one was the Iowa River Sanitary Sewer Interceptor. This interceptor parallels the existing sanitary sewer interceptor that serves the northern part of the city, an area of approximately 2,220 acres. 

Challenges

  • location along the Iowa River
  • poor soil conditions (high groundwater, river backwater area)
  • depth in some areas >40 ft
  • existing streets and US Hwy 14
  • elevations
  • forested wetlands
  • permitting Army Corps of Engineers (ACOE), IDNR, IDOT
  • breaching the ACOE flood control levee and construction across an Iowa River oxbow area

DESIGN CHALLENGES

The FOX design met numerous challenges (sidebar) through project phasing which accommodated historical flood conditions as well as high water levels of the Iowa River. In addition, it was necessary to install the interceptor while maintaining the existing interceptor in service.

Complicating design efforts, a portion of the sanitary sewer was located under an old water plant building; this significantly limited space for construction for approximately 550 linear feet (lf) of the interceptor due to elevations, the Iowa River, shallow rock conditions, and high groundwater.  To complete the pipe installation, a cased bore was required in this area.

PHASING

The design team separated project construction into three phases to allow street reconstruction to be completed within a construction season as well as to allow construction in backwater areas of the Iowa River during historically low water levels. FOX completed design of the approximately 12,000 lf of 36-inch and 30-inch sanitary sewer interceptor project in the fall of 2013; it was released for bid in March of 2014.  Five bids were submitted in April of 2014. The low bidder, S.M. Hentges & Sons of Jordan, MN, was awarded a contract in the amount of $5.7 Million and construction began in June of 2014.

A significant effort was required to dewater, due to the sandy soil conditions and proximity to the Iowa River. Dewatering wells were installed by July of 2014. During the drawdown period of the dewatering process, the Iowa River saw a record crest of 22.5 feet. To allow construction to begin while maintaining water levels as required by the specifications, the contractor doubled dewatering efforts by installing dewatering wells at 50-foot intervals and increasing pumping capacity to 25 HP.

THE ROUTE

Construction of the 4,500 lf of 36-inch interceptor was completed from the WPCP to the ACOE levee west of Hwy 14 in October of 2014.  Construction continued west adjacent to the Iowa River for the remaining 6,400 lf of 30-inch interceptor while 550 lf of cased bore was installed in rock under N. Center Street adjacent to the Iowa River. The bore pit for this work required an area 50 feet in length, 40 feet in width, and 25 feet deep located within the equipment storage yard of a private commercial business. The bore continued until January of 2015 when the bore head could no longer advance; it was a mere 100 feet short of the final bore length.

The contractor excavated the bore head to a depth of 45 feet just west of N. Center Street and completed the remaining bore length by open trench construction in rock.

Work continued west through an existing Iowa River oxbow area in difficult soil conditions as well as during Iowa River flooding that occurred in March of 2015.

SCHEDULE/CONTRACT COST

Even with the many challenges, the project was considered substantially complete by the required date of June 3, 2015. The final contract cost of $5.5 million was 3% under the original contract price. The project was operational by the spring of 2015 and is functioning as designed.

River Oxbow Construction

River Oxbow Construction

FROM THE CONTRACTOR: Gary Zajac, S.M. Hentges and Sons, Inc.

“The Marshalltown Iowa River Interceptor was a challenging and highly difficult project with many specialized scopes of work. The project had 3 segments that had to be completed by critical dates … areas of pipe installation in the floodplain were soft and would not support equipment. The third phase was again down in the floodplain and had 2 crossings of a backwater of the Iowa River. We were caught in early March and flooded out for a week when an early snow melt occurred. We were able to finish the project before spring runoffs flooded this area again and have the interceptor online in April of 2015.  The design of the project worked well with permit requirements, areas constructed in summer months and utilizing frozen conditions to install pipe in the low flood plains with minimum risk of flooding.”  

FOX Engineering is an environmental engineering firm based in Ames, Iowa. We specialize in water and wastewater solutions for our diverse municipal and industrial clients. Our work varies in size and scope and can be found throughout the Midwest and beyond.