Marshalltown, IA

Iowa River Sanitary Sewer Interceptor

  • Client City of Marshalltown
  • Type Municipal
  • Services Infiltration/Inflow and Collection System Improvements
  • Construction Cost $5.5 Million
  • Project Manager Steven P. Soupir, P.E.

Challenge

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, the city initiated a program to reduce infiltration/inflow (I&I) which would provide 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 evaluation indicated that significant changes were necessary to convey a 10-year event to the Water Pollution Control Plant (WPCP).

As a result, several projects were initiated; one was the Iowa River Sanitary Sewer Interceptor which parallels the existing sanitary sewer interceptor serving the northern part of the city, an area of approximately 2,220 acres.

Solution

Phasing the project was essential. The FOX design met numerous challenges through 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 the existing interceptor was in service.

Challenges

  • location along the Iowa River
  • flooding of 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

Levee Construction

Levee Construction

Complicating design efforts, a portion of the sanitary sewer was located under an old water plant building. This significantly limited construction space 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 allowed street reconstruction to be completed within a construction season and 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.

River Oxbow Construction

River Oxbow Construction

Outcome

Even with the many challenges which included flooding and poor soil conditions, 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.

Dewatering

Dewatering

“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.” 

Gary Zajac, S.M. Hentges and Sons, Inc., Contractor

Working around WPCP construction

Working around WPCP construction