Town Branch sewer rehabilitation

Project Summary

Springfield, Ill., like most cities founded in the 1800s, grew around its original and aging sewer system.  More than 100 years ago, two natural streams ran through the city, serving as an open sewer and eventually emptying into the Sangamon River.  As the city and the odor grew, these natural drainage channels were enclosed and one of them became the Town Branch Sewer.  The Town Branch Sewer District underlies the central portion of the city.  Through its collection system, it serves an area of approximately 1,800 acres.  The district contains some of the oldest sewers in the city — some dating as far back as 1865.  The majority of the sewers were constructed of brick and stone in arch or oval configurations.  Over the years, streets and buildings were built over the sewer.

Springfield city officials knew there were problems with the Town Branch Sewer, but they were not certain about the extent of its deterioration.  They chose Hanson to carry out the sizable task of finding the problems and bringing the sewer to a safe and usable level.  We looked at the sewer from a comprehensive system standpoint and determined the most effective solution.

Hanson’s initial involvement began with a condition survey of the Town Branch to document its size, condition and the location of all entering main sewers and lateral lines.  We also checked for structural integrity.  To perform this survey, a five-person crew received special training to work in the confined spaces encountered as they walked through and observed the sewer.  During the survey, we discovered severe failures, including collapsed walls and washouts of backfill in three locations. 

Hanson provided quick-response engineering services to expedite the repair of the failed sections.  We provided sketches and guidance on temporary bracing of walls to inhibit further collapse of the sewer and prepared plans for the needed repairs from within the sewer, avoiding an open-cut repair.  Hanson then contacted contractors capable of completing the repair work and solicited bids for the work.  After the contracts were awarded, Hanson provided construction observation and documented the contractor’s work while repairs were made.  The repairs were completed less than 30 days after the failures were first observed.

After the survey was completed, Hanson conducted a comprehensive hydraulic analysis of the sewer (the first one since the 1940s).  From this data, we developed a detailed computer model of the sewer and a runoff simulation model to determine system overload.  From the model, we were able to determine where the sewers were under capacity so we could address the worst areas first.

In keeping with our goal to restore structural integrity and increase system capacity of the sewer at the least cost, we decided to replace specific sections (where most needed), repair and rehabilitate the remaining sections, and construct new relief sewers where possible.  We used the following techniques to reach our goal:

• Coated the original brick walls with welded wire fabric and shotcrete, a sprayable concrete, to increase structural integrity in the sewer’s weakened areas.
• Replaced a portion of the sewer, deficient in both size and capacity, using tunneling techniques. 
• Used pipe-jacking techniques to construct about 1,000 feet of new sewer in the downtown Springfield area.  This was the longest and most extensive tunnel project in the city’s history.
• Rerouted a portion of the sewer to allow for the fill and abandonment of a failing segment that was located beneath two large buildings.