WASTEWATER/Project stabilizes sewer pipeline and riverbank

In 1999, spring rains near Kansas City, Mo., eroded a section of riverbank in the Blue River exposing a 120-foot-long section of 60-inch diameter concrete sanitary interceptor. The pipe, which transports sewage from the city to a treatment plant, was left unsupported by the riverbank. Officials at the city’s Water Services Division (WSD) knew that, if the pipe were to crack, raw sewage would spill into the Blue River.

The WSD contracted with locally based Burns & McDonnell to design a solution. The company presented two options: design a new sanitary line further inland, or leave the line where it was, protecting it and armoring the river channel to prevent future problems. The WSD chose to protect the existing line because that option would result in fewer trees being removed and would not involve the high cost of clearing a site for a new pipe.

WSD contracted with locally based Garney Construction to complete the job. First, the contractor needed to stabilize the sewer line and prevent it from failing during construction. H-piles were used on either side of the pipe at each pipe joint using a template constructed from wood. The template fit over the sanitary sewer and allowed the placement of piles in a uniform manner. Then, the contractor fastened cables under the pipe and attached them to a cross-member to act as a sling for support.

With the pipe supported, sheet piling was driven into the bedrock on the riverward side of the pipe. The sheet piling served a dual purpose: to prevent additional erosion and damage from high-water events during construction and to serve as forms for the concrete used to encase the pipe.

While the sheet piling was being placed, a cofferdam was built to divert the flow of the river to the left bank, leaving the right half of the river ready for construction. Rock fill was then placed on the riverward side of the sheet piling, partially to begin shaping the side slopes, and to provide weight to offset the weight of the concrete encasement to be placed around the pipe.

The area under and around the sewer line was dewatered, excavated and mucked out, and cables were attached to scrap pieces of H-piling and thrown over the top of the sewer line. (The pieces of H-piling rested on the excavated bottom beneath the pipe.) The city then poured concrete over the small pieces of H-piling to anchor the cables and protect against the pipe “floating out.”

Because there was no way to get adequate compaction beneath the pipe, the contractor poured concrete around the pipe to a point one foot over the top of the pipe. Next, the sheet pilings were cut off a foot above the concrete; rock fill was placed over the pipe; and the slopes of the bank were shaped to the invert of the channel. The slings were cut loose so that, if the concrete settled, the pipe could settle with it rather than producing point loads on the pipe at the slings.

Starting at the downstream end of the project, the contractor began shaping the channel side slopes and bottom to receive riprap. A solid wall of steel, concrete, dirt and stones now protects the section of pipe that was exposed and in danger of breaking. At the end of 2001, the project received national project-of-the-year honors from the Kansas City, Mo.-based American Public Works Association in the category of Emergency Construction/Repair. Since completion, there have been several heavy rains that caused high river levels, and the bank has shown no signs of erosion.