Grinder pumps provide cost-effective sewer solution near historic road
Mark Gahry, Director of Public Works, Charter Township of Brownstown, Michigan
Brian L. Woodworth, P.E., Project Manager, Wade Trim, Taylor, Michigan
Infrastructure components can have different existences—disappearing before their useful life is over or reappearing centuries later when they are no longer needed. The strength and fragility of public works becomes painfully clear when these events happen on the same project like the Harbin Drive sanitary sewer in Brownstown, Michigan. While a sanitary force main was being designed on an island where only a single iron monument could be found, a Revolutionary War road surfaced near the project at the mouth of the Huron River.
Ironically, the conditions that allowed the historic corduroy road to survive—high water levels and wet soils—challenged engineers while designing the sewer to serve 16 residences along a narrow island at the confluence of Smith and Silver Creeks and the Huron River, where they empty into Lake Erie. Pollutants from failing septic tanks reaching these waters were a major concern. The soils were not stable enough to support traditional construction methods for a gravity sewer so a pressurized alternative had to be developed that was affordable and easy to maintain.
Located south of Detroit, the Charter Township of Brownstown is a rapidly growing community of approximately 23,000 that experienced a 22% population increase between 1990 and 2000. The Township has proactively managed its infrastructure providing water and sewer for development since the 1960s. However, there are pockets of development that started out as seasonal homes in the late 1950s and later became year-round residences like Harbin Drive.
Completely surrounded by water, especially when the lake water levels are high, Harbin Drive is protected through a series of dikes and seawalls constructed by the U.S. Army Corps of Engineers in 1974, as part of Operation Foresight. Soil borings revealed that the peninsula's gravel road was built on peat and fill material; the bedrock was 27 feet below the road. The soil conditions and limited work area necessitated that a construction process be selected that could maintain the shallow depth of bury and keep the road open to the traveling public. These soil conditions along with the dike construction had not adequately protected the iron monuments and property irons placed by the builder years earlier. Only one iron monument could be located along the roadway, making an easement agreement with property owners essential.
Close coordination with the 16 property owners was critical throughout the project. Before detailed design began, meetings were held with homeowners to explain how existing soil conditions would impact design and construction. While the single iron monument could be used for design, identifying right-of-way would be extremely difficult. By developing an agreement with property owners, concerns were reduced about working in the right-of-way, allowing engineers to focus on developing the best solution, even if that included construction on private property.
Since extensive fill materials would be required to support a traditional gravity sewer, this option was eliminated based on cost. A vacuum system was investigated next. Surprisingly, the manufacturer determined the project was too small to cost-effectively use the system. Traditional grinder pumps were then evaluated under a scenario where the township would pay for installation and maintenance of the pumps and residents would pay for electricity to operate the pumps. Residents were receptive to this type of agreement and housing grinder stations on their property.
A system was designed where individual grinder pumps grind sewage and pump it to a 2,000-foot force main under Harbin Drive that connects to a 3,000-foot force main along Jefferson Avenue that feeds into an existing gravity sewer. Pump cycling times were analyzed to determine force main diameter—the farther the distance from the gravity sewer connection and the lower the flows, the smaller the pipe diameter. The final force main ranged in diameter from 1.25 to 3.0 inches and in depth from 6 to 15 feet. Together, the 16 grinder pumps perform the task that would have been required with a single pump station if a traditional gravity sewer had been feasible.
Force mains were installed using directional drilling where a passageway was drilled through the ground and then the pipe was pulled through. This enabled the pipe to be inserted into the existing soil without compromising its stability. The existing water main could also easily be avoided. Four drilling pits were needed along the project to perform the work and to use for future maintenance access. Three air release and flushing points were also added on Harbin Drive to aid in operations.
The force main along Jefferson Avenue had to be installed above a recently discovered corduroy road. The timber roadway had been constructed for troops traveling to Fort Detroit during the War of 1812. Low water levels in the Great Lakes, that feed the Huron River, had brought this treasure to the surface. State historians were contemplating what to do with the remnants of the area's largest road construction project from the period. A letter was sent to the State Historical Preservation Office stating the force main drilling project would not impact the roadway and approval was given to proceed with construction.
The $194,000 construction project has been operating smoothly since completion in April of 2005. Homeowners were assessed $3 a front-foot, based on values the Township assigned for service in 1965, which equaled half the cost of an 8-inch sewer pipe at that time. The average cost to the homeowners was about $500 a lot. With a system in place to maintain the grinder pumps, this is sure to be an infrastructure solution that protects homeowners and the surrounding water for decades to come.
Mark Gahry has 18 years of experience working on municipal public works projects and can be reached at (734) 675-4000 or firstname.lastname@example.org; Brian L. Woodworth has worked closely with Brownstown Township since 1996 and can be reached at (734) 947-9700 or email@example.com.