Much of our nation’s underground infrastructure is reaching or functioning beyond expected lifespans.
This critical network of pipes and mains serve our most basic drinking water and sanitary sewer needs. Many municipalities are faced with the challenging task of replacing or lining deteriorating facilities while minimizing service disruptions for residences and businesses.
The City of Bowie, Maryland, recently completed a successful pilot project that replaced 830 ft of water main using pipe bursting. Although this trenchless approach is used to replace sewer lines, this project offered proof of concept for upgrading and replacing the city’s aging water network.
Pipe bursting is a trenchless method that has traditionally been used for more than 30 years for replacing sanitary sewer pipes, but more recently is being employed for water main replacements. Essentially the process involves placing a pipe bursting head within an existing pipe to be burst. This bursting head is then pulled using a hydraulic unit back through the existing pipe. As it is pulled back though, the blades on the bursting head cut apart the existing pipe. An expander head then follows and pushes the pipe fragments out into the surrounding soil. Following this expander, a new pipe is pulled into place in the space once occupied by the original pipe. Bursting enables pipes to be upsized by as much as 3 pipe sizes, depending on the depth of the pipe. Typically, pipes installed up to 12 ft deep are either replaced size for size, or upsized to the next highest diameter.
The process itself provides several advantages over the standard “cut-and-cover” trenching method. These benefits include significantly reduced design and construction costs, shortened design and construction duration, limited pavement disturbance, lower risk of damage to adjacent properties and nearby infrastructure, and reduced environmental concerns such as impacts to streams and natural resources. The realization of these advantages depends on the specific site being considered. For instance, a pipe segment with a large number of house connections may not produce a lower construction cost because of the excavation work associated with relinking each service connection to the new main. In addition, the geotechnical conditions of an area can restrict the applicability of the technology. This is the case in areas of dense soil conditions, such as various types of clay, which may transfer the force of the bursting to adjacent structures. This situation could also lead to possible heaving of surface soils and pavement. It is important to understand the subsurface conditions in an area being considered for bursting.
City of Bowie Case Study
The City of Bowie owns and operates a water and sewer utility system that serves 7,800 residents and 100 commercial businesses. The system, which was installed in the 1960s, includes approximately 90 miles of water distribution piping. While the 10-in. and larger pipes are concrete-lined cast iron, most of the piping is 6- or 8-in. diameter, unlined cast iron pipe, which has experienced significant corrosion problems. The most common corrosion problem is tuberculation, which is the formation of small mounds of iron oxide (tubercles) inside the piping. Tuberculation accumulation over the past 50 years has reduced the hydraulic capacity of the pipes, and when high flows are experienced, suspended iron oxides discolor the water.
After investigating different strategies to replace its water mains, the Bowie Department of Public Works initiated a prototype pipe bursting project in one of the neighborhoods experiencing very low flows. This particular section of 6-in. main is approximately 830 ft long and serves 23 homes that are not supplied by any other water mains. The water main extends beneath trees, asphalt roadways, paved driveways, sewer and gas easements, and a stream.
The City wanted to avoid any unnecessary interference with the daily activities of the residents who lived along the section of water main. Pipe bursting was optimal because it required limited excavation, thereby reducing cost, shortening the construction schedule, and minimizing impacts to property owners.
The design effort was able to be streamlined, as well. Since pipe bursting is a means for rehabilitating existing pipe in the same location, the design drawings could be produced without the need for a full topographic survey, which is required for standard pipe replacement engineering drawings. The design utilized the existing city GIS mapping as a basis for the construction plans to indicate the location of the bursting efforts, as well as other accompanying details. One particular challenge faced by the design team was the lack of a profile drawing for the existing water main. The profile would have indicated the water main depth, as well as the configuration of the water main beneath the existing stream. Because of this, the design team undertook several test pits at the stream crossing to verify the absence of pipe fittings or concrete encasement, which would potentially prohibit the bursting head and pull rods from passing through this area.
The contractor set up a water bypass and dug entrance and exit pits at approximately 500-ft intervals for the bursting set up. The new 8-in. pipe was pulled behind the expander head, within 14 in. of a nearby gas main and beneath the stream with no environmental impacts. Eight water services were then reinstated after testing and disinfecting the new water main.
The success of this project offered proof of concept for replacing the City’s aging water distribution system while minimizing disturbance to the daily activities of the residents. In Bowie, this pipe bursting project proved successful with numerous benefits including:
- Reducing design and construction cost by approximately 40 to 50 percent
- Shortening design and construction durations by between four to six months.
- Limiting pavement disturbance
- Lowering the risk of damage to adjacent properties and nearby infrastructure
- Minimizing environmental concerns and permitting associated with the stream crossing
Like many approaches for upgrading or replacing underground infrastructure, pipe bursting may not be appropriate in all locations and must be evaluated versus other methodologies for constructability, efficiency and cost-effectiveness. For the City of Bowie and many other municipalities, it is another tool in the toolbox for managing its aging assets and providing the best possible level of service to its customers.