Notes from the underground
Every day in the United States, 32 billion gallons of wastewater are transported through sewage collection systems, in pipes that vary in degrees of structural integrity and capacity as well as material makeup (e.g., brick, concrete, clay, plastic, etc.). As the systems age and populations grow, local governments are faced with the task of rehabilitating and expanding existing pipelines, but few have the financial resources to revamp entire networks. (According to EPA, a nationwide overhaul of wastewater collection systems would cost $73 billion.)
Even when communities are able to repair pipelines, they often incur costs beyond those for planning, material and labor. For example, a study by Jason Consultants, Washington, D.C., analyzed pipeline repair in the United Kingdom and found that, in densely populated, high-traffic areas, the social costs of traffic delays, business interruption and neighborhood disruption can reach three to four times the cost of the actual repairs.
Consequently, many wastewater managers are diagnosing the most critical pipe sections in their communities and using trenchless methods to alleviate their problems. Applying the technology to a broad range of pipe materials and sizes allows local governments to minimize costs as well as disruption.
Pipeline triage”Treat the most critical first,” is the logical strategy for cities with urgent priorities and limited funds. Factors such as age, infiltration and inflow rates, external loading and risk (in terms of pipeline failure consequences) help determine inspection priorities.
In 1993, the Phoenix Water Service Department commissioned a study of the city’s 4,000-mile collection system, which includes 30- to 60-year-old sewers. In Phoenix, unlined concrete pipes and the flat desert topography create the perfect environment for the bacteria that cause hydrogen sulfide corrosion.
The water service team inspected manholes, measured flow and videotaped a representative sample of pipelines from 24 to 90 inches in diameter. The results were then combined into a pipe “grade” that helped identify the portions of the system most vulnerable to failure.
In one year, the city rehabilitated approximately 18,000 feet of large diameter pipe and 70,000 feet of small diameter pipe, using sliplining and modified sliplining techniques.Project Manager Bob Webb is renewing and updating his inspection program for the next tier of graded pipelines.
Webb notes that sonar inspection, in addition to video assessment, provides a reliable picture of pipeline conditions. “Today, sonar inspection helps us compare actual conditions to a superimposed pipeline profile, giving an accurate picture of the entire structure,” he says.
In addition to following an ongoing inspection schedule, the Water Services Department has instituted a four-year pipeline maintenance program. It includes annually spraying more than 37 miles of unlined concrete sewers with magnesium hydroxide to ward off corrosion. “It’s a life-cycle approach that takes some of the pressure off the capital dollars,” Webb says.
Options for rehabilitationIn addition to identifying needed repairs for a system, complete assessment provides the information necessary to analyze the feasibility of trenchless rehabilitation options: * sliplining — installing a new pipe within an existing one; * modified sliplining — inserting a reduced diameter pipe liner into the host pipe and expanding it for a tight fit; * cured-in-place lining — inserting a flexible pipe liner and heating it to closely fit the host pipe; * pipe bursting — using radial forces to break out a deteriorated pipe and install a new pipe in the same location.
Selecting the best technology starts with an examination of the structural condition of the original pipeline and an analysis of the trenchless options. For example, how will the rehabilitation affect system hydraulics? How will it affect the pipeline’s longevity? What sort of maintenance will be required? Are access points, traffic and safety parameters conducive to a trenchless installation?
In Phoenix, city staff members are using software to help them evaluate trenchless rehabilitation options. Using the program, they are able to examine hydraulic variables such as flow, slope, friction factors and velocity, as well as cost.
Sliplining. The sliplining process inserts a new pipe into the old one by pulling or pushing. The remaining annular space typically is filled with grout.
Sliplining is simple and relatively inexpensive, and it provides full structural rehabilitation for pipes that are not excessively deteriorated. However, the liner pipe requires a sizable installation pit and usually creates a significant loss of hydraulic capacity.
Modified sliplining/cured-in-place lining. Modified sliplining includes a range of technologies that use reduced-diameter or deformed plastic liners to overcome sliplining’s installation and capacity limitations. The liner is inserted through a manhole and returned to its original diameter with hot water or steam, forming a snug fit within the existing pipe.For cured-in-place rehabilitation, a flexible fabric tube liner impregnated with resin is inserted into the host pipe and positioned by injecting hot water or steam. The heat cures the resin, shaping and hardening the tube to match the existing pipe.
Modified sliplining and cured-in-place technology offer several advantages. For example, liners are installed through existing manholes; lateral location and reconnection are simplified; and lining can be completed within a few hours, reducing the need for bypass pumping.
In Nashville, Tenn., Metro Water Services (MWS) is using both methods to rehabilitate approximately 20 percent of its 2,000-mile sewer system. “Some cured-in-place systems offer the capability to rehabilitate both the main line and the service line without requiring excavation at the service connection point,” says Greg Ballard, an engineer for MWS. Polyester needled felt fabric and PVC linings have worked well for Nashville, he adds.
The cost of modified sliplining and cured-in-place lining make them attractive options for cities and counties. “Greater competition among contractors has improved quality and made cured-in-place liners significantly less expensive than they were even five years ago,” Ballard says. In Phoenix, Webb estimates that using cured-in-place lining has saved 40 to 50 percent of the cost of pipeline replacement.
Pipe bursting. The pipe bursting process removes existing pipe and simultaneously installs a new, sometimes larger, pipe at the same location. A pneumatic or hydraulic bursting tool breaks the existing pipe, compresses the surrounding soil, and pulls or pushes the replacement pipe into position.
When the replacement pipe is larger than the original pipe, adjacent utilities and/or stiff, rocky soils may limit pipe bursting’s usefulness. Also, lateral connections must be excavated immediately to reinstate service, causing some surface disruption.
On the other hand, the method allows communities to rehabilitate pipelines in congested areas without locating new pipe routes. Also, it eliminates the problem of structural defects in the old pipe because a new pipe is installed.
Recognizing those advantages, cities used pipe bursting to install more than 100 miles of pipe between 1990 and 1996. Typically, they used the technology to enlarge pipes measuring less than 18 inches in diameter by 20 to 50 percent.
For example, in Central Point, Ore., the Bear Valley Sanitary Sewer Authority used pipe bursting to replace nearly 2,000 feet of 16- and 18-inch diameter concrete pipe with 20-inch high-density polyethylene (HDPE) pipe. And in Shreveport, La., a contractor burst a 15-inch concrete pipe and installed a new 30-inch HDPE pipe.
Installation methodsAs the underground utility network grows, the available pipeline installation space decreases while the density of the above-ground neighborhood increases. Sewer installations present particular challenges because of their depth, long reaches between manholes, and requirements for precise line and grade installation.
As they have done in pipeline rehabilitation projects, local governments are using trenchless technology for new installations. With pipe jacking/microtunneling, pilot tube microtunneling and directional drilling, they are able to extend collection systems while minimizing costs and disruptions.
* Pipe jacking and microtunneling are trenchless techniques for installing gravity pipes to precise line and grade. Pipe jacking uses hydraulic rams to push preformed pipe sections to line the hole formed by a cutting head or shield. Microtunneling is a miniaturized, remote-controlled form of pipe jacking, typically used with pipes smaller than 36 inches in diameter, where access is limited.
Pipe jacking and microtunneling can provide the same line and grade accuracy as open cut methods and offer additional benefits. For example, they may be used below the water table; they reduce the potential for damage to adjacent structures and utilities; and they decrease surface disruption in high-traffic areas.
Community disruption was central to the decision to use microtunneling in Fremont, Calif. There, the Union Sanitary District (USD) had to contend with shallow groundwater and contaminated soil during a recent 1-mile installation of 21-inch polymer-concrete pipe. “Heavy traffic volumes, conflicting utilities and unstable soils were important factors in our decision to opt for microtunneling,” says Jesse Gill, senior engineer for USD. Microtunneling at 17- to 18-foot depths avoided existing utilities and eliminated the need to treat the contaminated soil and groundwater.
As part of its construction planning process, USD included requirements for noise and traffic control, as well as community information programs. Phoenix had similar requirements in its rehabilitation project, and Webb recommends that traffic control and bypass plans be incorporated into all project specifications.
* As part of pilot tube microtunneling, 6- or 8-foot diameter, steel-lined, vertical shafts are augured into the ground, allowing installation of clay pipe horizontally from the shaft. The method is most effective when used for small diameter sewer lines, and it minimizes street cuts and settling, protecting construction crews and the public from the dangers of open trenches in unstable soil.
Cape Girardeau, Mo., developed a new pilot tube microtunneling application in 1996, when it separated its combined sewer system and installed 9.3 miles of new sanitary sewer mains. The city faced several potential problems, including a tight budget, high-density residential and commercial neighborhoods, and unstable soil conditions. To overcome those obstacles, the project designer and contractor used a remotely guided pilot tube and a modified European house connection process to install 6- to 15-inch diameter pipes.
With that installation method, several house connections were made from the same vertical shaft. Furthermore, because the installation occurred below other utilities and on a precise line and grade, construction and testing were performed on the new sewer and laterals as a single unit, prior to connecting the houses. Finally, each connection was smoke-tested, allowing the city to identify and eliminate more than 2,000 points of infiltration and inflow.
For Cape Girardeau, pilot microtunneling proved to be community-friendly and cost-effective, says City Engineer Mark Lester. “For project bids to date, the city has saved about 15 percent in construction costs, compared to open cut methods,” he notes. * Precision guidance systems make directional drilling a significant trenchless success story. Steering from a surface-launched rig, operators use a rotating drill string to pilot bore and then enlarge the opening with a back reamer. The method eliminates the need for access pits; has low mobilization costs; accommodates a wide range of rig sizes; and speeds pipe installation.
The electric, telecommunications and gas industries pioneered directional drilling. Wastewater utilities are exploring new guidance capabilities that improve accuracy and cut costs. For example, Everett, Wash., is installing approximately 630 feet of 12-inch diameter HDPE pipe on a 1 percent slope. According to Project Manager Mark Sadler, directional drilling, if successful, could save the city nearly 50 percent over conventional installation methods.
The benefits of trenchless installation also have spurred innovation among water utilities. For example, in Flower Mound, Texas, town officials will use directional drilling to install new HDPE water mains beneath the lawns and driveways of a 280-home subdivision. A feasibility study demonstrated that guided drilling could replace the existing pipes at a lower cost than open cut installation.
Regardless of how it is used — in wastewater or water applications, for rehabilitation or expansion projects — trenchless technology is allowing communities of all sizes to renew and extend their underground infrastructure. While the process remains hidden, its benefits, ranging from relative cost to neighborhood impact, are clearly visible.
Steven Kramer is principal for trenchless technology, Daniel Nichols is project manager, and Ann Dettmer is the writing and design manager for Sverdrup Civil, Maryland Heights, Mo.
When San Diego began rehabilitating a trunk sewer along East Mission Gorge, watchdogs fixed their gaze. Because the area is rich with historic and natural resources, the project was scrutinized at all stages by multiple groups. Their concerns were calmed, however, when the city employed trenchless techniques for pipe restoration, thereby minimizing disruption.
In service since 1963, the East Mission Trunk Sewer carries millions of gallons of wastewater each day from San Diego and its surrounding suburbs to a plant at Point Loma for treatment. En route, the sewer passes through historic Mission Trails Regional Park, a popular scenic venue that contains Native American and California Mission artifacts.
Crossing the San Diego River at several points, the sewer line also runs near a dam built by Native Americans in 1805 to supply water to the mission. Finally, it touches a riparian habitat that is home to a variety of endangered plant and animal life.”Because of the sewer’s location, this project had the attention of every government, civic and special interest group you could think of,” says Jeffrey Shoaf, a senior civil engineer for the city. “Everyone had a different — and perfectly valid — reason why they wanted to protect the area from disruption.”
A 1994 report by consulting engineers Dudek & Associates, Encinitas, Calif., indicated that the East Mission sewer needed major repairs. Over time, the river had washed away much of the sewer’s cover, exposing it to large, falling rocks in some areas. As deterioration, root intrusion and debris deposits escalated, flow capacity dropped from 29 million gpd to 25 million gpd.
While the sewer’s capacity was shrinking, the area’s population was not. “The communities served by the sewer are all growing,” Shoaf explains. “As time passed, the problem would only get worse.” Additionally, more than 1 million gallons of river water were infiltrating the pipeline each day, burdening the city’s treatment plant.Because of the area’s sensitive makeup, and because the city needed to maintain sewer service during the course of rehabilitation, officials chose a trenchless method for restorative work. “Digging up and replacing this deteriorated sewer was really not an option,” Shoaf says.
“From day one, we had a lot of constraints on how and when we could perform our work,” he notes. For example, construction could not occur between March 15 and Sept. 1 because it would interrupt the mating season of two endangered bird species: the Least Bell’s Vireo and the California Gnatcatcher.
The contract was awarded to the Santa Fe Springs, Calif., office of Chesterfield, Mo.-based Insituform Technologies. Using cured-in-place technology, the company began work on the first phase of the project in Fall 1996.
(Cured-in-place rehabilitation involves the insertion of a custom-made, felt tube into the deteriorating pipe from an existing manhole. The tube, filled with resin, is then inverted, and hot water is circulated through it to cure the resin and create a jointless, corrosion-resistant pipe.)
Working only during fall and winter months for two years, the company restored more than 7 miles of 36- and 42-inch pipe. The project required 25 inversions, costing the city $11 million.
The job was not without obstacles. For example, the water level within the river gorge was at its highest during the project, and much of the pipe was remotely located. “Two-thirds of the pipe didn’t allow vehicular access,” Shoaf says, noting that temporary and permanent river crossings were installed to link workers to the construction site.
The East Mission project was completed in late 1998 with no complaints, Shoaf says. “We made great efforts to communicate with everyone from the citizen advisory groups and federal agencies to the park rangers and visitors about what we were doing,” he explains. “Then we did what we said, and it really paid off.”
While the 1990s have been a boom time for housing construction, the down side is that new houses and increasing sewage volume have put pressure on sanitary distribution systems. That was the case in Rockford, Ill.,where the Rock River Water Reclamation District (RRWRD) recently replaced 2,300 feet of an existing 10-inch line.
In 1981, the RRWRD lined the sanitary trunk line in the southeast part of Rockford to eliminate root intrusion. As a result, the pipe’s diameter was reduced to 8 inches. At the same time, housing expanded to the east of the trunk line, increasing the effluent that flowed through the pipe.
By 1997, the town needed to double the pipe’s diameter, and officials soon began examining the feasibility of replacement methods. They determined that open excavation would not work for several reasons: * The pipeline ran through a residential area, and open excavation would increase the probability that the town would have to remediate damage to lawns and property. * At 10 to 17 feet, open excavation would have been costly. * Each owner had sign-off authority on the restoration, and open excavation increased the possibility for service calls months after the project’s completion.
Instead of open excavation, officials chose pipe bursting (cracking existing pipe and inserting new pipe in its place). The trunk line had no laterals connected to it, but merely picked up the wastewater from other 8-inch lines that collected effluent. Because no lateral pits would have to be dug, excavation could be minimized, and long burst runs would be possible.
Rockford Blacktop Construction was awarded the replacement contract and began work in June 1998. It divided the project into several burst runs, first removing the liner from the existing clay tile pipe.
Each burst was made using the Hammerhead Mole from Pella, Iowa-based Vermeer Manufacturing. A 10-foot section of 20-inch diameter polyethylene pipe was used at the lead end of the burst; it was then reduced and fused to the new 16-inch polyethylene pipe, which was pulled through the existing line.
The first two runs, measuring 200 and 190 feet, were made with a single winch, and longer runs (400, 410 and 635 feet) required two winches. (A single large winch could not be used because it would have been difficult to place into the backyards. Also, the replacement plan called for the retention of the existing old masonry block manholes, and officials feared that the downrigger bracing of a large winch would break the manhole structures.)
New pipe was pulled through the shorter runs at a rate of 5 feet per minute. The 410-foot run took a little over an hour to complete, and the 635-foot run took two-and-a-half hours.
At its deepest, the project traveled 18 feet beneath a water main and a garage, as well as part of an apartment building. Workers used a seismograph to monitor vibration levels and to ensure that the pneumatic tool would not damage nearby structures.
Workers also took precautions to prevent blockage within the pipeline. Because of the amount of soil compaction expected during the longer burst runs, a half-inch lubrication line feeding a water and polymer mixture to the burst area was strung inside new pipeline.The Rockford replacement was completed last August and cost the town $295,000. Although officials expect to replace additional pipe sections, there are no such plans for the near future.
The forecast for Eugene, Ore.: lots of rain, and a high probability of more stringent water quality regulations. In other words, perfect conditions for a pilot program to test the feasibility of an in-line stormwater treatment system.
Nestled in the Willamette River valley on the western slopes of the Cascade Range, Eugene (population 133,000) receives an average of 47 inches of rain annually. The city has held a federal national pollutant discharge elimination system (NPDES) permit since 1993, and it has an ongoing stormwater management program.
As the federal government introduces new standards for local water quality, city officials have set aside funds for water quality pilot programs, including a project that focuses on stormwater treatment. “It’s forward thinking to get experience [in maintaining and monitoring] stormwater treatment structures as we prepare for the potential that the bar will be raised for future water quality standards,” says Tom Larsen, a civil engineer for Eugene’s Public Works Transportation Division.
Eugene officials chose the Ferry Street Bridge, which is undergoing a $30 million, multi-year renovation, as their pilot site. The drainage basin measures just under 3 acres, and the site marks a transportation and commercial corridor that accommodates an average of 64,000 cars per day; several automobile dealerships; and an active gas station.
Grease, oil, suspended solids and other removable loadings run off parking lots and roadways with rainfall. Removing those pollutants is crucial, as the drainage area discharges directly to the Willamette River, which flows through the center of Eugene.In treating the site’s stormwater, city staff set the following criteria: * Treatment must be provided in line; * Maintenance must incorporate existing equipment; * The treatment must be designed so that, even if maintenance were not supplied, conveyance would not be diminished from the level provided by a standard in-line manhole.
Multiple factors affected the specifications for the treatment unit. For example, the city needed a shallow design because land availability was limited; the more vertical the unit, the higher the excavation costs and the likelihood that dewatering would be needed to combat fluctuating groundwater levels.
With those considerations in mind, Eugene officials purchased the Vortechs Stormwater Treatment System 3000 for installation at the Ferry Street Bridge. Manufactured by Vortechnics, Portland, Maine, the unit is constructed of precast concrete and aluminum. Mowat Construction, Kirkland, Wash., completed the project and made the first connections to the new system last November.
For the next several years, the Eugene Public Works staff will monitor the amounts of sediment and other pollutants removed from the stormwater structure. It also will evaluate maintenance efficiencies and determine if other sites may be suitable for similar structures. Meaningful monitoring results are expected after the bridge construction is completed in early 2000.
For more information about the Ferry Street Bridge project and the water quality pilot program, visit the city of Eugene web site at www.ci.eugene.or.us.