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Trenchless sewer rehabilitation plays role in Boston Harbor cleanup.

Updating municipal infrastructure with the latest environmentally sensitive technology isn't always easy. Just ask engineers with the Massachusetts Water Resources Authority (MWRA), who set out to rehabilitate the corroded and structurally unsound Wellesley Extension Relief Sewer (WERS). Their effort was part of a larger plan to modernize the wastewater collection and treatment system that serves 43 communities in the Boston area.

As agency engineers began to make plans for the refurbishment of the extensive system, they were faced with a dilemma. Ignoring the problem could lead to contamination of the Charles River, the Boston Harbor, and nearby public and private water-supply aquifers. On the other hand, excavating and replacing the pipes would mean disrupting sensitive wetlands on the surface and private property owned by suburban residents.

MWRA selected Insituform New England, a subsidiary of Insituform Technologies, Inc. to handle this delicate task. Insituform rehabilitated the badly deteriorated large-diameter existing sewer pipelines with their jointless, non-disruptive pipe rehabilitation process. In addition to rehabilitating the pipeline, the Insituform[R] process required minimal surface disruption.

Begun in 1993, the project was completed in 1995. When the last truck pulled away, over 7.5 miles of reinforced concrete pipe of varying diameters had been rehabilitated - the largest sewer rehabilitation contract ever undertaken in the United States.

The Wellesley sewer is comprised of a 35-year-old 48-, 54-, and 60-in. diameter reinforced concrete pipe, and also contains a 3,000-ft tunnel and nine concrete structures consisting of flow connection chambers and siphon inlet and outlet chambers. The system is one of four major interceptors that serve 21 communities south and west of Boston and eventually lead to the Nut Island wastewater treatment facility in Boston Harbor.

Due to unusually rapid economic growth and suburban development in the west Boston metropolitan area, increased flows caused the interceptor system to become overloaded, especially during periods of high groundwater and stormwater inflows. The pipes simply could not handle the extra infiltration and inflow, and overflows of untreated raw sewage spilled directly into the Charles River during heavy rain storms.

In addition to the overload, the system's decaying condition was causing problems. Closed-circuit television and visual inspection of the WERS revealed that hydrogen sulfide gas had caused extensive structural deterioration of the pipes and manholes, most of which had been installed in the 1950s. Over the years, the hydrogen sulfide problem was exacerbated when manhole and chamber covers were sealed to minimize odors drifting onto nearby private properties. Other factors had contributed to the corrosion of the pipeline, including a continuous flat pipe slope over the entire length with a 6-ft drop per 10,000 ft of length. The corresponding low velocities - and tributary force mains along the interceptor - had resulted in septic sewage due to long detention times in pumping station wet wells and force mains. Video inspections also revealed loss of concrete, extensive exposed aggregate, and visible reinforcing steel throughout the pipeline and manholes.

As a result of the condition survey, it was determined that the rehabilitated pipe design would be based on a fully deteriorated condition of the host pipe. This design basis provides for the rehabilitated pipe to support all dead loads, live loads, and groundwater load imposed - including the 100-year flood elevation - with the assumption that the existing sewer pipe could not share any loading or contribute to the structural integrity of the completed pipe.

Environmentally Sensitive Site

One of the most urgent aspects of the project was the environmentally sensitive nature of the site. The Charles River, the delicate wetland areas adjacent to it, and a number of private properties were facing possible serious contamination. Nearly 75 percent of the targeted sewer line ran through the flood plain of the river, which includes extensive wetlands and crosses more than 100 private properties. This portion of the river also runs in close proximity to both public and private water supply wells.

While the system's environmentally sensitive location made the rehabilitation critical, it also limited many of the possible solutions. Various rehabilitation methods were considered by SEA Consultants, Inc., the firm contracted by MWRA for the design and construction administration services on the project.

Because of the surrounding wetlands, excavation was eliminated from the outset. The techniques considered for rehabilitation of the pipes were coating systems, slip lining, and cured-in-place pipe rehabilitation. Each method was evaluated based on its infiltration prevention, resistance to abrasion and hydrogen sulfide corrosion, service life (40 years was optimal), structural support, ease of installation, and cost.

After the design evaluations were complete, SEA recommended that the pipeline be rehabilitated by either Insituform pipe rehabilitation or slip lining, since either method could provide corrosion resistance, structural support, and a long-term solution. Competitive specifications were prepared, and seven bids were received. Six of the bids were for sliplining, however, the low bid came from Insituform. That process proved more cost-efficient, mainly because sliplining would have required the use of 36 insertion shafts, which would have impacted the surrounding wetlands and resulted in additional costs. As a result, the slip lining bids submitted were significantly higher than the bid submitted by Insituform - in fact, the difference between the first two bidders alone was $1.1 million.

Because the Wellesley project encompassed approximately 36 square miles of land and open water primarily within the Charles River basin, a large number of permits were needed from various federal, state, local, and private agencies before the work could take place. The non-disruptive nature of the Insituform process met all of the specific work area and access restrictions established by the permitting agencies. The long installation lengths - up to 2,352 ft - minimized the number of sites at which the surface had to be disturbed.

Pushing the Limits of Trenchless Technology

This commitment to minimizing surface disruption took some planning. The lengths of pipe between the insertion points often contained multiple bends, and most challenging of all was the presence of long pipe reaches that regularly exceeded 1,000 ft and in some cases exceeded 2,000 ft.

The process uses a custom-manufactured non-woven felt tube impregnated with a polyester thermosetting resin. The tube is inserted into the damaged section of pipe, usually through an existing manhole, and installers use gravity and water pressure to propel the tube through the host pipe, inverting it while pressing it radially outward into intimate contact with the walls of the existing pipe. When the tube reaches a termination point, the water inside is heated with a mobile heat exchanger that accelerates the curing of the thermosetting resin. The result is a jointless pipe-within-a-pipe.

The rehabilitated pipe is as strong or stronger than the pipe it replaces and can restore the structural integrity of collapsing underground pipes. In addition to restoring original flow capacity (or, in some cases, surpassing original flow capacity), the rehabilitation helps to prevent dangerous washouts and cave-ins.

The process proved especially useful in the WERS rehabilitation. In addition to successfully navigating the system's many bends, the Insituform process proved itself in record-length installations. In fact, the first inversion of the project measured 1,400 linear ft, which was a record length for the regional Insituform installer. Another inversion - a 2,352-ft length of 48-in. diameter pipe - set a world-wide company record.

The aspect of the project that most challenged the limits of trenchless technology was the rehabilitation of a 4,200-ft, 60-in. diameter segment called the Riverdale Tunnel. The pipeline not only featured a 3,000-ft tunnel with limited access points, but also challenged rehabilitation efforts with a 55-ft head of groundwater over the crown of the pipe. Insituform installers successfully rehabilitated the pipe with two record inversions that overlapped near the center of the segment. A customized polyester resin system was developed to give the new pipe a minimum modulus of elasticity of 350,000 psi and a record 200,000 pounds of resin was used. A 2,050-ft upstream inversion was completed first, and was followed by a 2,150-ft downstream inversion.

Although these unusually long lengths were considered by the industry to be exceptions to the rule when first completed, SEA and Insituform have made them all but routine: more than 30 "long-length" inversions (greater than 1,110 ft) were completed over the course of the project. Insituform perfected a number of unique processes to make these installations possible, including the use of sealed overlaps and hold-back lines.

The uniqueness of the WERS project also proved the adaptability of the process. While most Insituform tubes are impregnated with resin at an Insituform facility and brought to the job in a refrigerated truck, the length, diameter, and weight of the tubes custom-designed for this large project required on-site impregnation. And while fluctuating temperatures frequently require rescheduling of trenchless inversions, Insituform managed the WERS installations in a variety of adverse conditions: the first seven inversions actually were completed during snow storms and -5 [degrees] F temperature conditions. On-site heating of the resin in the winter months and submersion of the impregnated liners in ice-baths during the summer heat-wave were scheduled into the process to minimize delay.

In addition to the actual pipe rehabilitation, Insituform performed pre- and post-installation inspections, established flow diversions during inversions to prevent service interruptions, and applied epoxy coating on the concrete flow control structures. The company also rehabilitated manholes, filled 17,250 ft of various-sized sewers for abandonment, and conducted post-installation environmental restoration.

A Successful Partnership

MWRA, SEA, and Insituform have proved with this enormous project that environmental sensitivity and cost-efficiency can both be accommodated in underground pipeline rehabilitations. While a prior excavation and replacement of a parallel large-diameter pipeline cost MWRA $38 million over 2-1/2 years, the rehabilitation project (similar in scope) was bid and constructed at a cost of $12.2 million over roughly two years.

The enormous savings have not led to a sacrifice in quality - the trenchless pipe rehabilitation technology has the strength and versatility to succeed in projects of scopes never undertaken before. Infiltration and inflow have been reduced, already resulting in less overflow of raw sewage into Boston Harbor.

In addition to durability, efficiency had also been improved when the project was completed - flow capacity was increased due to the decreased coefficient of friction. The WERS rehabilitation was designed to abate overflow and help the system meet future peak flows during a 40-year planning period, which will considerably improve the quality of water in the Boston Harbor.

As trenchless processes like those continue to rise to the challenges presented by large-scale and complicated tasks, they will continue to earn the confidence of engineers that has fueled the explosive growth of the industry to date.
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Author:Struzziery, John J.; Spruch, Arthur A.
Publication:Public Works
Date:Sep 1, 1996
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