Record Trenchless Sewer Rehabilitation Project Plays Leading Role in Boston Harbor Cleanup

As agency engineers began to make plans for the refurbishment of the extensive WERS 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 underground pipes in the collection network would mean disrupting sensitive wetlands on the surface, as well as private property owned by suburban residents.

MWRA selected <%=company%> to handle this task. The firm's joint-less, non-disruptive, well-proven, 25-year-old process was used to rehabilitate the existing badly-deteriorated large-diameter sewer pipelines. Begun in 1993, the project was completed in 1995. When the last truck pulled away, over 7.5 miles (12 km) of reinforced concrete pipe of varying diameters had been restored to like-new condition-as a result of the largest sewer rehabilitation contract ever undertaken in the United States.

A Commitment to the Environment
The Wellesley sewer comprises a 35-year-old 48, 54, and 60-in. (122, 137, and 152 cm) diameter reinforced concrete pipe, a 3,000-ft (915m) 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 which 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 ground water 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 collection system's decaying condition was causing problems. Closed-circuit television and manual inspection 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 by the sealing of manhole and chamber covers to minimize odors drifting on to nearby private properties. Other factors had contributed to the corrosion of the pipeline, including a continuous flat pipe slope over the entire length with only a 6-ft drop per 10,000 ft of length (1.8-m drop per 3,048 m of length). The resulting low stream velocities-and tributary force mains along the interceptor-had caused sewage to become septic, 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 in manholes.

Out of the condition survey came the decision that the design work for the rehabilitation project would be based on a fully-deteriorated host pipe condition. This design basis provided for the rehabilitated pipe to support all dead loads, live loads, and groundwater loads imposed-including the 100-year flood elevation-with the assumption that the existing sewer pipe could not share any loading on, or contribute to the structural integrity of the completed pipe.

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 public and private water supply wells.

While the system's environmentally sensitive location made the rehabilitation critical, it also limited the choice of 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 rehabilitation techniques considered were coating systems, slip lining, and the cured-in-place pipe process. 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. Because coating systems would have required additional rehabilitation and were projected to eventually cost twice the price of other alternatives for a 40-year planning period, it was concluded that this approach would be too time consuming and costly.

After the design evaluations were complete, SEA recommended that the pipeline be rehabilitated by using either the cured-in-place or slip lining systems, since both methods 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 Technologies, Inc., of Memphis, Tennessee, which proposed the use if its cured-in-place system. This was projected to be more cost-effective, mainly because sliplining would have required the use of 36 insertion shafts. The work to prepare these would have affected the surrounding wetlands and added to the costs. This was evident in the bids, which were significantly higher than the cured-in-place proposal. In fact, the difference between the first two bidders alone was $1.1 million.

Because the Wellesley project encompassed approximately 36 square miles (58 square kilometers) of land and open water primarily within the Charles River basin, a large number of permits and approvals were needed from various federal, state, local, and private agencies before the work could take place. But the non-disruptive nature of the rehabilitation method selected met all of the specific work area and access restrictions established by the permitting agencies. The long installation lengths-up to 2,352 ft (720 m)-minimized the number of sites where 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 (305 m)-and in some cases more than 2,000 ft (610 m).

Insituform's system has been used successfully around the world in many hundreds of pipe restoration projects, municipal and industrial. It is based on 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 pipe. When the tube reaches a termination point, the water still contained in it is heated with a mobile heat exchanger unit that accelerates the curing of the thermosetting resin. The result is a joint-less pipe-within-a pipe, as strong or stronger than the original pipe within which it now rests, and which restores the structural integrity of collapsed sections. In addition to recovering the original flow capacity of the pipeline ( in some cases surpassing it, due to a decreased coefficient of friction achieved by the smooth surface of the cured lining), the rehabilitation helps to prevent dangerous washouts and cave-ins.

The system proved especially useful in the WERS project. In addition to successfully navigating the pipeline's many bends, the method produced record-length installations. In fact, the first inversion of the project measured 1,400 ft (450 m), which was a record length for the regional installing contractor. Another inversion-a 2,352-ft (720-m) length of 48-in. (122-centimeter) diameter pipe-set a worldwide company record.

The aspect of the project that challenged the limits of trenchless technology the most was the rehabilitation of a 4,200-ft (1,280-m), 60-in. (152-cm) diameter segment called the Riverdale Tunnel. This part of the pipeline not only included a 3,000-ft (0.91km) tunnel with limited access points, it posed other problems since the groundwater level was 55 ft (17 m) above the crown of the pipe. The supplier's installation crew completed this segment successfully 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 (2,415 MPa), and a record 200,000 lb (74,600 kg) of resin was used. A 2,050-ft (625-m) upstream inversion was completed first, and was followed by a 2,150-ft (655-m) downstream inversion.

Although these unusually long lengths were considered by the industry to be exceptions to the rule when first completed, SEA's engineers and the Insituform staff proceeded to make them appear routine. More than 30 "long-length" inversions (greater than 1,110 ft/335 m) were completed over the course of the Boston project. The supplier developed several innovative techniques to make these installations possible, including the use of sealed overlaps and hold-back lines.

The uniqueness of the WERS undertaking also demonstrated the adaptability of this process. While the normal procedure is to impregnate the felt tubes with resin at a company plant and transport them 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, the WERS installations were accomplished in a variety of adverse conditions. For example, the first seven inversions were completed during snow storms and -5 F temperatures (-20 C). On-site heating of the resin in the winter months, and submersion of the impregnated liners in ice-baths during a summer heat-wave minimized weather-related delays.

The contract also called for the supplier to perform pre-and post-installation inspections, establish flow diversions when necessary to prevent service interruptions, apply epoxy coatings to the concrete flow control structures, rehabilitate manholes, fill 17,250 ft (5,258 m) of variously-sized sewers for abandonment, and complete post-installation environmental restoration.

A Successful Partnership
The MWRA, SEA, and Insituform demonstrated with this very large project that environmental sensitivity and cost-effectiveness can be accommodated simultaneously in underground pipeline rehabilitation projects. While the prior excavation and replacement of a parallel large-diameter pipeline cost the MWRA $38 million over 2 1/2 years, the WERS job, which was similar in scope, was bid and constructed for $12.2 million in roughly two years.

These enormous savings have not led to a sacrifice in quality. The cured-in-place technology produces the strength and versatility to succeed in projects like this one, which was of unprecedented scope. Infiltration and inflow have been reduced, already resulting in less overflow of raw sewage into Boston Harbor. And the life of the rehabilitated pipeline is not in question. This supplier's system has been shown in a federally-funded study to have a design life of at least 50 years, and for each separate Wellesley installation, Insituform was required to provide two flat-plate or unrestrained samples that had been tested in accordance with ASTM D-638 and ASTM D-790.

The WERS rehabilitation project was designed to abate overflows, and to help the collection system meet future peak flows during a 40-year planning period. It will contribute significantly to the water quality improvements expected to be realized in the Boston Harbor.

About the Authors: John J. Struzziery, P.E., is an associate at S.E.A. Consultants, Inc. in Cambridge, Massachusetts; Tel. 617-568-9100. Arthur A. Spruch, P.E., is a principal of the firm. This paper represents the opinions of the authors and not necessarily those of the Massachusetts Water Resources Authority. Insituform can be contacted at Tel. 314-532-6137 or Fax. 314-530-0751.

Edited by Ian Lisk