Microtunneling Shows Its “Green” Strength in Cleveland
The Northeast Ohio Regional Sewer District (NEORSD) evaluated replacement and repair alternatives for a 30-year-old pump station and associated sewer force main that conveyed domestic and industrial sewage from the Valley Belt Industrial Parkway to a downstream interceptor.
The local sewer system consisted of approximately 2,000 ft of gravity sanitary sewer, a pump station and 750 ft of force main. The pump station and force main were required to convey flows to a 40-ft high spot along the Valley Belt Industrial Parkway. As part of its evaluation, NEORSD considered eliminating the pump station and abandoning its force main through construction of a deep gravity sewer, consistent with its “Green” philosophy of replacing aging pump stations with gravity sewers, where feasible and cost-effective. Preliminary results of this evaluation suggested that a gravity sewer was more cost-effective than installing and maintaining a replacement pump station. However, an open-cut installation of new gravity sewer through the 40-ft high hill was considered to be economically unattractive. Hence, a trenchless solution was preferred.
NEORSD retained the engineering services of Hatch Mott MacDonald Inc. (HMM) to complete a comprehensive study to identify the anticipated geotechnical conditions and construction risks, develop the design criteria for the new gravity sewer and applicable trenchless construction method(s) and perform construction administration and management services. Based on the results of the comprehensive study, microtunneling was identified as the preferred trenchless method to complete the construction of the new gravity sewer. The site-specific advantages of this method included significantly reduced disruption and social impacts to local residences, businesses, and commerce and reduced environmental impacts.
While microtunneling was identified as the preferred construction method, NEORSD had concerns due to a claim on a previous microtunnel project. To manage the potential for claims more effectively, HMM used its risk-based design approach, including the use of a risk register to capture/document identified risks and monitor/track the level of each risk as the design matured. Mitigation measures were developed and incorporated into the design for the identified risks.
The project involved constructing approximately 1,560 ft of new 18-in. PVC sewer pipe at installation depths of between 12 and 55 ft. Microtunneling was used to install 42-in. reinforced concrete casing pipe in two drives measuring 761 and 688 ft. A two-pass microtunnel installation strategy was adopted to reduce construction risks associated with the required near-flat grade of new gravity sewer, the potential for encountering cobbles up to 11 in. in diameter on the eastern portion of the alignment and to allow placement of intermediate jacking stations within the casing string. Connections to the existing sewer at each end of the project were completed using traditional open-cut construction methods. The eastern microtunnel drive passed beneath three large diameter gas transmission mains with 10 ft of cover. Stand pipes were installed on one of the pipelines to closely monitor potential ground movements in response to microtunneling operations during construction.
The project is located on the adjacent slopes of the southern and western banks of the Cuyahoga River and West Creek. The ground conditions along the microtunnel alignment featured two distinct geological settings. Glacio-lacustrine deposits of silts and clays are present along the majority of the alignment. These clay soils are characterized as being very soft to very stiff. Laminations and interbeds of silt and very fine sands up to 4 in. in thickness are locally present within the clay at random intervals. The west microtunnel drive was located solely within these materials.
Alluvial deposits of sand and gravel are present within the floodplain of West Creek on the eastern portion of the alignment. The sand layers are characterized as being very loose to loose and flow readily when unsupported. Gravel layers up to 2.5 ft in thickness occur at various depths within the sand. These water-bearing alluvial deposits are hydraulically connected to West Creek and the Cuyahoga River. Cobbles up to 11 in. were anticipated intermittently within the alluvial soils. The east microtunnel drive initiated within the alluvial materials and transitioned into the glacio-lacustrine deposits approximately two-thirds of the length into the drive.
Jacking shafts were constructed on both ends of the microtunnel alignment, due to limitations on available space and proximity to a 10-story building adjacent to the reception shaft location. The east jacking shaft measured approximately 28.5 ft in length, 17 ft in width, and 17 ft deep below the ground surface. Sheet piles were extended to a depth of approximately 40 ft below ground surface to prevent basal heave within the shaft. This shaft was located within the flood plain of West Creek. Significant flooding occurred on two occasions during the design phase of the project. As a result, the east jacking shaft was designed as a sheet pile shaft with the sheets extending to an elevation above the 100-year flood level. Flooding did occur during shaft construction but the extent of flooding was not as severe as observed during the design phase. No delays or additional costs were incurred by the contractor as a result of flooding.
The 22.5-ft deep, 28.5-ft diameter west jacking shaft was constructed using steel liner plates and ribs. Special consideration was given to the design of the thrust block to properly transfer the required jacking loads to the shaft support system and adjacent soils to limit induced pressures to less than 3,000 psf. Grout was used to fill voids behind the liner plates as the shaft was constructed from the top downward.
The 60-ft deep, 16-ft diameter common reception shaft was designed as a steel liner plate and rib support system. This shaft was constructed in a similar manner to the west jacking shaft.
Construction of the project was awarded to Jay Dee Contractors Inc. Its microtunnel machine consisted of a refurbished RVS 600S Soltau machine. The contract documents allowed for the contractor to select the appropriate cutter head for the anticipated site soils. Jay Dee’s cutter head was equipped with disc cutters, carbide teeth and scraper teeth. While this cutter head was not necessarily matched to the fine grained glacio-lacustrine soils, the cutter wheel tools were selected by the contractor for the microtunnel drive through the alluvial deposits.
Both microtunnel drives were completed without adverse incident. The western drive through the stiff silts and clays proved to be a challenge with respect to balancing the amount of material excavated with the advance rate. Difficulties passing the relatively dry clayey soils through limited openings of the cutter head and entraining the clay rich soils into the slurry system resulted in under excavation of the site soils and subsequent minor ground surface heave.
The 18-in.PVC carrier pipe was installed within the RCP casing. Steel spacers were used to position the PVC pipe on grade. Once installed, low density cellular grout was used to backfill the annulus between the casing and carrier pipes.
The new gravity sewer continues to convey sanitary and industrial flows. The old pump station and associated force main have been decommissioned and abandoned to complete the sustainable green solution offered by this project. No claims were requested by the contractor, alleviating NEORSD’s previous concerns from a previous microtunnel project. The final contract amount was approximately $5 million, which was 15 percent lower than the engineer’s estimate.
Glenn Duyvestyn is a senior project engineer with Hatch Mott MacDonald Inc. and Stephan Janosko is with the Northeast Ohio Regional Sewer District.