A greener solution through trenchless microtunneling
Northeast Ohio Regional Sewer District / Cleveland, Ohio
For thirty years, a pump station and force main conveyed domestic and industrial sewage from Cleveland’s Valley Belt Industrial Parkway to a downstream interceptor.
In keeping with its philosophy of replacing aging pump stations with gravity sewers where feasible and cost-effective, the Northeast Ohio Regional Sewer District (NEORSD) considered creating a gravity sewer that would bypass the pump station and force main.
The new gravity sewer would be located near the confluence of the Cuyahoga River and West Creek, where the western slope of West Creek adjoins the southern slope of the Cuyahoga. The western portion of the alignment would pass through clay soils varying in consistency from very soft to very stiff. The eastern portion would pass through loose to very loose layers of sand, and water-bearing gravel deposits including cobbles.
As reported in “Microtunneling Shows Its ‘Green’ Strength in Cleveland,” an article published in Trenchless Technology,
NEORSD retained the engineering services of Mott MacDonald 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….
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, Mott MacDonald 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 scope of work included study, design, and construction service phases.
- The study phase included a comprehensive geotechnical investigation and a detailed study of alternatives. Evaluations covered hydraulics, odor control, sedimentation, flood plain, wetlands, permits, land acquisition, pipe materials, construction schedule, agency coordination, operations issues, maintenance issues, and cost.
- The design phase included a detailed design of the preferred alternative, decommissioning of the pump station, preparation of geotechnical data and baseline reports, and preparation of Contract Documents.
- Construction services included compliance to Contract Documents, progress payment assessments, and on-site microtunnel monitoring services.
The contractor used a refurbished RVS 600S Soltau microtunneling machine to install 42-inch concrete casing pipe in two drives measuring 761 feet and 688 feet long. Connections to the existing sewer at each end were completed using traditional open-cut construction.
The project required work adjacent to an environmentally sensitive area, and the eastern drive crossed beneath three large-diameter high-pressure gas lines with clearances of less than two pipe diameters.
Trenchless Technology reported that the site-specific advantages of microtunneling “included significantly reduced disruption and social impacts to local residences, businesses, and commerce and reduced environmental impacts.”
The final contract amount for the new sewer was approximately $5 million, 15% lower than the engineer’s estimate. The contractor did not experience any delays, and made no claims.
According to the District, “replacing the pump station and force main with a new gravity sewer will prove less costly in the long term than continuing to maintain the pump station and force main. It will also eliminate the potential for service disruptions, which could occur in the event of pump station failure.”