For nine days in May 2010, a Herrenknecht micro-TBM mined a 600ft (83m) section of interceptor sewer with a 1,800ft (549m) radius curve in the middle. It arrived in the receiving shaft with an accuracy of less than 12mm.

The Homestead Avenue Interceptor Extension (HAIE) in Hartford, Connecticut, made history as the first curved microtunnel in the United States. It’s the first trenchless project ever for the city, and to be followed by many more that the Metropolitan District (MDC) will tender for its Clean Water Project. The USD 1.6bn infrastructure program will meet state and federal consent decrees for sanitary and combined sewer overflows (SSOs and CSOs) by 2020, and will be delivered in two phases.

Approximately 26,250ft (8,000m)—half of which will be deep rock tunnelling, and the other half microtunneling—are planned for better conveyance and greater storage capacity. Although not all the projects have been designed yet, and that number doesn’t include the final North Tunnel, of which the dimensions will be determined later (Figure 1).

South Hartford
The 3,900ft (1.19km) HAIE was one of the program’s first projects, and broke ground in 2009. It’s part of Phase One, budgeted at USD 800M, along with the 13,500ft (4100m) South Hartford Conveyance and Storage Tunnel (SHCST), sewer separation and other pipeline work.

At the end of February, the MDC issued a request for qualification for the final design of the SHCST and the consolidation conduits that will convey CSO and SSO flows into the tunnel.

Initially the MDC wanted to take advantage of existing tunnels for the Clean Water Project’s storage requirements, but ultimately a tunnel in the southern part of the city would be the best solution to pick up overflows in the west.

Scheduling has been influenced by improvements at the Hartford Water Pollution Control Facility to enable the plant to handle the additional flows. The SHCST cannot be operational until the plant is expanded and operational itself. “That is about to go in design this summer,” says Bob Moore, chief administrative officer of the MDC. “It will be preceding the design of the tunnel by probably six to nine months.”

Three major alignments were investigated during the preliminary design, each with one end at the treatment plant and the other end in the general vicinity of the CSOs to be discharged into the tunnel. Ground conditions near the tunnel route include four major bedrock units: Portland Arkose (arkose, siltstone and shale), Hampden basalt, East Berlin formation (siltstone, sandstone and shale) and Holyoke basalt. One significant challenge in the design will be to address an artesian condition discovered at two borings along the route. A Geotechnical Data Report will be made available to prospective designers.

The preliminary design report recommends a 26ft (7.9m) diameter, concrete-lined tunnel approximately 13,500ft (4115m) long, constructed with a TBM. Final design is anticipated to take two years, starting in October.

During this period the number of construction contracts will be determined, with the expectation to advertise them in February 2014.

More microtunneling
Overall, the largest amount of microtunneling will be on the west end of the SHCST, which will pick up overflows from the area’s SSOs and CSOs. Although not all the projects have been designed yet, there will likely be between 13,000 to 14,000ft (3,960 to 4,270m) of microtunnel.

“We’re undergoing the sewer separation,” says Bill Hogan, project engineer with the MDC. “We probably have about two-thirds of the contracts in design at this point in time. But there are some significant areas we have not started. So there may be additional microtunneling in those sewer separation areas.”

One of the main components of the Clean Water Project is sewer separation in six different areas of the city. These projects will be delivered gradually. For example, Granby Street, in the north, is in various phases of construction (Figure 2). For this particular project approximately 2,510ft (765m) of sewer pipe (48-inch and 60-inch) will be installed by microtunnelling, and 16,240ft (4,950m) sewer and drain pipe (8- inch to 60-inch) by open cut excavation.

Later this year, Northeast Remsco, as a subcontractor to Paganelli Construction, will undertake the microtunnelling portion of the Garden Street Relief Sewer. Approximately 900ft (274.3m) of 48in (1.22m) reinforced concrete drain pipe will be installed, and to minimize road disruption, 750ft (228.6m) will be installed using a closed face slurry micro-TBM.

Microtunneling accounts for approximately 25 per cent of the USD 4.5M total cost for the Garden Street Relief Sewer Project. Open-cut work for sewer pipes, installation of a new junction chamber at the Homestead Avenue Interceptor and rehabilitation of 5,000ft (1,524m) of existing combined sewers are also part of the project.

North Tunnel
During wet weather events, consolidation conduits will pick up combined wastewater from throughout the system and discharge it to the North Tunnel. This final step of the program will be determined in further detail once all of the consolidation conduits, sewer separation projects and the SHCST are constructed. Only preliminary work, based on computer modeling, has been done for this deep rock CSO storage tunnel, estimated to be 30ft (9m) in diameter.


Figure 1: Some 8km of microtunneling is planned for Hartford to increase sewerage network capacity Figure 2: Works on the Granby Street area sewer separation are in various phases