The existing 1-m (42-inch)-diameter water main serving East Boston and Boston's Logan Airport crossed beneath the Chelsea River with little cover and was located on piles for most of the East Boston side of the river when the U.S. Army Corp of Engineers (USACE) was about to undertake its Boston Harbor Navigation Improvement Project.  This project would enable double-hulled oil tankers to access Boston's oil terminals, many of which were located on the Chelsea River.

The USACE project meant that the water main had to be lowered and/or relocated, especially as a major feature of its service to the airport was for fire-fighting.  The oil tankers use the river almost continuously, and the water main work could not affect their operations in any way. 

The decision was to install a new water main, and PB was retained by the Massachusetts Water Resources Authority to provide design and construction management services.  The new main was 1.2 m (48 inches) in diameter.

Alignment Selected

The alignment analysis quickly indicated that the new pipe must be located adjacent to the existing one to make use of the existing cut-off valve systems and the existing pipes on either side of the river.  Moreover, both sides of the river upstream and downstream were being used for salt piles, oil tank terminals, a yacht club, marine contracting and storage buildings.  There was also concern about the possibility of extensive contamination in some of these areas.

Space at the site was tight because the existing water main had to remain in service until the new facility was completed, and there was a high voltage cable at this crossing.  The only space available for staging was the easement for the existing pipe from Marginal Street on the Chelsea side to the river. 

This proposed alignment from Marginal Street in Chelsea to Condor Street in East Boston was surveyed, underwent a full geotechnical investigation program, and was assayed for possible hazardous materials.  Full details of the existing piping and valving system were determined and all other utilities plotted.  Easements were identified and procedures set in motion to obtain the necessary properties for construction of the pipeline and future maintenance.

Microtunnel Is the Selected Trenchless Technology

Two trenchless technologies were considered for the crossing-directional drilling and microtunneling.

• Directional drilling is a trenchless construction technique that uses guided drilling for long distance crossings, such as under rivers, lagoons, landfills or highly urbanized areas.  Its three main stages are the drilling of a pilot hole, enlarging it by back reaming, and then installing the pipe.  The diameter and pipe material used determine the radius of the drill and the length of easement required.      
• Microtunneling uses a remote-controlled micro tunnel boring machine (MTBM) combined with pipe jacking to drill and install pipe in one pass.  It requires a mining shaft and a reception shaft with the tunneled pipe in between. 

When the profile of the proposed crossing was made available and geological information was indicated, however, it became clear that there was not sufficient length to apply direction drilling techniques within the confines of Marginal Street and Condor Street.  Microtunneling between two deep shafts was the selected alternative.  The associated discussions and proposals were presented at public meetings in both Chelsea and East Boston.

The actual center of the shipping channel did not fully agree with the geological data, which better indicated the original channel.  The ground was an exceptionally dense glacial till with cobbles and possible boulders.  The density was less and the soil weak at depth beneath the center of the river.  This depth defined the final profile for the pipe crossing and the corresponding depths of the shafts.

A small park on the Chelsea side that was the biggest area available to us was used as the mining site.  The reception shaft was in a small parking lot on the East Boston side with a lane providing access to Condor Street.  The shafts were installed using slurry wall techniques to minimize any groundwater and possible contaminant movement.  Groundwater treatment facilities were on-site but required little use.

The 1.5-m (6-foot)-diameter MTBM was the first of its kind to be used in North America.  The specification required a slurry shield machine with a combination head to drive through the tills, with spades and picks to cut any cobbles or boulders on the way.  The head was articulated and contained an air lock to allow for replacement of cutters, if required, since a 911 (emergency) shaft was not permissible.

All necessary permits were obtained.  A truck haul route system was established and construction of the 305-m (1,000-foot)-long crossing was completed.  Permalok pipe was placed behind the MTBM.  This is a screwed steel liner that is watertight.  The 1.2-m (48-inch)-wide ductile iron water main pipe was installed with fully restrained joints in the shafts and tunnel, and it was connected to the existing system.  The annular space in the tunnel was grouted.  Care had to be taken on the river side sites as the ground there was very poor.  The existing pipe on the Chelsea side was on facenes (horizontally laid logs to spread the load over very weak soil) and on piles on the East Boston side.  The new pipe was placed on imported fill.

Conclusion

This new water main has now been in service for five years.  Without this microtunneling technology and shaft construction methodology it would not have been possible to construct this new pipe beneath the Chelsea River in such a deep profile, in challenging ground conditions, and without problems from groundwater, pollution movement or surface settlement.¹