Manchester Water Works Takes Advantage of Trenchless Technologies for Water Service Reliability
- By Robert McCoy
- Jun 13, 2011
Under the river, across the railway and through an environmentally sensitive area...
While this may sound like the beginnings of Little Red Riding Hood’s trek, it’s in fact the path that Manchester Water Works (MWW) followed to improve water reliability to thousands of customers.
MWW is a publicly run water utility that serves approximately 160,000 residential, commercial, industrial and wholesale customers in the greater Manchester, N.H., area. The existing distribution system includes two “dead-end” sections in the northern section of Manchester separated by the nearby Merrimack River. Since water is currently supplied from only one direction, the water users along these dead-end legs of the distribution system are vulnerable to unexpected shutdowns.
The utility sought to install a water main line that would go under the 900-foot Merrimack River (New Hampshire’s largest river) and cross the Pan Am Railway that runs adjacent to the river, with the least possible impact to the environment. The utility also sought to minimize the construction duration and project costs.
To accomplish the utility’s goal, its engineers and contractors turned to trenchless technologies and creative solutions to install one of the most significant fusible PVC™ pipe installations to date in New England using horizontal directional drilling (HDD).
Initial studies indicated that the new water main installation would require 20-inch diameter pipe over a length of 5,000 feet. Approximately 4,000 feet would pass through upland areas, utilizing conventional open-cut methods with minimal environmental impact. During the preliminary design study, Kleinfelder/SEA, the engineer-of-record for MWW, had to look beyond conventional solutions to cross the remaining 1,000 feet under the river and across the railroad.
The engineering team evaluated dry and wet open-cut methods to cross the river. However, both methods would require extensive measures to prevent adverse environmental impacts and significant permitting that would extend the project schedule.
They also screened several trenchless technologies, including pipe jacking, micro-tunneling and HDD. Pipe jacking and micro-tunneling would require deep sending and receiving pits, which would need extensive shoring and dewatering.
HDD rigs, on the other hand, are typically set up at grade, avoiding the need for deep sending and receiving pits. Since the river is not directly disturbed or impacted, many of the permits that would have been required for the open-cut method would not be necessary. Another advantage is that horizontal directional drilling is often used for crossings of several thousand feet and through varying subsurface conditions, including bedrock.
The Pan Am Railway crossing had slightly different design and construction considerations. The railway owner stipulated that MWW would need to install a steel sleeve within the limits of the railroad right-of-way without disrupting railroad operations. The surrounding topography and limited area between the railway and shoreline introduced additional challenges and limited the number of potential options for installing the sleeve.
Kleinfelder/SEA assessed potential trenchless methods for installing the new 42-inch sleeve under the railroad, including horizontal auger bore, micro-tunneling, pipe ramming and pipe jacking.
Insufficient area for the required exit pit between the railway and the steep shoreline of the river disqualified both the horizontal auger boring and micro-tunneling methods. Pipe jacking created a series of challenges, including the inability to install a dewatering system within the railroad right-of-way, groundwater conditions, potential instability of the face of the jacking operation, and the relatively steep incline of the sleeve profile.
Ultimately, pipe ramming proved the optimal solution because it does not require an exit pit, can be installed at any angle and does not require dewatering along the proposed alignment of the sleeve.
Constraints and criteria
With HDD and pipe ramming methods in mind, engineers moved forward with the preliminary design study, development of contract drawings and specifications, and construction administration for the river and railroad crossings.
For the railway, engineers needed to align the 42-inch steel sleeve within the limits of the railroad to allow crews to guide the initial HDD pilot bore into and beyond the sleeve on a proper trajectory. The sleeve had to be installed deep enough to provide sufficient cover below the railway, but shallow enough to stay above bedrock.
In the river, construction crews needed to achieve adequate separation between the proposed 20-inch diameter water main and the bottom of the riverbed along the entire alignment to minimize the risk of a frac out (escape of drilling slurry to the river).
Beginning in spring 2010, crews installed 4,000 linear feet of new 20-inch diameter ductile iron water main by conventional open-cut methods in the upland areas.
The next step was to install the 42-inch diameter steel sleeve by pipe ramming prior to the HDD pilot boring.
Following placement of the cable/wire guidance tracking system (necessary to guide the pilot bore along the proper trajectory), several rain events resulted in rising water levels and currents in the Merrimack River. This created tension and displacement of the cable/wire. Therefore, crews repositioned and anchored the cable/wire more securely with concrete blocks. However, it was not until the river level and current dropped slightly that the crews were able to secure and survey the cable/wire in preparation for the pilot bore.
Crews completed the pilot bore in one week. As anticipated based on the geotechnical investigations conducted during the design phase, the contractor encountered solid rock along approximately two-thirds of the crossing alignment during drilling of the pilot bore. The contractor then proceeded to pre-ream the hole to 24-inch diameter and then 36-inch diameter.
After several challenging pullback attempts, the MWW opted to complete the project using fusible PVC™ pipe material, which was a bid option in the original contract documents. Fusible PVC™ pipe includes pipe joints that are fusion-welded on site. The fused joints are low profile, which minimizes raised obstructions along the outer surface of the pipe. Furthermore, the pipe is flexible along its entire length, providing a consistent curvilinear profile that matches the borehole alignment. These pipe characteristics proved effective in meeting the constraints associated with the project.
In March 2011, the new 20-inch water main was successfully pulled back. By April 2011, the final connections between the 20-inch ductile iron pipe, installed in the upland areas, and the 20-inch Fusible PVC™ pipe installed by HDD were successfully connected, tested and placed into service.
The new 20-inch diameter Merrimack River Crossing will improve water pressure, reduce potential for nitrification, enhance water quality, and improve the overall level of service in this section of MWW’s distribution system for years to come.