Project Profile: The Methuen Readiness Center: Going Underground to Gain Additional Space

Sept. 1, 2010

The Massachusetts Army National Guard (MAARNG), stationed in Methuen, MA, needed to redesign its existing Methuen Readiness Center to meet the MAARNG mission, readiness, and training requirements. The existing Readiness Center was inadequate in size and did not meet the current Anti-Terrorism and Force Protection (AT/FP) standards. The replacement facility had to support the MAARNG new force structure and enable the planned relocation of two battalions into the facility, one of which was to replace the unit previously stationed there.

Designed by OMR Architects, the new Readiness Center consisted of a 42,000-square-foot, two-story facility in the center of the site. It included an assembly hall; administrative, training, and support spaces; and kitchen and maintenance bays. The existing armory building remained onsite, and the motor vehicle storage building was demolished. The project also included military and civilian parking areas, force protection features, an entrance gate, and a chain link fence to control the perimeter.

Addressing stormwater management was an important step in the redesign of the Readiness Center. Some areas of the site were located within the 100-year floodplain of the Merrimack River, so the MAARNG had to ensure compensatory storage to offset floodplain filling caused by the project to comply with the Massachusetts Department of Environmental Protection’s Wetlands Protection Act.

To comply with the requirements of Executive Order 11988, Floodplain Management, the MAARNG obtained a floodplain waiver from the National Guard Bureau for the construction of the Readiness Center in a floodplain. The project did not require any work within wetland resource areas, but it included development of approximately 23,400 square feet of previously developed land within the buffer zone of a bordering vegetated wetland located on the adjacent property. The site also bordered a portion of the land subject to flooding.

As the redesigned site featured an increased number of impervious surfaces, bringing the total of impervious areas to 4.9 acres, the engineers from Boston, MA-based Nitsch Engineering Inc. conducted a pre- and post-development hydrologic analysis for the two-, 10-, 25-, and 100-year storm events. They had to ensure that the proposed stormwater system would limit the post-development rate of runoff from the site to no greater than the predevelopment rate.

Besides the new facility, the 5.5-acre site housed about 3.4 acres of parking spaces and the 148-foot AT/FP site boundary setback and related stand-off distances. The space constraint issue posed a challenge in terms of a stormwater system design, making aboveground solutions less desirable.

“With limited space and the absence of a municipal connection, an underground solution was the only way to go,” says Anthony Donato, P.E., LEED-AP, of Nitsch Engineering.

Nitsch Engineering selected the Cultec Recharger 180HD chamber system to control stormwater onsite. Cultec manufactures nine different chamber sizes to accommodate almost any site parameter; the Recharger 180HD is 20.5 inches high and 36 inches wide and has a chamber storage capacity of 3.45 cubic feet per linear foot. According to Donato, the Cultec model provided a greater storage volume per linear foot for the Methuen Readiness Center site than a conventional perforated pipe system did.

Additionally, the stormwater system had to be located outside the 100-year floodplain area, limiting the space available for infiltration. The Recharger 180HD parameters accommodated the design constraints of the site, including the elevation of the existing water table. Moreover, the Recharger 180HD, as all Cultec chambers, met H-25 wheel load requirements, an important consideration in selection of the stormwater system. As an added benefit, the units are lightweight, stackable, and easy to ship, and do not require heavy installation equipment, resulting in a cost-effective installation.

“We used the Cultec system on our projects in the past and know it as a reliable and economic solution,” says Donato. “The arch chamber design ensures the system’s structural integrity, and the heavy-duty models work well in high-traffic applications, as in this case, where the system is located under the parking lot.”

In addition to allowing for the maximum use of land, the subsurface infiltration chamber system eliminates potential liabilities inherent in aboveground water storage, including accessible standing water for breeding insects such as mosquitoes. It also removes a high percentage of phosphorus, nitrogen, lead, zinc, suspended solids, and organic compounds from runoff through infiltration.

The engineers placed the Cultec system on the northern end of the site. The system was designed to detain runoff during small storm events and retain it during large storm events to promote groundwater recharge and reduce runoff to local streams and rivers. The chambers offer significant groundwater recharge in areas with a high percentage of impervious surfaces and allow for more controlled infiltration.

The stormwater retained onsite is infiltrated into the ground, and during large storm events, an overflow pipe from the infiltration system discharges runoff to an outlet pipe and then into a drainage swale at the northwest corner of the site. The stormwater sheet-flows to the northwest corner of the site, where it empties into a wetland area adjacent to the site.

To design the system, the engineers used HydroCAD modeling software. The program already included the Recharger 180HD parameters, so the engineers needed to specify only the 6-inch layers of stone above and below the chambers.

To address the facility’s stormwater storage needs, the Cultec retention and detention system provided 11,052 cubic feet of storage. The storage volume per installed chamber was 40 cubic feet, including the storage capacity of the stone. The project required 300 Recharger 180HD units, which were installed in a 6,530-square-foot bed and backfilled with 573 tons of stone.

The installation of the system began with excavating a bed, laying the Cultec nonwoven polypropylene filter fabric along the sides and the bottom of the bed, and then adding a 6-inch layer of crushed stone. The chambers were arranged in the bed and fed using the company’s unique internal manifold feature. Developed by Cultec, the internal manifold is created by inserting the Cultec HVLV FC-24 Feed Connectors into the portals located on the sidewalls of the chambers. The inclusion of two side portals on every chamber allows manifolding to take place at any point within the stormwater management system. The internal manifold feature eliminates the need to build external manifolds, thereby condensing the system by decreasing the required footprint.

After the chambers were in place and covered with 6 inches of crushed stone and a layer of filter fabric, the site was backfilled and prepared for asphalt. The entire system was installed in two days, with the excavation process taking half of that time.

The maintenance of the system is minimal, required only of the preliminary collection system before it feeds the bed or other filtering devices that may be employed.

At the Methuen Readiness Center, the Cultec subsurface system was designed to capture, retain, and detain a large amount of stormwater runoff in a relatively small footprint. The system became an ideal solution to the site-specific challenges of land constraints and freed up space for the new building and parking places–space that otherwise would have been taken up by aboveground stormwater solutions.