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Underground Storage and Detention Systems

Underground storage and detention systems help developers carve out extra space.

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Credit: Site Design Concepts
Whether a site is undeveloped, densely developed, or somewhere in between, it can face challenges in detaining stormwater runoff. When aboveground space is at a premium, underground stormwater detention systems are an efficient way to store, detain, or infiltrate stormwater runoff while the ground above is used for parking, parks, or other ­features. These detention systems also may include filters to remove sediment and debris before the water enters the system.

The three projects below show how three very different properties used an underground system to solve their own stormwater detention challenges.

Lyman Terrace Revitalization Project
A set of 1940s-era apartment buildings in Center City of Holyoke, MA, escaped demolition a few years ago. Instead, the buildings are being rehabbed and are expected to become part of the economic renewal of the downtown area, according to the Lyman Terrace Revitalization Study prepared by MassDevelopment and the Massachusetts Housing Partnership.

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The 5.8-acre development consists of three city blocks, each approximately 2 acres in area, bounded by four municipal streets. Each block contains six red-brick buildings surrounding a large courtyard space. Because the footprints of the existing living spaces didn’t meet current building standards, site work is being done to add an extra room to each apartment. Federal, state, and city funds are being used for the project.

The three blocks lie parallel to a nearby canal. From the top of the first block to the bottom of the third is a 60-foot drop. Because of the slope, the blocks are terraced.

Before the project began, stormwater ran from the roofs, roadways, and parking areas within the blocks straight into the city’s sewer system, says John Furman, the engineer of record on the project.

Furman, managing director of the VHB office in Springfield, MA, designed the stormwater system and handled the permitting process. VHB is a civil engineering company with 24 offices along the East Coast. It specializes in land development, transportation planning and engineering, and environmental services.

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“Holyoke’s criteria are to reduce stormwater runoff from the two- and 10-year design storm by 25% when there’s redevelopment,” says Furman.

The site posed many challenges. There was no room for aboveground detention, and the only available areas for an underground system were very small, narrow spaces between the buildings.

In addition, he says, “Part of design process was testing the soil where each system would go for the soil type and infiltration. When we got the results back, they showed either ledge or extremely poor soils. There was nothing we could do with the soils, so we couldn’t use an infiltration system.”

The poor soils were partly a result of demolition of ­previous apartment buildings on the site in the early 1940s to allow for the construction of these existing ones. The debris included degraded brick and clay.

“Now we have a situation with the design parameters being hold and release, and reduce the stormwater by 25%,” says Furman.

The designers evaluated three or four different options, including StormTrap’s underground stormwater detention system manufactured of reinforced, high-strength precast concrete blocks. Although the Springfield office hadn’t used StormTrap before, other offices in the company had.

“VHB has a very elaborate system of problem-solving,” he says. “We all take advantage of others in the company who have experience in using different products.”

Furman used five of StormTrap’s SingleTrap systems to temporarily store the runoff and release it at a controlled rate to mitigate erosion and flooding. SingleTrap’s high void ratio met the site’s needs and it was affordable, he says. StormTrap has offices across the US as well in other countries.

“StormTrap was an excellent partner,” he notes. “We communicated with them as the system was being designed, and they had a representative onsite during the installation to make sure their systems were installed properly.”

He designed the systems to run parallel to the canal. Three of the systems run lengthwise between two sets of apartment buildings down one side of the three blocks, with one system in each block. The other two systems run lengthwise between two other sets of buildings down the other side of two blocks.

This project was one of three taking place in the same very limited spaces. Work also was being done on the buildings and some site work was taking place, as well as work on the roadways for a rehab of the utilities.

“We were lucky,” says Furman. “The same contractor was awarded the other two contracts. We had good cooperation between that contractor and ours.”

There was one more challenge: The city was upgrading its sewer system from a combined system to one that separated its stormwater and sanitary sewer systems.

“It added a few restrictions to our project,” he says. “We had to look at the water quality from our site. Under state law, water from a roof doesn’t need treatment as long as it isn’t a metal roof, but water from roads and parking lots does need to be treated.”

He added a Stormceptor from Imbrium Systems Inc. in Whitby, ON, to each of the SingleTrap systems. Manufactured locally in Westfield, MA, Stormceptors include a single chamber where sediments settle and oils and debris rise. Both sediments and floatables remain trapped until they are removed.

Because the three blocks of apartment buildings are separated by city streets, most of the stormwater is generated onsite. In the newly designed system, stormwater from the roofs goes through underground piping to the SingleTrap systems, and from there, it goes to the city stormwater system.

Stormwater from the small parking lots and roadways within each block goes to the Stormceptors through inlet pipes or grates and from there to the SingleTrap systems. Treated stormwater leaves the systems through the outlet pipes and enters the city’s stormwater system.

Credit: VHB
StormTrap installed at Lyman Terrace

David G. Roach & Sons Inc. of Hardwick, MA, installed the systems. This installation was the first of its kind for Roach, which performed extremely well in unfamiliar territory, says Furman. “They were a great contractor to partner with. They were very diligent in keeping to the schedule that was set, and the quality of their workmanship was excellent.”

The deepest of the five excavations were 8 to 10 feet deep. They had to be shallow because existing utilities needed to remain connected while the installation was going on, says Furman. Older piping systems that were still in use were relocated around the excavations.

Crews set the 4-foot SingleTrap basins, which have no base, on concrete foundations for support. The foundations were depressed at the outlet pipes to allow the pipe inverts to be flush with the tops of slabs. Each basin holds 13,018.32 cubic feet of stormwater.

Roach & Sons installed the inlet and outlet pipes with a 1-foot concrete collar on a cradle of aggregate. They applied joint tape around the perimeter of each system, and then sealed all the exterior joints between adjacent modules with an elastomeric resin bonded to a woven, highly puncture-resistant polymer liner. Wrapping each system in the liner protects the stormwater in the system from contacting the potentially contaminated soils.

Starting at the bottom of the concrete pads, Roach & Sons backfilled around each system with clean, crushed, angular #5 aggregate and compacted it in lifts a maximum of 12 inches thick, taking care to apply the backfill evenly around each system and not to wedge the backfill against the structure or disturb the joint wrap around the structure. They extended the fill 24 inches above the tops of the systems and compacted it.

Finally, crews covered and seeded the disturbed areas. Because of the slope, the final amount of cover ranged from 2 to 4 feet.

It took approximately 1 week to excavate the hole for each system, a week to form and install the concrete slab, and a week to get a crane and lift the StormTrap components in place and secure them together, says Furman.

Once stormwater is flowing into the system, there is virtually no maintenance. Every year, the access ports to the system should be opened to see if any sediment is building up inside the units, and if so, it can be vacuumed out easily.

“It was an excellent project,” says Furman.

Aquatic Center Parking Lot, Village of Allouez
In May 2017, work began on an addition to and renovation of the Cerebral Palsy Aquatic Center in the Village of Allouez, WI, which is bordered by Green Bay. The center, which has two warm-water pools and a whirlpool, is increasing its services for adults, therapy, and childcare, as well as its office space.

The center lies on 4.78 acres on a ridge. The slopes are gentle, and there are no serious rain events, flooding, or contaminated stormwater. The East River, on the eastern side of the site, is a tributary of Fox River, on the western side.

The center is in an older part of town with a residential area on one side, a commercial area with classic homes on another, and a highway and a prison on the two other sides, says Wally Wildenberg, who oversaw the installation of the underground stormwater detention system for Keller Inc.

The approximately 21,404-square-foot addition was designed by Somerville Inc. in Green Bay and built by Keller in Kaukauna, WI. Keller also built or reconstructed the driveways, parking lots, sidewalks, and utilities that serve the facility.

The project impacts approximately 2.2 acres of the site, according to the stormwater management and erosion control plan provided by Patrick Kuehl, who designed the detention system. Kuehl is a civil engineer with Robert E. Lee & Associates Inc. in Green Bay.

Among the factors that Kuehl considers when he’s designing a stormwater detention system are the soil type, the slope, the volume of water the site is expected to receive during a 2-year and 100-year storm, the cost, and how the system fits geometrically into the space.

“Finding a system that would fit into the space available at this site was a challenge,” he says. “There is virtually no green space.”

HydroStor HS75 chambers and GoldFlo WT pipes from Prinsco Inc. in Willmar, MN, were chosen. According to Prinsco, the HDPE chambers hold 75 cubic feet of stormwater each and are designed for maximum land utilization and minimum environmental impact. The dual-wall HDPE pipes form the inlet and outlet manifolds. They have watertight integral gasket bell and spigot coupling systems for optimum flow.

The detention chambers are located under the existing parking lot. Their area is 2,673 square feet and the chambers provide a volume of 5,332 cubic feet of storage. They will be largely empty most of the time.

Stormwater from the roof of the building and from the roadway around the building enters catch basins and storm manholes with sumps that collect sediment and phosphorus.

“There was no need for a sediment retention chamber,” says Wildenberg. “It’s basically a clear water system. All of the surface areas that will ultimately shed water to this system are not subject to accumulating sediment. All water that will be collected will have hard surfaces to run on—that is, the private paved roadway around the building, and the roof of the building.”

After this sump treatment, water enters a flow control manhole that directs runoff from up to the 2-year design storm to a 12-inch pipe that discharges to the underground storage chambers. This stormwater is released slowly to a Village of Allouez storm sewer system that carries the water to village-owned Heritage Hill Stormwater Pond for further treatment.

Once the 12-inch pipe reaches maximum capacity, any additional flow is diverted to a 15-inch pipe which discharges to a roadside ditch that carries all of the runoff exceeding the capacity of the underground chambers to the East River.

“We really had to coordinate with the village regarding pretreating and reducing the flow of the stormwater,” says Kuehl. “It was challenging, but working with the village, we came to a solution. The village benefits, too. They were able to redirect stormwater from an adjacent parcel to the pond, increasing the area of development treated by the Heritage Hill Stormwater Pond.”

The site posed its own challenges. Space was limited and the center had to remain open during the installation, he says.

In addition, notes Wildenberg, there was little difference between the elevation of the site and of the stormwater treatment pond, so the system had to be as shallow as possible. Fortunately, the soil is mostly clay and tested out as more load-bearing than was needed.

Harold Tauschek & Sons Excavating & Snow Removal Inc. of New Franken, WI, excavated to a depth of 5.5 to 6 feet for the installation of the stormwater chambers. The bottom and sides of the excavations were lined with a non­woven geotextile to prevent the soil from migrating into the backfill.

The crew placed and leveled 6 inches of clean 3/4-inch aggregate from offsite around the fabric-encapsulated chambers in order to provide additional storage. A 4-inch perforated perimeter drain-through was connected to the discharge control manhole on the downstream end of the chambers.

Tauschek & Sons then laid a 15-foot-long layer of geotextile on the aggregate on the inlet ends for scour protection from the incoming stormwater. They placed a total of 68 chambers in four runs and then connected the inlet and outlet ends to manholes with Prinsco’s GoldFlo WT pipes.

It’s important to align the chambers and to get the spacing between them right because they interlock, says Wildenberg. In addition, the crew had to be sure to set the plumbing parts properly.

Tauschek & Sons added more backfill around, between, and above the chambers, and wrapped the entire system with the geotextile. “This creates a capsule so the stone also filters the water,” says Wildenberg.

The final layer was a bedding layer of crushed gravel on the geotextile, which was covered with pavement.

The installation took about three weeks in September 2017. The work went very well, both Wildenberg and Kuehl agree.

The system requires very little maintenance, says Wildenberg. The catch basins and the caps can be cleaned out if necessary.

WellSpan Health Care Facility
When WellSpan Health was planning to construct a new outpatient healthcare facility near Hanover, PA, the company wanted to make the most of the site, says Adam W. Anderson of Site Design Concepts Inc. in York, PA.

“It was a new development project on an existing green site located on the outskirts of Hanover Borough,” explains Anderson. “The client desired to maximize the proposed building area, which dictated the parking and associated infrastructure requirements. The maximized design took up most of the property.”

Anderson, project site engineer, and his team at SDC designed the stormwater management systems to allow maximum development of the 15.7-acre site.

National Pollutant Discharge Elimination System (NPDES) permit regulations require sites that disturb more than 1 acre of land to provide volume and water-quality control for up to the 2-year runoff volume increase, and runoff controls to reduce the runoff rate for up to and including the 100-year storm event, says Anderson. Municipal ordinances are somewhat different, but the goal of both is for a developed site to mimic predevelopment, meadow conditions once the site is stabilized.

“We needed an efficient underground system with consistent 5:1 loading ratios,” he says. “That is, for every five square feet of impervious area, the bed area had to be at least one square foot.”

CREDIT: Site Design Concepts
StormKeeper units installed at a new health care facility

He chose the underground chamber retention system StormKeeper SK75 from Lane Enterprises, whose corporate headquarters is in Camp Hill, PA.

“We’d used similar chambers before,” he says. “Lane was helping us with other amenities, and we gave them a chance. They’re comparable and their customer service is good. They made themselves available as they were needed.”

StormKeeper chambers are made of virgin polypropylene and are injection molded, which provides an extreme level of structural integrity and arch stiffness, according to StormKeeper. The chambers also have tight tolerances.

Anderson put in controls to isolate runoff from a development upslope and a roadway culvert. The only runoff from offsite is from a portion of an adjacent commercial site.

He designed three StormKeeper systems, which are under parking lots around the proposed building. They’re connected to the storm sewer in the parking areas and will be connected to the building roof drain system when the building is constructed. Each system infiltrates a portion of the calculated 2-year runoff volume increase.

Two of the three systems are connected in line. The third system is parallel to the other two and is in line with three rain garden and bioretention areas.

While there’s only an approximately 5% slope across the property, there is a 45-foot elevation change, which posed challenges for the stormwater management designs. The flexibility of the StormKeeper chamber system allowed designers to meet all the requirements and conditions, he says.

One challenge was to meet the minimum cover requirements for the chamber systems while not exceeding the maximum allowable cover.

Anderson modified the grade at the bed areas to satisfy this requirement. For example, based on the existing grade to the invert of one bed, the maximum depth ranged from 2 feet to 10 feet, a difference of 8 feet. The proposed depth after the grade modification was 6.5 feet to 13 feet, a difference of 6.5 feet.

Anderson also had to consider the existing grade and the earthwork required to balance the cut and fill, giving consideration to the excavation required for the beds.

The biggest challenge of all was getting access to the property, which is connected to a congested state roadway.

York Excavating Company in York, PA, is installing the three chamber systems. Site work, mass grading, and installation began in September 2017.

“The York Excavating crew had mostly installed perforated pipe in stone bedding, although it was somewhat familiar with the chamber system,” says Anderson. “It appears the system installations are moving along very well. As construction progresses, the contractor is becoming more efficient at the installations as the field crews become familiar with the specific designs, system components, and installation requirements.”

It was critical for the bed system inverts to be installed on virgin ground to facilitate infiltration in the subgrade. During the excavations, York Excavating used a large track hoe with a toothed bucket. Crews were aware that there might be shallow bedrock, and they avoided deep cuts wherever possible. They then scarified the ground, a silt loam.

“Because these are infiltration beds, scarifying the ground is important,” says Anderson. “If you smear or compact the ground, the water won’t be able to infiltrate.”

York crews placed a permeable nonwoven geotextile fabric on the soil to stop fines from migrating into the system, a 6-inch layer of stone from a local quarry, and then the chambers. They connected the pipes, which range in size from 8-inch-diameter connections to 24-inch-diameter manifold pipes, to each side of the systems. It’s important to make sure the connecting pipes and the chambers fit together tightly, says Anderson.

As they backfilled, they pinned the chambers together through the stone with 8-inch nails or staples to make sure the chambers wouldn’t move, and compacted the stone. They added more backfill until they had a 12-inch cover on top and compacted again.

They wrapped the entire system in the geotextile fabric, overlapping the chambers, and placed and compacted soil on top until it was up to grade.

The first flush of stormwater enters the inlet of a chamber system and goes through a pipe into the Sediment Strip, a row of chambers where sediment and silt concentrate and settle out. The Sediment Strip limits the maintenance to a single row of chambers in each system, explains Anderson. From there, the stormwater enters the rest of the chamber system, where it infiltrates slowly into the ground.

One chamber system has 569 chambers, can hold 42,618 cubic feet of stormwater, and is designed to ­dewater 18,110 cubic feet within 24 to 72 hours. The second has 370 chambers, can hold 27,713 cubic feet, and is designed to dewater 14,009 cubic feet in the same amount of time. The third system has 624 chambers, can hold 46,738 cubic feet, and is designed to dewater 29,928 cubic feet in the same amount of time.

During larger storms, such as a 100-year storm event, a portion of the runoff is retained for infiltration. As the remaining volume fills the Sediment Strip, it backs up into the inlet structure, which is connected to the manifold pipes by a 90-degree turned-up elbow on the inside of the structure.

When the water flows over the top of the elbow, the runoff spills into the manifold pipes and is conveyed and distributed to the rest of the chamber system, where it is detained. It discharges at a reduced rate through the outlet control structure, a precast concrete box containing a removable aluminum control plate with orifice openings at varying elevations. Anderson is proposing to place riprap at the outlets to prevent erosion.

From the outlet control structure, swales allow the water to infiltrate as it makes it way to an unnamed tributary, where it flows to Plum Creek.

The Sediment Strips have precast concrete inlets or manholes at either end to allow easy access and cleaning, which consists of jetting or washing out the silt and sediment. SW_bug_web

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