DC Water is in the midst of a massive infrastructure upgrade aimed at improving its water and sewer systems for now and years to come. Notably, the authority is building a series of massive tunnels aimed at collecting combined sewer overflows in an effort to improve waterways including Rock Creek, the Anacostia River and the Potomac River.
These large-scale tunnels range up to 23 ft in diameter and reach depths of more than 150 ft. While perhaps more visible in the public eye, the large-scale tunnels, however, are not the only focus of DC Water. In fact, DC Water is actively addressing its smaller diameter sewer lines as part of a proactive infrastructure upgrade plan.
These lines form the nucleus of the collection system and are vital to its overall efficiency. Disruption of these lines can lead to costly repairs and disruptions to residents and businesses alike. To address aging infrastructure in the city’s Northwest, DC Water recently began construction of the Oregon Avenue and Bingham Drive, NW Sewer Rehabilitation Project.
The project involves the construction of a new 24-in. sewer line to replace aging sewers in the area, as well as abandon sewers that run under sensitive areas of Rock Creek Park. The popular park, owned by the National Park Service (NPS), is a 1,754-acre city park officially authorized in 1890, making it the third national park to be designated by the federal government. It offers residents and visitors the opportunity to find refuge from the city’s hustle and bustle just a stone’s throw from the major attractions, government centers, shopping and entertainment. One of the objectives of the project was to keep the new sewer within the footprint of the roadway within park lands, so that any future maintenance or repair work would not infringe upon the natural features of the park.
“The Oregon Avenue and Bingham Drive project is part of our ongoing rehab projects across the city aimed at rehabbing our aging infrastructure,” said Willis Thomas, project manager for DC Water. “It is part of our proactive plan to assess, repair and replace sewers ahead of time before we have any major issues. The objective of this project is to design and build replacement interceptors throughout the park to allow abandonment of the deteriorated interceptors to better facilitate future maintenance and access.”
Of critical importance is minimizing impact on the area. Because of this, trenchless construction was chosen for a large portion of the project. “The project zone is in an area that is popular for people walking and biking, as well as along residential areas,” Thomas said. “We did not want to open-cut the entire length of the sewer so that we could stay out of the way as much as we can.”
The Oregon Avenue-Bingham Drive project involves the rehabilitation of sewers on Oregon Avenue, NW from Beech Street, NW to Bingham Drive, NW and on Bingham Drive, NW from Nebraska Avenue, NW to Beach Drive, NW. DC Water is responsible for four sanitary sewer interceptors that cross the western half of Rock Creek Park.
The sewers along Oregon Avenue, NW drain into one of four sewers that cross the western half of Rock Creek Park, conveying flow from Oregon Avenue, NW to the Rock Creek Main Interceptor (RCMI).
Three of these park crossings are in natural stream channels and are subject to erosion; two of those crossings are severely exposed and require abandonment. The fourth sewer is within the paved area of Bingham, Drive NW.
The objective of this project is to design and build a replacement sewer interceptor through the park to allow the abandonment the deteriorated interceptor. To better facilitate future maintenance and access the NPS allowed the sewer to remain through the only paved roadway in the area – Bingham Drive, NW. Benefits of this project include the rehabilitation of an aging sewer system, improving structural integrity while maintaining adequate hydraulic capacity, and reducing stream and groundwater infiltration and sanitary sewer overflows.
“One of the key challenges to this project was the need to stay within the footprint of the roadway,” said Lisa Franke, designer for the consulting firm JMT. “The park service didn’t want any disruption to the park. We also had to adjust the alignment to avoid existing utilities as well as maintain flow during construction.”
The $16.8 million project consists of approximately 4,300 lf of 24-in. sewer, including 2,600 lf installed by three microtunneling drives. The two major drives include one section north-south along Oregon Avenue adjacent to Rock Creek Park, and one section east-west along Bingham Drive within the park.
The portion along Oregon Avenue was designed as a series of microtunnels with intermediate pits to serve as future access points. However, in working with the tunneling contractor, Bradshaw Construction, of Sykesville, Maryland, the project team decided to combine three tunnel drives into one 1,866-ft drive. Drop shafts were constructed following tunnel construction to provide access points.
“The initial design called for three separate tunnel drives, but considering how restricted we were with the park, power lines and the need to maintain traffic, we decided to combine those tunnels into a single drive,” said Todd Brown, project manager for Bradshaw Construction.
To help contend with the narrow work area within the roadway, Bradshaw Construction designed an elliptical launch pit – 33-ft long by 22-ft wide – to limit the impacts on traffic and adjacent facilities. Bradshaw Construction work with Jennmar and its partner Contech to supply elliptical rib and liner plate for shaft support.
“Typically, we use circular shafts, but we wanted to minimize out footprint while at the same time having a shaft that was long enough for us to jack 20-ft pipe segments,” Brown said. “We considered driving sheet piles, but the proximity to power lines led us to consider other options. Additionally, the tight space would have made it difficult to construct a soldier pile-and-lagging shaft. After talking with Jennmar, we decided to use the elliptical shaft approach and it worked out well.”
In addition to allowing longer pipe segments lengths for jacking, the elliptical shape allowed an excavator to work in the hole, which would have become increasingly difficult in smaller diameter circular shafts. Bradshaw Construction used a conventional circular shaft for the reception shaft.
With the decision made to combine the tunnel drives into a single 1,866-ft drive and the elliptical launch shaft in place, Bradshaw Construction turned its attention to completing the drive. Bradshaw used a Herrenknecht AVN-1200 to install the 20-ft lengths of Permalok steel pipe.
Additionally, Bradshaw used an HKS 150 slurry plant from Herrenknecht for solids control. “The HKS 150 has the shaker sets stacked on top of each other and discharges at the rear, allowing us to stay within one lane of traffic,” Brown said.
The primary ground conditions consisted of decomposed rock with portions of more competent rock. While the decomposed rock may be fairly easily mined by the MTBM, it is not without challenges, especially at longer distances, Brown said.
“Decomposed rock is almost the worst of two worlds,” he said. “Hard rock tunneling would have been slower, but you are left with a stable hole. In sands, the hole tends to collapse around the pipe, but it is displaceable which can be an advantage if you encounter obstacles. We found that decomposed rock, while not hard to mine, really contributed to high jacking loads, especially with longer jacking distances. Weak rock creates fines that make it more difficult to keep the pipe string lubricated, particularly closer to the launch shaft where the hole has been open the longest.”
To help deal with the jacking loads, Bradshaw used four interjack stations along the 1,866-ft stretch. Initially, they had planned to use two interjacks but added the extra stations as jacking continued.
Even though the final carrier pipe was a 24-in. ID PVC pipe, Bradshaw opted to use the 60-in. MTBM to deal with the ground conditions in combination with the length of the drive.
“The specs allowed for a machine as small as 36-in. in diameter, but we opted to go with the larger machine,” Brown said. “The AVN-1200 is the smallest machine with a planetary drive that will allow access to face to change cutters, which was something we needed at this distance. We didn’t need to do a whole lot of cutter changes, but it was necessary to complete the drive.”
Bradshaw Construction also used a theodolite guidance system from VMT to ensure the machine stayed on path for the gravity flow sewer.
Bradshaw was given NTP on Sept. 30, 2016, and began construction of the 1,866-ft drive on May 2, 2017. Crews finished the drive on Aug. 14, marking the longest microtunnel drive Bradshaw Construction has completed to date.
“The mining was actually the easy part,” Brown said. “The whole time we were fighting jacking loads and pushing all our systems to their limits.”
Following completion of the drive, crews have turned their attention to installing the carrier pipe and constructing the two access points using elliptical shafts.
Next up for Bradshaw will be another company first – its first curve microtunnel drive. This will be a 400-ft S-curve that will combine three short drives and eliminate two intermediate shafts while keep the alignment underneath Bingham Drive.
Crews will use the same Herrenknecht AVN-1200 but will jack 48-in. Vianini RCP. To account for the curves, Bradshaw will use a Jackcontrol hydraulic joint system to avoid point loading on the concrete pipes.
Construction on the curved tunnel is expected to begin in early 2018, with completion of the total project expected by late fall.