Microtunnel Project Overcomes Macro Challenges DC Water’s First Street Tunnel

The Clean Rivers Project for DC Water is a $2.6 billion program that will help improve water quality in the Potomac River and its tributaries when its massive tunnel program is completed by 2022. The project includes 13 miles of 23-ff ID concrete lined storage and conveyance tunnels, as well as shafts and related infrastructure.

The Clean Rivers Project includes several large-scale tunnel projects, including the Blue Plains Tunnel ($330 million), the Anacostia River Tunnel ($253 million) and the First Street Tunnel ($157 million). The Northeast Boundary Tunnel – the program’s largest in terms of footage –– has yet to bid.

It is interesting to note that with all the complications of building massive tunnels within a bustling urban setting that perhaps one of the most complicated portions of the job was an adit. Built as part of the First Street Tunnel, the Adams Street Adit involved a short run of microtunnel completed under very challenging and unique circumstances. The depth (100 ft), diameter (104 in.)  and tightly constricted working area all contributed to the demands of the project.

First Street Tunnel

The construction for the First Street Tunnel includes about 2,700 lf of 20-ft ID tunnel approximately 80 to 160 ft deep; three drop shafts, three adit connections, four diversion chambers, and a 6 mgd pumping station housed in a fourth below-grade shaft. The four shafts range from 20 to 65 ft. in diameter with invert depths ranging from 100 to 160 ft below the ground surface. The three adits range from 20 to 400 ft. in length and are 8 to 10 ft ID. The invert depths for the diversion chambers are 25 to 35 ft below the ground surface.

The First Street Tunnel is unique in that is was fast tracked in response to severe flood events that occurred in summer 2012 in the Bloomingdale and LeDroit Park neighborhoods. As a result of a task force recommendation, the First Street Tunnel design and construction schedule was significantly accelerated to mitigate flooding in the area.

One of the challenging aspects of the First Street Tunnel project is the close interaction with neighborhood in this densely populated neighborhood. DC Water did its best to lessen impacts on area residents and businesses during construction.

A $157 million design-build contract was awarded to a joint venture of Skanska-Jay Dee in October 2013 with completion scheduled for 2016.

Adams Street Adit Microtunnel

The First Street Tunnel project included the construction of three adits to convey flows into the mainline tunnel. To minimize impact on residents, the adits were constructed via underground construction methods. Two of the three adits were short and were built using ground freezing in conjunction with roadheader excavation. The excavation used shotcrete as initial support with a final concrete lining.

One adit, however, spanned approximately 370 ft and required an alternative approach. This adit began at a drop shaft at the corner of Adams Street NW and Flagler Place NW and followed the Adams Street right of way eastward to the mainline tunnel under First Street. The Skanska/Jay Dee team, in conjunction with WSP-Parsons Brinckherhoff, the consultant for the design-builder, decided that microtunneling offered the best solution.

“Given the tight jobsite conditions and the ground conditions we expected to encounter, it was determined that TBM-type tunneling was the best way to go,” said AG Mekkaoui, project manager for Jay Dee. “Considering that dewatering was not allowed along the alignment and the tunnel’s proximity to buildings, a pressurized face machine was needed to minimize the risk of settlement.”

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Jay Dee performed the microtunneling work for the joint venture using its mts Perforator 2000M3 20C Microtunneling System with mtsSlurryTec, which the company had purchased for a project it completed in Cincinnati, Ohio. The slurry mts Perforator system included power pack, jacking unit, automated bentonite lubrication system and TACS guidance equipment. The machine, with a cutting diameter of 104 in., was used to install 84-in. ID concrete jacking pipe. Originally built in 2008, the MTBM was updated with the latest in guidance and operator controls by mts Perforator prior to launch.

The shaft was constructed using ground freezing and conventional excavation and concreted from the bottom up. The shaft, at the intersection of Adams and Flagler, reached a depth of 100 ft and was limited to a diameter of about 20 ft due to the narrow right of way and proximity to homes. Making the shaft even more constricted was the fact that it included a concrete structure that will serve as a permanent ventilation structure for a deaeration chamber situated at the base of the shaft. The narrow shaft meant that precise handling of the pipe segments as they were lowered into the shaft was a must.

Expected ground conditions included sand and gravel with some fines below the groundwater table.

Challenges Abound

Due to the residential environment and the proximity of the houses to the jobsite, special measures were taken to minimize the impact on residents. The entire jobsite area was enclosed by chainlink fence, and was lined with sound-absorbing materials. Some equipment, including the slurry separation plant, was enclosed within a noise-dampening structure. Additionally, restricted work hours and limited lay-down area for equipment and materials posed logistical challenges.

One of the more unique aspects of the project was the presence of the deaeration chamber at the base of the shaft. This facility was a horseshoe-shape chamber – approximately 75 ft long –excavated through frozen ground from the shaft along the same alignment and depth as the microtunnel. Because the dimensions of the chamber were larger than that of the microtunnel, that meant the MTBM had to be jacked through the chamber before it could begin excavating. Crews jacked 7 pipe segments to get the MTBM though the chamber and to the excavation face.

Once the machine was jacked through the chamber, excavation commenced. But crews were faced with another challenge on the receiving end where the microtunnel dead-ended at the mainline First Street Tunnel. Here, the MTBM needed to proceed through the already-constructed segmentally lined tunnel and recovered from within the tunnel.

In preparation of the reception, crews created a grout block in frozen ground on the outside of the mainline tunnel and precut a reception ring in the mainline tunnel. In the mainline tunnel, which has a diameter of 20 ft, crews constructed a concrete pad on which the MTBM would come to rest. Crews then grouted a block of the mainline tunnel roughly 10 ft on either side of the alignment to create a sealed environment for the MTBM.

As the machine mined through the pre-cut tunnel wall into the grout block, the area outside of the mainline tunnel was grouted to seal off any ground water. Then crews began to expose the machine by tearing down the lean concrete reception block. Once exposed, the MTBM was lifted off the concrete pad, transported to a trailer, and carried out to the main work shaft near the intersection of First Street and Channing Street.

Successful Completion

With the variety of unique challenges facing the crew, planning and preparation were of the utmost importance. “It took a tremendous effort, but everything worked out,” Mekkaoui said. “The owner was also of great help. They were flexible and able to extend work hours during the critical portion of the drive to ensure that the mining run was completed. Everyone involved pulled together and dealt with the conditions to the best of our abilities, and the end result is that we completed the run successfully.”

Andreas Thiele, an engineer for mts Perforator who was onsite for the MTBM update and mining, said that the Adams Street Adit microtunnel was one of the most challenging jobs he has been involved with. “What was so interesting about this project was all the unique and challenging aspects that came together,” he said. “On a typical job you might see one or two unique aspects, but here you had the combination of the constricted job site, the narrow shaft, the need to jack the MTBM through an existing structure, and a reception into a grout block within an existing mainline tunnel. It was a very interesting and very challenging piece of work.”

There are about 150 MTBMs and 300 thrust boring machines built by mts Perforator ranging in dimension from 200 to 3000 mm diameter (8 to 118 in.) in use worldwide. Mts Perforator is also able to provide service and upgrade for Soltau MTBMs.

In the end, crews were able to complete the mining in less than one month. The machine broke into the reception block on April 15 and the tail can recovered through the main mining shaft on May 4. With completion of the microtunneled adit, residents are one step closer to realizing the goal of reduced flooding and a cleaner environment.

Jim Rush is editor/publisher of TBM: Tunnel Business Magazine.

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