August 2021 Vol.76 No. 8
Features
Creative Construction Sequencing Aids Light Rail Construction
The Los Angeles County Metropolitan Transportation Authority – LA Metro – is in the final stages of constructing an underground light rail trunkline that will allow riders to travel through downtown LA with far fewer transfers and more direct service. Known as the Metro Regional Connector Transit Project, it will connect three existing light rail transit (LRT) systems, reducing commute times by up to 20 minutes.
Regional Connector Constructors (RCC), a joint venture of Skanska USA Civil West California District Inc. in partnership with Traylor Bros. Inc., was awarded the design-build contract by LA Metro to construct this 1.9-mile completely underground transit way through downtown Los Angeles, the second most highly populated metropolitan area in the U.S.
The new line will increase downtown transit options with three new stops at state-of-the-art underground stations –
the existing, but reconstructed, Little Tokyo/Arts District Station; the completely new Grand Av Arts/Bunker Hill Station; and the Historic Broadway Station located at Broadway and 2nd Street.
While the tunnels for the twin rail lines were excavated with a 400-foot tunnel boring machine, the stations were excavated by open cut. In order to open cut to the needed depths in a high-density metropolitan area, the contractors first had to safely get past numerous older utilities, many of which needed repair or replacement. The Historic Broadway Station was the most challenging to construct, due to having the most congested existing underground utilities in the area, including a 10-foot-diameter, reinforced concrete (RCP) storm drain that had been constructed around 1960.
Unique concept
A creative construction sequencing plan devised by RCC would expose the existing utilities along the 400-foot station zone to make any repairs or replacements. These exposed utility lines would then be suspended for perhaps as long as four years over what would soon be 50 to 60 feet of air.
The electrical, telephone, fiber optic and natural gas lines were easily enclosed in a suspended casing, but the 10-foot diameter RCP storm drain belonging to the LA Flood Control Department was a larger and heavier challenge. The contractors immediately realized that due to its weight and uncertain structural integrity, the existing concrete storm drain would have to be either re-routed or replaced with something lighter.
Hobas centrifugally cast, fiberglass reinforced, polymer mortar (CCFRPM) pipe was chosen as the lighter, stronger, less expensive, and more efficient option for the project. Hobas pipe is six times lighter than RCP, and its ease of installation is due partly to its push-on joints that allow fast assembly. Skanska crews had recently installed large-diameter Hobas pipe in the Washington, D.C. Clean Rivers First Street Tunnel project, and were familiar with its characteristics and installation.
Work began on the Historic Broadway Station in 2016. Soldier piles were drilled into the Fernando Formation bedrock along 2nd Street and deck beams were set to support the decking for the restoration of the 2nd Street roadway for continued public use during construction below. The traffic decking, constructed with an entrance structure, enabled crews to access the subterranean construction zone.
With preparations completed, workers began excavation to expose utilities down to the invert of the existing storm drain at 30 feet below grade. The contractor had taken delivery of 320 linear feet of 126-inch-diameter, 46-psi stiffness rated pipe in an off-site staging yard. Then, during non-peak traffic time, RCC crews were able to take a lane closure to bring in and lower the sections of Hobas pipe through the access window.
Making it happen
A temporary support system for the pipe installation was devised using vertical supports and a series of threaded rods to attach nylon web slings to suspend the pipe from the temporary ceiling/supporting deck beams for the 2nd Street decking. As each 20-foot section of pipe was lowered through the deck opening, it was moved into place with telescoping material handlers, suspended below the traffic decking with web slings along the north side of the existing RCP storm drain, and jacked into connection with the previous section.
The pipe was secured at grade with turnbuckles and ultimately with lateral and longitudinal braces. There it would remain suspended for the remainder of the excavation, station build-out and backfill to the pipe’s grade.
As the new 300-foot line neared completion, the existing RCP storm drain was removed up to a couple of feet above its invert. This work was done during Los Angeles’ dry season, from April to October, to take advantage of the low flow at that time of year. A sandbag dam and pump were able to handle the minimal base flow during the construction of cast-in-place transition structures to connect the new line to the existing storm drain at either end of the cavern. Upon completion of the connection, the remaining lower portion of the RCP was removed.
Below these suspended utilities, more than 70 workers used compact and mid-sized excavating equipment to dig down another 54 feet to the invert of the already bored rail tunnels, creating a cavern approximately 440 feet long by 55 feet wide. The 31,500 cubic yards of material was removed through the access opening in the decking and trucked offsite. More extensive than the excavations associated with the other two underground stations, this “cavern” under 2nd Street will accommodate cross-over switches between the tracks. All the while, the suspended storm drain system remained in service.
With build-out of the station’s concourse level and installation of the station roof concrete slab, the cover phase of the “cut and cover” operation commenced with replacement of the fill material to within inches of the pipe, just to the point where a skid steer could safely perform compaction. At this point, the fill material was changed to 100 psi slurry surrounding the new pipe by approximately 18 inches, which after curing, provided an equivalent E’ (Msb) of at least 25,000 psi per AWWA M45.
Split installs were not recommended due to the potential of stress concentrations at the interphase of rigid to flexible surround. Therefore, incorporating a full-surround slurry for pipe support was a sound engineering decision. Above the slurry, the backfill material reverted back to compacted soil, topped with the pavement base course for the reconstruction of 2nd Street.
The new section of 126-inch Hobas CC storm drain, totally and permanently encased, will continue serving the LA Flood Control Department and the citizens of Los Angeles indefinitely.
FOR MORE INFORMATION:
Hobas Pipe USA, (800) 856-7473, (281) 821-2200, hobaspipe.com
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