September 1, 2008Drivers on a busy Fountain Valley, Calif., thoroughfare may have been surprised earlier this year when they saw crews on the street wearing life jackets while constructing five shafts for an Orange County Sanitation District tunneling project.
The joint venture (JV) of Barnard Construction Co. Inc. and Soletanche Inc. recently completed these shafts as part of the $32.8 million, 66-in. Ellis Avenue Trunk Sewer Project for the OCSD. To reduce time in the street and traffic interruption, the JV devised a one-pass method of excavating and building the 11-ft diameter shafts and the 60-in. diameter access manholes encased within them. The life jackets? They were part of the crew’s personal protective equipment (PPE).
One reason owners and engineers opt to tunnel in urban environments is to eliminate or reduce work in the streets. Yet, it’s difficult for them to explain to local residents why tunnel projects create traffic issues. In fact, nearly every tunnel project has some need or requirement for shafts along the tunnel alignment: transit tunnels have mandatory exit shafts and/or ventilation shafts; water and sewer tunnels require manholes for maintenance and/or isolation valves, etc.
The Ellis Avenue Project shafts allow for maintenance access to the 66-in. HOBAS sewer pipe installed inside the 109-in. tunnel constructed by Barnard Soletanche as part of the same project. The contract required five access manholes at 1,000-ft centers over a one-mile stretch in Ellis Avenue, a five-lane street with an entrance ramp onto the I-405 Freeway. Barnard Soletanche’s innovative scheme took only eight to 10 work days per shaft location, reducing traffic delays and minimizing impacts to the surrounding businesses and community.
The general project location lay just west of the Santa Ana River Channel in the southeastern Los Angeles basin, approximately four miles inland from the Pacific Ocean. Soils encountered on the project consisted primarily of silty clays and clayey, silty sands. The crews observed groundwater approximately 18 ft below the existing ground surface throughout the tunnel alignment.
Manhole Detail — Two-Pass Method
The more traditional original design called for construction of the intermediate manhole shafts in a two-pass method. This process included performing the initial excavation of the drilled shafts the full 11-ft diameter to a depth approximately 3 to 4 ft below the design invert of the tunnel (up to 40 ft deep). Upon completing the initial excavation, the shafts were to be completely backfilled with a low-strength concrete/sand slurry material (approximately 1,000 psi). The 11-ft diameter shaft would accommodate the TBM passing through the low-strength concrete while maintaining a minimum section outside the tunnel sufficient to transfer loads to the undisturbed soil below.
After excavating the tunnel through the backfilled shafts, the JV was to re-establish its presence within the street to perform the second pass installation of the final manhole. This step included drilling through the low-strength concrete backfill material and exposing the top of the initial tunnel lining. This second pass drilling was to be performed with a drill diameter just large enough to accommodate the 60-in. fiberglass manhole, which would ultimately tie into the 66-in. HOBAS final tunnel lining. The 60-in. manhole was to be installed and grouted into place before the street surface could be restored and reopened to traffic.
Manhole Detail — One-Pass Method
To minimize impacts on traffic, the surrounding neighborhood and local businesses, Barnard Soletanche engineered a one-pass method for this portion of the work. Generally, the modified plan consisted of constructing the drilled shaft and 60-in. fiberglass manhole in a single step from the surface, later completing the tie-in from within the tunnel.
The intermediate manholes were located in the first two lanes of the five-lane Ellis Avenue. The JV installed traffic control around the work site, reducing Ellis Avenue to two lanes through the work zone. This afforded the project three lanes in which the JV could construct the drilled shafts. In addition, the drilled shafts were to be located immediately north of a 30-in. asbestos cement waterline. To prevent damage to this existing line, the first 6 ft of excavation had to be performed via traditional open-cut methodology. They used a trench box shoring system to support the surrounding ground and adjacent existing utilities.
Once this initial excavation had been completed, Barnard Soletanche set up a polymer slurry batching and storage system that would be used to stabilize the excavation during the drilling process. When the slurry system had been established, the JV’s drilling subcontractor, Barney’s Hole Drilling Service, mobilized a LoDrill mounted on a 30-ton excavator to commence drilling of the shafts. They piloted the excavations with a 60-in. can bit, and later reamed them to the completed 11-ft diameter. To keep the excavation open, they used a polymer slurry. The shaft was excavated approximately 3 to 4 ft below the future tunnel invert. Excavation spoils were deposited adjacent to the shaft inside the containment area constructed of steel sheets, k-rail and HDPE membrane liner. Dump trucks backed up to the containment area where they were quickly loaded with an excavator.
Upon completing the drilled shaft excavation, the crew backfilled the shafts via tremie with low-strength concrete, up to an elevation of approximately 1 ft above the crown of the future tunnel. The team collected the displaced polymer drilling slurry at the surface and recycled it for use on drilling the next manhole. The next step involved lowering an 8-ft diameter corrugated metal pipe (CMP) down the 11-ft diameter hole through the drill slurry and pushing the bottom of the CMP approximately 1 ft into the low-strength concrete. When the CMP had been anchored, additional low-strength concrete was tremied into the annulus between the outside of the CMP and the drilled hole. At this point, the CMP was still full of drilling slurry on the inside and surrounded with low-strength concrete on the sides and bottom. After allowing the low-strength concrete to set up, the drill slurry was pumped out, and the secured smaller shaft was ready to receive installation of the fiberglass manhole.
The team then set the 60-in. diameter fiberglass manholes inside the 8-ft diameter CMP-lined shafts and positioned them atop the initial lift of low-strength concrete. The secured shafts allowed surveyors to verify the exact location of the lower section of the fiberglass manhole, to ensure that it was plumb and to verify its location with respect to the design alignment of the 66-in. sewer line. Once a manhole had been placed, the bottom 2 ft around the outside of the fiberglass manhole were backfilled with sand before finally backfilling the balance of this annular space with more low-strength concrete. The temporary box shoring was then removed and the area backfilled and restored to its original condition.
The team constructed the drilled shafts and installed the 60-in. manholes in advance of the tunnel heading. Upon completing of the tunnel, Barnard Soletanche crews removed the initial tunnel lining within the stabilized drilled shaft locations and hand-mined up to the 2-ft sand pocket, exposing the end of the fiberglass manhole. As the 66-in. HOBAS sewer pipe was installed, crews installed saddle sections, which tied the new sewer line into the previously installed fiberglass manholes. The annulus around the tee connections was finally grouted using grout and vent tubes installed during the initial placement of the fiberglass manhole.
The life jackets? Well, they attracted a little attention in the street — worn by the crew to prevent drowning in the 40-ft deep polymer slurry-filled hole.
With its method to install the drilled shaft and fiberglass manhole in a single pass, Barnard Soletanche was able to minimize impacts to the surrounding area by completely eliminating a second mobilization and traffic detour. The JV was able to complete each manhole in just a two-week cycle — the last of which took only 7 1/2 working days.
The joint venture team of Barnard Construction Co. Inc. (Bozeman, Mont.) and Soletanche Inc. (Miami, Fla.) successfully completed the Orange County Sanitation District’s tunnel project in April 2008. The project was completed ahead of schedule, within budget and without a lost-time accident. The success of this project was attributable to the cooperation and teamwork of the owner, OCSD; design engineers, Malcolm Pirnie Inc. and CDM; construction manager Parsons-CH2MHill; and Barnard Soletanche JV.
Ben Campbell, Jordan Hoover and Shelley Burg, P.E., are with Barnard Construction Co., Bozeman, Mont.