May 2010 Vol. 237 No. 5
Features
Gas Pipeline Construction Relied Heavily On HDD To Minimize Environmental Impacts
The recently completed Line 108 Replacement Project, an 11 mile long natural gas pipeline for Pacific Gas & Electric (PG&E) in northern California, featured the use of horizontal directional drilling (HDD) for portions of the pipeline installation beneath two rivers and two extremely sensitive natural areas.
Without this innovative approach, as well as other measures designed to minimize environmental impacts, the project likely never would have come to fruition.
Extensive growth throughout the Sacramento area necessitated the replacement of an existing 16 inch diameter gas transmission line that dated to the 1930s. PG&E’s Sacramento Local Gas Transmission System serves about 600,000 customers in some of the fastest growing counties in the state, including Sacramento County. PG&E anticipates this transmission system will serve approximately 22,330 new customers annually during the next several years. Most of them will be served by Line 108.
To take advantage of PG&E’s existing land rights, the alignment of the 24 inch diameter replacement line generally follows the existing pipeline. The pipeline connects the Thornton Meter Station, located just south of the Mokelumne River in San Joaquin County, to the Elk Grove Station, located just south of Elk Grove Boulevard in Sacramento County.
Beginning at the Thornton Meter Station, the replacement pipeline crosses the Mokelumne and Cosumnes rivers and extends north for slightly more than three miles along the east side of the tracks of the Union Pacific Railroad (UPRR). The pipeline then crosses under the UPRR line and Franklin Boulevard, before continuing north for roughly another three miles. At this point, Franklin Boulevard veers west, and the pipeline passes beneath it and continues north for nearly three miles along the west side of the UPRR line.
At the community of Franklin, the pipeline turns west, passing under a short stretch of Bilby Road before turning north and continuing for roughly another mile under Franklin Boulevard. After the intersection of Franklin Boulevard and the UPRR line, the pipeline runs along the west side of the tracks for the final mile, where it ends at the Elk Grove Station.
In addition to the replacement of Line 108, PG&E installed a pressure limiting station at the Elk Grove Station. At press time, the utility was in the process of removing a bridge that once supported the pipeline where it crossed the Cosumnes River. Removal of the bridge is the final element of the overall project.
Evaluating The Impacts
Although most of the pipeline extends through agricultural areas, the alignment also crosses numerous environmentally sensitive areas, including the two rivers, two critical natural areas and a variety of other habitats. As a result, the Line 108 Replacement Project required extensive environmental analysis and careful planning to avoid or minimize environmental impacts.
PBS&J prepared the Project Environmental Assessment for the Line 108 Replacement Project and assisted PG&E in complying with the requirements of the California Environmental Quality Act (CEQA), which requires that state and local agencies identify significant environmental impacts associated with their activities and take steps to avoid or mitigate those impacts, if feasible. In addition to consulting with the various agencies overseeing the project, PBS&J handled such tasks as special status plant surveys, avian surveys, habitat mapping, wetlands delineation and wetland permitting.
Among the more fragile natural areas through which the pipeline passes are the Mokelumne and Cosumnes rivers, the Cosumnes River Preserve and the Stone Lakes National Wildlife Refuge (NWR). The pipeline crosses the two rivers and the Cosumnes River Preserve at its southern terminus, while the Stone Lakes NWR is located near the project’s northern end. Comprising parcels of land owned by several federal and state agencies and nonprofit organizations, the Cosumnes River Preserve is administered by the U.S. Bureau of Land Management. The Stone Lakes NWR is administered by the U.S. Fish & Wildlife Service.
Within the Cosumnes River Preserve, the pipeline crosses riparian forest, riparian scrub, grasslands and seasonal wetlands. Meanwhile, within the Stone Lakes NWR, the pipeline traverses annual grasslands, seasonal wetlands, drainage features and vernal pools. These pools are depressions that remain inundated during the rainy season and provide habitat to a unique assembly of plants and animals, such as the federally listed vernal pool fairy shrimp, that complete their life cycles before the pools dry out during the dry season.
The vernal pool fairy shrimp is one of many sensitive species whose presence had to be accounted for during the design and construction of the project. Other species that are protected by either federal or state mandate and known to inhabit areas along or near the pipeline alignment include the giant garter snake, Swainson’s hawk, western pond turtle, valley elderberry longhorn beetle and burrowing owl, as well as 11 special status plants.
Given the protected status of several landforms and species along the alignment, extensive evaluations of potential environmental impacts were required to secure the necessary permits and approvals from various resource agencies, including the California State Land Commission, the U.S. Army Corps of Engineers, the California State Water Resources Control Board, the California Department of Fish & Game, the U.S. Fish & Wildlife Service, the California Office of Historic Preservation and the Sacramento Metropolitan Air Quality Management District.
Avoiding The Impacts
Key issues addressed as part of the planning and permitting for the project included minimizing environmental impacts associated with the pipeline’s route and the temporary use areas in which pipe sections are connected before installation. During consultations with the various regulatory agencies, several mitigation measures were developed to avoid disturbing wetlands and other waters under the jurisdiction of the Clean Water Act that are located on or adjacent to the construction area.
By far, the main technique used to avoid impacts to wetlands and other jurisdictional waters of the United States involved installing pipeline in sensitive areas by means of HDD. This technique was used to install pipeline beneath the two rivers, portions of the Cosumnes River Preserve and Stone Lakes NWR, and other sensitive areas.
A bore of 2,600 feet was used to pass beneath the rivers and related riparian habitat, requiring a pull back area of the same length along agricultural land south of the Mokelumne River. Within the Cosumnes River Preserve, HDD was used to install 3,300 feet of pipe beneath freshwater marsh, annual grassland, seasonal wetlands, and riparian habitat. Elsewhere along the pipeline alignment, another 1,400 feet of pipe were installed by HDD underneath an unnamed tributary to Snodgrass Slough, while a 1,500 foot section of pipeline was placed by HDD beneath a dairy.
The project’s longest pipeline section installed by means of HDD occurred within the Stone Lakes NWR. Here, 6,500 feet of pipe were installed in this manner to avoid disturbing vernal pools and seasonal wetlands. A pullback area of the same length extended to the north of the Elk Grove Station between an existing sound wall and the UPRR line.
Pipe installed via HDD was bored a minimum of 60 feet underneath the bed and banks of the navigable waterways and roughly 25 feet below any other features. HDD activities beneath the rivers were scheduled to be conducted between June 1 and Nov. 30 to avoid impacts to protected fish species. Similarly, HDD work within the two natural areas was scheduled for summer to minimize impacts to wetlands and the giant garter snake.
Meanwhile, other mitigation efforts were designed and implemented to address certain temporary and permanent impacts to various habitats. Such measures included conducting construction during the dry season, fencing off special status plants or wetland habitat near the construction zone, employing measures to control erosion, and training construction workers regarding the environmental measures to be employed on the project. Furthermore, a PBS&J biologist was present during construction to ensure compliance with the required conservation measures and restrictions.
Construction
Of course, other construction techniques in addition to HDD were used to install the pipeline. Where it passes through agricultural lands, the pipeline primarily was installed by means of trenching. Pneumatic pipe ramming, also known as hammer boring, was used for shorter bores and when room for a HDD pipe string was limited because of sensitive habitat.
Construction technique summary
Overall, open trenching was used to install 69% of the pipeline while HDD was used to install about 30% (see table). The hammer-bore method was used to install the remaining 1%.
For the portion of the pipeline that was buried directly, 24 inch outer diameter API 5L steel pipe with 0.375 inch wall thickness (Grade X 60 DSAW) with 16 mils of fusion bonded epoxy was used. For the portion that was installed by means of HDD or hammer boring, 24 inch outer diameter API 5L steel pipe with 0.5 inch wall thickness (Grade X 60 DSAW) with 16 mils of fusion bonded epoxy plus 40 mils of abrasion resistant overcoating.
Designed by PG&E and Trigon EPC, the pipeline was installed by Southwest Construction. Work began in June 2008 and was completed the following October. During construction, PBS&J provided environmental construction monitoring services, including biological, archeological and paleontological monitoring. PG&E constructed the pressure limiting station.
In addition to the regulatory agencies involved with the permitting, the following organizations contributed to the development of the project: the County of Sacramento, the city of Elk Grove, and the conservation organization the Nature Conservancy.
Final Element
The last element of the Line 108 Replacement Project involves removing a suspension bridge constructed by PG&E in the 1930s to support the original pipeline where it crossed the Cosumnes River. PG&E, which owns the bridge, was asked by the Nature Conservancy, which owns part of the Cosumnes River Preserve, to remove the span to help prevent trespassers from reaching an island with sensitive habitat.
The 630 foot long structure is supported by two piers and two anchor blocks, one of each of which is located on either side of the river. Along with the bridge, PG&E is removing the north anchor block and north pier to one foot below the natural grade and backfilling the area. Because the bridge originally had been painted with lead based paint, special measures must be taken during its demolition to prevent the introduction of contaminants into the environment. Work on the bridge removal began in June 2009 and was completed shortly thereafter.
Three years were spent completing the environmental reviews and obtaining the various permits required for the project. Overall, the project team negotiated approvals on 19 local, state and federal permits and 98 land owner/purchase agreements. Given the number of environmental hurdles, this project succeeded in large part because of the cooperation of the Bureau of Land Management, the U.S. Fish & Wildlife Service, the California Department of Fish & Game, and the Nature Conservancy. Within PG&E, Michael Gunby, the principal land planner for the utility, led the efforts related to the environmental aspects of the project.
Pipeline construction required only three months. With more than 82,000 man hours of construction, the workers achieved triple zero safety performance.
Thanks in part to the careful planning and preparations, construction of the $41 million pipeline was completed two months ahead of schedule and under budget. PG&E and its partners helped to ensure that the utility has the necessary infrastructure in place to serve the growing demand within the Sacramento metropolitan area while minimizing the project’s untoward environmental effects.
Author
Ron Walker is a senior scientist in the Sacramento office of the consulting engineering firm PBS&J.
Comments