Construction of Gas Pipeline Relied Heavily on HDD to Minimize Environmental Impacts

Minimal Impact, Maximum Outcome
By Ron Walker | November 2009 Vol. 64 No. 11
Successful hammerjack bore connecting to area previously trenched. Photo: Travis King.

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 the summer months 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 besides HDD also 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.

Overall, open trenching was used to install approximately 69 percent of the pipeline, while HDD was used to install about 30 percent (see table). The hammer bore method was used to install the remaining 1 percent.

Construction Technique Summary

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.