Utilizing Horizontal Directional Drilling to Deal with Deep Scour and Severe Environmental Restrictions in a 1700-Foot River Crossing

The Santa Margarita River Crossing is part of a design-build project to replace or update three sewage pump stations and replace their associated forcemains within the Santa Margarita River watershed of the Marine Corps Base at Camp Pendleton (MCBCP) in San Diego County, California. It was determined that the portion of the sewer force main which originally hung from the Stuart Mesa Road bridge across the Santa Margarita River was ideal as a trenchless crossing and could be bored utilizing horizontal directional drilling (HDD). The existing geology and available data were reviewed and an exploration and laboratory-testing program was developed for geotechnical design of the HDD alignment. Scour was critical for the 100-year flood event, dictating the sewer be installed about 50 feet below the streambed. The sewer force main consisted of an 18-inch diameter HDPE pipe installed within a 28-inch diameter HDPE casing in a 42-inch diameter borehole. The HDD design was performed to evaluate the stability and the potential of a collapse of the drilled hole during the HDD installation, as well as the effect of a "frac-out" during HDD installation. There were several environmental challenges associated with this project including: Army Corps permitting; U.S. Fish & Wildlife approvals; schedule restrictions including installation of the pipeline under the river during late fall, but before winter rains; construction and seasonal restrictions within occupied California gnatcatcher and Riparian habitats; Drilling Mud Controls and Frac-out Avoidance Procedures; use of measures to protect the tidewater goby and aquatic habitat at the Santa Margarita River; and minimization of disturbance to wetlands and other waters of the U.S. The horizontal directional drill was successfully completed in November 2006. This paper will discuss the environmental, geotechnical, stream-bed hydrologic and permitting challenges that led to the design and construction decisions for this important component of Camp Pendleton's wastewater infrastructure.