Twin 30-Inch Ductile Iron Pipe HDD Crossings of the Historic San Marcos River

Population growth and, consequently, increased demand on the water distribution system in San Marcos, Texas, created the need for increased water flow to the southeast section of the city. The City's Master Plan defined the size and established preliminary termination points for the proposed transmission main. Although the Master Plan defined the pipe size as 30-inch (762mm), it was the responsibility of the City of San Marcos's consultant, Carter & Burgess, Inc. (Engineer), to confirm the size and to provide a final design for the water transmission main. The project final design called for approximately 19,500 feet (5.9 km) of 30-inch (762mm) and 5,200 feet (1.6Km) of 24-inch (610mm) water line. Mitigating the impact to historical and cultural resources was paramount to this project. A major portion of the proposed pipeline alignment followed the El Camino Real, or Kings Highway, a trail from Monterrey, Mexico, to San Marcos, to the Caddo Indian villages in East Texas. This trail was used by the Indians of yesteryear and many Spanish explorers who traveled along the San Marcos River. One site within the project corridor was the Villa of San Marcos de Neve, established in 1808. Previous archeological excavations near this site found artifacts dating back more than 11,500 years. Today, the old Camino Real is one of Hays County's most valuable historical assets. To ensure preservation of this protected area, the Engineer specified horizontal directional drilling (HDD) for construction of the pipeline under the San Marcos River and through the area. The HDD installation portion of this project presented several unique opportunities and challenges. This paper will discuss several of these, including the world's first installation of twin parallel 30-inch (762mm) flexible restrained joint ductile iron pipelines, each approximately 1,200 LF (369m). The project also involved the Ductile Iron Pipe Research Association (DIPRA) and Corrpro, a leading provider of corrosion control engineering services, systems, and equipment, to evaluate the long-term effectiveness of polyethylene encasement applied to 30-inch (762mm) pipeline installed using HDD. Finally, this paper will provide designers and end users with information about the proper HDD application, design, and installation of flexible restrained joint ductile iron pipe.