Design of Mid-Path Intercept HDD Crossing of the Santa Maria River

Nipomo Community Services District is located on the central coast of California. The District‘s Santa Maria River Crossing of the Supplemental Water Project Bid Package #1 includes installation of over 2,000 feet of 30-inch diameter waterline installed by a multiple pass directionally controlled drilling operation known as horizontal directional drilling (HDD). The work was completed as designed using a mid-path intercept installation for the pilot bore. Located in and across the Santa Maria River, the Project was conducted amid sensitive habitats. The preliminary engineering for the Supplemental Water Project recommended construction of a 30-inch OD HDPE pipeline crossing the Santa Maria River via HDD in order to reduce disturbance of environmentally sensitive areas, avoid construction on the bluff, and streamline permitting for the project. The HDD River Crossing alignment begins within the riverbed at the HDD Entry Point located approximately 880-feet northwest of where the pipeline passes under the levee adjacent to the river. From this point, the alignment extends northward towards the south facing bluff of the Nipomo Mesa, traversing approximately 2,100 feet of riverbed and gaining approximately 110 feet in elevation as it rises to the top of the bluff. At the top of the bluff, the alignment extends an additional 500 feet northward towards the HDD Exit Point near the reservoir and pump station site. The carrier pipe vertical alignment (profile) includes an inverted arc with a radius of 5,000-feet. The soils encountered generally consist of alluvium derived from the Paso Robles Formation and dune sand. The groundwater levels vary significantly seasonally, and thus the HDD construction was specified to take place during periods of low flow because the HDD entry point is within the flood plain. A surface casing was required to stabilize the soils at the entry and exit points during drilling. The significant surface elevation difference between the entry and exit pits presented a unique challenge to the designers of the Project. This paper addresses the design considerations that led to a successful mid-path intercept HDD installation, and the construction challenges that were overcome.