San Vicente to Second Aqueduct Pipeline

GEI provided planning and geotechnical engineering services to the San Diego County Water Authority (Authority) for an 11.7-mile long water supply pipeline to connect San Vicente Reservoir with the Authority’s Second Aqueduct.

Two alternatives were considered for the pipeline – a primary trench route and a primary tunnel route. GEI was the prime consultant responsible for developing alternative tunnel alignments for the project. Initial evaluations of the alignments included review of regional geologic information, developing conceptual level subsurface profiles and tunnel designs, and preparing budgeting level cost estimates. Both shallow and deep tunneling alternatives were considered, incorporating various intermediate access shaft locations. Shallow tunnel alternatives would penetrate conglomerate formations, while deep alternatives would avoid the conglomerates and stay within igneous rocks.

GEI performed a Phase I field exploration program and prepared an initial geotechnical data report (GDR) and a geotechnical interpretive report (GIR). The field exploration program included geologic reconnaissance and the completion of seven borings up to 540-feet-deep, totaling 2190 lineal feet of core drilling. The program included extensive pressure packer tests as well as installation of vibrating wire piezometers. GEI assisted the Authority in obtaining access rights to boring locations, many of which were in environmentally-sensitive areas. Laboratory testing of rock core included petrographic analyses, compressive and tensile strength, and Cerchar abrasion.

GEI prepared feasibility level designs for alternative tunnel alignments. This work included evaluating alternate methods to advance the tunnel, providing tunnel support, and placing and supporting a steel liner pipe or concrete lining for use as a water conductor. Evaluations also included preliminary siting of portals and shafts, assessing staging area needs, and investigating possible uses for tunnel muck. Estimates of probable construction costs were also developed.