Challenging Geologic Investigations for the First Large New Dam in 21st Century California

While permitting, funding, and addressing public concerns may represent the largest challenges to building a modern dam, they are thankfully not the engineering geologist’s main concerns; quantifying foundation properties and addressing geologic issues are the key responsibilities of a dam geologist. The Pacheco Reservoir Expansion Project site lies within a portion of the Coast Ranges geomorphic province of California dominated by problematic Franciscan Complex geology and dynamic tectonics between two active fault zones. The recently completed first phase of investigations in this exciting geologic setting faced non-geologic challenges as well such as a short project schedule, remote site location, rugged topography, obtaining private property access, as well as wildfire-related concerns and Covid-19 delays. Goals of this investigation were to define the bedrock foundation characteristics at two separate sites under consideration for a proposed 350-ft high dam, spillway, outlet tunnel and structures, as well as a conveyance pipeline and multiple borrow sites. Like many new projects in remote locations, there was little existing geologic information and a lack of access roads and water for drilling; all of this affected the site investigation. The investigations included: lidar and bathymetric surveying, geologic mapping, borings, borehole hydraulic conductivity testing, dilatometer testing, piezometers, geophysical surveys, test pits, and laboratory testing of soil and rock samples. Preliminary results indicate that the site geology is surprisingly straightforward for Franciscan Complex terrane. There are no active faults on the site. (And this is California?) Our first impressions of surficial geologic units from reconnaissance mapping were incorrect in some areas. From subsurface investigations we identified several ancient landslides on valley side slopes that were not reflected by current topography. We also recognized likely Pleistocene-age stream channel and landslide deposits in the valley bottom that were not reflected by the bathymetric survey. Senior level review of all field data proved extremely useful to align and unify the field logging and descriptions of geologic units by a diverse team of geologists. Results of this initial phase of investigation have supported preliminary design of the project. The second phase of site investigation will concentrate on the footprint of the preferred dam site and selected alternative alignments of related infrastructure in a focused and cost-effective way.