Drilling deep into the ground at Pacheco Reservoir

Building a modern-day dam to help provide safe, clean drinking water for residents of Santa Clara and San Benito counties is a tremendous undertaking.

The proposed expansion of Pacheco Reservoir, located approximately 60 miles southeast of San Jose, includes the construction of an earthen dam upstream of the existing dam, a pump station, a pipeline and other supporting facilities.

Geologic explorations to identify optimum design parameters for the dam started with drilling boreholes as deep as 340 feet into the ground to gather information about the soil and rock characteristics at the site. Once each bore hole is complete, samples of the rock and soil are taken to a laboratory and tested to determine their engineering properties.

This effort is all about dam safety. Valley Water is working to identify where strong rock is located to provide a stable foundation for the dam. Drilling is also identifying on-site sources of materials that are appropriate for safe dam construction.

In 2019, Valley Water completed 63 borings ranging from 30 feet to 340 feet in depth at the site of the proposed expansion. Altogether, 6,776 feet of drilling has been completed. That’s about 1.25 miles. The drilling work at Pacheco paused in December, and restarted in early spring.

This geologic exploration will help Valley Water and its partner agencies, the San Benito County Water District and Pacheco Pass Water District, build a dam that is safe for future generations. Once complete, the project will boost Pacheco Reservoir’s operational capacity from 5,500 acre-feet to up to approximately 140,000 acre-feet, enough to supply up to 1.4 million people with safe, clean drinking water for one year in an emergency.

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