Salt Springs, owned by Pacific Gas & Electric Co. (PG&E), is the 5th oldest concrete-faced rockfill dam (CFRD) in the world and the first CFRD to reach 100 meters in height.  As an example of state-of-the-art dam engineering, a geomembrane system was installed to control dam seepage.

Background

The Salt Springs Dam is located in California, southeast of Sacramento, high in the Sierra Nevada Mountain Range.  Construction of the dam was halted at mid-height in the 1930s. When the rock placement resumed, the sluicing technique was improved.  This created a zone of differential settlement in the dam where the concrete face has required continual repairs over its 70+ year history.

Malfunction

In 2001, a leak occurred in this zone resulting in a significant increase in seepage beyond the level set by the California Department of Safety of Dams (CA DSOD).  As a temporary measure, PG&E undertook immediate repairs with divers at considerable expense to decrease the seepage. The owner also investigated how to permanently fix the upstream face at Salt Springs.

Partial Solution

The conclusion was to install a PVC geomembrane system over the upstream face with the goal of reducing seepage to 12 cfs. In the spring of 2004, Phase I repairs began with the installation of the CARPI geomembrane system over the center of the face in the transition
zone (3790 to 3810 elevations), in order to mitigate the risk of significant seepage increases, as occurred in 2001.

The membrane system arrested seepage in this zone during the filling of the reservoir from snow runoff in 2004. At the same time, an Gannett
Fleming was hired to perform a seepage analysis of various geomembrane configurations.

In-depth Analysis

The study results showed that in order to maximize the seepage reduction from a geomembrane installation, the area of the membrane system installation should be altered and extended down the face instead of to the crest.  The geomembrane installation plan was altered and the geomembrane system was completed in the spring of 2005 encompassing approximately 200,000 square feet on the face of the dam.

This paper describes the background of the dam and the seepage problem in depth, as the authors discuss the analysis and installation.  It also includes seepage readings and costs.