ASSESSMENT OF GEOLOGIC CONDITIONS AT THE THERMALITO DAM COMPLEX, CALIFORNIA

The characterization of site conditions is a critical part of dam risk assessments and provides context for evaluating the stability of a dam, and its potential for failure. We analyze the geologic stratigraphy and structure underlying the foundations of facilities in the Thermalito Dam Complex in Butte County, California: Thermalito Forebay and Afterbay, Diversion and Fish Barrier Dams.

Review of pre-construction geologic maps, extensive drill hole logs (1959-2020), regional geologic relationships and geophysical datasets, shear-wave velocity data, and deep oil/gas wells reveal highly variable site conditions. These range from Quaternary fluvial terraces of the Feather River, other locally derived alluvium, late Cenozoic volcaniclastics of the Tuffs of Oroville (Tto), hard and strong Miocene Lovejoy Basalt, poorly consolidated sediments of the Eocene Ione and Capay Formations, to Mesozoic basement. Over the years, DWR has collected shear wave velocity measurements of each of these units using downhole suspension logging, MASW, cross-hole surveys, and other downhole survey techniques. Data were synthesized in a series of 2D geological profiles, extending from the ground surface to basement to inform geologic models, which may be used to calculate site response and support detailed analyses of dam stability.

Additional 2D profiles were prepared at the Forebay Main Dam where Lovejoy Basalt in the foundation consists of large, intact blocks. The blocks have previously been interpreted as being down-faulted from the main Lovejoy Formation of the Campbell Hills along a northeast-trending fault, requiring careful consideration in the risk assessment process. A fault model can be used to explain Lovejoy geometries along parts of the sections, but not all. Based on the geometry and extent of the blocks, we propose that they were separated from the main exposure of the Lovejoy Formation by landsliding processes, with slide planes potentially detaching along the top of the Ione Formation, and movement ceasing prior to deposition of the Tto. While a landslide origin appears to be more likely than a fault origin, we cannot fully preclude the latter within the resolution of the available data and therefore can’t eliminate that as a consideration for risk assessments.