CS3A - Concurrent Session 03A: Hardfill Dam Feasibility in a High Seismic Area

The feasibility of a 300-foot high hardfill dam alternative was evaluated for a site located in California with relatively high seismicity. Hardfill dam construction is similar to roller compacted concrete (RCC) but can include lower quality aggregate and higher fines content. The resulting hardfill material can be weaker than RCC but has higher stiffness and strength than earthfill. Hardfill dam geometry has a symmetrical trapezoidal shape resulting in lower dynamic tensile stresses within the body of the dam and lower static and dynamic shear stresses on the foundation compared to RCC or concrete gravity dam which has a vertical or near-vertical upstream face. This allows hardfill dams to be founded on competent, yet relatively lower strength foundation than RCC or concrete dams. Over 75 hardfill dams with heights ranging from 75 to 500 feet have been constructed around the world since the late 1980’s, including in areas of high seismicity.

Field investigation conducted for the proposed dam site included core drilling and testing, test pits, and seismic refraction surveys. The investigations revealed that the foundation consists primarily of Franciscan sandstone with interbedded siltstone that was judged adequate to support a hardfill dam and the rock quality was good enough to provide hardfill aggregate. Based on geologic mapping and investigations, no active faults were identified at or close to the proposed dam site. Seismic loading criteria for use in design were developed based on the results from deterministic and probabilistic seismic hazard analyses. A 10,000-year return period ground motion with PGA of 0.6 g was used in the feasibility level analysis.

The 30% level design analyses focused on internal stresses within the dam, global stability, differential settlement along the dam and foundation interface, and dam and foundation interaction. Both 2-D finite element linear elastic and nonlinear analyses were performed, using ANSYS and LS-DYNA, respectively. Based on the results of the geologic investigations, the design analyses, and a constructability assessment, the hardfill dam type was concluded to be a technically feasible dam type alternative for the proposed site. The eventual decision not to proceed with hardfill dam considered the lack of precedent for this dam type in California, and the lack of documented performance of existing hardfill dams over 300 feet high under strong earthquake shaking. This paper will discuss potential next steps designers and dam safety regulators could take to construct a hardfill dam in highly seismic areas.