Cornforth Consultants was retained by the Barney Joint Commission to evaluate the seismic stability of a 50-foot raise of the existing Trask River Dam (subsequently renamed Eldon Mills Dam). Potential seismic ground motions were evaluated for three seismic sources: an interface subduction earthquake; an intraplate subduction earthquake; and various crustal earthquakes. The ground motion parameters assigned to each source included the maximum credible earthquakes (MCE), peak bedrock accelerations (mean and 84th percentile values), and acceleration time histories. Probabilistic estimates of return periods were also developed for the peak motions.
The bedrock acceleration time histories were propagated through the soil in a site-specific ground response analysis. The seismic stability of the dam was performed using finite element methodology. Initial static shear stresses within the embankment and foundation soils were calculated, and the free-field time histories were input into a 2-dimensional finite element program to calculate the resulting dynamic shear stresses and accelerations within the embankment.
The dynamic shear strength of the slightly clayey silt foundation soil was tested in the laboratory using a cyclic triaxial apparatus. The dynamic shear strength was compared with the calculated shear stresses to obtain an estimate of the increase in pore water pressures that would be induced by earthquake motions. The resulting increase in pore water pressure was modeled in limit-equilibrium stability analyses. Finally, an estimate of the crest deformation was determined using a Newmark-type analysis.