Comparative evaluation of foundation modeling for SSI analyses using two different ABC approaches: Applications to dams

Abstract

The numerical modeling of the unbounded domain is one of the most challenging issues both for site response and soil-structure interaction analyses. The viscous-type absorbing boundary conditions (ABCs) are commonly used for the modeling of the unbounded domain due to their low computational cost and simplicity. However, in cases, their application can lead to substantial errors. In this paper, a time-domain viscoelastic perfectly matched layer (PML) is implemented for modeling the unbounded domain in the dynamic response evaluation of a concrete gravity dam. The performance of the viscoelastic PML and the viscous ABCs are examined through numerical examples under different types of dynamic excitations including uniform and spatially variable seismic ground motions. The numerical results highlight the excellent performance of the PML approach to model the unbounded domain under all types of excitations. It is also shown that the viscous-type ABCs perform poorly under spatially variable excitations and reflected waves from the structures, as they cannot properly absorb inclined waves.The nonlinear response of a concrete gravity dam subjected to deconvolved seismic excitations at depth resulting from five surface seismic records was then evaluated. The effect of the finite foundation depth on the dam response was also examined by considering a shallow (approximately equal to the height of the dam) and a deep (approximately double the height of the dam) finite foundation length. The results indicate that the PML is capable of maintaining its accuracy even if the size of the computational finite domain is reduced. On the other hand, it was shown that the shallow finite foundations modeled with viscous ABCs substantially underestimate the dam response under uniform excitations, whereas the use of a deep finite foundation model with viscous ABCs can lead to more conservative results. An important conclusion of this study is that the use of foundation models with viscous-type ABCs for the dynamic analysis of dams, and, consequently, other long structures, subjected to spatially variable excitations, should be avoided as it can significantly underestimate the crest displacement and the total damage in the dam.