UC Berkeley

Engineers find value in using simplified procedures to estimate seismically induced permanent displacements in their preliminary assessments of the seismic performance of earth structures and buildings. This research develops new, robust simplified procedures to estimate shear-induced displacements of earth and waste structures and natural slopes and to estimate shear-induced settlement of structures founded atop liquefiable soils.

Most simplified procedures for estimating seismic slope displacements are based on variations of the Newmark (1965) sliding block analyses. These procedures were developed largely using earthquake ground motions from shallow crustal earthquakes along active plate margins (e.g., California earthquakes). These semi-empirical procedures should not be applied directly to other seismo-tectonic settings, such as subduction earthquake zones, without evaluating their applicability for tectonic settings for which they were not originally developed. In this study, a simplified procedure for estimating seismic shear-induced permanent displacements in slopes located in subduction earthquake zones is developed. The primary source of uncertainty in assessing the likely performance of an earth slope or system during an earthquake is the input ground motion. Hence, a comprehensive database containing 810 recorded ground motions from subduction zone interface earthquakes was developed and used to compute seismic slope displacements. The proposed seismic slope displacement model captures the primary influence of the system’s yield coefficient ky, its initial fundamental period Ts, and the ground motion’s spectral acceleration at a degraded period of the slope taken as 1.5 Ts. The new procedure uses the framework of the widely used Bray and Travasarou (2007) method developed for shallow crustal settings. The model separates the probability of “zero” displacement (i.e., < 0.5 cm) from the distribution of “nonzero” displacement, so that low values of calculated seismic displacement do not bias the results. The new seismic displacement model better captures the unique seismic setting of subduction zone earthquakes. It has been validated using observations of 12 case histories of seismic slope performance during recent earthquakes, including the 2011 Moquegua, 2007 Pisco, 2010 Maule, 2011 Tohoku, and 2016 Muisne earthquakes.

Current state of practice procedures typically separate the estimation of the ground motion intensity measure (IM) from the estimate of seismic displacement (D), given the selected IM hazard level. Thus, D is estimated based on a single IM value. A straightforward performance-based seismic slope assessment procedure is proposed which considers the full range of potential IM values to estimate seismic slope displacements directly related to a hazard level. Seismic performance is assessed through either a Newmark-type seismic displacement estimate or a calibrated seismic coefficient that can be used in pseudostatic slope stability analyses. The procedures were developed for a wide range of earth systems for shallow crustal earthquakes and subduction zone earthquakes. Currently employed simplified slope displacement procedures do not provide consistent assessments of the actual seismic slope displacement hazard. The proposed procedures can be readily used in practice to perform rigorous performance-based seismic slope displacement hazard assessments.

Liquefaction-induced settlement of shallow-founded buildings continues to produce significant damage during earthquakes. The primary mechanisms of liquefaction-induced building settlement are shear-induced, volumetric-induced, and ejecta-induced ground deformation. The state-of-the practice still largely involves estimating building settlement using empirical procedures developed to calculate post-liquefaction, one-dimensional, reconsolidation settlement in the free-field away from buildings. These free-field analyses cannot possibly capture shear-induced deformations in the soil beneath shallow foundations. Performance-based design requires an improved assessment of liquefaction-induced building settlement. Nonlinear dynamic soil-structure-interaction (SSI) effective stress analyses have shown to be able to capture shear-induced liquefaction building settlement mechanisms.

Dynamic SSI effective stress analyses are performed to identify key trends in the settlement of buildings with shallow foundations affected by soil liquefaction. Over 1,300 dynamic SSI effective stress analyses are performed by systematically varying subsurface conditions and building properties while applying 36 earthquake motions. Shear-induced soil deformation mechanisms govern during strong shaking; whereas volumetric-induced deformation mechanisms contribute more significantly after shaking. The analytical results provide salient insights regarding the key parameters controlling liquefaction-induced building settlement. The relative density of the liquefiable layer is the key soil property, and its thickness is an important soil profile characteristic. Building contact pressure is the most important building parameter, and building width is also important. The ground motion intensity parameters that correlate best with building settlement are standardized cumulative absolute velocity, Arias intensity, and spectral acceleration at 1 s. The post-liquefaction bearing capacity factor of safety indicates when large building settlements are possible.

Volumetric-induced free-field ground deformation may be estimated with available empirical procedures. Although challenging to estimate, ground failure indices and experience can be used to estimate roughly ejecta-induced building settlement. Nonlinear dynamic SSI effective stress analyses are required to estimate shear-induced ground deformation. Results from these analyses identified earthquake, site, and building characteristics that largely control liquefaction-induced building settlement during strong shaking. A simplified procedure is developed based on the results of these analyses to estimate the shear-induced component of liquefaction building settlement. The standardized cumulative absolute velocity and 5%-damped spectral acceleration at 1 s period capture the ground shaking. A new parameter called the liquefaction building settlement index, which is based on the shear strain potential of the site, captures in situ ground conditions. Building contact pressure and width capture the building characteristics. Field case histories and centrifuge test results validate the proposed simplified procedure. Recommendations and an example for evaluating building performance at liquefiable sites are shared.