UC Berkeley

Recent earthquakes in Chile, New Zealand, and Japan have re-emphasized the damaging consequences of liquefaction on infrastructure. Due to the complexity of the problem and limited well-documented field case histories, liquefaction-induced building settlements are often estimated using empirical correlations developed for free-field sites on level ground that account for post-liquefaction volumetric strains only. Additional effects due to the presence of a structure are not accounted for with these procedures. The earthquake performance of structures founded on liquefiable ground depends on a complex interaction between the soil properties, the ground motion characteristics, and the structural properties. This thesis presents three related research projects that address aspects of the effects of soil liquefaction including near-fault sites. This research thesis is focused on characterizing and selection of near-fault ground motions, geotechnical centrifuge testing of model buildings affected by liquefaction, and the development of field case histories in Chile following the 2010 Maule, Chile earthquake.

Earthquake ground motions are important in liquefaction-induced building performance. Ground motions in the near-fault region frequently have intense, double-sided pulses in the velocity-time series that can be very damaging to structures; forward directivity is a leading cause of these pulses. However, pulses do not always occur in the forward directivity region, and some pulses are not caused by forward directivity. The present study used a new, automated algorithm to classify a large database of records as pulse or nonpulse motions. A straightforward model was developed to estimate the proportion of pulse motions as a function of closest site-to-source distance and epsilon of the seismic hazard.

Geotechnical centrifuge tests provide valuable insight into the performance of structures affected by liquefaction. An area particularly lacking understanding is the interaction of closely spaced structures subjected to liquefaction. Two well-instrumented centrifuge tests were performed to investigate the response of three types of model structures founded on liquefiable ground in isolated and adjacent configurations. Acceleration, pore water pressure, and settlement measurements indicated that liquefaction-induced settlement of structures depends on a complex interaction of ground motion, soil, and structural characteristics. For the particular scenarios examined in this study, adjacent structures experienced moderately lower foundation accelerations, tended to tilt away from each other, and settled less than isolated structures.

The 2010, MW = 8.8, Maule, Chile earthquake caused substantial damage, including liquefaction-induced damage to infrastructure and provides an important opportunity to learn from these field case histories. This project focuses on improved characterization of the subsurface conditions using penetration testing (i.e., SPT and CPT) at a hospital and two bridges that suffered liquefaction-induced damage. The recently constructed hospital has 10 structurally isolated wings varying in height from one to six stories, which provides a unique opportunity to examine the differing response of varying wings. Liquefaction of plastic, silty soils at the hospital resulted in differential settlement, whereas liquefaction of clean, medium-dense sandy soils resulted in lateral spreading and damage to bridge piers.