Liquefaction of Soils in the 1989 Loma Prieta Earthquake

The Loma Prieta Earthquake of October 17, 1989 was the most costly single natural disaster in U.S. history, resulting in losses of $7 to$9 billion, and claiming 63 lives. These damages were concentrated mainly at a number of distinct sites comprising a relatively small fraction of the affected region, as local site conditions and related geotechnical factors exerted a major influence on damage patterns and loss of life in this catastrophic event. This paper discusses one of these geotechnical factors, the widespread occurrence of soil liquefaction during the earthquake, as well as the associated damages and the resulting lessons learned. Additional significant geotechnical factors which exerted a strong influence on damage patterns during this event, including site-dependent dynamic response and seismically-induced slope instability, are discussed in companion papers in these proceedings

Centrifuge Modeling of Pile-Supported Wharves for Seismic Hazards

Recent earthquakes have highlighted many seismic hazard concerns for western U.S. ports. Port waterfront structures are commonly constructed utilizing pile-supported wharves in combination with rock dike structures retaining a hydraulically placed backfill. Seismic damage is generally attributed to weak soils that are often prevalent in the marine environment (e.g. liquefiable sands, sensitive cohesive soils). In response to past damage, many ports are instigating soil improvement strategies to eliminate or minimize potential occurrences of liquefaction and to increase the strength of cohesive soils. The design of a seismically resilient wharf requires an understanding of its performance during design level earthquakes. Due to the complex nature of pile-supported wharves, state-of-the-art centrifuge modeling techniques are being used to better understand their seismic performance. The authors used the large-scale centrifuge facility at the University of California at Davis. This paper presents details on the construction, instrumentation, and testing of the models. Results from the tests are also included, such as the seismic pile behavior, effect of soil improvement, and the overall behavior

Inertial and Liquefaction-Induced Kinematic Demands on a Pile-Supported Wharf: Physical Modeling

Results of a centrifuge test on a pile-supported wharf were used to investigate the time-, depth-, and row-dependent nature of kinematic and inertial loading on wharf piles in sloping rockfill. P-y models were calibrated against recorded bending moments in different piles and different depths. It was found that full kinematic demands and full superstructure inertia should be combined to estimate bending moments at pile head and shallow depths (less than 10 diameters below the ground surface). On the contrary, it was found that applying full kinematic demands alone was adequate to estimate pile bending moments at large depths (greater than 10 diameters deep)

Pile-Supported Wharves Subjected to Inertial Loads and Lateral Ground Deformations. I: Experimental Results from Centrifuge Tests

Five dynamic, large-scale centrifuge tests on pile-supported wharves were used to investigate the time- and depth-dependent nature of kinematic and inertial demands on the deep foundations during earthquake loading. The wharf structures in the physical experiments were subjected to a suite of recorded ground motions and imposed superstructure inertial demands on the piles. Partial to full liquefaction in loose sand resulted in slope deformations of varying magnitudes that imposed kinematic demands on the piles. It was found that the wharf inertia and soil displacements were always in phase during the critical cycle when bending moments were at their maximum values. The test results were analyzed to provide the relative contributions of peak inertial loads and peak soil displacements during critical cycles, and the data revealed the depth dependency of these factors. The results of this study are used in a companion paper to provide recommendations for the design of pile-supported wharves subjected to foundation deformations

Pile-Supported Wharves Subjected to Inertial Loads and Lateral Ground Deformations. II: Guidelines for Equivalent Static Analysis

An equivalent static analysis (ESA) procedure is proposed for the design of pile-supported wharves subjected to combined inertial and kinematic loads during earthquakes. The accuracy of the ESA procedure was evaluated against measurements from five large-scale centrifuge tests. The wharf structures in these tests were subjected to a suite of recorded ground motions and the associated superstructure inertia, as well as earthquake-induced slope deformations of varying magnitudes. It is shown that large bending moments at depths greater than 10 pile diameters were primarily induced by kinematic demands and can be estimated by applying soil displacements only (i.e., 100% kinematic). In contrast, the large bending moments at the pile head are primarily induced by wharf deck inertia and can be estimated by applying superstructure inertial loads at the pile head only (i.e., 100% inertial). Large bending moments at depths shallower than 10 pile diameters are affected by both inertial and kinematic loads; therefore, evaluation of pile performance should include soil displacements and a portion of the peak inertial load at the pile head that coincides with the peak kinematic loads. Ranges for inertial and kinematic load combinations in uncoupled analyses are provided for different soil profiles. The details on the back-calculated load combination factors are provided in the companion paper

Seismic Performance of Pile-Supported Piers and Wharves Subjected to Foundation Deformations

The interaction of inertial and kinematic demands is investigated using data from five physical models of pile-supported wharves using a large-scale geotechnical centrifuge. The wharf structures in this study were subjected to a suite of recorded ground motions, therefore associated superstructure inertia, and earthquake-induced slope deformations of varying magnitudes. The observations from these tests were used to provide insights on how to estimate large bending moments that developed at pile head and at depths significantly below a commonly assumed point of fixity that are associated with deep-seated ground deformations. Design recommendations are proposed on how to combine inertial and kinematic demands in a manner that is representative of the global structure

Effects of Long Duration Earthquakes on the Interaction of Inertial and Liquefaction-Induced Kinematic Demands on Pile-Supported Wharves

Nonlinear dynamic analyses were performed to evaluate the effects of ground motion duration on the dynamic response of a pile-supported wharf subjected to liquefaction-induced lateral ground deformations. The numerical model was first calibrated using recorded data from a well-instrumented centrifuge test, after which incremental dynamic analyses were conducted using a suite of spectrally matched motions with different durations. The nonlinear dynamic analyses were performed to evaluated three loading scenarios: combined effects of inertial loads from the wharf deck and kinematic loads from ground deformations, deck inertial loads only in the absence of liquefaction (with minimal kinematic loads), and kinematic loads only in the absence of deck mass inertia. The analysis results were evaluated to provide insights on the relative contribution of inertial and kinematic demands on the response of the wharf with respect to motion duration. It was found that the contribution of peak inertial and peak kinematic loads to the maximum total demand increases only slightly with motion duration and intensity. The response of the wharf was found to be primarily governed by kinematic demands when subjected to long-duration motions for the type of foundation analyzed in this study which is commonly used in the port industry

Experimental P-Y Curves from Centrifuge Tests on Pile Foundations Subjected to Liquefaction and Lateral Spreading

The results of five centrifuge models were used to evaluate the response of pile-supported wharves subjected to inertial and liquefaction-induced lateral spreading loads. The centrifuge models contained pile groups that were embedded in rockfill dikes over layers of loose to dense sand and were shaken by a series of ground motions. The p-y curves were back-calculated for both dynamic and static loading from centrifuge data and were compared against commonly used API p-y relationships. It was found that a significant reduction in ultimate soil resistance occurred in dynamic p-y curves in partially/fully liquefied soils as compared to static p-y curves. It was also found that incorporating p-multipliers that are proportional to the pore water pressure ratio in granular materials is adequate for estimating pile demands in pseudostatic analysis