August 4th, 2014

Pooled Fund: Performance-Based Assessment of Liquefaction

Preserve Infrastructure, by Guest Post.

A new study led by UDOT and funded through the FHWA Transportation Pooled Fund Program began in March and is progressing well. The study is number TPF-5(296), entitled “Simplified SPT Performance-Based Assessment of Liquefaction and Effects.” A research team from Brigham Young University (BYU) is performing the two-year study. Other state DOTs participating in the study include Alaska, Connecticut, Idaho, Montana, and South Carolina.

Liquefaction of loose, saturated sands results in significant damage to buildings, transportation systems, and lifelines in most large earthquake events. Liquefaction and the resulting loss of soil shear strength can lead to lateral spreading and seismic slope displacements, which often impact bridge abutments and wharfs, damaging these critical transportation links at a time when they are most needed for rescue efforts and post-earthquake recovery.

Most commonly used liquefaction and ground deformation evaluation methods are based on the concept of deterministic hazard evaluation, which is related to the maximum possible earthquake from nearby faults. Recent advances in performance-based geotechnical earthquake engineering have introduced probabilistic uniform hazard-based procedures for evaluating seismic ground deformations within a performance-based framework, from which the likelihood of exceeding various magnitudes of deformation within a given time frame can be computed. However, applying these complex performance-based procedures on everyday projects is generally beyond the capabilities of most practicing engineers.

The objective of the new study is to create and evaluate simplified performance-based design procedures for the a priori prediction of liquefaction triggering, lateral spread displacement, seismic slope displacement, and post-liquefaction free-field settlement using the standard penetration test (SPT) resistance. Many of the analysis methods used to assess liquefaction hazards are based on SPT resistance values since the SPT is commonly used in site soil characterization for building, transportation, and lifeline projects.

This study represents a worthwhile pilot study which could prepare the way for additional research with the U.S. Geological Survey to further the use of the simplified, performance-based method.

Figure 1: Liquefaction loading map (return period = 1,033 years) showing con-tours of CSRref (%) for a portion of Salt Lake Valley, Utah

Figure 1: Liquefaction loading map (return period = 1,033 years) showing con-tours of CSRref (%) for a portion of Salt Lake Valley, Utah

The key to the simplified method is the use of a reference soil profile in development of liquefaction loading maps which are then used with the site’s soil data to estimate effects of liquefaction. An example map is shown in Figure 1, where CSRref represents a uniform hazard estimate of the seismic loading that must be over-come to prevent liquefaction triggering, if the reference soil profile existed at the site of interest.

Derivations for simplified performance-based liquefaction triggering and lateral spread displacement models have been completed in the study. Validation efforts have shown that the simplified results approximate the full performance-based results within 5% for most sites that were evaluated.

A summary of the study work plan and copies of current reports from the study are available at the TPF-5(296) study website.

This guest post was written by Kevin Franke, Ph.D., P.E., from BYU, and David Stevens, P.E., Research Program Manager, and was originally published in the Research Newsletter.

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