Tag Archives: earthquake

Vertical Earthquake Drains for Soil Liquefaction Mitigation

Photo of a verticle earthquake drain

Vertical earthquake drains developed by Nilex, Inc.

Limited blast liquefaction testing, vibration testing, and centrifuge testing suggest that vertical drains can be effective in preventing earthquake-induced soil liquefaction and associated settlements or lateral spreading. However, no full-scale drain installation has been subjected to earthquake-induced ground motions. This lack of performance data under full-scale conditions has been a major impediment to expanding the use of this technique for mitigating liquefaction hazards.

To determine the viability of large diameter (4 in.) prefabricated vertical drains for preventing soil liquefaction and associated settlements under full-scale conditions, the pooled fund study no. TPF-5(244) was initiated in 2013 by UDOT, Brigham Young University (BYU), and other state DOTs from California, New York, and Alaska, in conjunction with the National Science Foundation’s George E. Brown Jr. Network for Earthquake Engineering Simulation (NEES) Facility at the University at Buffalo (UB) of The State University of New York.

Photo

NEES-UB 20-ft high laminar box with hydraulic actuators.

In August and September 2014, two test series with vertical drains in liquefiable (loose and saturated) sand were completed using the laminar shear box and high speed actuator system at NEES-UB. Tests involved level ground conditions with two drain spacings: 4 ft for the first series and 3 ft for the second series. For each drain spacing, the soil profile was subjected to a total of nine sinusoidal motions at increasing peak base accelerations of 0.05g, 0.10g, and 0.20g. The settlement of the soil profile was measured using surface settlement plates, string potentiometers, and Sondex profilometers. Pore pressure transducers were used within the sand at various depths to measure pore water pressures. Accelerometers and LVDTs were located along the height of the shear box to define the acceleration and deflection profiles induced by the shaking at the base. Example data plots are shown below.

A few video recordings from the first series of tests at NEES-UB are available for viewing at this link. Progress reports and the overall scope of work for the study are provided on the web page for study no. TPF-5(244).

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Settlement and excess pore pressure ration versus depth plots during the first shaking test at 4-ft drain spacing, with 15 cycles of shaking and 0.05g peak acceleration.

Remaining tasks on the project include data analysis, comparison with previous tests on untreated sand, evaluating predictive methods, and preparing the final report regarding drain effectiveness. If full-scale tests prove the effectiveness of the drainage technique, significant time and costs savings can be achieved for both new construction and for retrofit situations, as compared to other mitigation techniques.

This guest post was written by Kyle Rollins, PhD with Brigham Young University and David Stevens, PE with UDOT Research Division and was originally published in the UDOT Research Newsletter.

Pooled Fund: Performance-Based Assessment of Liquefaction

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.