Abstract

Soil liquefaction during earthquakes is one of the most widely recognized geohazards. It has direct impact for offshore projects in terms of pipeline floatation, slope stability, bearing capacity and lateral pile design. While a number of researchers have developed methods to assess liquefaction potential, these approaches are usually limited to defining a factor of safety or probability of liquefaction for a given design earthquake. This paper presents a methodology to combine the results of seismic hazard studies, site response modeling and conventional geotechnical analyses, and their respective uncertainties, to define the annual probability of liquefaction. The methodology mirrors the probabilistic basis for the 2000 SAC/FEMA Steel Moment Frame Guidelines developed by Cornell and coworkers [1]. This powerful probabilistic framework can be extended to a wide range of geohazard problems where it is necessary to define the overall risk level for a project.

Introduction

Soil liquefaction is caused by the buildup of excess pore pressures during an earthquake. Ground shaking causes deformations of the soil skeleton which result in rearrangement of the soil grains. In loose sands and silts these deformations cause the soil to reduce in volume. As the earthquake takes place over a short time period, drainage is not possible and the tendency to compact results in increased pore pressures. If the pore pressures become equal to the total stress full liquefaction occurs and the soil behaves as a liquid, loosing virtually all strength. In this event buried pipelines float out of trenches, gentle slopes can experience large deformations, gravity bases can fail in various modes, and piles loose significant portions of their lateral resistance.

The usual method to evaluate liquefaction potential is to compare the seismic demand on a soil layer to the capacity of the soil to resist liquefaction. Both these quantities are expressed as the ratio of average cyclic shear stress to vertical effective stress. Following the NCEER nomenclature [2], seismic demand is the cyclic stress ratio (CSR) while the capacity is denoted as cyclic resistance ratio (CRR). The induced stresses (CSR) are determined by the intensity of earthquake shaking and the dynamic response of the soils at the site. The capacity (CRR) depends on the density and nature of the deposit and is usually estimated from in-situ test data. In a standard analysis, the ratio of capacity (CRR) to demand (CSR) gives the factor of safety against liquefaction for a specific return period design earthquake.

From the probabilistic point of view there are three aspects to the problem:

  • the probability of an earthquake causing a specific level of ground acceleration at the site;

  • the probability of a particular acceleration level inducing a critical cyclic stress ratio in the soil;

  • given the cyclic stress level in the soil, the probability of liquefaction.

Each of these three elements carries both intrinsic randomness and a degree of epistemic (modeling) uncertainty which must be included in the assessment.

The first point can be addressed by a probabilistic seismic hazard assessment (PSHA).

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