Steven L. Kramer

Ground Motion Intensity Measures for Liquefaction Hazard Evaluation

The requirements of performance-based earthquake engineering place increasing importance on the optimal characterization of earthquake ground motions. With respect to liquefaction hazard evaluation, ground motions have historically been characterized by a combination of peak acceleration and earthquake magnitude, and more recently by Arias intensity. This paper introduces a new ground motion intensity measure, CAV5, and shows that excess pore pressure generation in potentially liquefiable soils is considerably more closely related to CAV5 than to other intensity measures, including peak acceleration and Arias intensity. CAV5 is shown to be an efficient, sufficient, and predictable intensity measure for rock motions used as input to liquefaction hazard evaluations. An attenuation relationship for CAV5 is presented and used in an example that illustrates the benefits of scaling bedrock motions to a particular value of CAV5, rather than to the historical intensity measures, for performance-based evaluation of liquefaction hazards.

Geotechnical Characterization and Random Field Modeling of Desiccated Clay

AbstractAn extensive set of in situ and laboratory test data is presented for a footing load test site east of Houston, Texas, in desiccated Beaumont clay. The in situ test program included standard and cone penetration tests (CPTs), the latter of which was selected for statistical analysis to produce vertical and horizontal random field model parameters for corrected cone tip resistance. Given the relatively high sampling frequency of the cone tip resistance in the vertical direction, the vertical random field model parameters were determined using the modified Bartlett’s test statistic with fitted autocorrelation models subject to a strict fitting criterion. Horizontal random field model parameters were generated by collapsing the two-dimensional distribution of the CPTs to a one-dimensional representation and by using less stringent evaluation of the autocorrelation. The results of this study indicate that Beaumont clay exhibits greater inherent spatial variability than previously reported at other cla...

Investigation of Cyclic Liquefaction with Discrete Element Simulations

AbstractA discrete-element method (DEM) assembly of virtual particles is calibrated to approximate the behavior of a natural sand in undrained loading. The particles are octahedral, bumpy clusters of spheres that are compacted into assemblies of different densities. The contact model is a Jager generalization of the Hertz contact, which yields a small-strain shear modulus that is proportional to the square root of confining stress. Simulations made of triaxial extension and compression loading conditions and of simple shear produce behaviors that are similar to sand. Undrained cyclic shearing simulations are performed with nonuniform amplitudes of shearing pulses and with 24 irregular seismic shearing sequences. A methodology is proposed for quantifying the severities of such irregular shearing records, allowing the 24 sequences to be ranked in severity. The relative severities of the 24 seismic sequences show an anomalous dependence on sampling density. Four scalar measures are proposed for predicting th...

Triggering of liquefaction flow slides in coastal soil deposits

Abstract Liquefaction flow slides have been observed in coastal soil deposits throughout the world. The causes of many of these slides have never been well understood. The materials involved in and circumstances surrounding eight reported liquefaction flow slides are reviewed and compared. Many of these slides have occurred in very similar materials shortly following small to moderate drawdowns of the adjacent water level. Liquefaction susceptibility of sands may be evaluated by steady state methods. Recent research indicates that liquefaction of loose sands similar to those typically involved in these slides may be triggered by very small changes in stress conditions if the sand is initially in equilibrium under high shear stresses. The triggering of liquefaction may lead to flow sliding if the unbalanced driving stresses exceed the steady state strength of the liquefied soil. Elasticity methods are used to estimate the initial stress conditions in a gentle, infinite slope, and the effect of unloading due to drawdown. It is shown that liquefaction flow slides in coastal soil deposits may be initiated by small to moderate drawdowns consistent with those observed in actual slides.

Reliability of Spread Footing Performance in Desiccated Clay

AbstractTo advance the use of reliability-based design procedures, it is necessary to evaluate the sources of the design parameter uncertainty including inherent variability, measurement error, and transformation uncertainty. The results of a probabilistic evaluation of undrained footing bearing performance are discussed in the context of an extensive test site characterization described in a companion paper. Kriged cone tip resistance values are transformed into design parameters using a second-moment probabilistic approach and compared with the parameters obtained from the laboratory test analyses on specimens retrieved from the test site. The spatial, measurement, and transformation uncertainty are incorporated into probabilistic finite-element and bearing capacity analyses in which the results are compared against a full-scale load test performed at the test site. The results indicate that the reliable assessment of the spread footing response depends to a large degree on the assumed strength anisotro...

Procedure for the Empirical Evaluation of Lateral Spread Displacement Hazard Curves

AbstractLiquefaction-induced lateral spread has caused substantial damage to buildings, bridges, embankments, buried utilities, and other critical components of infrastructure in numerous past earthquakes. Although many practitioners use analytical Newmark-based seismic slope displacement models to estimate liquefaction-induced lateral spread displacements, empirical regression models remain popular among most practicing professionals today. However, all lateral spread estimates have a significant level of uncertainty. This paper introduces a procedure to develop performance-based estimates of liquefaction-induced lateral spread displacement by incorporating a widely used empirical model into a probabilistic framework. Utilizing the same principles used in probabilistic seismic hazard analysis, the empirical lateral spread equation is modified and inserted into a performance-based framework modeled after the framework introduced by the Pacific Earthquake Engineering Research Center. Lateral spread hazard ...

Performance-based design methodologies for geotechnical earthquake engineering

Performance-based design offers a number of advantages over historically common design approaches. As currently practiced, performance is most commonly evaluated in terms of system response, typically displacement-related, using traditional ground motion intensity measures defined at two or more discrete hazard levels. Such approaches do not necessarily allow all of the potential benefits of performance-based design to be realized. More recently, integral performance-based procedures that account for ground shaking hazards at all hazard levels have been developed. This paper reviews basic concepts of performance and different approaches to the implementation of performance-based design. An load and resistance factor-like methodology is described and illustrated with an example.

Performance-Based Earthquake Engineering: Opportunities and Implications for Geotechnical Engineering Practice

Great advances have been made over the 40-some years in which geotechnical engineers have actively been involved in the practice of earthquake engineering. Most recently, advances have come through the development of performance-based earthquake engineering, which seeks to predict the seismic performance of structures and facilities in ways that are useful to a wide variety of stakeholders. Performance-based earthquake engineering requires the integrated, collaborative efforts of several groups of earthquake professionals, including geotechnical engineers; as such, it will affect the practice of geotechnical engineering in seismically active areas. This paper reviews the evolution of performance-based earthquake engineering, discusses the notion of performance and its description, and describes a recently developed framework for performance evaluation. The nature and effects of the many uncertainties that apply to the prediction and description of ground motions, system response, physical damage, and loss are described. The paper gives examples of different manners in which performance-based earthquake engineering can be implemented into practice. Finally, a series of challenges and opportunities presented by performance-based earthquake engineering for geotechnical engineering practitioners are identified and discussed.

Simplified Approximation Procedure for Performance-Based Evaluation of Liquefaction Potential

Performance-based procedures for evaluation of liquefaction potential have been shown to provide more consistent and accurate indications of the actual likelihood of liquefaction in areas of different seismicity than conventional procedures. The process of performing a complete site-specific performance-based evaluation of liquefaction potential, however, requires numerous calculations involving quantities that many geotechnical engineers are not familiar with. This paper shows how the results of complete performance-based analyses can be expressed in terms of a scalar parameter corresponding to a particular element of soil in a reference soil profile, and presents procedures for adjustment of that parameter to account for site-specific conditions that differ from those of the reference profile. The procedures are shown to closely approximate the results of complete site-specific performance-based evaluations. Engineers can then use mapped values of the scalar parameter, along with the recommended adjustment procedure, to realize the benefits of a performance-based evaluation without having to actually perform the performance-based calculations.

Performance-Based Evaluation of Bridges on Liquefiable Soils

................................................................................................................................. iii ACKNOWLEDGMENTS ........................................................................................................... iv TABLE OF CONTENTS............................................................................................................. v LIST OF FIGURES ..................................................................................................................... xi LIST OF TABLES ....................................................................................................................xvii