Normal accidents. Living with high-risk technologies

Time-Frequency Analysis.

The concept of the suction stress characteristic curve (SSCC) for unsaturated soil is presented. Particle-scale equilibrium analyses are employed to distinguish three types of interparticle forces: (1) active forces transmitted through the soil grains; (2) active forces at or near interparticle contacts; and (3) passive, or counterbalancing, forces at or near interparticle contacts. It is proposed that the second type of force, which includes physicochemical forces, cementation forces, surface tension forces, and the force arising from negative pore-water pressure, may be conceptually combined into a macroscopic stress called suction stress. Suction stress characteristically depends on degree of saturation, water content, or matric suction through the SSCC, thus paralleling well-established concepts of the soil–water characteristic curve and hydraulic conductivity function for unsaturated soils. The existence and behavior of the SSCC are experimentally validated by considering unsaturated shear strength data for a variety of soil types in the literature. Its characteristic nature and a methodology for its determination are demonstrated. The experimental evidence shows that both Mohr–Coulomb failure and critical state failure can be well represented by the SSCC concept. The SSCC provides a potentially simple and practical way to describe the state of stress in unsaturated soil.

Suction Stress Characteristic Curve for Unsaturated Soil

A simple stress-ratio controlled, critical state compatible, sand plasticity model is presented, first in the triaxial and then in generalized stress space. The model builds upon previous work of the writers, albeit the presentation here is made with extreme simplicity in mind, and three novel aspects are introduced. The first is a fabric-dilatancy related quantity, scalar valued in the triaxial and tensor valued in generalized stress space, which is instrumental in modeling macroscopically the effect of fabric changes during the dilatant phase of deformation on the subsequent contractant response upon load increment reversals, and the ensuing realistic simulation of the sand behavior under undrained cyclic loading. The second aspect is the dependence of the plastic strain rate direction on a modified Lode angle in the multiaxial generalization, a feature necessary to produce realistic stress-strain simulations in nontriaxial conditions. The third aspect is a very systematic connection between the simple triaxial and the general multiaxial formulation, in order to use correctly the model parameters of the former in the implementation of the latter. The simulative ability of the model is illustrated by comparison with data over a very wide range of pressures and densities.

Simple plasticity sand model accounting for fabric change effects

ural and Political Observations Made upon the Bills of Mortality (1662) stands as the first great example of modern statistical data analysis, Bernstein also spends some time telling us about the genesis of Lloyd’s of London, the famous insurance firm. This weaving of topics that are standard in the history of probability and statistics with many that are not is one of the strengths and attractions of the book. Another is the obvious zest with which Bernstein describes the many intriguing people and tantalizing mysteries so peculiar to the mathematics of chance.

Against the Gods: The Remarkable Story of Risk

Over the past decade, major advances have occurred in both understanding and practice with regard to assessment and mitigation of hazard associated with seismically induced soil liquefaction. Soil liquefaction engineering has evolved into a sub-field in its own right, and engineering assessment and mitigation of seismic soil liquefaction hazard is increasingly well addressed in both research and practice. This rapid evolution in the treatment of liquefaction has been pushed largely by a confluence of lessons and data provided by a series of major earthquakes over the past dozen years, as well as by the research and professional/political will engendered by these major seismic events. The overall field of soil liquefaction engineering is now beginning to coalesce into an internally consistent and comprehensive framework, and one in which the various elements are increasingly mutually supportive of each other. Although the rate of progress has been laudable, further advances are occurring, and more remains to be done. As we enter a “new millenium”, engineers are increasingly well able to deal with important aspects of soil liquefaction engineering. This paper will highlight a number of important recent and ongoing developments in soil liquefaction engineering, and will offer insights regarding research in progress, as well as suggestions regarding further advances needed. 1 Dept. of Civil and Environmental Engineering, University of California, Berkeley. 2 Dept. of Civil Engineering, Middle East Technical University, Ankara, Turkey. 3 Fugro Engineering, Santa Barbara, California. 4 Arup, San Francisco, California. 5 URS Corporation, Oakland, California. 6 U.S. Geological Survey, Menlo Park, California. 1.0 INTRODUCTION Soil liquefaction is a major cause of damage during earthquakes. “Modern” engineering treatment of liquefactionrelated issues evolved initially in the wake of the two devastating earthquakes of 1964; the 1964 Niigata (Japan) and 1964 Great Alaskan Earthquakes. Seismically-induced soil liquefaction produced spectacular and devastating effects in both of these events, thrusting the issue forcefully to the attention of engineers and researchers. Over the nearly four decades that have followed, significant progress has occurred. Initially, this progress was largely confined to improved ability to assess the likelihood of initiation (or “triggering”) of liquefaction in clean, sandy soils. As the years passed, and earthquakes continued to provide lessons and data, researchers and practitioners became increasingly aware of the additional potential problems associated with both silty and gravelly soils, and the important additional issues of post-liquefaction strength and stressdeformation behavior also began to attract increased attention. Today, the area of “soil liquefaction engineering” is emerging as a semi-mature field of practice in its own right. This area now involves a number of discernable sub-issues or subtopics, as illustrated schematically in Figure 1. As shown in Figure 1, the first step in most engineering treatments of soil liquefaction continues to be (1) assessment of “liquefaction potential”, or the risk of “triggering” (initiation) of liquefaction. There have been major advances here in recent years, and some of these will be discussed. Once it is determined that occurrence of liquefaction is a potentially serious risk/hazard, the process next proceeds to assessment of the consequences of the potential liquefaction. This, now, increasingly involves (2) assessment of available post-liquefaction strength, and resulting post-liquefaction overall stability (of a site, and/or of a structure or other built facilities, etc.). There has been considerable progress in

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Recent Advances in Soil Liquefaction Engineering: A Unified and Consistent Framework

Compression model for cohesionless soils

A simple four-parameter elasto-plastic model describes the non-linear volumetric behaviour of freshly deposited cohesionless soils in hydrostatic and one-dimensional compression. It expresses the tangent bulk modulus as a separable function of the current void ratio and mean effective stress using natural strains. Specimens compressed from different initial formation densities approach a unique response at high stress levels—the limiting compression curve (LCC)—which is linear in a double logarithmic void ratio-effective stress space. The model describes irrecoverable, plastic strains which develop throughout first loading and represent mechanisms ranging from particle sliding and rolling at low stresses to crushing—the principal component of deformation for LCC states. The three input parameters describing plastic deformation can be readily estimated from a hydrostatic or one-dimensional compression test loaded to high stress levels; the elastic bulk modulus requires accurate small strain measurements in...

This paper presents a new generalized effective stress model, referred to as MIT-S1, which is capable of predicting the rate independent, effective stress–strain–strength behaviour of uncemented soils over a wide range of confining pressures and densities. Freshly deposited sand specimens compressed from different initial formation densities approach a unique condition at high stress levels, referred to as the limiting compression curve (LCC), which is linear in a double logarithmic void ratio, e, mean effective stress space, p′. The model describes irrecoverable, plastic strains which develop throughout first loading using a simple four-parameter elasto-plastic model. The shear stiffness and strength properties of sands in the LCC regime can be normalized by the effective confining pressure and hence can be unified qualitatively, with the well-known behaviour of clays that are normally consolidated from a slurry condition along the virgin consolidation line (VCL). At lower confining pressures, the model characterizes the effects of formation density and fabric on the shear behaviour of sands through a number of key features: (a) void ratio is treated as a separate state variable in the incrementally linearized elasto-plastic formulation: (b) kinematic hardening describing the evolution of anisotropic stress–strain properties: (c) an aperture hardening function controls dilation as a function of ‘formation density’; and (d) the use of a single lemniscate-shaped yield surface with non-associated flow. These features enable the model to describe characteristic transitions from dilative to contractive shear response of sands as the confining pressure increases. This paper summarizes the procedures used to select input parameters for clays and sands, while a companion paper compares model predictions with measured data to illustrate the model capability for describing the shear behaviour of clays and sands. Copyright © 1999 John Wiley & Sons, Ltd.

Formulation of a unified constitutive model for clays and sands

Seismically induced settlement of buildings with shallow foundations on liquefiable soils has resulted in significant damage in recent earthquakes. Engineers still largely estimate seismic building settlement using procedures developed to calculate postliquefaction reconsolidation settlement in the free-field. A series of centrifuge experiments involving buildings situated atop a layered soil deposit have been performed to identify the mechanisms involved in liquefaction-induced building settlement. Previous studies of this problem have identified important factors including shaking intensity, the liquefiable soil's relative density and thickness, and the building's weight and width. Centrifuge test results indicate that building settlement is not proportional to the thickness of the liquefiable layer and that most of this settlement occurs during earthquake strong shaking. Building-induced shear deformations combined with localized volumetric strains during partially drained cyclic loading are the dominant mechanisms. The development of high excess pore pressures, localized drainage in response to the high transient hydraulic gradients, and earthquake-induced ratcheting of the buildings into the softened soil are important effects that should be captured in design procedures that estimate liquefaction-induced building settlement.

Juan M. Pestana

Papers

Recent Advances in Soil Liquefaction Engineering: A Unified and Consistent Framework

Compression model for cohesionless soils

Compression model for cohesionless soils

Formulation of a unified constitutive model for clays and sands

Mechanisms of Seismically Induced Settlement of Buildings with Shallow Foundations on Liquefiable Soil