Prueba de fiabilidad

Youd and Holtzer, 1994, with permission ASCE)

• Liquefaction is a soil behaviour associated with excess pore water pressure, but it is not necessarily undrained and the movement of excess pore pressures throughout the soil over time may be crucial (e.g. Lower San Fernando dam).

• Excess pore pressures can arise from cyclic loading of soil, whether by earthquakes (e.g. Niigata) or by external forces (e.g. Molikpaq).

• Excess pore pressures can arise through static loading if the soils are loose enough (e.g.

Fort Peck, Nerlerk). Even though straining may be evident for days before the failure, the transition to high excess pore pressures is normally very rapid. Any attempt at an Observational Approach is likely futile and certainly dangerous.

Introduction 37

• Reducing mean effective stress because of water seepage can trigger liquefaction (e.g.

Aberfan).

• Liquefaction involves increasing strains, and may become a flowslide if the soil is loose enough. Even if not a flowslide, strains can be large enough to cause functional failure of structures (e.g. the buildings in Niigata).

• Soil strata have naturally variable density, and the distribution and structure of these natural variations can play a crucial role (e.g. at Nerlerk).

1.4 OUTLINE OF THE DEVELOPMENT OF IDEAS

The overview of liquefaction given by the few case histories just discussed provides the context to develop the theoretical aspects needed to properly understand liquefaction.

Chapter 2 introduces the critical state and how it may be measured. The state parameter ψ is used to show how many soil behaviours (e.g. peak friction angle) are unified regardless of soil type, fines content, etc, Aspects such as fabric and overconsolidation and how they affect behaviour are also considered.

At this point the framework for liquefaction is defined, but not computable. So Chapter 3 introduces NorSand, a generalized critical state model based on the state parameter ψ. NorSand is presented in three steps (starting with triaxial monotonic loading, followed by the generalization to 3-D stress space and finally the general model for cyclic loading) as this makes things a lot easier to understand and explain. Chapter 3 gives the first two steps. Calibrations are presented against laboratory data, for triaxial and plane strain drained tests. Undrained conditions illustrated in Chapter 6 while cyclic loading aspects are left to Chapter 7.

Having introduced the state parameter and what it can do, from normalization of soil behaviour through to the computable NorSand model, the next logical step is to measure the state parameter in situ, which is the subject of Chapter 4. The usual difficulty that undisturbed samples are impossible to obtain, at least practically and on a routine basis, means that penetration tests are the basis for most work. Determining ψ from penetration tests is covered in considerable detail as the method relies on the CPT. Other needed parameters (such as shear modulus) and alternatives to the CPT are also considered.

Chapter 5 moves into the realm of real soils, as opposed to the largely clean quartz sands used in research testing and, in particular, addresses the issue of how to select a characteristic state for design. Calculations are normally done using a single value for state or strength throughout a domain. The domain might be broken into a few layers or zones, but that is usually the limit of design idealizations. The reality is that soil state and properties vary in situ even in the same geologic stratum—there is a distribution of properties. Correspondingly, one of the issues in design is the value to be chosen as representative (or characteristic in limit states jargon) for design calculations or analysis.

Often guidance would be sought from a code of practice on this choice, but there is precious little such guidance for foundation design, never mind liquefaction problems.

However, the situation with characteristic values for liquefaction is not totally grim as there are three interesting and important studies that give some clues on selecting characteristic values. These studies are summarized and discussed in Chapter 5, which is a bit of a leap as these results depend on advanced simulation methods which are not

covered in this book. The reader is asked to accept this leap in order to appreciate the effect of variability of state to understand the limitations of the various case histories used to underpin design methods for liquefaction.

Chapter 6 presents liquefaction and large-scale deformations under essentially static loads, as these fall within the same theoretical framework. The triggering mechanism may be static, cyclic or hydraulic. This chapter builds on the triaxial theory based on laboratory experience of drained tests to the undrained conditions usually encountered in liquefaction. Key case histories are included. The outcome of this chapter is both an understanding of flaws in some existing approaches and a calibrated methodology for going forward with liquefaction engineering.

Chapter 7 gets to cyclic loading and liquefaction, or cyclic mobility as it is more usually called. First, laboratory test data are used to examine trends in the data from different kinds of cyclic tests. Then NorSand is extended from monotonic to cyclic loading and in particular the effect of rotation of principal stress directions is introduced.

This sets the theoretical framework for cyclic liquefaction. Current design practice is dominated by correlations to case histories, so this practice is presented before showing how empirical factors are predicted from theory and, importantly, where theory suggests existing experience is being wrongly extrapolated (mainly with depth effects and effect of soil compressibility).

At this point the intended critical state view of soil liquefaction has been delivered, so Chapter 8 contains a summary of the material presented and looks to the future of how the subject may develop.

To keep the book more readable, some details are presented in appendices. One thing needed to work with the material in this book is a consistent set of stress and strain measures and Appendix A defines those. Laboratory testing procedures are important to obtain accurate, consistent critical state data, so Appendix B details our experience in this regard. Appendices C and D are concerned respectively with the variation of critical friction angle with Lode angle and NorSand derivations (in particular the full 3D version). Appendix E contains CPT calibration chamber test results as these are not readily found in the published literature, while Appendix F documents many of the case records needed to evaluate residual strength after static liquefaction.

Introduction 39