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Probabilistic Methods in Geotechnical Engineering Executive Summary Probabilistic methods have been less widely used in geotechnical engineering than might be expected, given the variable nature of soil and rock. In response to requests from the Geomechanical, Geotechnical and Geo-Environmental Systems Program of the National Science Foundation; the U.S. Army Corps of Engineers; the Federal Highway Administration; and others, in 1992 the National Research Council's Geotechnical Board formed the Committee on Reliability Methods for Risk Mitigation in Geotechnical Engineering to examine the reasons for the seemingly slow acceptance of formal probabilistic methods in geotechnical engineering practice and to explore the potential for their wider use. This report is the result of that study. Central to the committee 's deliberations was the convening of a workshop, attended by thirty geotechnical probabilists and determinists, in July 1992 in Irvine, California. This report draws on the discussions at the workshop and on the experiences of the committee members. The committee examined the role probability plays in geotechnical engineering today and what actions can be taken to derive greater benefit from an improvement in the use of probabilistic methods in geotechnical engineering. The committee undertook its assignment with two principal audiences in mind: (1) federal agencies that have active geotechnical programs for which a greater understanding and use of probability theory might be beneficial and (2) the general geotechnical community, including practitioners, researchers, and educators. In geotechnical practice and education, the terms “reliability” and “probability” are often used interchangeably. However, strictly speaking, the definition of “probabilistic method” is different from that of “reliability method”. The former comprises any method using probability theory. In engineering, the probabilistic method is often used for, but not limited to, assessing the reliability (or likelihood of success) of a given strategy. The reliability method is used solely for the purpose of evaluating reliability. The methods for reliability evaluation can involve theories other than probability. In this report, the above definitions are adopted; the committee has chosen to focus on those applications involving probabilistic considerations.
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Probabilistic Methods in Geotechnical Engineering CONCLUSIONS The committee's principal finding concerning the role of probability in geotechnical engineering is that probabilistic methods, while not a substitute for traditional deterministic design methods, do offer a systematic and quantitative way of accounting for uncertainties encountered by geotechnical engineers, and they are most effective when used to organize and quantify these uncertainties for engineering designs and decisions. Probability theory should not be used to supplant comprehensive site investigations, laboratory test programs, mechanistic analyses, or engineering judgment. Probability theory should be used along with these more traditional geotechnical engineering tools. The committee finds that for a wide range of geotechnical problems, probabilistic methods can provide valuable insights and perspectives beyond those normally obtained from deterministic methods, especially in those cases where experience is inadequate. Probabilistic methods are appropriate for many applications where sufficient data exist that these methods can be used effectively to assist decisions between alternative actions and to justify any acquisition of additional data prior to final design decision. The committee also identified some types of situations in which the use of probabilistic methods for solving geotechnical problems is not necessarily effective. There are instances where the problem considered is not well understood, and the parameters, as well as their uncertainties, are not well defined. For some problems, the basis needed for probability estimation, including both measured data and factors based on judgment, is not available. In other geotechnical problems, deterministic methods are so well developed and well understood that probabilistic methods do not offer any special benefit. For example, although probabilistic methods have the potential for use in conventional foundation design, traditional deterministic techniques are so widely accepted and effective for these problems that there has been little motive for change as only minimal benefits will be gained by using probabilistic methods in this case. Geotechnical engineers are increasingly asked to tackle many types of nontraditional problems for which there are no precedents. Probabilistic methods can provide a systematic means of assessing the reliability of engineering designs in those cases. At the same time, society also demands explicit assessment of risk related to new technological systems developed by engineers. There is an urgent need for effective communication of risk between engineers, regulatory agencies, and the public. The committee concludes, therefore, that probability theory has potential for wider application in geotechnical engineering, with attendant benefits for the profession, its clients, and the public. Currently, even in cases where probabilistic methods are appropriate, they are not often used, because the geotechnical engineering community, by and large, has not developed the skills and experiences to apply such methods. Many factors related to the training and experience of geotechnical engineers, and to the expectations of their clients, have combined to inhibit use of probability theory in some areas of geotechnical engineering. Where codes and regulations require it, for example in the geo-environmental area, probability theory has been adopted readily and applied effectively by the geotechnical community. Probability theory has also been used effectively in connection
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Probabilistic Methods in Geotechnical Engineering with the exploration and sampling of soil and rock masses and in economic analysis of alternative courses of action, where the information derived from deterministic approaches is less useful for decision making. RECOMMENDATIONS The principal recommendation of the committee is that education of new geotechnical engineers, as well as of practicing geotechnical engineers, in probabilistic methods should be undertaken if the advantages and benefits of using probabilistic methods are to be fully exploited. Probabilistic methods and their pertinent applications should be included in geotechnical engineering programs at universities, so that graduates will be versed in the techniques and be able to make informed choices regarding the use of probabilistic methods in their practice. Continuing education programs are needed for practitioners and teachers of geotechnical engineering, so that they can incorporate the methods in their practice and in their geotechnical instruction. Inclusion of probability experts in geotechnical projects should be encouraged, when feasible, to accelerate the effective assimilation of probabilistic procedures. As graduates of geotechnical engineering programs and experienced practitioners become more knowledgeable about the techniques and potential benefits of probabilistic approaches to their problems, the committee expects that the use of probability theory in geotechnical engineering will expand. The committee recommends that the geotechnical engineering community be more actively involved in the development of codes and regulations, especially in the growing geo-environmental fields. As more areas of geotechnical engineering become codified and regulation-driven, expanded use of probability methods in geotechnical practice will likely be required. Failure of the profession to engage itself in the development of these probability-based codes and regulations will result in dictation by outsiders of the methods and standards for geotechnical work. The committee urges federal agencies—whether research-oriented or mission-driven—to play a role in integrating probabilistic methods in geotechnical practice. For example, the U.S. Army Corps of Engineers, charged with maintaining thousands of miles of levees, can greatly benefit through the use of probabilistic methods in setting priorities for maintenance within limited resources. Such use is illustrated by Example 1, described in Chapter 2. Other agencies involved with writing codes for geotechnical practice can adopt probability-based codes such as the Load and Resistance Factor Design (LRFD) procedures. Further research on the incorporation of site-specific factors in design codes can enhance the next generation of probability-based codes and regulations. Finally, the National Science Foundation can play at least two roles. First, an initiative on improving the use of probability theory in geotechnical engineering would serve to stimulate the educational community to develop and implement course materials. Secondly, a National Science Foundation initiative that emphasizes more problem-oriented research, examining cases such as those offered in Chapter 2 of this report, can provide the engineering community and, in turn, the U.S. construction industry with a stronger competitive position.
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