National Academies Press: OpenBook

Manual on Subsurface Investigations (2019)

Chapter: Chapter 2. Geotechnical Uncertainty and Risk

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Suggested Citation:"Chapter 2. Geotechnical Uncertainty and Risk." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 2. Geotechnical Uncertainty and Risk." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 2. Geotechnical Uncertainty and Risk." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 2. Geotechnical Uncertainty and Risk." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
×
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Suggested Citation:"Chapter 2. Geotechnical Uncertainty and Risk." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Page 11

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7 C H A P T E R 2 Geotechnical Uncertainty and Risk Introduction For many civil infrastructure projects, uncertainties regarding subsurface soil and rock conditions are a significant contributor to the overall technical and financial risks for the project (National Research Council [NRC] 1984, Institution of Civil Engineers [ICE] 1991). The broad purpose of a subsurface investigation is to inform geotechnical engineers, contractors, and other professionals about soil and rock conditions to aid with identifying and mitigating the geotechnical-related risks (FHWA 2017). The objective of the investigation is to gather sufficient information regarding subsurface conditions to reduce technical and financial risks due to subsurface conditions to a level that the stakeholders of the project find tolerable. Uncertainty The uncertainties related to subsurface conditions are typically divided into two categories: (i) natural or aleatory variability, and (ii) knowledge or epistemic uncertainty (Baecher and Christian 2003). Natural variability is the inherent randomness in geologic materials that manifests itself on both spatial and temporal scales. In principle, natural variability cannot be reduced; however, it can be estimated more accurately by collecting additional data. Knowledge uncertainty is related to a lack of data or information about geologic materials and a lack of understanding of the physical laws that govern their behavior and, thus, limit the ability to model subsurface conditions. In geotechnical engineering, knowledge uncertainty is often considered to include (i) site characterization (or statistical estimation) uncertainty, (ii) model uncertainty, and (iii) parameter (or measurement) uncertainty (Lacasse and Nadim 1996, Phoon and Kulhawy 1999, Baecher and Christian 2003). Unlike natural variability, knowledge uncertainty can in principle be reduced collecting additional data and improving the quality of the data and models. Geoprofessionals infer or estimate the uncertainties regarding the geometry (i.e., stratigraphy and groundwater level) and properties of subsurface materials using inductive reasoning based on limited data, judgment, and experience (Baecher and Christian 2003) when performing analyses and preparing designs. A subsurface exploration program can be viewed as a critical element to identifying and reducing the knowledge uncertainties and to more accurately estimating the natural variability regarding soil and rock conditions at the project site. A thorough subsurface exploration program is a means to an end of identifying, characterizing, and reducing geotechnical risks associated with knowledge uncertainty and natural variability. In many cases, the marginal costs of a more thorough subsurface exploration program are small compared to the risk-reduction benefits. Geotechnical Risks Geotechnical risks comprise both technical and financial risks. Technical risks are related to the inability of a structure to satisfy the desired performance requirements for one or more limit states, including service, fatigue and fracture, strength, and extreme event. Financial risks are related to claims, change orders, and cost and schedule overruns attributed to subsurface conditions that differ from those anticipated based on the preconstruction site investigation, as well as an overly conservative design.

8 2.3.1 Technical Risk Geotechnical-related technical risks are often attributed to weak or soft soils, loose sands, expansive clays, and jointed rock among other causes. Geotechnical failures may be related to the following: • Global or local stability (i.e., exceeding the strength limit state) • Excessive deformation (i.e., exceeding the service limit state) • Internal piping and erosion • Shrink and swell of foundation soils • Collapse of metastable soils (e.g., loess) • Collapse of subsurface voids due to natural phenomena (e.g., karst) or human activity (e.g., mining) • Extreme events, including earthquakes (e.g., soil liquefaction, lateral spreading) or floods (e.g., scour) Sowers (1993) examined nearly 500 failures of civil infrastructure, including foundations, embankment dams, excavations, tunnels, highways, waste disposal facilities, port and marine structures, and heavy construction. Most of the failures he examined involved geotechnical engineering issues. Based on analysis results presented by Sowers, it can be inferred that approximately 20 percent of the failures he considered involved inadequate or missing data, factors that Sowers attributes often to an engineer or owner’s decision in the early stages to postpone or forego a detailed site investigation to save time and money. The timely recognition of the benefits related to detailed site investigation may have reduced the technical risks and the number of failures. 2.3.2 Financial Risk There have been numerous studies in recent years to examine geotechnical-related financial risks (Hoek and Palmeiri 1998, Prezzi et al. 2011, Boeckmann and Loehr 2016, Neupane 2016). Hoek and Palmeiri (1998) observed that on all major civil engineering projects, geotechnical risks are a “serious factor in cost and schedule control.” They also concluded that the inadequacy of information obtained from the site investigation program is a leading contributor to geotechnical risks on large projects. This was found to be particularly true for those projects that are linear in nature (e.g., highways and tunnels) because boreholes are often widely spaced and the interpolation between boreholes introduces additional knowledge uncertainty. Prezzi et al. (2011) examined more than 300 contracts issued by the Indiana Department of Transportation (DOT) and found that 41 percent of road contracts and 37 percent of bridge contracts experienced geotechnical change orders. These change orders comprised approximately 1.3 percent of the total estimated construction costs per year. Based on interviews with engineers from Indiana DOT and their consultants, Prezzi et al. (2011) concluded that one of the main reasons for the large numbers of geotechnical change orders was the failure to identify areas of poor subgrade soil due, in part, to an insufficient site investigation. Prezzi et al. (2011) found that the geotechnical change orders for Indiana DOT agreed with a survey the authors conducted of other State DOTs that found a primary reason for change orders in other states were unexpected site conditions due to insufficient site investigations. Boeckmann and Loehr (2016) surveyed 55 transportation agencies and found that construction claims, change orders, and cost overruns resulted most frequently from differing site conditions (DSCs), including (i) pile overruns, (ii) higher than expected groundwater, (iii) misclassified or mischaracterized subgrade soils, (iv) unanticipated rock encountered during foundation construction, and (v) mischaracterized rock for drilled shaft construction. Boeckmann and Loehr (2016) estimated that the costs associated with claims, change orders, and overruns caused by DSCs were likely much greater than 7 percent of the project budget for some projects. Neupane (2016) surveyed geotechnical engineers from State DOTs and consulting firms to evaluate the causes and impacts of geotechnical problems on bridge and road construction projects. Responses were received by 53 engineers representing 42 different State DOTs and 43 consulting engineers. The survey

9 respondents indicated geotechnical-related problems frequently caused cost overruns, schedule overruns, and construction claims resulting in additional costs ranging from at least 5 percent to more than 25 percent of the project cost. Among the most common geotechnical-related causes of cost and schedule overruns, claims, and change orders were (i) an insufficient number of borings or soundings, (ii) misclassified or mischaracterized subgrade soils, and (iii) poorly defined groundwater conditions (i.e., higher groundwater table than expected). The results of these studies are consistent in drawing attention to the significant financial risks caused by a poor subsurface investigation that fails to sufficiently reduce the knowledge uncertainties and accurately evaluate the natural variability in the subsurface soils. 2.3.3 Risk Mitigation The previously cited studies also provide recommendations to mitigate geotechnical risks. In general, these recommendations fall into one of four categories: • Provide an adequate budget to enable more borings, soundings, and tests to be conducted. • Retain suitably qualified and experienced consultants to plan and perform the site investigation, evaluate potential risks, and prepare a geotechnical baseline report (GBR) consistent with the risks. • Allocate sufficient time and resources to prepare a thorough GBR. • Improve subsurface investigation practices to include developing and implementing minimum standards. The results of the referenced studies and surveys imply that technical and financial risks can be readily mitigated by conducting a more thorough subsurface exploration to reduce knowledge uncertainties and better characterize natural variability. While this is true, it should be recognized that (i) these risks cannot be eliminated altogether and (ii) there may already be an implicit acceptance of these risks as tolerable and, thus, a more thorough subsurface exploration is not considered justified. This latter point is called the normalization of deviance, defined by Vaughn (1996) as “the gradual process through which unacceptable practices or standards become acceptable.” In many cases, the normalization of deviance associated with an adequate subsurface investigation can be attributed to human and organizational factors that are not included in engineering analysis and design methods (Sowers 1993, Baecher and Christian 2003, Bea 2006). Sowers (1993) attributes it to “ignorance of prevailing practice” or “rejection of current technology.” Bea (2006) calls the failure to take advantage of available information an “unknown knowable,” and Bazerman and Watkins (2004) call the consequences of not using available information a “predictable surprise.” From these descriptions, it is clear that many geotechnical-related risks can also be reduced by becoming familiar with best practices for geotechnical site investigations and avoiding the normalization of deviance. 2.3.4 Geotechnical Risk for Design-Build Projects Mitigating the risk of differing geotechnical site conditions is more difficult for design-build contracts awarded before a complete subsurface investigation is conducted. Typical design-build highway projects often provide only a small fraction of the necessary geotechnical investigation at the time of procurement. The design-build contracts require the design-build team to conduct a full subsurface site investigation and prepare a GBR as part of the final design. As a result, construction costs could increase due to unforeseen site conditions at the time the contract is awarded. While this risk is intended to be assumed by the contractor, it is likely that it will be assigned ultimately to the owner. Essex (2007) provides guidelines for using the GBR as a risk-management tool for design-build projects. In addition, the forthcoming NCHRP Report 844: Guidelines for Managing Geotechnical Risks in Design-Build Projects being prepared by Iowa State University is intended to assist public agencies in managing geotechnical risk on highway construction projects that are delivered using the design-build contracting mechanism. The guidelines are intended to provide strategies for aligning the DOT’s and its design-builder’s perception of geotechnical risk and

10 provide geotechnical risk management tools that can be used to implement those strategies on typical design-build projects. Load and Resistance Factor Design and Subsurface Investigation LRFD is a reliability-based approach that replaces the single factor of safety that is used in allowable stress design (ASD) with individual factors applied independently to loads and resistances. Various combinations of factored loads are used to define limit states corresponding to service, fatigue, strength, and extreme events. The individual factors are intended to reflect the natural variability and knowledge uncertainties associated with both loads and resistances and can be chosen to replicate conventional factors of safety or, preferably, to result in a target probability of failure or reliability index based on reliability theory. For geotechnical analyses (e.g., driven piles), the resistance factors depend primarily on the method used to calculate the nominal resistance. However, consideration is also given to the type and extent of the subsurface exploration program to account for knowledge uncertainty and the natural variability of the subsurface materials. For example, resistance factors in Article 10.5.5 of AASHTO (2017) for the strength limit state were calibrated using the average values of soil parameters from in situ and laboratory tests, accounting for the typical variability in the property. It is noted in Article 10.5.5 that smaller resistance factors should be used if “site or material variability is anticipated to be unusually high.” Conversely, higher resistance factors may be used if they are based on “substantial statistical data combined with calibration” via reliability theory. Similarly, Article 10.5.5 notes that when, for example, static or dynamic load tests are used for driven pile and drilled shaft foundations, the number of tests used to develop design resistance factors should be based on the variability in subsurface conditions. In this regard, LRFD represents a step forward in explicitly recognizing the important roles of uncertainty and risk in geotechnical practice. LRFD also provides a framework for achieving a more efficient and reasonable design via the use of improved resistance factors that are directly linked to the thoroughness of the subsurface investigation.

11 Chapter 2 References AASHTO. 2017. AASHTO LRFD Bridge Design Specifications. US Customary Units, 8th Edition. American Association of State Highway and Transportation Officials, Washington, DC. Baecher, G.B., and J.T. Christian. 2003. Reliability and Statistics in Geotechnical Engineering. John Wiley & Sons, West Sussex. Bazerman, M.H., and M.D. Watkins. 2004. Predictable Surprises, the Disasters You Should Have Seen Coming, and How to Prevent Them. Harvard Business School Press, Boston. Bea, R. 2006. “Reliability and Human Factors in Geotechnical Engineering.” Journal of Geotechnical and Geoenvironmental Engineering. Vol. 132, No. 5, pp. 631–643. Boeckmann, A.Z., and J.E. Loehr. 2016. Influence of Geotechnical Investigation and Subsurface Conditions on Claims, Change Orders, and Overruns. National Cooperative Highway Research Program Synthesis 484, Transportation Research Board, Washington, DC. Essex, R.J. 2007. Geotechnical Baseline Reports for Construction: Suggested Guidelines. The Technical Committee on Geotechnical Reports of the Underground Technology Research Council. ASCE, Reston, VA. FHWA. 2017. “Geotechnical Site Characterization.” Geotechnical Engineering Circular No. 5. National Highway Institute Course No. 132031, FHWA NHI-16-072, United States Department of Transportation, Federal Highway Administration. Washington, DC. April. Hoek, E., and A. Palmeiri. 1998. “Geotechnical Risks on Large Civil Engineering Projects.” International Association of Engineering Geologists Congress. Vancouver, British Columbia, Canada, September 21–25. ICE. 1991. Inadequate Site Investigation. Institution of Civil Engineers. Thomas Telford, London. Lacasse, S., and F. Nadim. 1996. “Uncertainties in Characterising Soil Properties.” Uncertainty in the Geologic Environment: From Theory to Practice. C.D. Shackleford, P.P. Nelson, and M.J.S. Roth, Eds. Geotechnical Special Publication No. 58, American Society of Civil Engineers, New York. pp. 49–75. NRC. 1984. Geotechnical Site Investigations for Underground Projects. Vol. 1: Overview of Practice and Legal Issues, Evaluation of Cases, Conclusions, and Recommendations. National Research Council. National Academy Press, Washington, DC. Neupane, K.P. 2016. “Causes and Impacts of Geotechnical Problems on Bridge and Road Construction Projects.” MS Thesis, Department of Civil and Environmental Engineering and Construction, University of Nevada, Las Vegas, December. Phoon, K.K., and F.W. Kulhawy. 1999. “Characterisation of Geotechnical Variability.” Canadian Geotechnical Journal, Vol. 36, pp. 612–624. Prezzi, M., B. McCullouch, and V.K.D. Mohan. 2011. Analysis of Change Orders in Geotechnical Engineering Work at INDOT. Publication FHWA/IN/JTRP-2011/10. Joint Transportation Research Program, Indiana Department of Transportation and Purdue University, West Lafayette, Indiana. Sowers, G.F. 1993. “Human Factors in Civil and Geotechnical Engineering Failures.” Journal of Geotechnical Engineering, Vol. 119, No. 2, pp. 238–256. Vaughn, D. 1996. The Challenger Launch Decision: Risky Technology, Culture, and Deviance at NASA. University of Chicago Press, Chicago, Illinois.

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TRB's National Cooperative Highway Research Program (NCHRP) Web-Only Document 258: Manual on Subsurface Investigations provides an update to the American Association of State Highway Transportation Officials (AASHTO) 1988 manual of the same name. This report reflects the changes in the approaches and methods used for geotechnical site characterization that the geotechnical community has developed and adopted in the past thirty years. The updated manual provides information and guidelines for planning and executing a geotechnical site investigation program. It may also be used to develop a ground model for planning, design, construction, and asset management phases of a project.

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