United States Department of Transportation
i
Incorporating the Site Variability and Laboratory/in-Situ Testing Variability of Soil Properties in Geotechnical Engineering Design: Research Project Capsule [23-3SS]
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2016-04-01
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Abstract:While structural engineering deals with mostly homogeneous manmade materials (e.g., concrete and steel), geotechnical engineering typically involves highly varied natural materials (e.g., soil and rock). As a result, high variance of the resistance of geotechnical structures is expected due to the horizontal and vertical spatial variation of soil properties at the site. Compounding the variability problem is the fact that the accuracy and reliability of design data is sometimes unknown or not controlled. If the uncertainties are not properly considered, under-design may result in failure of geotechnical structures or over-design may result in extra costs. Geotechnical variability is a complex attribute that results from many disparate sources of uncertainty such as spatial variability due to natural geologic deposits, equipment and operator measurement variations, statistical errors due to limited information, and model bias. To account for uncertainty, the Louisiana Department of Transportation and Development (DOTD) changed from Allowable Stress Design (ASD) to Load and Resistance Factor Design (LRFD) for deep foundations and other geotechnical engineering work. Since LRFD treats both the loads and the resistances of geotechnical structures as random variables, the resistance factor (φ) is assessed based on a reliability analysis. Current LRFD guidelines specify constant resistance factors for a project, regardless of the fact that soil properties vary across a project site or along its depth. Research is needed to evaluate site variability, measurement errors, and model transformation bias. While site variability is inherent and cannot be reduced, measurement uncertainty can be reduced by increasing the number of measurements or improving the method of measurement (standard operating procedures).
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