C+S March 2023 Vol. 9 Issue 3 (web)

Global Stability Analysis Methods, Common Pitfalls, and Strategies for Successful Performance

arc, spiral, noncircular). LEMs utilize the material’s shear strength and do not rely on the soil’s stress-strain behavior. Vertical, horizontal, and moment forces induced along the failure surface (Figure 2) and exter - nal loads acting on the rigid body (e.g., structures), live loads (e.g., vehicles), and temporary loads (e.g., construction loads) are typically considered explicitly in these analyses. In global LEM analyses, the mass of the rigid body is divided into slices. Various global limit equilibrium analysis methods are frequently used throughout the geotechnical engineering industry, including Ordinary Method of Slices, Simplified Bishop (SB), Spencer, Morgenstern and Price (MP), to name a few of the most common. Each global LEM has limitations that the engineer must consider before selecting one or more methods for use. These limitations can include assumptions or simplifications that are only applicable to isotropic soil conditions, circular failure surfaces, etc. Furthermore, some global LEMs, such as Ordinary Method of Slices or SB, are often over-simplified, inaccurate, or both due to various inherent assumptions. GSA Software Considerations, Common Pitfalls, and Strate- gies for Successful Performance Engineers frequently perform GSAs using commercially available, user-friendly limit equilibrium software packages, allowing them to perform multiple analyses quickly and efficiently. Some of these software packages allow engineers to utilize simplified analyses that only partially satisfy force and moment equilibrium (e.g., SB method). Other software packages allow engineers to utilize more robust analy - ses that are considered complete equilibrium procedures (e.g., Spencer, and MP). Further, these software packages can quickly analyze many failure surfaces and loading conditions for multiple LEMs simultane - ously. However, a failure to understand the mechanics of the software combined with a lack of understanding of the inherent assumptions or simplifications of certain LEMs can lead to the use of inappropriate as - sumptions, unconservative analyses, and potentially unsafe conditions. LEM Analysis Methodologies Many LEM analysis methodologies rely on various simplifying as - sumptions or limitations that require careful consideration when performing GSAs. For example, one of the more widely utilized analysis procedures is the SB method. This method uses the forces in the vertical direction and the moments about the center of the circular failure surface to evaluate equilibrium conditions of the failure wedge, whereas the shear forces between individual slices are not considered. While many software packages allow the engineer to analyze slopes for noncircular failure surfaces using the SB method, this method is only appropriate for circular failure surfaces.

By J.E. Hughes, Ph.D., P.E. 1 , Z.K. Boswell, P.E. 2 , B.P. Strohman, P.E., G.E. 3

Introduction Many construction projects incorporate fill or excavated slopes, utilize support of excavation systems, or require retaining walls as part of the necessary site development. These structures create unbalanced loads, requiring a professional engineer to perform global stability analyses (GSA) to evaluate the potential failure surfaces that can develop be - hind and below the structure due to the unbalanced loading condition (Figure 1). Global stability is often described in terms of the factor of safety against failure along a failure surface. The factor of safety is defined as the ratio of the soil shear strength (resistance) to driving shear stress acting along the failure surface. GSAs are a crucial part of the design process for these structures. If per - formed incorrectly, unexpected movement or potential catastrophic failure can result, posing a significant risk to public safety and adjacent property. This paper describes some common global stability limit equilibrium analysis methodologies, software considerations and common pitfalls, and strategies for successful performance. Most of the content herein applies to two-dimensional analyses; however, many of the consid- erations, pitfalls, and strategies are applicable to three-dimensional analyses as well. Common Limit Equilibrium Analysis Methodologies The Limit Equilibrium Method (LEM) is one of the most utilized procedures to perform GSAs. Limit equilibrium is a static analysis technique that evaluates force and moment equilibrium conditions as- suming the soil and/or structure above the failure surface acts as a rigid body above an assumed failure surface of a given shape (i.e., circular/



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