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

Unlike the SB method, the MP and Spencer methods satisfy force and moment equilibrium conditions and are appropriate for both circular and noncircular failure surfaces. In the authors’ experience, methods that satisfy force and moment equilibrium conditions provide more reliable results. When performing these analyses, it is imperative that the engineer understand the benefits and limitations of the available methodologies and evaluate their appropriateness for each project. Soil and Groundwater Conditions The soil and groundwater conditions play an important role in deter- mining the global stability of slopes, support of excavation systems, retaining walls, and other structures. Thus, it is essential to utilize and accurately depict the site-specific soil and groundwater conditions for use in the GSAs. The authors recommend the engineer rely on location- specific subsurface investigations and field and laboratory testing, if feasible, to assess the necessary analysis parameters. As part of the evaluation of the soil and groundwater conditions, a detailed understanding of the material’s shear strength is required. Drained and undrained strengths are used to define the behavior of soils during shearing. Drained strength is the strength when the soil is loaded slowly or over a long period of time such that no excess pore - water pressures develop, or when the excess porewater pressure fully dissipates after the soil is loaded. Undrained strength is the strength when the soil is loaded faster than the porewater can flow in and out of the material during shearing. During undrained shearing, excess porewater pressure develops within the soil. Over time, the excess porewater pressures dissipate and the drained strength is achieved. The construction type (temporary vs. permanent), the design life of the structure, and the soil type impact whether an evaluation of the drained condition, undrained condition, or both is necessary. Further, for sites with loose, saturated sands or sensitive clay soils, an evaluation of the potential for post-peak strength loss during shearing is required. The groundwater conditions at a site can also directly influence the shear strength of the soil and the driving forces acting on the failure surface. An understanding of the static groundwater levels is neces - sary and should also consider seasonal groundwater fluctuations and less frequent extreme groundwater events, if necessary. Further, the use of parametric studies to evaluate the sensitivity of the results with respect to the input and analysis parameters is an essential measure to understand the reliability of the analysis results. External Loading Conditions External loading conditions can also significantly impact the response of a structure or slope, both during and after construction. For example, during construction, analyses are typically required at various excava - tion depths, at differing stages of fill placement, or when other unbal - anced loads are placed or removed from the areas near the slope or structure. For analysis of the permanent, long-term condition, loads from adjacent structures and seismic loads should be considered. Of critical importance for GSAs are the magnitude of the load, its location, depth, and extent relative to the structure or slope. Fur - thermore, the authors recommend that engineers exclude temporary loads in GSAs if those loads have a stabilizing effect. These tempo -

rary loads can unconservatively increase shear resistance along the failure surface. Anisotropic Soil Behavior and Shape of the Failure Surface Two key considerations that require detailed attention when evalu- ating the global stability of a structure or slope are soil anisotropy (e.g., soil that has properties that are directionally dependent) and the shape of the critical failure surface. The United States Army Corp. of Engineers (USACE) Engineering and Design Manual for Slope Stability (EM 1110-2-1902), emphasizes the importance of these items, stating, “Stability analyses based on general slip surfaces are now much more common and are useful as a design check of critical slip surfaces of traditional shapes (circular, wedge) and where complicated geometry and material conditions exist. It is especially important to investigate stability with noncircular slip surfaces when soil shear strengths are anisotropic.” To consider the effects of anisotropy of the undrained shear strength, different strength zones are often utilized in GSAs: triaxial compres - sion (TXC) behind the slope or structure, direct simple shear (DSS) below the slope or structure, and triaxial extension (TXE) in front of the slope or structure. While the USACE emphasizes the importance of analyzing non circular slip surfaces for anisotropic soil conditions, it is often necessary to analyze both circular and noncircular failure surfaces using appropriate global LEM methodologies to determine the critical failure surface. The authors recommend that engineers evaluate soil anisotropy and the importance of the different modes of shearing (TXC, DSS, and TXE) when performing GSAs. Evaluation of Failure Surfaces Most global limit equilibrium software packages allow the engineer to either manually define the failure surfaces, or the program automati - cally generates the failure surfaces. If the engineer chooses to define the failure surfaces manually, they should have sufficient experience performing GSAs to assess the suitability of the results. For manu - ally defined failure surfaces, it is typically necessary to define mul - tiple failure surfaces, compare the factors of safety, and evaluate the reasonableness of the results. This process is often both monotonous and time-consuming to identify the critical failure surface and factor of safety. In lieu of manual definition of failure surfaces, it is often more common to have the limit equilibrium software automatically generate the failure surfaces utilizing a user-defined search criterion. This approach often results in the analysis of significantly more failure surfaces. For automatically generated failure surfaces, the engineer should still perform a thorough review to evaluate the suitability of the results and slip surfaces. Engineers performing GSAs either manually or automatically specified failure surfaces should understand how the software defines the failure surface to ensure the critical failure surface and factor of safety are iden- tified. If the program features multiple search algorithms, each algorithm needs to be considered, and the results compared for consistency. Regardless of the failure surface selection approach (manual or auto - matic), the use of narrow model extents typically leads to misleading and often unconservative results. For example, if the model’s edges


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