Slope stability analysis is required for bridge approach fills and soil slopes that contain a foundation that is greater than 10 feet in height or steeper than 2H:1V. For bridge approach fills 10 feet or less from the profile grade to the toe of the embankment, the geotechnical engineer has the option to perform a stability analysis, if judged necessary.
Stability analyses must be performed for roadway embankments and cut slopes within soils that are greater than 20 feet in height or steeper than 2H:1V. The stability analysis cross-sections should be selected from locations that include side-hill fills and worst-case combinations of fill height and/or weak foundation soil strength throughout the project. Normally, the cross-sections need to cut the slope perpendicular to the contours at the worst-case locations. For roadway embankments 20 feet or less, the engineer has the option to perform a stability analysis, if believed necessary.
For embankment design, the geotechnical engineer should select appropriate soil strength parameters based on laboratory testing using drained and/or undrained loading. Based on whether prior movement has occurred or whether the material can break down over time, either peak, fully softened, or residual strengths must be selected. In the case of rapid loading/drawdown both total and effective stress strength parameters may be used based on the materials’ drainage potential. Where prior movement or creep has occurred, residual friction angles can be estimated based on accurate geometry and the anticipated phreatic water surface at the time of failure, using back analysis.
When the geomaterials cannot be anticipated for the fill, then conservative presumptive soil parameters can be used (click here for WVDOH presumptive soil parameters
). In the absence of laboratory strength testing, or back-analysis, residual presumptive values must be used when prior movement, creep, or fully softened materials are suspected. For rock fills greater than 50 feet in height, the presumptive friction angle must be reduced based on the effective normal stress as presented in Figure 10.4.6.2.4-1 of the AASHTO LRFD Bridge Design Specifications. If presumptive soil parameters other than those published by the WVDOH are selected, then they must be reviewed by the WVDOH prior to their use. The geotechnical engineer must include in the geotechnical report a discussion of the selected presumptive parameters or the type of sample preparation and laboratory testing used to obtain the selected strength parameters and why.
The geotechnical engineer must recognize that many of the mudstones (shales and claystone) found in the Conemaugh, Monongalia, and Dunkard Groups weather to fully softened shear strengths when not properly broken down in the field during compaction. These shales and claystone must be broken down and compacted in thin lifts, wet of optimum moisture, to be suitable for random fill embankment. These materials must not be placed as rock fills. When these shales and claystone are present in cut slopes, the slope must be no steeper than 1H:1V. When slickensides are present, then the 1H:1V slope should be flattened to the dip of the flattest slickenside. Similarly, the lakebed soils from the Teays Valley, Fairmont, and Morgantown ancient lake deposits can weather and lose their strength to fully softened values when exposed in cut slopes. When lakebed soils are present in cut slopes, the slope must be flattened to the fully softened or residual friction angles to maintain stability. These lakebed soils are sometimes highly plastic and should be avoided in fills due to their shrink-swell behavior.
Fills or cut slopes that support or include a foundation element must be designed to have a minimum long-term factor of safety of 1.5 for compliance with AASHTO LRFD Bridge Design Specifications. The portion of an embankment that requires a factor of safety of 1.5 is where any failure surface can pass through, under, or touch the front of the foundation or element of the foundation. As a practical matter for new structures, this portion of the embankment with higher factor-of-safety compacted materials includes the fore-slope area and the area behind the foundation defined by a 1H:1V line from where fill is needed under the back of the foundation element up to the embankment surface and then projected vertically down. For example, if the depth from the profile grade to the natural ground is 10 feet, then all the fill behind the foundation for a minimum distance of 10 feet and extending in front of the foundation to the toe, must possess the strength needed to have a factor of safety of 1.5. The geotechnical engineer must discuss in the geotechnical report the material types allowed and their compaction procedures to achieve the 1.5 factor of safety in this structural fill area around foundations. For existing bridges that do not display distress due to past movement or creep of embankment soils, the existing embankment can be used in its current geometry.
New roadway embankments or cut slopes must be designed with a minimum long-term factor of safety of 1.3. Reuse of existing embankments that do not display movement or creep are acceptable at their current factor of safety. Structure foundation embankments subject to flooding should have a stability analysis check for rapid drawdown conditions using the design storm elevation down to the normal pool elevation and must result in a minimum factor of safety of 1.1, provided the fill height is 50 feet or less. When the fill height is greater than 50 feet, a short-term factor of safety of 1.2 should be used. Granular layers can be drained or partially drained during the drawdown period. Multi-staged drainage is normally not required for rapid drawdown.
When fills are placed on soft/wet clays and some silts, undrained loading may occur. Total stress analysis should be performed and should achieve a minimum factor of safety if 1.1 for short-term loading, provided the fill height is 50 feet or less. When the fill height is greater than 50 feet, a short-term factor of safety of 1.2 should be used. In addition, the geotechnical engineer must check for lateral squeeze when these conditions are present or anticipated. It is difficult to estimate pore pressures for design, and it is more difficult to measure them during construction and, consequently, we generally limit fill placement to a rate of 6 vertical feet per week to avoid overloading soft foundation soil as a practical matter. Otherwise, the geotechnical engineer must demonstrate to the WVDOH through stage construction or ground modification that overload will not occur. When loading from surcharges or traffic is anticipated, it must be accounted for in the stability analysis. For traffic loading, the WVDOH normally considers 250 psf over the entire traveled way.
In addition, global stability analysis is required for retaining structures that are not keyed into bedrock such as MSE walls and RSS slopes. All retaining structures that are designed to resist the driving force from a pre-existing landside (including those imbedded into rock) must have a demonstrated factor of safety of 1.5. Tieback anchored walls should be modelled in software and demonstrate a factor of safety of 1.5.
Normally, stability analysis using seismic acceleration is not required by the WVDOH for roadway and bridge embankments. The practitioner may be required to perform stability analyses based on the USGS horizontal acceleration for the zip code in question when an embankment impounds water and falls under Dam Control authority.
Limit-equilibrium analysis using either the Bishop, Spencer, or Morgenstern-Price method is normally expected. Stability analysis programs using either hydrostatic, steady-state, or transient flow models can be used, where appropriate. Two and 3-D Finite Element (FE) stability analysis can be used where the shape of the slide is unusual, or the soil-structure interaction is complex. When elastic parameters are used in FE analysis, they must be validated by assuming a 2-D FE analysis or a 3-D model that has very little thickness and is smooth on its sides and fixed on its bottom. This validation must compare the factors of safety against a 2-D limit-equilibrium analysis with the same geometry and must result in similar factor of safety.
Failure surfaces that are circular, spiral, wedge shaped, irregular, planar, or combinations thereof should be used to model the envisioned failure considering the influence of relatively shallow bedrock found in most of West Virginia. When rock slope failures can be modeled as planar, limit-equilibrium methods can be used; otherwise, stability analysis techniques such as kinematic analysis and stereographic plotting are required for rock slopes (refer to the Cut Slopes Guidelines
for further information).