Slope Stability Analysis
1. Stability Analysis Applicability & Requirements
Stability analyses are required based on specific geometry and height thresholds. Cross-sections for analysis should be selected from locations including side-hill fills and the worst-case combinations of fill height and/or weak foundation soil strength throughout the project. Normally, cross-sections must cut the slope perpendicular to the contours at these worst-case locations.
| Structure / Slope Type | Mandatory Analysis Criteria | Optional Analysis Criteria |
|---|---|---|
| Bridge Approach Fills & Soil Slopes (containing a foundation) | Greater than 10 feet in height OR steeper than 2H:1V | 10 feet or less from profile grade to toe of embankment (at Geotechnical Engineer's discretion) |
| Roadway Embankments & Cut Slopes (within soils) | Greater than 20 feet in height OR steeper than 2H:1V | 20 feet or less (at Geotechnical Engineer's discretion if believed necessary) |
2. Soil Parameter Selection & Laboratory Testing
The geotechnical engineer is responsible for selecting appropriate soil strength parameters based on laboratory testing using drained and/or undrained loading. Peak, fully softened, or residual strengths must be selected depending on whether prior movement has occurred or if the material is prone to breaking down over time.
- Rapid Loading/Drawdown: Both total and effective stress strength parameters may be used, based on the materials' drainage potential.
- Prior Movement or Creep: Residual friction angles can be estimated via back-analysis, using accurate geometry and the anticipated phreatic water surface at the time of failure.
- Presumptive Parameters: When geomaterials cannot be anticipated for the fill, conservative presumptive soil parameters can be used (refer to WVDOH downloads). In the absence of laboratory testing or back-analysis, residual presumptive values must be used if prior movement, creep, or fully softened materials are suspected.
- Rock Fills (> 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.
3. Material-Specific Guidelines (Shales, Claystones, & Lakebed Soils)
Mudstones (Shales and Claystones)
Many of the mudstones 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 materials must be broken down and compacted in thin lifts, wet of optimum moisture, to be suitable for random fill embankments.
- These materials must not be placed as rock fills.
- When present in cut slopes, the slope must be no steeper than 1H:1V.
- When slickensides are present, the 1H:1V slope should be flattened to the dip of the flattest slickenside.
Lakebed Soils
Ancient lake deposits from the Teays Valley, Fairmont, and Morgantown areas can weather and lose 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 soils are sometimes highly plastic and should be avoided in fills due to their shrink-swell behavior.
4. Required Factors of Safety (FS)
All slopes, foundations, and retaining structures must meet or exceed minimum design factors of safety in compliance with WVDOH and AASHTO LRFD specifications.
| Application / Condition | Minimum Long-Term FS | Minimum Short-Term FS |
|---|---|---|
| Fills or Cut Slopes supporting/including a Foundation Element | 1.5 | N/A |
| Retaining Structures not keyed into bedrock (e.g., MSE walls, RSS slopes) | 1.5 (Global) | N/A |
| Retaining Structures resisting driving forces from pre-existing landslides | 1.5 | N/A |
| Tieback Anchored Walls (modeled in software) | 1.5 | N/A |
| New Roadway Embankments or Cut Slopes | 1.3 | N/A |
| Structure Foundation Embankments subject to Flooding (Rapid Drawdown, Fill ≤ 50 ft) | N/A | 1.1 |
| Structure Foundation Embankments subject to Flooding (Rapid Drawdown, Fill > 50 ft) | N/A | 1.2 |
| Soft/Wet Clays and Silts (Undrained Loading, Fill Height ≤ 50 ft) | N/A | 1.1 (Total Stress) |
| Soft/Wet Clays and Silts (Undrained Loading, Fill Height > 50 ft) | N/A | 1.2 (Total Stress) |
Structural Fill Around Foundations (FS = 1.5 Zone)
The portion of an embankment requiring a 1.5 factor of safety includes any zone where a failure surface can pass through, under, or touch the front/element of the foundation. For new structures, this higher factor-of-safety zone includes:
- The fore-slope area.
- The area behind the foundation defined by a 1H:1V line extending from where fill is needed under the back of the foundation up to the embankment surface, and then projected vertically down.
Example: If the depth from the profile grade to natural ground is 10 feet, all fill behind the foundation for a minimum distance of 10 feet, extending in front of the foundation to the toe, must possess the strength required to achieve an FS of 1.5. The geotechnical report must outline the allowed material types and compaction procedures to achieve this.
5. Loading and Construction Constraints
- Rapid Drawdown Operations: Checked using the design storm elevation down to the normal pool elevation. Granular layers can be drained or partially drained during drawdown. Multi-staged drainage is normally not required.
- Soft Foundations & Lateral Squeeze: The geotechnical engineer must check for lateral squeeze when soft/wet clays or silts are present. Due to pore pressure tracking complexities, fill placement is practically limited to a rate of 6 vertical feet per week to avoid overloading. Alternatively, stage construction or ground modification must be demonstrated to the WVDOH to prevent overload.
- Surcharge & Traffic Loading: Must be accounted for in the stability analysis. For traffic loading, the WVDOH normally considers a uniform surcharge of 250 psf over the entire traveled way.
- Seismic Loading: Stability analysis using seismic acceleration is normally not required by the WVDOH for roadway and bridge embankments. However, analysis based on the USGS horizontal acceleration for the project zip code may be required if an embankment impounds water and falls under Dam Control authority.
6. Analytical Methods & Failure Modeling
Limit-Equilibrium & Finite Element Analysis
Limit-equilibrium analysis using either the Bishop, Spencer, or Morgenstern-Price method is normally expected. Software utilizing hydrostatic, steady-state, or transient flow models can be used where appropriate.
Two and Three-Dimensional Finite Element (FE) stability analyses can be used where the shape of the slide is unusual or soil-structure interaction is complex. When elastic parameters are used in FE analysis, they must be validated against a 2-D limit-equilibrium analysis with identical geometry (assuming a 2-D FE analysis or a 3-D model with very little thickness, smooth sides, and a fixed bottom) and must yield a similar factor of safety.
Failure Surfaces
Failure surfaces—including circular, spiral, wedge-shaped, irregular, planar, or combinations thereof—should model the envisioned failure mechanism, taking into account the relatively shallow bedrock found throughout most of West Virginia.
- Rock Slopes (Planar): Limit-equilibrium methods can be used if rock slope failures can be modeled as planar surfaces.
- Rock Slopes (Non-Planar): Techniques such as kinematic analysis and stereographic plotting are required (refer to the Cut Slopes Guidelines for further information).
