Understanding Global Stability in Retaining Walls
Global stability refers to the overall stability of a retaining wall system, including the reinforced soil mass, slopes, and surrounding site conditions. Unlike simple sliding or overturning checks, a global stability analysis evaluates potential slip surfaces that extend behind or below the wall, ensuring the entire system remains intact under various conditions.
For small residential retaining walls, homeowners and contractors often assume that following basic construction steps is enough. However, certain site conditions demand a global stability analysis to prevent large-scale failures that could compromise property and safety.
Situations Requiring Global Stability Analysis
Global stability is recommended in most cases to ensure long-term performance and safety. However, the following conditions require a global stability check:
1. Groundwater Considerations
- If the groundwater table is above or within the height of the retaining wall, or within 0.66H of the bottom of the wall, it can significantly weaken the soil and reduce stability.
- Water pressure behind the wall can lead to hydrostatic forces, increasing the risk of sliding or overturning failures.
2. Steep Slopes at the Toe or Top of the Wall
- If the ground slopes at 3H:1V (or steeper) at the top or toe of the wall, the stability of the entire system is compromised.
- Slopes below the wall can undermine support, while slopes above can exert additional lateral pressure.
3. Tiered Walls
- When constructing tiered retaining walls, each wall must be analyzed as part of a larger system.
- A common mistake is treating each tier as a separate wall, rather than accounting for the influence of the upper wall on the lower one.
- Tiered walls often require additional reinforcement lengths or geogrid to ensure overall stability.
4. Surcharges Above the Wall
- Surcharges include any additional weight applied above the wall, such as:
- Driveways or parking areas
- Retaining walls supporting a building
- Heavy landscaping features (large trees, patios, or pools)
- These loads increase lateral earth pressure and demand a global stability check. In addition, failure of a wall supporting a building or vehicles increases the cost of property damage and risks to loss of life.
5. Seismic Considerations
- In seismic regions, retaining walls must be designed to withstand additional dynamic forces.
- Global stability analysis ensures the wall system does not experience deep-seated rotational failures during an earthquake.
6. Poor or Unstable Soil Conditions
- If geotechnical exploration finds soft soils, organic matter, peat, high plasticity clay, or expansive soils, a global stability analysis is necessary. These soils include those defined as CH, OL, OH, or PT by the USCS and loose soils classified as ML, MH, or CL.
- In addition, soils with plasticity index (PI) greater than 20 or liquid limits (LL) greater than 40 require global analysis.
- These materials are highly compressible and weak, making them prone to global failures without proper design considerations.
7. Critical Structures and Slopes
- Walls over 10 feet tall where failure could affect buildings, roads, or utilities must undergo global stability analysis.
- Any wall greater than 15-ft, even with the most favorable conditions.
- Slopes exceeding 25 feet, especially near infrastructure, need additional assessment to prevent catastrophic failures.
8. Design Factors to Trigger Global Stability Analysis Requirement.
- After the initial wall design, if more than 50% of the Internal Compound Stability (ICS) slip arcs converge at the back of the wall design envelope. Internal Compound Stability takes into account how external loading affects the internal components of the retaining wall system, including the facing elements and reinforced zone.
Responsibilities in Retaining Wall Projects
In small residential projects, there is typically no civil engineer involved, making it crucial for homeowners to understand their responsibilities:
1. Soil Investigation (Owner’s Responsibility)
- Before building a retaining wall, the owner (or civil engineer hired by the owner) should obtain a soil report to identify any poor soils or groundwater issues.
- A geotechnical engineer can conduct subsurface exploration and provide necessary soil parameters for design.
2. Wall Design (DIY Retaining Wall’s Responsibility)
- The wall designer (DIY Retaining Wall) provides retaining wall designs that account for global stability where necessary.
- Design elements such as reinforcement lengths, geogrid spacing, and embedment depth will be based on site conditions.
- Determine the Factor of Safety to use in the global stability analysis based on the criticality of the structure:
- FS > 1.3 typically for low risk structures
- FS > 1.5 typically for walls supporting buildings, whose failure would impact traffic, or those on critical slopes or otherwise considered a critical structure
3. Global Stability Analysis (Separate Geotechnical Engineer if Needed)
- While DIY Retaining Wall handles retaining wall design, homeowners must hire a geotechnical engineer to perform a a subsurface exploration and provide the necessary soil properties for global stability analysis when required. DIY Retaining Wall can perform a global stability analysis only if a geotechnical engineer provides the necessary soil properties for the analysis.
- The wall engineer (or civil engineer) must clarify if global stability analysis is needed based on their site’s conditions.
4. Inspection During Construction (Owner or Third-Party Inspector)
- An independent testing agency or inspector may be required to verify that soil conditions, compaction, and reinforcement are installed correctly. This third party inspector should be hired directly by the owner (not the contractor) to avoid conflicts of interest.
Conclusion
Many owners and engineers alike believe that retaining wall failures only occur due to poor construction, but in reality, ignoring global stability can be a major cause of failure. If your project includes slopes, groundwater, tiered walls, or poor soil conditions, a global stability analysis is necessary.
For small walls without slopes or surcharges, a detailed analysis may not be needed, but when in doubt, it’s always best to have a licensed professional assess the risks.
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