Global Stability Analysis

Global stability analysis, drainage, and reinforced design overcame poor soils and steep slopes at the Ridge at Broomfield using the Allan Block system.

Global Stability Analysis Saves the Day

Global stability can be significantly impacted by poor soil conditions, steep slopes, and the presence of groundwater. The Ridge at Broomfield development presented all three challenges, requiring a carefully engineered retaining wall solution.

The site featured large slopes both above and below the proposed retaining walls, combined with weak native soils. Recognizing the complexity, a local professional engineer was brought in to develop a reliable design solution.

Steve Kelley of SK Wall Design led the engineering effort, working closely with Allan Block Master Wall Builder Slaton Brothers to ensure all walls were constructed in accordance with design specifications. Early evaluation of the site identified several critical concerns:

• Slopes above the wall extended at a 4:1 ratio over long distances, increasing surcharge loads.
• Slopes at the toe of the wall also extended at a 4:1 ratio for approximately 40 ft (12 m), creating significant global stability challenges.
• Geotechnical analysis revealed a low soil friction angle of just 22 degrees.
• Excavation exposed groundwater in multiple areas, requiring extensive water management.

Engineering the Solution

After completing a comprehensive analysis, the design team selected the AB Classic block from Allan Block as the optimal solution. The flexibility of the segmental retaining wall system allowed for several critical design enhancements:

• Increased geogrid reinforcement lengths
• Additional buried block for toe stability
• Expanded reinforced foundation zones
• Integrated drainage systems

To manage groundwater, Slaton Brothers installed both chimney and blanket drains during construction, ensuring the reinforced mass remained dry. Additional geogrid reinforcement was incorporated into the foundation to improve base stability.

The Ridge at Broomfield project highlights the versatility of the Allan Block system. Using AB Walls design software, the engineering team efficiently developed detailed construction plans, while the experienced Slaton Brothers crew executed the work to overcome the site’s challenging conditions.

Understanding Global Stability

Global stability refers to the ability of a system—such as a hillside—to resist large-scale failure when subjected to external loads. In retaining wall design, this typically involves evaluating how added loads from structures or walls may trigger deep slip surfaces within the slope.

When weight is added to a hillside, the soil mass may attempt to redistribute along a failure plane. For this reason, it is critical to evaluate global stability using specialized Global Stability Analysis (GSA) software to ensure long-term performance.

While AB Walls software does not perform full global stability analysis, it allows designers to export cross sections directly into programs such as ReSSA. This capability streamlines the design process and can save significant modeling time.

AB Walls does include Internal Compound Stability (ICS) analysis, which evaluates potential slip surfaces within the reinforced soil mass using a modified Bishop’s method. However, ICS does not account for deeper failures beneath the wall or beyond twice the wall height, making full GSA essential for complex sites.

Managing Groundwater

When groundwater is encountered during excavation, proper drainage becomes critical. Installing a French drain system allows water to be safely diverted away from the wall foundation, ensuring proper construction and long-term performance.

Effective drainage was a key component of success at the Ridge at Broomfield, preventing hydrostatic pressure buildup and maintaining soil strength.

Strengthened Foundations

In sites with clay soils, steep slopes, or heavy surcharges, increasing foundation capacity is often necessary. This can be achieved by:

• Expanding the size of the reinforced foundation zone
• Incorporating geogrid reinforcement within the foundation

Geogrid helps distribute loads more evenly through the structural fill, improving bearing capacity.

Proper compaction is equally important. For clay soils, equipment such as a sheep’s foot roller is ideal due to its ability to compact fine-grained materials effectively.

Overcoming Global Stability Challenges

When global stability analysis indicates potential failure, several design adjustments may be required:

• Extending geogrid reinforcement lengths
• Reducing vertical spacing of geogrid layers (e.g., from 16 in. to 8 in. lifts near the base)
• Combining both approaches for maximum stability

At the Ridge at Broomfield, geogrid lengths approaching 170% of wall height were required to achieve acceptable stability.

Best Practices for Sloped Sites

When a slope exists at the toe of a wall, designers should incorporate additional embedment to improve stability. Recommended practices include:

• Providing a 5 to 7 ft (1.5–2.1 m) horizontal bench in front of the wall
• Maintaining minimum embedment of 1 in. (25 mm) per 1 ft (300 mm) of wall height

These measures reduce the likelihood of deep slip surfaces forming beneath the wall.

Conclusion

The Ridge at Broomfield project demonstrates how proper engineering, advanced analysis tools, and experienced construction teams can overcome even the most challenging site conditions. By combining global stability analysis, effective water management, and reinforced foundation design, the project team delivered a long-lasting retaining wall solution.

For more detailed guidance, refer to Chapter 9 of Allan Block’s Best Practices document, which provides in-depth strategies for designing and constructing retaining walls in globally sensitive conditions.