Geogrid Overlapping in MSE Walls in the Transverse Direction: Best Practices, Recommendations, and Manufacturer Guidelines

Mechanically Stabilized Earth (MSE) walls are an essential component in modern civil engineering, offering a cost-effective and robust solution for soil retention and structural stability. A crucial aspect of MSE wall construction is the use of geogrids, which provide the necessary reinforcement to the soil. This article focuses on the practice of geogrid overlapping in the transverse direction, outlining best practices, recommended scenarios, and instances where overlapping should be avoided. Additionally, we will review some manufacturers' recommendations regarding the non-overlapping of geogrids.

The Importance of Transverse Geogrid Overlapping in MSE Walls

Transverse geogrid overlapping refers to the side-by-side placement of geogrid sheets to ensure continuous reinforcement across the width of the wall. Proper overlapping in this direction is vital for maintaining structural integrity, ensuring load distribution, and preventing weak points in the wall.

How Proper Overlapping in MSE Walls Shall Be Performed

1. Overlap Length: The standard transverse overlap length typically ranges from 12 to 18 inches, though this can vary depending on the geogrid type and project specifications. It is crucial to adhere to the manufacturer's guidelines for the recommended overlap length.
2. Alignment and Placement: Ensure that the geogrids are aligned correctly in the transverse direction without any gaps, wrinkles, or folds. Misalignment can lead to weak points and compromise the wall's performance.
3. Securing Overlaps: Use mechanical fasteners, pins, or other securing methods to hold the overlapped sections in place. This prevents movement during the backfilling process, ensuring a continuous reinforcement layer.
4. Staggering Layers: When using multiple layers of geogrids, stagger the overlaps in each layer to avoid creating a vertical plane of weakness. This staggered arrangement helps distribute loads more efficiently.

When Overlapping is Recommended

1. Insufficient Geogrid Width: When the width of the geogrid rolls is narrower than the required wall width, overlapping is necessary to cover the entire area.
2. Joint Locations: Overlapping in the transverse direction is recommended at joint locations to ensure continuity and avoid weak points in the reinforced soil structure.
3. Corners and Curves: In areas where the wall changes direction, transverse overlapping provides additional reinforcement to manage stress concentrations effectively.
4. Variable Soil Conditions: In soils with varying properties, transverse overlapping can help maintain the stability and performance of the MSE wall.

When Overlapping is Not Recommended

1. High Tensile Strength Geogrids: Some high-tensile strength geogrids are designed for use without overlapping. Overlapping these geogrids in the transverse direction can lead to excessive stiffness and potential failure points.
2. Specific Soil Conditions: In highly cohesive soils, transverse overlapping might not be necessary and could cause issues with soil-geogrid interaction.
3. Manufacturer Specifications: Always refer to the manufacturer's guidelines. Some geogrid manufacturers specify that their products should not be overlapped in the transverse direction due to unique design and material properties.

Manufacturer Recommendations Against Overlapping

1. Tensar International: Tensar recommends a minimum overlap of 12 inches for their BX and UX geogrids but advises against overlapping their high-strength uniaxial geogrids designed for longer spans. According to Tensar, overlapping high-tensile geogrids, particularly in the transverse direction, can lead to unnecessary complications and potential points of failure.
2. Huesker Inc.: Huesker’s Fortrac geogrids generally require overlaps in specific conditions, but they emphasize that their high-strength geogrids are designed to function optimally without overlapping. Overlapping these geogrids, especially in the transverse direction, can interfere with their intended performance.
3. Strata Systems: Strata suggests a 12-inch overlap for their StrataGrid products in certain conditions. However, they note that in specific applications, particularly where high-strength is not required, transverse overlaps can be minimized or eliminated. Strata advises consulting their technical guidelines for specific recommendations.

Conclusion

Proper transverse geogrid overlapping is a critical aspect of constructing MSE walls, ensuring structural integrity and longevity. While overlapping is often recommended for maintaining continuous reinforcement, it is essential to follow manufacturer guidelines and consider specific project conditions. By understanding when and how to overlap geogrids in the transverse direction, engineers and contractors can significantly enhance the performance of an MSE wall.

Always consult with a geotechnical engineer and refer to the specific manufacturer's recommendations for tailored best practices to your project. Following these guidelines will help ensure the success and durability of your MSE wall construction.