Advances in the design of geosynthetic reinforced soil (GRS) retaining walls

Benefits of GRS retaining walls

GRS retaining walls can offer economical and practical solutions for bridge abutments, piers, foundations for low-volume roads, highways, etc. The benefits of choosing GRS retaining walls over their counterparts include:

• High load-carrying capacity
• Reinforced soil is more ductile and less likely to suddenly collapse
• Flexible and tolerant to differential settlement
• Better seismic resistance ability
• Adaptable to a variety of backfill material
• Faster and easier to construct
• More economical to construct and lower life-cycle maintenance costs
• Environmentally friendly

Difference between GRS walls and MSE walls

Both GRS and MSE walls generally consist of a facing element (vertical or near-vertical) and a reinforced soil mass; however, they include the following differences:

• The facing elements of GRS walls consist of flexible material, such as gabion baskets filled with aggregates, while MSE walls usually have rigid facings, like concrete panels. The facing of GRS walls is not considered a structural member and is non-load bearing.
• The reinforcements (metallic strips or geosynthetic) in MSE walls are “attached to” the rigid facing to keep the face elements and soil mass together with a tied-back approach. In contrast, the geosynthetic reinforcements (high strength woven geotextiles or geogrids) are “placed in” the soil mass in a GRS wall to increase the tensile strength of the soil and carry the tensile loads. A tighter reinforcement spacing (less than 0.3 m) is usually used in GRS walls. The reinforced soil mass is a soil-geosynthetic composite.

The differences between reinforcement mechanisms of MSE and GRS walls have received considerable attention from design engineers and researchers. Efforts have been made to optimize the design of the GRS walls in recent decades.

GRS retaining wall design

Current design methods of GRS retaining walls generally include external and internal stability assessments. The external stability assessment for GRS walls is similar to conventional retaining walls and MSE walls. The following aspects are considered for external stability:

• Overturning at the toe
• Sliding resistance along the base
• Bearing resistance of the base
• Settlement
• Global stability

The internal stability evaluation of GRS walls should account for the behaviour of soil geosynthetic composite. The following are taken into consideration:

• Rupture resistance: the reinforcements are not overstressed or ruptured.
• Pullout resistance: the bonding between soil and reinforcements must be maintained.

Recent advances suggest that the commonly used tied-back method for assessing the rupture resistance, such as in AASHTO LRFD and FHWA-NHI design guidelines for MSE walls, is not appropriate to evaluate the reinforcement strength in GRS walls. Instead, the tensile strength and stiffness of the geosynthetics should be selected based on the load-carrying capacity of the composite resulting from soil-reinforcement interaction or the prescribed limiting lateral wall movement, whichever is larger. The ultimate strength of the reinforcement must ensure sufficient ductility and satisfactory long-term performance. The required reinforcement stiffness should meet at a minimum service strain of 2% and 3% for load-bearing and non-load-bearing applications, respectively. The GRS approach has been adopted in FHWA-HRT-11-026 and FHWA-HRT-17-080 GRS retaining wall design guidelines.

Advantages of the GRS design approach

The main advantage of the GRS approach is that it considers the actual soil-geosynthetic composite behaviour of the GRS wall, so the estimated strength requirement is more realistic than the tied-back MSE approach. Field-scale experimental tests indicate that the reinforcement strength calculated from the tied-back MSE approach can be 47–74% smaller than the measured results. However, the largest difference in reinforcement load between the GRS approach and measured results is 13–16%. The tied-back MSE theory tends to underestimate the amount of reinforcement required for GRS walls, whereas the GRS design approach matches measured results more closely.

Conclusion

GRS retaining walls are flexible, environmentally friendly, economical, easy to construct, and have high load-carrying capacity along with many advantages over other options. In addition, properly designing the wall can maintain its performance, long-term stability and cost efficiency.

BBA’s geotechnical team have the expertise in designing several types of retaining walls such as MSE and GRS walls in various projects, from site investigation, foundation assessment and detailed design to operational specifications, quality assurance and site support. BBA will support clients with all their wall design needs!