Why Proper Geogrid Installation Matters for Allan Block Retaining Wall Stability

08 October 2025

Allan Block retaining wall systems are renowned for their durability, ease of installation, and aesthetic appeal. In many standard applications, the weight and setback of Allan Block retaining wall systems alone provide sufficient stability. However, in certain conditions - taller walls, slopes, or sites with additional loads - geogrid reinforcement becomes essential. Proper geogrid installation ensures your wall remains safe, stable, and long-lasting. 

How Does Geogrid Work?  

Geogrid works by creating a composite structure between the soil and the wall, distributing lateral pressures and allowing taller, more stable walls. When site constraints limit geogrid use (e.g., tight property lines or restricted excavation), alternative methods such as No-Fines Concrete cores or Double-Width Gravity Walls may be considered. 

Materials Needed for a Reinforced Allan Block Retaining Wall System 

To install a geogrid-reinforced wall correctly, you will need: 

• Geogrid  

• Well-graded, compactible aggregate (6–38mm, <10% fines) 

• Drainage Stone 10mm – 20mm aggregate  

• Allan Block retaining wall systems 

• Perforated drainage pipe 

• Capstone adhesive for finishing 

For project-specific guidance and quantities, Colinwell’s Technical Department can provide an estimating guide. 

Step-by-Step Geogrid Installation  

Step 1: Cut and Position the Geogrid  

• Cut the Geogrid to the specified lengths provided in your wall design.  

• Place the cut edge of the Geogrid against the back of the raised front lip of the Allan Block.  

• Roll the Geogrid back over the fully compacted and level excavation area.  

 

Step 2: Stack the Next Course of Blocks  

• Place the next course of Allan Block units directly on top of the Geogrid.  

• Push the block from the rear into place for Allan Block to lock in. A perfect running bond is not required.  

• Ensure each block is tight against the front edge of the block below.  

 

Step 3: Fill Cores and Backfill  

• Fill the hollow cores of the blocks and the space 300mm behind the wall with 10mm – 20mm drainage stone.  

• Place approved infill soil behind the drainage zone.  

 

Step 4: Compact the Fill  

• Compact all materials within 900mm of the wall face.  

• Compact in 200mm maximum lifts, beginning on top of the blocks and working backward.  

• Never compact directly on the Geogrid.  

 

Step 5: Repeat the Process  

• Continue installing additional courses, adding Geogrid layers as specified.  

• Install cap blocks once the desired wall height is reached.  

• Finish by grading the surface away from the wall to ensure proper drainage.  

 

Risks of Improper Geogrid Installation 

Even when geogrid is required, mistakes can compromise wall stability: 

• Bulging or leaning walls 

• Soil settling or voids behind the wall 

• Increased repair costs and maintenance 

• Reduced lifespan and safety hazards 

 

Conclusion 

Geogrid reinforcement is a powerful tool for building taller, stronger Allan Block retaining wall systems but only when needed and installed correctly. Following best practices for placement, compacting, and anchoring ensures your wall performs as designed, protecting your investment and ensuring safety. 

For technical advice or project-specific guidance, contact Colinwell’s Technical Department.