Allan Block Retaining Walls and No-Fines Concrete

Learn how No-Fines concrete backfill allowed the Woodland Heights retaining wall to be built successfully where space limited traditional geogrid reinforcement.

Woodland Heights Retaining Wall Replacement Using No-Fines Concrete

Woodland Heights, a 30-unit townhome association built in the 1980s, faced the need to replace its second timber retaining wall. However, several site constraints prevented the installation of traditional geogrid reinforcement within the property boundaries. Limited excavation space meant that a conventional reinforced soil wall was not a feasible option.

To address these challenges, Duffy Engineering and Associates specified the use of No-Fines concrete backfill combined with an Allan Block AB Classic retaining wall system. Using the AB Walls 10 design software, Duffy incorporated the alternative reinforcement option to design the wall and verify that all required safety factors were exceeded.

Understanding No-Fines Concrete

No-Fines concrete has been used in the construction industry for decades and has been utilized in the segmental retaining wall (SRW) industry for more than 15 years. Like conventional concrete, No-Fines concrete contains Portland cement and coarse aggregate. However, sand (fine aggregate) is intentionally omitted from the mix.

Without fine aggregate, void spaces develop within the concrete, creating a material that is both strong and highly permeable. This permeability allows water to move freely through the structure while maintaining structural integrity.

Construction Challenges and Phasing

After receiving approval from the City, The Lawn Ranger Inc. began construction using a phased approach. This method minimized potential water management and slope stability issues during construction.

The phased approach proved beneficial when the site received 4 inches (100 mm) of rain over a 48-hour period midway through the project. Fortunately, the contractor had anticipated weather risks and protected the work area with large tarps, preventing damage to the excavation and partially constructed wall.

Wall Construction Process

The retaining wall was constructed by placing No-Fines concrete within the block cores and extending 2.5 ft (0.76 m) behind the wall. The concrete was placed in two-course increments.

Once the No-Fines backfill was placed, the crew stacked the next two courses of block. By the time the blocks were positioned, the previous lift of concrete had cured sufficiently to support the next placement. This process continued until the wall reached its final height.

In total, approximately 40 cubic yards (30.6 cubic meters) of No-Fines concrete were placed within the retaining wall structure.

Project Benefits

Because the No-Fines concrete retaining wall required a smaller structural footprint than a traditional geogrid-reinforced wall, the amount of excavation was significantly reduced. This minimized the cost of hauling excavated soil offsite.

The Woodland Heights project demonstrates how No-Fines concrete can provide a cost-effective solution for segmental retaining walls where space behind the wall is limited.

History of No-Fines Concrete

No-Fines concrete gained popularity in England after World War II during the construction of homes known as Wimpey Houses.

With skilled labor in short supply, builder George Wimpey developed a construction system that allowed unskilled workers to form and pour No-Fines concrete walls for multi-story homes. The mix was easier to place than traditional concrete and exerted less pressure on formwork.

Over time, the term “No-Fines concrete” evolved into “pervious concrete”, particularly as the material became widely used in pavement applications due to its drainage capabilities.

No-Fines Concrete Backfill for Segmental Retaining Walls

Using No-Fines concrete as backfill significantly expands the range of locations where reinforced retaining walls can be constructed. Projects with property line constraints or limited excavation space can often be built successfully using this method.

When Allan Block units are used with No-Fines concrete backfill, the permeable concrete bonds to the back of the blocks and increases the effective depth of the wall mass. This added mass allows for taller walls with less excavation compared to traditional geogrid-reinforced retaining walls.

Typical geogrid-reinforced walls require excavation depths equal to 60 percent or more of the wall height. In contrast, No-Fines reinforced retaining walls generally require excavation of only 30 to 40 percent of the wall height under similar site conditions.

Reducing excavation not only saves time and money but may also determine whether a project can be constructed within site constraints.

Additional Advantages

No-Fines concrete offers several additional benefits:

• Improved production rates with reduced labor requirements
• Elimination of soil compaction and compaction testing in the reinforced zone
• Excellent drainage due to the permeable concrete mass
• Elimination of drainage rock in block cores and directly behind the wall
• Reduced potential for settlement behind the retaining wall

Because the entire reinforced zone is permeable, water moves freely through the structure, creating a durable and stable retaining wall system.

Engineering Properties

No-Fines Concrete Backfill can be used with any Allan Block Retaining Wall Collection.

Typical mix characteristics include:

• Cementitious content: approximately 500 lb/yd³ (297 kg/m³)
• Water-cement ratio: 0.30 – 0.40
• Aggregate size: 3/8 in. to 3/4 in. (9.5 mm to 19 mm)
• Aggregate-to-cement ratio: approximately 6:1

The density varies depending on the aggregate used but typically ranges between:

• 100 – 135 lb/ft³ (1600 – 2160 kg/m³)

No-Fines concrete has little to no slump and behaves similarly to loosely placed aggregate until it cures. Once hardened, it forms a strong, permeable mass that also functions as the wall’s drainage zone.

Installation Steps

Refer to the AB Commercial Installation Manual for detailed instructions on preparing the base trench and installing the first course of block. After the first course is installed and leveled, follow these steps for placing No-Fines concrete backfill.

1. Place No-Fines Concrete Backfill

Fill the block cores and the specified backfill zone with No-Fines concrete. Methods for delivering the concrete will vary depending on site conditions.

For straight wall sections, it is recommended (but not required) that one back wing of the Allan Block unit be removed to help secure the block face to the concrete mass.

The vertical height of each placement should not exceed 16 in. (406 mm), or roughly two courses of block.

2. Allow Initial Set

Additional pours can typically be made once the previous lift has set, which usually occurs within 2 to 3 hours. During this time, additional block courses can be stacked in preparation for the next lift.

3. Install Additional Courses

Before installing the next course of blocks, brush the top of the blocks to remove excess concrete or debris. This step is recommended before the concrete hardens.

Install the next course of blocks, ensuring they are properly aligned and level. Continue placing No-Fines concrete backfill as described above until the wall reaches the specified height.