Allan Block® Segmental Retaining Wall

The only difference between the AB Classic and AB Vertical is the setback they create. The AB Classic creates a 6° setback while the AB Vertical generates a 3° setback.

Allan Block walls are normally designed to sit on a compacted granular base to allow for movements and settlement without compromising the wall. This saves time and money while providing a stable support for the blocks. However, an unreinforced concrete footing can be built instead.

Normally, unreinforced Allan Block gravity walls can be built up to 1.0 metres in clay soils provided that they are not surcharged. Geogrid reinforced Allan Block works well for retaining walls over 10 metres high. Always consult a chartered professional engineer on jobs of this scale.

We recommend you have a minimum base levelling pad size of 600 mm width and 150 mm depth. We believe that either GAP 20 or GAP 40 are probably the best options.

Levelling the first Allan Block course on top of the base levelling pad after compaction can be simplified by adding 15 mm of sand on top of the base levelling pad, this will help to level and place the first course of Allan Block blocks.

The SRW Best Practices recommend a minimum embedment of 150 mm. However, it is easier when constructing to use a minimum of 200 mm, which is the height of the Allan Block units. This minimum embedment applies to retaining walls with a total wall height of 1.8 m without a slope in front of the wall.

The Allan Block units are dry stacked, locking together without pins. The unique front lip creates a built-in setback; allowing the blocks to automatically interlock as they are stacked. This lip-notch configuration provides extra shear capacity, control the horizontal setback, and reduces overall costs.

We suggest you place inside the Allan Block open cavities, in the gaps between units, behind the units, and in the drainage column, a non-cohesive well-graded clean gravel such as drainage metal 25/7, 20/6. We suggest a minimum drainage column of 300 mm behind the Allan Block units. However, taller walls or water applications might require a wider drainage column.

It is always preferable to use a well-graded non-cohesive granular material. Examples of ideal aggregates are GAP 20, GAP 40, and GAP 65, because they provide the necessary shear strength and interlocking. Those materials also contain fine particles that help compact the soil.

Other granular material such as sand may also be appropriate. Cohesive soils such as clays would require extra care and compaction. They tend to change with moisture and retain water, so they should be avoided when possible.