Aiming for the sky with hollow core concrete blocks, but how high can you go? 

In the past, it was common knowledge that retaining walls “had” to be poured, reinforced concrete. Even though concrete retaining blocks (CRBs) have been on the market for decades, there are still some people who think blocks (especially hollow core units) cannot do the same job as poured concrete, especially for structures more than 4m high.  

Karin Johns, director of marketing and business development at Terraforce, explains that this is simply not the case.  

Aiming for the sky with hollow core concrete blocks requires an engineered structure by design engineers with the expertise to push beyond conventional boundaries.

Engineered for success 

As an engineered structure, concrete block retaining walls offer extensive flexibility at a range of heights and in varied soil conditions when designed by experts in this complex field. 

Although composite Terraforce walls have been built to a height of 11m with a single-skin facing, it is important to point out that there are certain limitations.  

As a rule of thumb, any wall (with or without extra reinforcing) exceeding 8m, and with a wall angle of more than 65 degrees, will require either a double layer or concrete filled blocks at the base to increase the crushing strength. This will avoid potential pressure cracks.  

Terracing taller 

Terracing a wall can seem like a tempting solution, as terraces effectively reduce the slope angle that the wall is resisting. But the foundation load from the upper wall can surcharge the lower one. 

This does not mean terracing can never be considered for achieving a greater wall height. It can be a good option, especially when space allows for it. As a basic guideline, the lower terrace must be approximately a third of the total wall height, and the distance between terraces depends on the overall wall height and overall horizontal space available. 

Generally, geofabric reinforced lower terraces will offer greater overall wall stability. Most importantly, the overall slope angle must be considered at the design stage. When building composite walls of any significant height, it is of crucial importance to consult with an experienced engineer.  

An extensive retaining wall at Montecasino in Fourways, Gauteng, boasts a face area of 2 700m² and an average height of 9m. Terraforce L13 blocks were placed on a 25m.p.a. concrete foundation and built to full height in chainages of about 60m.

Legal requirements 

The NHBRC requirement is very strict on these matters:  

• One can build a wall to a height of 1,2m without consulting an engineer, but as a general rule 1,5m for walls no steeper than 70 degrees is also possible.  
• Any steeper and especially load-carrying walls must be designed by a qualified engineer, even if only 1,2m high. 

Building high 

There are a variety of methods for building high, composite retaining walls using hollow core, interlocking blocks. Each of these will have to be carefully evaluated to establish suitability for any given situation: 

• Double skin: This effectively doubles the gravity mass of the wall and increases the lever arm for overturning moments. The wall mass can be further increased by spacing the front and back skins using a stabilised fill and a geotextile or grid to hold the two faces in juxtaposition. It is not always necessary to take the second skin to the full height of the wall.

• Geofabric: The use of horizontal, tensile layers increases the maximum height of the Terraforce wall significantly. The basis here is to determine the force of the “active” soil, then place enough fabric to counter this force, and to intersect the slip plane with sufficient depth to resist “pull-out” when forces in the soils are activated.  

The choice of fabric is important and is determined by the nature of the fill material, i.e., coarse free-draining or high-fines content. The last row of geofabric, usually three rows from the top, is mainly a needle-punched fabric placed to carry and distribute “line” loads close to the top of the wall and to limit stormwater damage.  

There are a variety of methods of building high with hollow core, interlocking blocks. Each of these will have to be carefully evaluated to establish suitability for any given situation.

Whilst the design is sometimes complex, the application is straightforward enough. BS8006 is considered the “Bible” when it comes to geosynthetic slopes, and there has been a move away from geotextiles in favour of higher tenacity products such as grids. 

• Concrete infill: Concrete fill has a similar effect to double skin, as the mass of the wall is increased per square metre. The concrete also improves shear resistance from block to block and boosts crushing strength. It is also possible to reinforce the concrete infill with steel or to incorporate vertical reinforced concrete (RC) pillars or horizontal RC beams into the blocks.

• Steel reinforcement: When considering the use of steel reinforcement in a concrete-filled wall, the block is considered to be a combination of a shutter and a spacer. The use of reinforcement would require structural input to assess the efficacy of the whole system.

• Interlocking keys: Concrete keys would only have value where there are higher than usual shear forces in a wall. They can either be plain coarse gravel infill, cement-stabilised soil infill or concrete keys. The soil inside the blocks should be tamped leaving a 50mm gap. Once the next row of blocks is placed, the soil is tamped through into the 50mm recess, effectively keying in the blocks. 

Basic guidelines 

Regardless of the methods described above, there are a few basic guidelines that need to be adhered to during the construction of a very high CRB/Terraforce wall: 

• A level foundation and an accurate first row are just the first of many steps to ensure a safe and stable wall. 
• Angled profiles need to be set up so that the wall angle will keep to the design slope. 
• Compaction must be 93% mod AASHTO or more. 
• If possible, use soil material with less than 15% passing a 0,075mm sieve. 
• If not possible, intermediate soil blanket drains and/or water-transmissive geotextiles with adequate strength, as well as drains both at the base of the wall against the cut face and at intermediate heights, need to be considered.  
• Drains should be placed against cut faces using either continuous sheets, if circumstances require, or “wick” drains placed at between 1m to 2,5m centres and at 45 degrees against the slope face. 

Terracing can be used to achieve a greater wall height when space allows for it, here a 12m backyard was transformed into a lush backdrop for a new home on a steep hill.

Although very difficult, all soils should be tested by a soils laboratory to determine the internal shear strength of soils, their cohesion, the percentage of fines and the plasticity index of both backfill soils and retained soils. 

Top tips for building high walls with hollow core concrete blocks from Terraforce. 

For more information, contact Terraforce: 

Tel: +27 21 465 1907 

Email: info@terraforce.com  

Website: www.terraforce.com