9/2014 – how to build a strong D.I.Y retaining wall

A strong wall is a well designed and built wall. You might consider the retaining wall you’re about to build to be solid from the front, but unless you give serious thought to what goes on behind and below the wall, it may not look good for long. A poorly built wall can lean, separate, even topple—and it’s out there in plain sight.

The following guidelines apply to Terraforce retaining blocks, manufactured by various concrete producers throughout South Africa, Canada, Australia, Spain, England, United Arab Emirates, Swaziland, Lesotho and Namibia.

They are designed to be as versatile and eco-friendly as possible, allowing horizontal and/or optional vertical interlock, seamless variations of wall inclination (90-35 degrees), and convex and concave corners and curves of unrestricted radius that make complex designs possible.

Added to that the blocks can be laid with either a round face, (plant supportive) or flush face (smooth or split version) finish to suit your specific aesthetic preference.

This is a collection of D.I.Y retaining walls in Western Australia, where Terraforce is manufactured by The Block Makers in Geraldton and sold in most Bunnings stores in the region:

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Planning your wall

Good preparation and planning is a substantial part of the process. Before you even start with installation, you need to consider the following:

        • Develop a precise plan for your Terraforce wall by analyzing your site, noting slopes, drainage and shape of wall. Plan the placement of your retaining wall by using stakes and string, leveling off to ensure an even height and using a tape measure to ensure an accurate measurement.
        • Contact your local public utilities office to ensure there are no pipes or cables in your digging zone.
        • Measure the length and the vertical height to obtain the vertical surface area and thus the number of units required.
        • Remember that most retaining walls require professional design and supervision input and must comply with local building regulations. This is only a guideline for walls under the height you are allowed to build yourself.
        • South African law is very strict on these matters, with the NHBRC requirement stating that 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 65 degrees is also possible. Any steeper and especially load bearing walls must be designed by a qualified engineer, even if only 1.2m high

Building your wall

1. Prepare a level foundation, well compacted gravel or concrete as directed by site conditions. The first course should be lower than the surface by one tenth the height of the wall to prevent it from being pushed forward by soil pressure. Compacted gravel foundations are usually sufficient for structures not higher than one (1) metre. On sloping sites the foundation may be stepped by block height at intervals to suit the slope.

2. Place the first row of blocks to required alignment and ensure that units are level in all directions. A small amount of mortar will assist with accurate leveling on a concrete foundation and a length of flexible plastic pipe will assist in setting out  smooth curves.

3. Install a drainage pipe with outlet and free draining back-fill as specified behind the first row of blocks.

4. Never back-fill with topsoil; it will break down and settle, creating a depression, that will create a problem by attracting and infiltrating storm-water behind the wall. Water trapped behind a wall may cause serious damage to the structure. Use sandy or gravelly materials, which compact much better, topped with a layer of clay to prevent water ingress.

5. It is advisable to install an open channel above the wall, to drain away any surplus surface water.

6. Fill the blocks with good quality soil or soil compost mix and tamp lightly.

7. Continue construction row by row, while back-filling and compacting free-draining material as each row is completed.

8. Insitu (cast in place) or precast interlocking keys to be installed when directed by the engineer.

10. When specified, install geogrid/geofabric on compacted back-fill and wedge between blocks (or cut and fold into blocks) as indicated by the engineer.