Planning alternatives

Introduction to Terraforce retaining blocks

TERRAFORCE Concrete Retaining Blocks (CRB) are factory made hollow units of uniform strength and dimensions. They have been substantially tested and analysed under field and laboratory conditions and are manufactured under licence by reputable manufacturers on five continents.

Terraforce retaining blocks interlock horizontally as well as vertically with optional keys or gravel infill, and they contain no air voids. They have a closed vertical and open horizontal surface structure. As such they lend themselves ideally to the construction of both light terrace walls as well as heavy gravity retaining walls. The units can be reversed so that a variety of versatile appearances and features can be created.

What are gravity retaining walls

There are various types of these systems on the market with elevations ranging from open checkerboard appearance to closed vertical surface structure and many are plant supportive to varying degrees. Some systems are interlinked, while others interlock on the vertical and/or horizontal plane. All systems rely mostly on interblock friction to derive a measure of sliding resistance and under ideal conditions that is sufficient to prevent collapse when constructing a gravity retaining wall.

Yet, when substantial active lateral earth pressure occurs, considering the slender cross-section of these walls clearly no amount of interlocking or interlinking will prevent such a structure from collapsing. The most economical solution under such conditions is often found in constructing a multi-skin gravity retaining wall. Such a wall relies upon weight and frictional resistance to obtain stability. Alternatively, earth reinforcing or cement stabilised backfill techniques may be employed.

Depending on a variety of factors, these drawings illustrate the various planning options of Terraforce retaining walls:


Factors influencing installation costs: as wall angles and heights increase so does cost. Due consideration has to be given to the presence of groundwater or unstable retained soil.

Light gravity wall
Heavy Gravity wall (multi-skin)
Light gravity wall
Heavy Gravity wall (multi-skin)
Composite structure (geofabrics) Vertical/ RC Filled wall
Composite structure(geofabrics)
Vertical/ RC Filled wall

Light gravity walls

Light gravity retaining walls normally consist of a single skin of blocks and should not exceed a certain height limit. Please note that the NHBRC requirement is 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 70º is also possible. Any steeper and especially load-carrying walls must be designed by a qualified engineer, even if only 1.2m high.

To check your possible wall height for a single skin wall, consult the design tables (on the design planning page) for Terraforce products.

The planning of a plant supportive retaining wall should be looked at from the environmental point of view as well as considering structural and cost factors.

type of retaining system: Resist the temptation to specify slender, stretched-out types or imitations of proven blocks. The system should be chosen for maximum structural mass (no air voids within wall) combined with optimal rooting conditions. This will help in binding the system and embankment together.

Climatic conditions: Exposure to factors such as coastal, salt-laden winds, long hours of direct sunlight, deep shadows, etc., should be taken into account.

Function and type of vegetation: Herbaceous, deep rooting (low maintenance) or attractive flowers (high maintenance) may be considered. Mixed complimentary species or uniform ground covers can be planted. Feeding values for birds and insects must be investigated for a complete bio-engineering approach.

Maintenance: The monitoring of soil fertility, irrigation and regular maintenance operations form an integral part of a growing investment. Indigenous plants are ideally adapted to prevailing climatic conditions. They generally require less water and maintenance, are deep rooting and can contribute considerably toward adding competence to these installations. Terraforce licensees in most areas have a list of suitable plants available.

 Spiral stair with stack up on left and stack back on right  Setback may vary
Spiral stairs: flush and sloped setback
Setback may vary
 Reverse the block and change the texture
Reverse the block and change the texture

Sharp or wide, convex or concave curves or corners are formed by simply turning each unit within the half-moon interlock. Design options are limitless with Terraforce because of the unique shape, which allows straight or staggered bond and wall angles from vertical to a 30˚ slope.

 Setback may vary  Setback may be equal
Setback may vary
Setback may be equal
 Always ensure blocks abut hard against each other  Semi circle retaining wall
Ensure blocks abut hard against each other
Semi circle

Heavy gravity walls

Heavy gravity walls are double skin walls, often with concrete reinforced backfill, for extra mass. A double skin, or layer, of blocks 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.

Often the designer is faced with a situation where the incline is steep and the mass of a wall system is insufficient to achieve a stable situation without resorting to additional measures. In most cases it will be found that the provision of a double skin wall for added mass is by far the cheapest solution, especially when it is not feasible to carry out additional excavation required to accommodate the installation of earth reinforcing techniques.

To view an example of a heavy-duty, composite installation, go to the case studies page where some examples of such installations are reviewed.

Heavy duty retaining walls
Heavy duty retaining walls

Composite structures

Composite retaining walls are geosynthetic reinforced soil segmental retaining walls. They utilise reinforcing sheets of geogrid or suitable woven geotextile which are attached to the fascia and are embedded in a body of engineered fill.

Vertical or near vertical retaining walls with a rock face block finish and a reinforced earth backfill design are a popular choice in many areas. The unique reversible nature of Terraforce blocks offers many advantages in this sector. While retaining all the Terraforce versatility, it is possible to build walls with seamless transitions from vertical to inclined elevations or from split face appearance to smooth moulded face appearance.

Composite retaining, single skin, with geosynthetic reinforcements
 Composite retaining, skingle skin, with geosynthetic reinforcemants

It is also possible to incorporate a plant supportive panel into a wall that is otherwise built in the split face appearance. Obtain a separate manual and design charts from your supplier for more information. See some case studies in colour.

The maximum wall heights for a single skin, mass gravity, block retaining wall system
The maximum wall heights (in block height, metres, feet) for a single skin, mass gravity, block retaining wall system – with additional geo grid reinforcement

Composite retaining wall design and construction procedures with Terraforce blocks have been subjected to rigorous laboratory tests. These were conducted on a large-scale test apparatus to evaluate the mechanical performance of, among others, the connection between blocks and grids.

In all tests, the primary mode of failure was rupture of the geogrid outside the blocks, and performance was found to be above average based on experience with a large number of systems tested over many years.

Vertical/RC filled walls


Occasionally, severe space limitations dictate the use of Terraforce blocks in combination with reinforced concrete infill or stabilised backfill. General planning: Develop a precise plan for your Terraforce wall by analysing your site, noting slopes, drainage and shape of the wall. Measure the length and the vertical height to obtain the surface area and thus the number of units required. Remember that retaining walls require professional design and supervision input and must comply with local building regulations.

reinforced concrete infill or stabilised backfil

This type of installation, while similar in nature to conventional reinforced concrete/masonry retaining walls, offers a number of added advantages:

    • Ease and speed of construction – no shuttering and less specialised construction skills required.
    • Aesthetic appeal – choose between split face blocks or smooth moulded appearance. It is even possible to retain a degree of plantability in some cases.

Design procedures are the same as those employed for traditional reinforced masonry retaining walls. See colour case studies.

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