Q & A

Please note that some of our licensees are located in countries where open-face retaining systems are still used, hence some of the questions and answers below have little or no meaning for consumers in more developed countries where these systems are no longer in use.

  1. Where can Terraforce concrete retaining blocks be purchased?
  2. What are the main differences between Terraforce and other systems?
  3. Are all retaining wall systems plant supportive?
  4. What is the difference between open-face and closed-face retaining systems?
  5. How does this difference affect long-term integrity of an earth retaining wall?
  6. How important is backfill compaction?
  7. How important is the drainage layer?
  8. How should I make my choices?
  9. What kind of foundation needs to be provided?
  10. At what inclination and to what height can my retaining wall be built?
  11. What kind of infill do I need for the blocks?
  12. Does the block infill improve stability, and if so, how?
  13. How do Terraforce retaining blocks interlock?
  14. Do the blocks have to be packed staggered, like brickwork?
  15. Is there a minimum radius for tight curves within a wall?
  16. Does the choice between round or straight face influence stability?
  17. Can Terraforce concrete retaining blocks be dismantled and re-used?
  18. Can Terraforce block walls be installed by keen do-it-yourselfers?
  19. Do I need mortar for the installation of concrete retaining blocks?
  20. Do I need a geo-fabric filter backing for Terraforce retaining walls?
  21. How can I create steps within the retaining wall?
  22. What is the recommended procedure for installing the wall?
  23. Are Terraforce concrete block walls cost-effective?
  24. Are Terraforce retaining blocks environmentally friendly?
  25. Is Terraforce truly a one-block system?
  26. To suit site requirements, what design options are available?
  27. Can I build terraces? What about converging terraces?
  28. Can I change the inclination within a wall, and if so, how?
  29. Can I match up to existing buildings or structures?
  30. Can damage due to handling and transport of the blocks be repaired on site?
  31. How can intermediate drainage outlets be installed in a long wall?
  32. Do I need an engineer to design my retaining wall?
  33. Are submitted plans required by local authorities?
  34. What about maintenance?
  35. Is Terraforce a proven retaining wall system? How long has it been on the market?
  36. What options exist to reinforce the backfill for high and/or steep walls?
  37. What about multiple rows of retaining blocks?
  38. How are the top rows of Terraforce retaining walls finished off?
  39. What are the most common shortfalls in segmented block retaining walls?
  40. How do I best avoid these shortfalls?

1. Where can Terraforce concrete retaining blocks be purchased? Top of page

Our licensees can be found in various locations on five continents. Go to the “where to buy”  section on our website to see their contact details and a list of the products that they make.

2. What are the main differences between Terraforce blocks and other systems?  Top of page

There are many different types of segmented concrete retaining blocks on offer worldwide. From heavy solid-cast concrete blocks to medium or small solid-pressed concrete blocks, via medium to small hollow blocks like the Terraforce range with a closed vertical surface. These are available in most parts of the world, while in some developing nations there are also a few loosely structured open-face systems available. Terraforce combines more advantages than any of the other systems: relatively light weight for transport, interlocking on both planes – horizontally and vertically – reversible to display smooth round-face or split straight-face, fully plant supportive, and closed vertical surface structure. For proper compaction of backfill and prevention of backfill erosion, wall angle can vary to suit site conditions and the blocks can be filled with soil, gravel or concrete as designed by the engineer. 
 
3. Are all retaining systems plant supportive?
  Top of page

No. Solid block systems are not plant supportive, and loosely structured, open-face systems are plantable to a degree, but excessive evaporation in these would require an increased input of irrigation water that could possibly increase backfill erosion. Terraforce blocks have an open horizontal surface structure, can be fully filled with garden soil and will allow root systems and irrigation water to penetrate deep into the installation. A continued aesthetic value is thus ensured. Waterwise plant list for drier regions.

4. What is the difference between open-face and closed-face retaining systems?  Top of page

Apart from what has been said under question 3, it can be stated that the main differences, as we see it, are as follows: Closed-face systems (meaning the vertical face) generally require more blocks per square metre of wall, all fully filled as specified and are thus heavier than open-face systems. Further compaction of the backfill is easier in closed systems and the backfill will retain it’s integrity, which is not always the case in open, loosely structured systems. Since, as far as we know, the pullout resistance of geogrids that have been clamped between blocks has never been tested in open systems, we can only state that Terraforce blocks performed in this test with results that are above industry standards. It also appears that a test to determine the full block crushing resistance of some open-face systems has yet to be devised.
 
5. How does this difference affect long-term integrity of an earth retaining wall? 
Top of page

Apart from the lesser constructed mass, generally, the long term integrity of drainage layer and backfill behind open walls remains a contentious issue. We are of the opinion that erosion of the backfill will occur over time.

6. How important is backfill compaction?  Top of page

Inadequate compaction of the backfill and of the drainage layer are the cause of some spectacular failures that we have seen in segmental block walls. Backfill for relatively high walls must be approved by the engineer and compacted to their specification, and care must be taken to prevent surface water ingress from above the wall. Closed-face Terraforce blocks will facilitate adequate compaction and ensure the long term integrity of the compacted backfill.

7. How important is the drainage layer?  Top of page

The wall itself and the foundation are important, and the drainage layer is at least as important. It should consist of a layer of coarse, well-draining material of not less than 300mm (1ft) behind the blocks as shown in our typical cross-sections, and be well-compacted. The engineer may also specify a permeable drainage pipe above foundation level. Channel surface water away from the top of the wall in an open swale.

8. How should I make my choices?   Top of page

Our blocks are available in different sizes to suit varying requirements. Your local supplier will gladly assist you in making the right choice. They will also refer you to an engineer where the need arises. Test results and evaluation reports are available on request. See the drop down menu on the top left for all the products by Terraforce. Where to buy.

9. What kind of foundation needs to be provided?   Top of page

Ultimately your engineer will make that decision, especially for walls above 1,6m (5ft). Low retaining walls can often be built on a compacted gravel foundation. In some instances even high walls can be built on gravel foundations, provided the walls are of the composite type (walls with geosynthetic reinforcing of the backfill). If the foundation has been specified as concrete, the first row of blocks should be set in mortar to facilitate levelling of blocks.

10. At what inclination and to what height can my wall be built?   Top of page

As a rule of thumb, it is safe to say that the cost of a wall goes up as it is planned to be steeper and higher. Low to medium walls at shallow inclinations can be built very cost-effectively, as can normal gravity walls that are stable by virtue of their self-weight. As height and/or inclination increases it becomes necessary to reinforce the wall by various means and consequently the cost increases as well. Terraforce walls have been built to heights approaching 20m (60ft) at an inclination of 70˚, and vertically up to 6m (18ft).

For cost-effective design service, go to Terrasafe, where we can help you design a robust retaining wall.

Questionnaires for design assistance for the following wall types:

For a Gravity wall
For a Near vertical wall
For a Terraced wall

11. What kind of infill do I need for the blocks?   Top of page

The blocks should be filled with material as specified by the engineer: garden soil, clean sand, gravel or concrete (reinforced if needed). Low garden walls that are meant to be plant supportive should be filled with the best garden soil available.

Refer to table of infill values.

12. Does the block infill improve stability, and if so, how?   Top of page

Gravel is heavier than garden soil and will also interlock the blocks better on the vertical plane, thus gravel infill improves stability considerably. Refer also to answer 13.

13. How do Terraforce retaining blocks interlock?
   Top of page

On the horizontal plane, the blocks interlock via the convex and concave shape of blocks that interact with one another. On the vertical plane, interlocking strength is gained by the block infill which functions like an interlocking core. Infill materials such as coarse gravel or concrete obviously have a higher core strength than sand or garden soil. No pins or interlocking lips are involved that might interfere with versatility in alignment and inclination.
(From 55˚ to near vertical)

14. Do the retaining blocks have to be packed staggered, like brickwork?   Top of page

Although this is standard practice, it is not a requirement and stack bond is also allowed. Our engineers agree that the integrity of a wall is not affected by switching from stagger bond to stack bond, as often happens on sites where the extreme Terraforce versatility is applied to create meanders at varying wall inclinations.

15. Is there a minimum radius for tight curves within a wall?   Top of page

Although very tight curves can be packed, especially with the bigger retaining blocks, it is recommended that smaller curves than 500mm (1,5ft) should be avoided. Right angle corners can be created by cutting interacting blocks with an angle grinder to suit. Turning a neat corner.

16. Does the choice between round face and straight face influence stability?

No, stability is not affected. However, poor construction techniques and cost-saving maneuvers will. Be sure to have suitably experienced workmen on site.

17. Can Terraforce concrete retaining blocks be dismantled and re-used?  Top of page

Yes they can, and on many occasions the blocks have been used for temporary installations.

18. Can Terraforce blocks be installed by keen do-it-yourselfers?   Top of page

As a rule of thumb, it is best to rely on experienced contractors who are committed to Terraforce standards and who will guarantee their workmanship (Recommended Installers). However, some of our award-winning installations have been built by home owners, one of whom likened this construction method to therapy for the soul. He created the most intricate details, with one block type, and thoroughly enjoyed it.

19. Do I need mortar for the installation of concrete retaining blocks?   Top of page

No, mortar is not required except when you want to level the first row of blocks on a concrete foundation. They are generally referred to as dry-stack blocks for that reason.

20. Do I need a geo-fabric filter behind my Terraforce retaining wall?   Top of page

Since Terraforce is a system with a closed vertical surface structure, a filter membrane is not normally required. There are exceptions, such as when very fine sand is used as backfill or when a high water table is present behind the wall. This will be decided by the engineer, who may decide to separate the drainage layer from the backfill with a geo-filter/geo-membrane to prevent migration of fine particles into the drainage layer. 

Terrasafe design service.

Questionnaires for design assistance for the following wall types:

For a gravity wall
For a near vertical wall
For a terraced wall

21. How can I create steps within the retaining wall?  Top of page

Steps can be built by simply reversing the standard Terraforce retainer blocks that you are using so that the flat area is facing forward. Alternatively, for a more up-market stairway, you can use the special Terraforce 4×4 step block to build very comfortable steps. Many stairways of both kinds are depicted on our web site. Steps in the right direction.

22. Is there a recommended procedure to follow when installing a wall?  Top of page

On our website and on our brochures you can find illustrated installation details or a step-by-step illustration of a Terraforce installation. We also have a design manual that deals with the issue in detail. Please always start building at the lowest point of a wall and follow our instructions carefully to ensure structural integrity and lasting performance.

23. Are Terraforce concrete block walls cost-effective?
  Top of page

They are, but not cheap. It is a product meant to have a good service life of 70 years plus, at a reasonable price. As a rule of thumb it can be assumed that they can be built at a cost of about 70% of  what a conventional retaining wall will cost.

24. Are Terraforce blocks environmentally friendly?  Top of page

They definitely are, our track record in this regard can leave no doubt. In addition, many of our licensees make use of recycled aggregates. The blocks are light to handle and transport, are free of toxic preservatives, and once completed can, with suitable planting, provide food and shelter for many birds and insects. Waterwise plant list.

25. Is Terraforce a truly one-block retaining system?  Top of page

Once you have chosen your Terraforce block, you can do most features that are normally required with that one block. Occasionally you may have to cut blocks to form sharp corners or to match up to existing buildings. Go to details on our website.

26. To suit site requirements, what design planning options are available? 
  Top of page

We can reaffirm that Terraforce remains unsurpassed in terms of versatility. Sections – planning alternatives and design alternatives – on our website deal with these issues. Terraforce also offers a cost-effective design service, Terrasafe.

27. Can I build earth retaining terraces? What about converging terraces? 
  Top of page

Terraces can be built with much success, as our case studies clearly show. The overall stability of the whole slope will have to be determined by a suitably qualified professional. Each terrace wall will have to be properly founded in accordance with accepted engineering principles. Walls can merge and separate again with ease.

28. Can I change the inclination within an earth retaining wall, and if so, how?   Top of page

Terraforce walls can merge seamlessly from near vertical to a low 55˚ inclination by simply shifting the blocks back or forward before filling. You must ensure that stability  parameters are still intact when doing this. Check with your engineer or other competent person.

29. Can I match up to existing buildings or structures? 
Top of page

Matching up to existing masonry walls with the stack bond method (refer question 14) is easy when using stagger (running) bond – some blocks need to be cut to match existing structures and the cut blocks should then be filled with concrete or mortar.

30. Can damage to blocks due to handling and transport of concrete retaining blocks be repaired on site? 
  Top of page

Minor cracks that are the result of the manufacturing process or minor chipping of blocks (up to 25 mm) during transport and handling are not to be rejected. Split or rock-face blocks are excluded from this requirement as their appearance is meant to look uneven.

Blocks with deep cracks should be rejected, but blocks with chips of up to 50mm in diameter have on many occasions been repaired successfully on site, provided that this is done in a professional manner and in a matching colour – some experimentation may be required.

31. How can intermediate drainage outlets be installed in a long wall?
   Top of page

The best method is to cut slots to suit the pipe diameter into the blocks and feed the outlets through these. These individual blocks should be filled with concrete or mortar.

32. Do I need an engineer to design my retaining wall?  Top of page

For low walls of up to 1m (3ft) you generally do not need an engineer. Terraforce has basic design tables for mass gravity walls (walls that rely on self-weight to achieve stability) and for composite retaining walls (walls that rely mostly on a body of reinforced earth behind the blocks to achieve stability) available in our brochures, our website and in two of our design manuals. We also have a design software package available free of charge from www.maxiwall.com. Designs are prepared in terms of accepted engineering principles and judgement.

Questionnaires are available for design assistance with the following wall types:

For a gravity wall
For a near vertical wall
For a terraced wall

Detailed recommended specifications:

Recommended specifications

Terraforce block input values

Recommended (full) bill of quantity

Terrasafe design service

33. Are submitted plans required by local authorities? 
  Top of page

Most building departments worldwide do require plans for approval for walls that are over 1m (3ft) high. Check with your local building inspector.

Submission sheets:

For gravity walls
For composite walls

34. What about maintenance?   Top of page

As with any project that has been designed to be aesthetically pleasing and environmentally sound, maintenance is unavoidable. On some walls planting will have to be kept weed free and rejuvenated occasionally, and the irrigation system may have to be maintained. On other walls, care has to be taken that larger invader plant species are removed, or sometimes no form of vegetation is welcome.

35. Is Terraforce a proven retaining wall system? How long has it been on the market?   Top of page

We have been planning and building earth retaining and erosion control measures for the past 30 years (since September 1979). Our products are now available on five continents – a record that we think speaks for itself.

36. What options exist to reinforce the backfill of high and/or steep walls?    Top of page

You could use a number of options and materials. Cement stabilised backfill has often been used as a cost-effective method of increasing stability, sometimes in conjunction with a double row of Terraforce blocks. Low strain (extruded) geogrids may be used in some instances and for critical installations one would specify high tensile (woven) geogrids for extra strength. Ground anchors of steel and wire-mesh sheets may on rare occasions be specified.

Design alternatives

37. What about multiple rows of retaining blocks?   Top of page

This method is becoming popular in areas where geogrids  are increasingly expensive due to the high oil price. Where blocks are made in pairs, for splitting to create the popular rock-face look, it is feasible to supply un-split pairs of blocks to the site for this application. The method is usually applied when available space is limited and reinforcing geogrids cannot be used. It is thus possible to fill the inner layer of blocks with reinforced concrete and the outer layer with garden soil.

38. How are the top rows of Terraforce retaining walls finished off?  Top of page

Machine-made or wet-cast capstones, rectangular or exactly matching the Terraforce block shapes, can be supplied by most of our licensees. Alternatively blocks in the top row may be capped/plugged with an approximately 40mm mortar layer and decorated with round pebbles or coarse brush strokes for a non-slip finish. Many users of our blocks prefer to simply fill the top blocks with garden soil to support attractive feature plants.

39. What are the most common shortfalls in segmented block retaining walls?   Top of page

The most common shortfalls are listed below. It should be noted that in some cases a combination of these can also cause a collapse.

a. Insufficient constructed mass – no or negligible design input
b. Saturated backfill – lack of drainage above or behind the wall
c. Design angle and height exceeded – lack of supervision
d. Settlement of backfill – substandard material, inadequate compaction/backfill erosion
e. Undermining – excavation close to wall foundations
f. Excessive loading – not accounted for in the original design
g. Limited bearing capacity – poor/saturated founding conditions or no foundation at all
h. Face connection failure – poor connection between blocks and geo-grid
i. Linear cracking of blocks – excessive lateral earth pressure or poor quality of blocks

40. How do I best avoid these shortfalls?   Top of page

If you wish to install a robust segmented block retaining wall that will do its job without fail while looking aesthetically pleasing, be advised to stick with the tried and tested. Avoid imitators who simply cotton on and who are strangers to terms such as: statutory requirements, surcharge loading, potential failure planes, interface shear capacity, pull-out and crushing resistance and progressive backfill erosion.

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