Main image: Concrete retaining block (CRB) walls correctly installed as per the Terraforce Design Guide
Concrete retaining block (CRB) walls have become increasingly popular. Not only do they offer various design aesthetics but adds value to a property and allows for building on otherwise hard to reach spaces.
CRB’s are not new, though we keep seeing failures due to bad design, incorrect installation and corner cutting.
Let’s look at how to avoid disappointment and keep the possible shortfalls in mind. Keep in mind that there is a minimum CRB block wall requirement and quality assurance during the manufacturing process.
Insufficient constructed massdue to no or negligible design input, use of incorrect block size/infill material, incorrectly conveyed site information or poor drawings/specifications.
Saturated backfill due to a lack of stormwater control above the wall. Inadequate sub-soil drainage.
Design angle and height exceeded due to a lack of supervision during construction. Unauthorised changes to the wall details.
Settlement of backfill due to substandard material, inadequate compaction, backfill erosion or a lack of supervision during placement of backfill.
Undermining caused by excavation close to wall foundations.
Excessive loading above the wall, which was not accounted for in the original design.
Limited bearing capacity due to poor or saturated founding conditions or no foundation at all.
Incorrect selection of supplementary reinforcement, choice of inadequate geo-grid. Poor connection between blocks and geo-grid. Inadequate concrete or reinforced concrete infill, or alternatively cement stabilised backfill.
Linear cracking of blocks due to excessive lateral earth pressure or poor quality of blocks.
Non-adherence to local Occupational Health and Safety Act (OHS Act).
6 steps to ensure a professional result
Decide what you want to achieve, possibly with a layout sketch and a cross-section drawing. Select from the block of your choice.
Contact your local supplier for prices and a reference to recommended installers with experience in the construction of retaining walls.
Ensure that a proper design has been prepared for submission to your local municipality, as approval may be required. Some installations are deemed pure landscaping measures and would not require the submission of plans. Confirm with the local authority what its regulations are. Should your recommended or preferred installer not be able to assist in these questions, find a new one.
During the construction phase, ensure that the designer of the approved plans or his representative visits the site to ensure that his specifications are met.
After completion of the wall, obtain a completion certificate as prescribed by SANS 10400 Form 4 or other local regulations (if required) and hand it to the building inspector.
Please ensure that the supplier of the blocks is a reputable, licenced manufacturer and is able/willing to provide design information or can refer you to a qualified engineer.
Assist the designer in conducting his task in a timely manner
The designer needs information to do his task without wasting time on clarifying site-specific details or on the desired appearance of the wall.
A cross-section sketch, or multiple cross-sections if the wall has varying heights (in 1m height intervals), should show the following information:
Height of wall and inclination of the fascia, in degrees. Walls at a 65-degree inclination are more cost-effective than steep walls at 85 degrees. Alternatively, a completed questionnaire for gravity, composite or terraced walls should be submitted.
Type of native soil found on site at the foundation level, and on the slope to be retained. Photos may help the designer to assess site conditions. A geotechnical report and/or density tests, if available, is a great help.
Is the ground behind the wall level or is it rising further? Photos will help if the designer is unable to visit the site.
Indicate any surcharge loading, such as a building or a driveway, above the wall and indicate the distance between the load and the top of the wall.
Steep and high walls often need to have geogrid reinforcing incorporated into the backfill for stability. Indicate the space available behind the block fascia and the cut face to incorporate such measures. Provide the designer with information pertaining to the type of geogrid that is available in your country.
Any groundwater visible? The most common cause of failure of retaining walls is the build-up of pore water pressure in the retained soil. Trapped water needs to escape via an agricultural drain, through spaced outlets.
Is there a possibility of surface water above the wall draining towards the wall? Ideally, all surface water needs to be directed away from the wall and is not allowed to spill over the walls. Drainage recommendations are to be followed diligently.
Any information on previously proposed designs involving other methods.
Appoint an experienced contractor/installer, even if his rates are higher than these offered by others who possibly lack the necessary know-how or commitment.
Ensure that a qualified professional is in charge to supervise the installation, to signing the completion certificate to be handed to the building inspector.
Always appoint a competent designer with experience and who has public liability and professional indemnity insurance cover.
For more information and to receive the Terraforce Design Guide, contact Terraforce (to whom acknowledgement is given for information in this article): Terraforce: Tel: +27 21 465 1907 Website: https://www.terraforce.com/