Against the wall
Retaining walls are a practical and decorative way to cope with sloping areas in the garden
By Lynne Yates
Steeply sloping areas in the garden are often a cause for concern – not only are they difficult to negotiate, but plants and soil may wash away during heavy rains due to erosion from properties or roads higher up.
A practical and attractive solution is to build a retaining wall against a steep slope as an embankment to provide stability, or to create a series of terraces on the property using retaining walls and steps.
A well-designed terraced garden usually has a main level area for the first terrace, often used as an entertainment area or planted up with lawn on which children or pets can run and play, and then a series of two to three smaller terraces, either with lawn or flower beds, or both.
Most commonly in home gardens, low retaining walls are built from brick and cladded with stone or other materials to suit the design of the garden, or are constructed using concrete retaining blocks (CRBs) that can be planted up with plants of your choice for a softer look.
It is necessary to call in an engineer to help with walls higher than 1.2m as retaining walls hold up tremendous weight and need to be properly designed to prevent them from collapsing.
Types of retaining walls
“The oldest known method of building retaining walls is by using stones or boulders, either randomly packed or shaped to obtain a better bond.
Thousands of years ago, some walls were built with a mixture of clay and straw while others were built by filling wicker baskets with loose rocks,” explains Holger Rust of Terraforce.
All these methods have been developed over the years into mass gravity concrete walls (the mass of the wall alone is usually sufficient to support the soil behind it); reinforced concrete retaining walls and their close relative, reinforced brick walls (usually involving a cantilever foundation to transfer pressure from the ground behind to the foundation); gabion walls (wire mesh baskets filled with rock); geotextile reinforced soil (a development of the straw reinforced clay method); brick walls (usually 220mm or thicker and sometimes with the core – a 10mm gap or more between brick wall skins – filled with steel reinforced concrete) and concrete retaining blocks – (CRBs), the latter two being the most commonly used on residential properties.
* Concrete retaining blocks (CRBs)
“Of all retaining wall methods currently being used, the concrete retaining block method is by far the most popular,” says Holger.
Various systems are available, interlocking to various degrees; some of these hollow blocks are plant supportive, some can be combined with reinforced concrete infill, while most can be combined with compacted reinforced soil and often combined with geosynthetic fabrics and grids placed horizontally to reinforce the fill behind.
Most common in gardens are plant supportive CRBs. They are laid staggered one above the other on a backward or tilted slope and the hollow centres are filled with soil and planted up with suitable plants.
“These plantable retaining walls are the most popular in a garden setting. Where serious terraces need to be built (garage access, property on a steep slope or ridge), a closed structural retaining wall may need to be constructed,” explains Taco Voogt from Technicrete.
When it comes to using CRBs on residential properties, Silvio Ferraris from ReMaCon lists their pros:
“They are quick and economical to construct, requiring no additional mortar as required with a brick wall, and they can be easily dismantled and either reused elsewhere or rebuilt when, for example, tree roots or erosion damage them.”
CRBs are available in different colours and designs. “There are specific retaining blocks, which are aesthetically pleasing (with a rock face texture in sand colour), economical and light, and that are DIY-friendly and can be constructed for walls not exceeding 1.2m high,” explains Silvio.
“For higher walls there are heavier blocks, but be aware that in terms of the national building regulations, walls higher than 1.2m are no longer ‘fit for purpose’ and require an engineer’s design, or no insurer will cover that wall should it ever fail. When selecting the type of blocks you want to use, choose a supplier that can provide you with technically good advice,” advises Silvio.
Things to consider
Height and slope
“Municipal by-laws require an Engineer’s Certificate when the retaining wall is over 1.2m high. Retaining walls erected on slopes less than 70⁰ are considered sloped embankments. Retaining structures are generally steeper than 70⁰. The height/slope ratio is important for the stability; it is best to seek advice from your supplier regarding this,” advises Taco.
Foundations, fill and drainage
“The foundation for walls up to 1.2m may be made of a well-compacted base, but should preferably have some cement mixed in, approximately one 50kg bag per 10m length of foundation, and it should be about 200mm deep so that the first row of blocks is actually under the ground (to give it some resistance at the base); higher walls require concrete foundations,” says Silvio.
Good construction practice with line and level using fish lines and spirit levels ensures a good looking wall.
Drainage should be provided behind the retaining blocks approximately one row above the foundation.
In certain cases, it is best to place a drain channel on top of the wall to control water flow coming from a higher neighbouring house.
Also make sure that water is diverted away from the wall’s foundation.
“If more than 1.2m high, an engineer’s design is required, but generally good construction practice requires the fill behind the retaining blocks to be an engineered fill (granular) that must be well compacted preferably with a mechanical compactor,” explains Silvio.
“These types of retaining walls are mostly maintenance free, except for the plants, which may need replacing seasonally,” explains Silvio.
However, keep in mind the following:
* Don’t allow big trees to grow above the walls.
* Check for possible storm water wash out.
* Check the wall angles. If there is a problem, remove the blocks and loose soil and replace them. If the problem is complicated, consult with the supplier of your blocks.
* Most importantly, do not put irrigation pipes too close to the backs of these walls – gardeners tend to puncture the pipes and leaking water into the fill behind these walls is a definite recipe for failure; rather set pipes and sprayers at least 1 200mm back.
* Brick walls
If you are building a low retaining wall (less than 1m), you can simply build one from a double layer of bricks, which you can plaster or clad with stone or tiles. This should be built on a concrete foundation to give the wall a solid base. Ensure that your retaining wall is strong and stable to hold the weight of the soil behind it. For walls that are higher than three times the wall’s thickness it is advisable to get the advice of an engineer.
Things to consider:
Ensure the interior of your brick wall is damp proofed with a suitable damp proofing product to avoid water damage to the wall, which may undermine its stability.
If water is unable to get through the retaining wall, pressure will build up, leading to cracking and even collapsing of the wall.
Water can also wash out the foundation of the wall. A well-built retaining wall will make provision for water to drain out via built-in drainage holes.
When backfilling the retaining wall, first layer gravel or stones at the bottom to allow water to move freely towards the drainage holes.
Then fill with soil and ensure that the topsoil or lawn behind the wall is even with the top of the wall to allow surface water to run over the top.
In the next article we'll look at installing a retianing wall.