A retaining wall is a structure that holds or retains earth behind it. Marc Worner deals with the possibility of a slippery slope.
It’s rare to find even ground on a landscape site. Retaining walls effectively level even the hardest of inclines or declines, allowing the transformation of an empty slope into an enclosed area for outdoor leisure, a decorative garden or playground.
The most important consideration in proper design and installation of retaining walls is to counteract the tendency of the retained material to move downslope due to gravity. If not properly addressed, earth pressures will push the wall forward or overturn it. Also, any groundwater behind the wall not dissipated by a drainage system will cause enormous hydrostatic pressure on the vertical surface.
First up
There are many materials that can be used to create retaining walls, like reinforced concrete blocks, poured concrete, treated timbers, rock, boulders, railway sleepers, and bricks.
Some are easy to use, others have a shorter lifespan, but all retain earth.
We need to know what type of soil we’re retaining before we know what type and height of retaining wall we can build, andthere are several questions to be answered before a project begins.
For example:
• Is the soil sand, clay, or something in between?
• What’s behind the wall – a slope, a driveway, a garden or something much heavier like a pool?
• Will the ground above or below the wall be level or sloped?
• What will be the height of the wall?
• All these factors will determine whether a gravity retaining wall or a reinforced retaining wall is needed.
Gravity or reinforced?
Gravity retaining walls depend on their own weight and setback/batter (the distance a course will be offset back into the slope from the course beneath it) and are typically shorter in height.
A good example is a gabion wall – effectively caged rock.
Reinforced retaining walls use some type of reinforcement to give more strength to the wall. One common method is to place geotextiles between courses. Manufactured high-strength reinforcement grid materials, geotextiles are available in various sizes and strengths and are rolled back into the slope during installation to create a stronger and more stable soil mass.
Sometimes this method isn’t a usable solution due to space or other site conditions, so alternative reinforcement materials need to be used. These might be things like soil nails or earth anchors, usually required in big commercial projects with steep slopes and limited space. This type of reinforcement requires the services of a geotechnical engineer for the wall’s design.
rock. Image: czarny/stock.adobe.com
Materials
While wood is often an inexpensive choice of material for a retaining wall, its short lifespan makes it a relatively poor choice. Timber walls are also vulnerable to timber-pest (termite) infestation and weathering. Dampness may cause timber to rot. As the wood decomposes, so does the retaining wall and the integrity of the usable space. However, rectangular hardwood sleepers or H4 treated pine logs are great economical choices for low-load applications like garden beds. They’re natural, low cost and low maintenance.
Stones, rocks and boulders can create beautiful retaining walls, but be mindful their installation is labour intensive.
Dry stack walls can typically be made from three types of stones: round field stones, relatively flat stacking stones, and uniformly cut dressed stones. Each type will give the wall a different look and feel. Flatter stones tend to be easier to work with than rounder ones as they’re more suited for stacking. Footings should be as wide as the wall is tall, so this type of wall may need plenty of room.
TIP: For the strongest possible wall, every stone set should be secure before another is placed on top of it. Always stagger joints between courses.
NB: By their nature, drystone retaining walls have excellent drainage.
As a wall is built, toss all unwanted stone scraps behind it. When the wall is complete, backfill behind it with gravel to just below the level of the cap stones or top course. For a more secure top, some landscapers set the cap stones into a bed of premixed mortar.
Importantly, these walls are self-draining.
Mortar
Mortared retaining walls are another option. These walls can be installed with a variety of veneers, bricks, flagstones, and so forth, which will add colour, style and texture to any landscape design.
Good mortar is vital in all brick or block walls as it bonds the units together, helps carry the weight placed on the wall and seals the joints to be weatherproof.
Lime is added to make mortar creamier or more workable and durable. It also helps minimise cracking as the mix dries out.
Aggregate sand is the fine aggregate component and the basis of the mortar, and only brickie’s sand should be used.
Water for use in making mortars should be clean, fresh and free from impurities. As a rule, if the water is suitable for drinking it will be okay for making mud. Pigments added to colour the mortar should not exceed 10 per cent of the weight of cement in the mix and should be thoroughly mixed with the other materials prior to the addition of water.
TIP: A sample of the coloured mortar should be made and allowed to completely dry before commencing work to ensure the desired colour is achieved.
Segmental
Segmental retaining walls offer modular and interlocking concrete masonry blocks for ease of use. Simply stack the mortarless blocks together and create a maintenancef-ree retaining wall. These types of retaining walls use similar construction techniques to those found in the mortarless construction of the Great Wall of China.
Segmental retaining walls come in a variety of sizes, shapes and colours, as well as different facing textures, and concrete blocks made from recycled materials are a sustainable option for building materials.
TIP: If space is an important consideration, filling the core of each concrete block with concrete mix and rebar (reinforcing steel rods) will add greater strength to a wall, and therefore height, and will reduce the need for any setback.
NB: Footings for reinforced concrete or reinforced concrete-block retaining walls can be extended either in front of or behind the wall. This can be helpful in circumstances where space is limited in one direction, such as at property boundaries.
A different slant
A slope above a retaining wall will add more pressure and weight, while slopes below the retaining wall may make the wall unstable due to sliding or erosion. Avoid slopes greater than three-to-one without first consulting a geotechnical engineer.
Any added weight behind a wall is called a ‘surcharge’. Patios, swimming pools and driveways are common residential surcharges. Get a geotechnical engineer’s opinion in these instances.
Water in all its forms is a major challenge for landscapers on a site. The enormous pressure water can exert on a wall can be destructive, so the potential problem needs to be dealt with before it destroys our work and our reputation.
The design and performance of most retaining walls is based on keeping the area behind the wall relatively dry. During wall layout, it’s important to evaluate the entire site to determine if water will drain into the area where the wall will be constructed. Temporary grading may be needed to ensure water won’t drain towards the construction site, and this may entail a discussion with the client and landscape architect to achieve the best outcome.
TIP: Incorporating berms and swales into the final landscape design is an easy and cost-effective strategy to direct surface water away.
In the margin
Prior to constructing a wall, review drainage plans to identify all potential sources of concentrated water, such as driveways, slopes above walls, grading of the site, water lines, mains or fire hydrants, roof downpipes and irrigation systems. The services of a hydraulic or civil engineer may be required to design the retaining wall when water plays a critical part.
To ensure a quality project, the soils used must not become saturated during construction and the final design must route water away from the back of the wall. To this end, always incorporate a toe drain and weep holes in the wall as minimum requirements.
Weep holes are small openings that allow water to seep out rather than build up behind the wall. Face the backside of weep holes with Geotech to block dirt but let water drain through. Vary the number and spacing of weep holes depending on the soil type. For instance, sandy soil may not require weep holes. Use metal or PVC pipes in the mortar joints or similar gaps in the wall.
TIP: Cover the infill soils and the entire wall project at the end of each day to prevent water saturation, especially if rain is forecast.
Before any quote, always check building requirements with a local council or private certifier in the first instance.
Knowing all this, it’s no wonder the project cost is always quoted high with a good-sized profit margin.