It is not unusual for properties to sit nestled in a steeply sloping garden, leaving no space for a pool or patio. It also not unusual for a wooden deck to provide that space for outdoor entertainment, considering that it, on the surface, seems to be one of the most straightforward solutions in providing more space for living, entertaining, sunbathing, barbecues, and relaxation.
Karin Johns, freelance Journalist, investigates whether segmental retaining walls (SRWs) offer a better solution.
Wood versus concrete
Wood decks may offer advantages in initial cost; however, even though it is treated wood, you will need to perform maintenance on the wood planks to keep them from degrading. Wood decks as they age may yield splintered boards, broken boards, structural weaknesses, decayed pieces, and discoloration which may not look very good. Another hazard is slick surfaces. Much of the “dirt” we see on a poorly maintained deck is living organisms. Just like the algae and moss that grow on rocks next to a stream, the organic material growing on a deck can make the deck very slippery when wet.
Instead, homeowners are increasingly turning to segmental retaining walls (SRWs) to create some outdoor space. Holger Rust from Terraforce, local retaining wall block manufacturer, describes segmental retaining walls “as gravity retaining walls that rely chiefly on their mass (weight) for stability, or alternatively composite/heavy duty retaining walls, with additional soil reinforcing. They consist of concrete masonry units which are placed without the use of mortar (dry stacked) and rely largely on inter block friction to derive sliding resistance. Used in combination with horizontal layers of soil reinforcement which extend into the backfill increases the effective width and weight of the gravity mass.”
Patios or garden extensions using this system can be easily installed (depending on the design of the wall and other situational factors), are known for their durability and are maintenance-free. Also, raised patios with segmental retaining walls create new space in sloped backyards and can extend an existing raised deck, creating entertainment spaces that give homeowners a spot to incorporate an in-ground hot tub or pool, a barbecue or fire pit and ground-level lighting.
They add that the SRW system is composed of units whose size and weight makes it possible to construct walls in the most difficult of locations. Curves and other unique layouts can be easily accommodated, providing an attractive, cost effective alternative to wooden decks or conventional cast-in-place concrete retaining walls. Savings are gained because on-site soil can usually be used, eliminating costs associated with importing fill and/or removing excavated materials, and because there is no need for extensive formwork or heavy construction equipment.
But don’t break the rules…
There are some ground rules to follow when considering building a raised patio or garden extension. Look for the following features in your materials:
• No cracks. Unlike rigid retaining wall structures, which may display cracks when subjected to movement, the flexible nature of segmental retaining walls allows the units to move and adjust relative to one another without visible signs of distress.
• Easy to repair. You can remove and replace the same units with no ugly patches after repairs to the base.
• Many shapes, colors and textures. For a comprehensive selection, homeowners should consider hiring a recommended segmental retaining wall installer.
• Site design consideration – Typical designs and specifications for segmental retaining walls should be prepared by a designer who has technical knowledge of soil and structural mechanics. Each SRW unit manufacturer can provide design information tailored to that product, which will indicate the wall heights and design conditions when an SRW should be designed by a qualified engineer. In addition, SRW systems should be designed by a qualified engineer when:
1. Structures will be surcharged
2. Walls will be subjected to live loads
3. Walls will be founded on poor foundations
4. The nature of the design conditions requires special consideration.
There are number of ways to design a raised patio or backyard extension to accommodate a pool and other outdoor entertainment areas. At Kanonberg Estate in the Western Cape, Terraforce L11 rock-face blocks provided a level, spacious platform — about 3m above road level — around a house built on a steeply sloping erf. To soften the walls that support the fill for this platform, various flowerbeds and steps were added into the design.
The 4m wall at the back (cut section) of the house was constructed in such a way as to provide the owner of the property with a large backyard area to allow him to install a swimming pool. The wall behind the swimming pool area is spiced up with an artificial rock waterfall, along with more plantable space to soften the hardness of the very high rock-face wall. The round curves and landscaping in and around the house ensure that this retaining installation provides not only for a practical product, but also an attractive one.
In Bishops Court, Cape Town, the leveling of a prime property situated on a slope required the execution of approximately 2020 m³ cut and fill earthworks. Engineers HKS Law Gibb designed a 7,5 m high and 90 m long Terraforce retaining wall at an angle of 65° to support the fill section. L13 type blocks were specified with insitu concrete keys. Construction was started on a 700 x 500 concrete foundation with lime stabilised clay backfill compacted to 90% AAASHTO.
Wet conditions during this period complicated this process somewhat. In addition, layers of restrain 110 high tensile geofabric, securely anchored into the blocks with concrete, where specified at certain levels. All necessary service pipes were installed simultaneously together with foundations and supporting columns for the swimming pool.
A half moon shaped protrusion for the pool was constructed vertically on a reinforced concrete foundation and RC block infill. Installed by Suburban Pools, this pool was acknowledged as an innovative and imaginative structural achievement, winning four prestigious NSPI gold awards:
• National pool of the year ’97 award
• Best pool in the Western Cape
• Award in the section of unusual structural or engineering skills
• Best pool in the residential category
Local retaining block fits the bill
In South Africa, there are a few segmental pre-cast interlocking retaining blocks to choose from, Terraforce blocks being the most reputable and well known. Manufactured by various concrete manufactures throughout South Africa and overseas since 1984, they are designed to be as versatile, durable and eco-friendly as possible:
1. Unlike monolithic concrete retaining walls, those constructed using the TERRAFORCE system are easily formed into complex curved shapes (either convex or concave) or into walls in which the upper and lower profiles are continuously changing. The walls have a flexible structure which is aesthetically pleasing, and produces a natural, environmentally friendly look.
2. Terraforce retaining blocks are designed to allow you a choice between round face, (plant supportive) or flush face (smooth or split version) to suit your specific requirements, and they present a closed vertical surface structure that provides maximum amount of soil mass within the wall and prevents backfill spillage, while at the same time offering uninhibited permeability.
3. Superior bearing capacity is provided by double wall thickness of adjoining elements which also means a broad base for substantial block-on-block friction resistance. Flush fitting, horizontal interlock over full width of the structure effectively avoids point loads and performs even when forming relatively sharp convex or concave curves.
4. Vertical interlock in the form of close fitting, in situ, concrete keys provides positive shear resistance at all wall inclinations. Continuous soil core and root systems within the elements provide additional strength. Where further strength is required the engineer may resort to reinforcing the backfill with geogrids or partially/fully filling the units with reinforced concrete. Double row thickness presents another possibility.
5. When the units are filled with soil the constructed mass of a single skin wall can vary between 800 kg/sq.m (S) – 440 kg/sq.m, depending on the choice of Terraforce block, which provides a substantial factor of safety with respect to overturning, compared to most modular retaining systems.
6. Open horizontal surface structure ensures uninhibited penetration of water and root systems for maximum growth. Optimal rooting conditions prevent plants from becoming pot bound. Vegetation rapidly transforms the block wall into a growing investment.
7. TERRAFORCE walls are not built in a checkerboard fashion and contain no air voids. Evaporation is thus reduced to a minimum. They offer minimal visual impact due to the choice of colours and natural boulder appearance.