Retaining Walls in a Garden
Any retaining wall design should consider the following:
1. Shape and Substance of the wall.
The wall must follow the shape of the embankment it is retaining, as close as possible, to avoid carting large quantities of soil either in or out of the work site.
The substance which the wall is built from is largely determined by what materials are available. Dry walls (without cementing) need a solid base/foundation.
A mortared wall needs a concrete strip foundation and weep holes for drainage.
The design should also consider that with different seasonal temperatures, there will be lateral expansion of the wall; therefore expansion joints should be incorporated.
If the wall is longer than 10 m then it may be necessary to provide expansion joints as it may be liable to subsidence and contraction or expansion and crack.
A gap of 1/2 inch, in a staggered fashion, should be sufficient to allow movement and not spoil the appearance of the wall.
2. Positioning of the wall to minimise soil movement.
All walls should slope back into the embankment. This slope is called "the batter".
A minimum batter should be approx. 1 cm for every 6 cm in height. Ideally, the ground at both the top and bottom of a wall should be fairly flat, to minimise erosion.
The batter is essential for wall stability in all retaining walls above 1 m height.
3. Drainage both above and below the wall.
This factor is obviously more critical in clay soils. A spoon drain may be built at both the top and bottom of the wall.
Subsurface drains and rubble back filling to facilitate drainage has also to be used in these positions.
If surface drainage is allowed to run over the top of the wall, it can cause bad erosion behind and at the base of the wall, very quickly.
There are different types of retaining walls. Gravity walls rely on the weight and size of the materials to retain the soil, and they may have batters to increase stability.
These are the most common ones nowadays in landscaping works. They are built with ‘classic’ materials like stone or masonry, or with new materials such as high strength polymers.
Gravity walls must be at least 50 to 60 % as thick as the walls’ height, or thicker if there are surcharges on the wall.
Cantilever walls rely on the weight of the soil on top of a large wall footage to compensate for the soil pressure against the vertical wall.
They may have additionally buttresses spaced to reinforce the wall. This technique is not as used now as it was before.
Sheet pilling is a technique offered to build waterfront retaining walls with materials like vinyl, polymer coated timber and steel fasteners.
Materials used for building retaining walls are masonry, concrete, timber, steel, stone, brick, high-strength polymers, UV-resistant vinyl and other new materials.
Latest trends in retaining walls construction has developed in favour of segmental walls instead of poured-over concrete, as they are quicker and more economical to put in place than the latter.
A mixture of classical materials and segmented construction is the modular retaining wall technology of stone-filled gabions. Other newly developed systems consist of timber crib walls and engineered reinforced soil systems.