Is a retaining wall really needed? Is a consent needed? Does the Building Code apply to the construction of retaining walls? Find out all this and more below
Some retaining walls are exempt from consent, while others are not. Read more about when a consent is required, what to consider from a sustainability perspective and design and construction requirements.
Consider whether a retaining wall is needed at all. Is it really necessary to disturb the earth? Stripping of topsoil and excavation may increase erosion and sedimentation entering waterways. Alternative measures may result in significantly less environmental impact.
Strategies for design and construction:
Minimise the need for high retaining walls by careful design of grading of ground levels in the overall project.
Consider using plant materials, erosion control matting or other measures to retain or stabilise slopes. Erosion control matting is often manufactured from recycled materials. Good proprietary systems are available.
Select materials with recycled content or with low environmental impact (crushed concrete aggregate, stone, timber) for construction of retaining walls.
Consider crib-type retaining walls that can be planted to help minimise run-off to waterways and increase the biodiversity of the site.
Use locally sourced materials such as stone and timber for retaining structures.
All materials have their pros and cons:
Timber retaining walls rely on timber treatment (usually CCA) for durability.
Dry stone walls can be built with low environmental impact, but they should be used with care because of the possibility of earthquakes.
Cement and reinforcing steel in concrete and masonry walls involve high energy use in their manufacture.
Design and construction requirements
Any retaining wall that retains not more than 1.5m depth of ground and that does not support any surcharge (any load additional to the load of that ground, such as the load of vehicles on a road) does not require a building consent, but all work must comply with the Building Code (Bl Structure, F4 Safety from falling) requirements. Retaining walls require specific design where:
They are outside the above parameters.
They are built on or support expansive soils, peat or very weak soil.
The water table is within 500mm of the underside of the wall foundation there is sloping backfill above the wall.
A number of retaining wall material manufacturers provide standard design and construction details.
The two main types of retaining wall are gravity wall and cantilever wall. Options for retaining wall design include:
Timber poles or posts with horizontal rails or poles – horizontal rails can be rectangular in profile (usually 150 x 50mm or 200 x 50mm). The use of TG&V profiled rails with a grooved finish results in a neater and more precise finish compared to rough-sawn timber.
Vertical timber poles or posts close spaced – usually buried for two-thirds of their overall length, but best restricted to retaining walls of under 1m (depending of course on the cross-sectional dimensions of the timber used) cast in situ concrete.
Concrete masonry (concrete block).
Reinforced masonry (brick).
Crib wall systems in timber or concrete.
Interlocking precast concrete units – a range of design options is available.
Stacked stone, mortared.
Stacked stone drywall (without mortar, but limited to low walls without surcharge).
Stone veneer applied to concrete or concrete masonry structural walls.
Gabions (river stone stacked inside a metal cage) – these can range in size from small domestic-scale retaining walls to those used for highway construction.
Specific design and construction requirements applying to all retaining walls:
Determine the loads acting on the wall – consider whether the wall will be subject to a load other than the retained material.
Ensure there is sufficient space for the wall and its foundation system to be constructed, particularly when close to a site boundary. Retaining walls must not cross any boundary. They must also be set back from any bank or sloped ground in front of the wall.
Use a retaining wall material or system suitable for the height, surroundings, space and the aesthetic requirements of the client.
Provide drainage behind the wall (draining to an approved stormwater disposal system) to prevent the possibility of water pressure build-up.
For specifically designed walls, use a chartered professional engineer and build in accordance with the design drawings – always consult the engineer early in the design process.
Ensure there is sufficient bearing capacity in the soil to support the wall and prevent overturning. Ground that is too soft must be removed and compacted till added.
For manufactured wall systems such as crib wall or interlocking blocks, follow the manufacturer’s installation instructions.
Where a retaining wall is being built on a boundary line, permission of the adjacent owner must be obtained in writing before commencing work. When considering low walls not requiring a building consent:
• Follow the construction details supplied by the manufacturer for proprietary retaining systems.
• Keep within the manufacturer’s height limit for the wall.
• For crib walls, ensure the correct batter or slope to the base of the wall is created – typically 1:4 – and footing is provided.
For low timber walls using posts or poles:
• Use H5-treated timber or timber rated as durable in ground contact – although H4 can
be used, BRANZ recommends using the higher specification material.
• Embed vertical members 100mm into the ground for each 200mm of height.
• Embed in well rodded concrete.
This article was first published in BRANZ’s Landscape Construction book, Second Edition and is reproduced with permission.
The book covers the design, building and planning rules as they apply to landscape construction and provides guidance on design and construction to ensure good practices are followed. It can be purchased at www.branz.co.nz.