A retaining wall includes any wall constructed of bricks, stone, reinforced concrete, concrete blocks, wood or concrete cribs, timber logs or any other materials designed to retain any excavation, earth fill or garden. Retaining walls may be constructed as sleeper walls (timber or concrete), crib walls (timber or concrete), reinforced walls (concrete or shotcrete) or weight walls (boulder or mass concrete or masonry). However, timber walls are vulnerable to timber pest (termite) infestation and weathering. Dampness may cause timber to rot significantly while it will increase the strength of young concrete and lime mortar.

Boulder retaining walls and crib walls occupy large spaces and the stability of a boulder retaining wall is highly dependant upon the skill of the excavator operator in choosing the right boulder for the right location. Footings for reinforced concrete or reinforced concrete block retaining walls can be extended either in front of or behind the wall. This can be extremely helpful in circumstances where space is limited in one direction such as in retaining walls at property boundaries.

The responsibility for the erection of the wall rests with the person who excavates or fills their land. It is not up to an adjoining landowner to contribute to the costs of a retaining wall, unless the wall is a common wall to support both cut and fill, for the benefit of both properties, on the common boundary.

In Queensland as per Section 21 of the Building Act 1975 (building work that is self-assessable for (IPA) and Schedule 1 of the Building Regulation 2006, a retaining wall is considered self-assessable i.e. it does not require a development approval for building work if it complies with

• There is no surcharge loading over the zone of influence for the wall where a surcharge loading is a load applied to soil that has the effect of consolidating it other than loads applied from people or vehicles moving over it or the effects of rain and the zone of influence for the retaining wall is the volume of soil retained behind the wall that affects the wall's structural integrity.
• The total height of the wall and of the fill or cut retained by the wall is no more than 1 meter above the wall's natural ground surface
• The wall is no closer than 1.5 meters to a building or another retaining wall.

Anything considered not self-assessable under the Building Act 1975 will require a Development Approval for Building Works to be issued by a Building Certifier and must be compliant with all relevant Building Codes. As the weight of soil behind a retaining wall can create massive hydrostatic pressures which may cause poorly designed walls to collapse, an engineering design should be prepared by a structural/civil engineer and/or the manufacturer's specification which includes engineered design specifications of the retaining wall. If an assessable retaining wall is constructed without first obtaining a building approval, an offence is committed against the Building Act (1975) and consequently, the Integrated Planning Act (1977), and heavy penalties apply.

Council Officers Consider

• Overall height of the proposed retaining wall
• Potential impacts on an adjoining property, particularly with respect to privacy or drainage
• Whether or not fencing is proposed on top of the retaining wall
• Potential for overshadowing of an adjoining property
• Potential impacts on drainage easements and/or overland flow paths

Material Change of Use (Planning Approval) will be required if the retaining wall involves cut and fill which

• Involves a total change of more than 2 meters relative to natural ground level at any point
• Occurs within 1.5 meters of any site boundary and a minimum of 4.5 meters clear of a canal boundary
• Exceeds 1 meter in height relative to natural ground level. Retaining walls below 1 meter in height can be built up to the side or rear boundary.
• Necessitates construction of a retaining wall exceeding 2 meters in height anywhere on the site.

Road Boundary Setback Provisions for Retaining Walls

On residential allotments over 450m, the minimum front road boundary setback is 6 meters if the retaining wall exceeds 1 meter in height above the natural ground surface.

On residential allotments under 450m the minimum front road boundary if the retaining wall exceeds 1 m in height above natural ground surface 3 meters for access streets and 4 meters for collector streets. Otherwise retaining walls below 1 meter in height can be built up to the front road boundary.

On allotments with a Plan of Development, setbacks are as per the Approved Plan of Development.

In all cases, retaining walls are also required to be located clear of any easements or Council infrastructure i.e. sewer mains, stormwater pipes, water mains (minimum setback from Council infrastructure is 1.5m for pipe diameters up to 150mm and 2m for pipe diameters over 150mm).

As per Section 75 of the Building Act 1975, provision must be made for drainage behind the retaining wall to prevent build-up of pressure which can be detrimental to the stability of the structure. Further, provision must be made to divert the surface water that would have naturally drained onto the allotment to an approved point of discharge. If soil conditions, ground levels, excavation or filling make it necessary to protect land, buildings or structures in the neighbourhood of building work then a retaining wall must be built or suitable methods used to prevent soil movement and drainage of the land, buildings or structures must be provided. Issues such as damp-proofing and waterproofing may also be critical.

Retaining walls perform a structural function and must be properly designed and constructed to avoid cracking, subsidence and collapse. All retaining walls require proper foundations, granular backfill and subsoil drainage including weepholes to avoid hydrostatic pressure build-up.

As an alternative to the construction of retaining walls, natural stabilisation techniques may be implemented. Naturally sloping land may be stabilised by trees, shrubs, grasses/ground covers, leaf litter, fallen timber and surface or subsurface rock outcrops. Grasses may be safely maintained on slopes of 3:1 or flatter while steeper slopes are best treated with appropriate ground cover vegetation.

Dry laid quarried rocks may be considered as an intermediate solution between random boulders or rock outcrops and solid masonry structures. However, use of small bits of broken bush rock set into coloured mortar should be avoided as this creates a totally unnatural effect. Besides, National Parks and Wildlife Service discourages use of bush rock in suburban gardens so as to preserve bush rock in its natural setting where it is important in protecting both plants and animals from bushfires and erosion.