Retaining walls are commonly used in Perth particularly on residential subdivisions and land development. If the construction is undertaken in accordance with the certified structural engineering documentation, there will be no issues with the performance or durability of the retaining wall. However, if even minor structural engineering details are omitted during construction it can lead to costly rectification works.
Visible leaning, cracking, movement or bulging in a retaining wall can indicate early signs of serviceability or structural failure. If the retaining wall is supporting additional structures such as a house, the consequences of structural failure can be significant. If you are concerned about leaning or cracks in a retaining wall, you should engage a professional structural engineer to inspect and to provide a written report in relation to its structural adequacy (for Perth Area projects we can provide a retaining wall inspection report within 3 business days following our site visit). As an example, common modes of failure for cantilever pole retaining walls (such as concrete panel and post) are as follows:
When designing and building a retaining wall, there are a variety of design options and materials to consider. Depending on the application, site condition and budget it is important to consider the construction methodology that would work best for the site.
Before designing the retaining wall & confirming the type (concrete panel and post, limestone retaining wall, mass brick et cetera), it is important to assess the location and environmental factors that could have an adverse effect on the retaining wall.
Building a retaining wall takes advanced planning and careful layout to reduce cracking and ensure long term performance.
This article will explore some of the factors that need to be considered early in the design phase along with guidance on choosing the best type of retaining wall for your project.
Check with your local council
It is recommended that the local council is approached initially for design regulations prior to the construction. Depending on the specific requirements, the council will advise if retaining walls higher than 500mm and walls located near surcharge loading such as a house or a roadway be designed and certified by a professional structural engineer. In some circumstances, if a retaining wall is constructed without council approval, a structural engineer inspection would be required in order to obtain a retrospective building approval.
Confirm location of existing services
When choosing a location for your wall, ensure that you have a detailed understanding of property lines and both above ground and underground utilities including stormwater management systems and reticulation. If you are using a professional structural engineer, it is standard practice to consider the location of the sewer line and as a precaution, a Dial Before You Dig enquiry should be submitted by the builder.
Consider drainage requirements
Depending on the wall size and height, a retaining wall can impede on natural drainage patterns and have environmental consequences downstream. Surcharge Loads need to be considered by the structural engineer. This would involve a compliance check with the relevant Australian Standards and assess if additional loading from fencing, guardrails, driveways, parking lots, or swimming pool needs to be factored into the designs. Loading from temporary construction equipment also needs to be considered.
A soil classification is required for common footing types - stump pads, strip, slabs that are often used in most housing construction. The soil that creates the foundation, or base, needs to be examined to ensure that it meets the strength required to support the wall. The soil classification will confirm if the soil is sand or clay which can have various levels of reactivity to moisture changes. The design and construction of retaining walls will be impacted by the soil classification. On clay sites, additional requirements are needed to ensure that the the retaining wall performs well with moisture changes.
A retaining wall with a height greater than 500mm usually requires engineering drawings to be certified by a practicing structural engineer. Subject to council requirements, after the certified engineering details are finalised, a building permit is obtained and consideration is given to aspects such as health and safety, structural engineering, environmental and heritage requirements in line with the local government and Building Code of Australia, the retaining wall can be constructed.
It is very important that compaction of the soil is checked before construction. For sand sites compaction is checked with a Perth Sand Penetrometer (PSP).
In some cases, the local shire or council will require a compaction test certificate. Engaging a professional structural engineering to undertake a compaction testing prior to the construction of the retaining wall is prudent. In our experience, we have encountered instances were the failure of the retaining wall was attributed to poor compaction. For more information refer to our article titled Compaction testing certificate using the Perth Sand Penetrometer (PSP).
Let’s take a look at some common retaining walls used in the Perth area:
Small retaining walls - up to 800mm
For small retaining walls there are a variety of proprietary products on the market design is ideal for low, vertical landscaping walls in garden and communal areas. It is often used to separate and highlight entertaining areas, BBQ areas, paths, garden beds, hedges, or to create and differentiate levels. A range of retaining wall blocks can be found at hardware stores or specialist brick manufacturers.
Concrete pre-cast panel and post retaining walls - up to 3000mm
If you are looking for a cost effective retaining wall solution you may consider a concrete panel and post retaining wall. Locally manufactured in Perth, posts and panels provide a range of colours, patterns, and textures to suit any architectural requirement. However, the posts are much more sensitive to surcharge loading and adequate compaction is required to prevent rotation of the footing and excessive leaning. Steel posts, typically 200UB18 sections can be used to retain up to 3.0m of soil.
These types of retaining walls are widely used throughout Perth and the South West. A panel and post concrete retaining wall is in most cases an economical alternative to brick and limestone retaining walls.
Often these types of retaining walls are installed along boundaries and they must be constructed in a way that prevents encroaching the neighbouring property unless consent if obtained in writing from the adjoining landowner.
These types of walls are generally quick to install and can perform quite well in sand sites provided that adequate compaction is achieved. Additional considerations will apply if existing structures are close to the walls or if the construction is to take place in reactive clay site.
Limestone Retaining Walls – up to approx. 8000mm
Limestone retaining walls are typically constructed from natural or reconstituted blocks which are 350mm x 350mm x 1000mm long. This is essentially a type of gravity retaining wall and are become very popular in the Perth Region due to the availability of materials.
Essentially these retaining walls use their own weight to hold the soil behind them. A cross-fall or a lean toward the soil being retained is a standard practice that counteracts any deviation from verticality that may occur over time.
Advantages of limestone retaining walls include long durability, easy maintenance and there is a range of colours and finishes. Form a structural engineering perspective, a major advantage is that the limestone retaining walls can be economically designed to withstand structures such as residential building or swimming pool either on top of the wall or in close proximity. It is recommended that a compaction test certificate is obtained before constructing the retaining wall and that the wall is inspected by a structural engineer before back-filling.
Construction of a swimming pool next to a retaining wall
Many land subdivisions in Perth implement the construction of boundary retaining walls to maximise the land and often result in flat, terraced lots. Construction of a swimming pool in close proximity to an existing structure, such as boundary retaining or screen wall, requires consideration to ensure that the excavation works are undertaken safely. RBC can assist with the structural engineering and construction feasibility of retaining walls or swimming pools in challenging site conditions.
In the case of placing a swimming pool in close proximity to an existing retaining wall, it is recommended that a retaining wall inspection is undertaken. The earthworks and retaining wall profile may restrict excavation, due to the possibility of undermining the existing retaining wall. With limestone retaining walls it is recommend to maintain a clearance of at least 1.5m from the retaining wall. Depending on whether the swimming pool is to be located on the high side or low side of the retaining wall, consideration in relation to the angle of repose and the angle of undermining is of critical importance. Generally, in Perth sand sites, the angle of repose is approximately 34 degrees. From our site inspection we may be able to assist with further optimisation and recommendations for your site.
In most situations, we can also produce the architectural drawings including the elevation(s) and plan view for the proposed retaining wall or swimming pool as part of our engineering certification which are suitable for Council submission.
Retrospective approval and structural certification for existing retaining walls
We have assisted many clients with retrospective approvals for existing retaining walls. In most cases it is very likely that an existing retaining wall can be approved if it was constructed well from the start. The best approach is the have a structural engineer to conduct a retaining wall inspection to assess the existing structure and to confirm if it can be certified from an engineering perspective (certification for compliance with the current version of the National Construction Code).
After the inspection a letter should be provided with advice regarding the current structural condition and recommendation for additional structural design work to achieve structural adequacy, as required. It is possible that modification to the existing retaining wall may be required subject to our site inspection.
If the retaining wall inspection confirms that the existing structure can be certified, Rotaru Building Consultants can undertake the necessary detailed structural analysis, calculations, mark-up specification and produce the required documentation with Chartered Engineer Certification. We can also assist with the engineering drafting / retaining wall profile and as-constructed certification.
The Local Government Authority would normally require an existing site plan which in most cases can be produced fairly basic format. An independent Building Surveyor Inspection is also required which we can coordinate to obtain the BA18 Certificate of Building Compliance.
During a pre purchase structural building inspection, it is not uncommon to find retaining walls that are in a state of failure or serviceability failure (for example when a retaining wall is leaning and exceeds the required tolerances). We have extensive expertise in designing, supervising construction and inspecting various types of retaining walls.
We have standard engineering drawings of typical limestone retaining walls, concrete panel and post, concrete blocks retaining walls and brick cavity filled retaining walls and we can provide the documentation certified / signed by a practising Structural Engineer. We can also assist if required with architectural plans or drawings such as plan view of the proposed retaining wall including all dimensions of the proposed structure. Elevation view (1:100 scale) of the proposed retaining wall, heights of the proposed retaining wall, existing ground levels at the boundary and the proposed finished ground levels and specifically tailored for each Council & Building Permit Application.
As always, if you are purchasing a home in Perth, it is essential to engage a qualified building inspector such as a structural engineer to undertake a pre purchase structural inspection to provide a written report and to determine if any structural issues are present. Our team is looking forward to assisting you with certified structural engineering details & structural or retaining wall inspections in Perth & the South West.
Rotaru Building Consultants
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