Perth retaining wall inspection, design guidance, structural engineering details & construction

In Perth, retaining walls are common in land development and residential subdivisions. The performance and durability of the wall will not be affected if it is constructed in compliance with certified structural engineering documentation. It is possible to incur costly rectification costs if minor structural engineering details are not included in the construction.

Early signs of structural failure or serviceability can be seen in retaining walls that show visible bulging, cracking or movement. Structural failure could be severe if the retaining wall supports additional structures, such as a house. A professional structural engineer should inspect and provide a written report on structural adequacy. (For projects in the Perth Area, we can provide a report within three business days of our site visit). Here are some common failures for cantilever pole-retaining walls (such as concrete panels or posts).

There are many design options available when building a retaining wall. It is important to determine the best construction method for your site based on the budget, site conditions and application.

Before you design the retaining walls and confirm the type (concrete post and panel, limestone retaining walls, mass brick, etc.), you must assess the environmental factors and the locations where they will be placed.

To build a retaining wall, you need to plan well and carefully layout. This will reduce cracking and ensure long-term performance.

This article will discuss some of the important factors to consider early in the design process and offer guidance on choosing the right type of retaining walls for your project.

Get in touch with your local Council.

Before any construction begins, it is good to contact the local Council for the design regulations. The Council will determine if retaining walls greater than 500mm or near surcharge loadings, such as houses or roads, should be designed and certified. A structural engineer inspection may be required in certain circumstances.

Confirm the location of services

You should have an accurate understanding of your property lines and underground and above-ground utilities such as stormwater management systems and the reticulation when choosing a location to build your wall. A professional structural engineer should consider the location and ensure that the builder submits a Dial Before you Dig inquiry.

Consider drainage requirements

Depending on its height and size, a retaining wall may have downstream environmental effects. The structural engineer should consider surcharge loads. The structural engineer will need to check compliance with Australian Standards. They will also assess whether additional loading from driveways, parking lots, fencing or swimming pools should be considered in the design. It is also important to consider temporary construction equipment.

Soil Classification

Common footing types, such as stump pads, slabs, and strips, require a soil classification. It must be examined to ensure the soil supporting the wall’s foundation (or base) is strong. The soil classification will tell you if the soil is clay or sand. Different soil types can react to moisture changes and have different levels of reactivity. Soil classification will impact the design and construction of retaining wall designs. Additional requirements will be required for clay sites to ensure that the retaining walls can withstand moisture changes.

Building Permit

For retaining walls with heights greater than 500mm, it is necessary to have engineering drawings certified by a professional structural engineer. The retaining wall may be built subject to local requirements.

Compaction Testing:

Compaction must be checked before any construction is begun. The compaction of sand sites is measured with a Perth Sand Penetrometer (PSP).

Sometimes, the local Council or shire will require a compaction certificate. It is a good idea to have a professional structural engineer conduct a compaction test before constructing the wall. We have seen instances where the failure of retaining walls was due to poor compaction. In our article, Compaction Testing Certificate Using the Perth Sand Penetrometer, you can find more information.

Let’s look at some of the most common retaining walls in the Perth region:

Small Retaining Walls – Up to 800mm

Many products can be used to make small retaining walls. These products are ideal for garden walls and other communal spaces. You can use it to divide and highlight BBQ areas, pathways, gardens, hedges, and garden beds. You can find various retaining wall blocks at hardware shops or specialist brick makers.

Precast concrete panel and post retaining wall – up to 3000mm

Concrete panels and posts are a great option for cost-effective retaining walls. Posts and panels are locally manufactured in Perth and come in various colors, patterns, and textures. The posts are more susceptible to surcharge loading, so it is important to ensure adequate compaction to prevent excessive leaning and rotation. You can retain up to 3.0m soil with steel posts (typically 200UB18)

These types of retaining walls are popular in Perth and the South West. In most cases, panel and post concrete retaining walls are an affordable alternative to brick or limestone walls.

These retaining walls are often installed along boundary lines and must be constructed to not encroach on neighboring properties unless written consent is obtained from the adjacent landowner.

These walls can be installed quickly and performed well on sand sites if adequate compaction is achieved. Considerations should be made if any existing structures are nearby or if construction is to occur in reactive clay.

Limestone Retaining Walls: Up to approximately. 8000mm

Keystones are required for limestone retaining walls. Limestone retain wall profiles have a particular cross-sectional profile. The height of the retaining walls and their base width are also important.

Limestone retaining walls can be constructed from either natural or reconstituted blocks that measure 350x350x350x1000mm. These walls are essentially gravity retaining walls. They have become extremely popular in the Perth Region because of the availability of materials.

These retaining walls rely on the soil’s weight to hold them up. This is a common practice to counter any vertical deviations over time.

Limestone retaining walls have many advantages, including long durability and easy maintenance. There are also a variety of colors and finishes. From a structural engineering standpoint, the major advantage of limestone retaining walls is their ability to be economically designed to resist structures such as swimming pools or residential buildings. Before constructing a retaining wall, it is important to obtain a compaction certificate and have the wall inspected by a structural engineer before back-filling.

Construction of a swimming pool next to a retaining wall

To maximize land, many Perth subdivisions use boundary retaining walls. This can often lead to flat or terraced lots. A swimming pool constructed near an existing structure such as a screen wall or boundary retaining wall must be considered to ensure safety. RBC can help with structural engineering and construction feasibility for retaining walls and swimming pools on challenging sites.

It is important to inspect the retaining walls before a swimming pool is placed within proximity of an existing one. Excavation may be restricted by earthworks or retaining wall profiles. It is recommended to keep a minimum of 1.5m between limestone retaining walls and the wall. It is important to consider the angle of repose and the angle to undermining depending on whether the swimming pool will be on a high or low side. The angle of repose in Perth sand sites is generally around 34 degrees. We may be able to help you with site optimization and other recommendations.

For most situations, we can produce architectural drawings, including the elevation and plan view. These drawings are acceptable for submission to Council.

Retrospective approval for an existing retaining wall and structural certification

Many clients have had us assist them with retroactive approvals of existing retaining walls. If the retaining wall in perth was well constructed from the beginning, it could likely be approved. A structural engineer should inspect the existing structure to determine if it is suitable for certification (compliance with the latest version of the National Construction Code).

A letter should be sent to the inspector with information about the structural condition and recommendations for structural design work that would improve structural adequacy. Our site inspection may require modification to an existing retaining wall.

Rotaru Building Consultants will conduct the required structural analysis, calculations and mark-up specifications if the retaining wall inspection shows that the structure is suitable for certification. We are also available to assist in the engineering drafting/retaining wall profile and as constructed certification.

The Local Government Authority would require a site plan. These plans can usually be created in a simple format. We can coordinate the inspection of a Building Surveyor to get the BA18 Certificate of Building Compliance.

Conclusion

It is not unusual to find retaining walls in a state that is unsafe or unserviceable during a pre-purchase structural building inspection. Our team has extensive experience in inspecting, designing and supervising the construction of various types of retaining walls.

We can provide standard engineering drawings for typical limestone retaining walls, concrete panels and posts, concrete blocks, and retaining wall brick cavities filled retaining walls. If required, we can provide architectural plans or drawings. This includes a plan view of the proposed wall and all dimensions. An elevation view (1:100 scale) showing the proposed retaining walls, the heights, and existing ground levels at the boundary. This is tailored to each Council & Building Permit application.

Hiring a qualified structural engineer is important to conduct a pre-purchase structural inspection. This will provide a written report as well as identify any structural problems. We look forward to helping you with certified structural engineering details and structural or retaining walls inspections in Perth & South West.

 

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