One reality of building in Coffs Harbour is managing sloping land with retaining walls. Retaining walls are structures designed to separate land into different levels – they literally “retain” earth in place so it doesn’t slide, fall, or wash down a hill.

Retaining walls may be used for aesthetic and convenience reasons, in applications like landscaping, property entries, and pathways. They may also be an integral part of a construction project, separating levels in a building, forming basement walls, and allowing for the use of sloping land.

Depending on the length and steepness of the slope involved and the nature of the material to be retained, a range of different retaining wall options are available. Each type of retaining wall has advantages and disadvantages. Some commonly used retaining wall materials are listed below:

Timber – pine or hardwood

Timber retaining walls are generally the cheapest option. Timber retaining walls may be constructed using hardwood or treated pine. Timber has an organic, natural appearance, so it complements gardens well.

Unfortunately, timber is a relatively unstable material when left fully exposed or buried in the ground, so timber retaining walls have limited lifespan, and may warp or move over time. Due to their limited lifespan, Coffs Harbour City Council does not permit the use of timber retaining walls more than 600mm high within 1m of a property boundary.

Dry stacked concrete block

For lower retaining walls, dry stacked concrete blocks are an excellent choice. These blocks are designed to mimic a natural stone block, and come in a range of colours. An Australian-made product like Hastings blocks from Austral may be used to create an attractive and effective wall.

As the name suggests, these blocks are designed to fit together like a lego set, without the need for mortar. They may be supported on a base of compacted road base or a concrete strip footing. These blocks can be used to create decorative curved walls or steps. For higher walls or walls near a boundary line, engineering design and council approval may be required. One drawback of this system is that it is difficult to install a fence directly on top of a dry stacked retaining wall.

Dry stacked concrete blocks are more expensive than timber, but are more durable.

Concrete sleeper

Concrete sleeper retaining walls are constructed by slotting precast concrete sleepers into steel rails that have been cemented deep into the ground. Concrete sleepers can be made to look like timber, sandstone, slate, or natural concrete.

With the use of special brackets, fencing may be installed directly on top of a concrete sleeper wall, so they are suitable for use along property boundaries (subject to obtaining the relevant approvals).

As with dry stacked blocks, engineering design and council approval may be required in certain circumstances. Concrete sleeper retaining walls are comparable in cost to dry stacked concrete block walls.

Concrete block retaining

Concrete blocks (sometimes known as “Besser blocks” or “cinder blocks”) are widely used throughout the construction industry in Australia to create strong, durable retaining walls. Each wall must be custom-designed by an engineer to suit the specific site conditions and purpose for the wall.

Concrete block retaining is definitely not a DIY task – it should be left to the professionals! A large and heavy reinforced concrete footing is poured first with reinforcing “starter bars” left protruding, then concrete blocks are laid over the starter bars. The blocks are designed with a slot to accommodate steel reinforcing running horizontally along each course. Once all the blocks are laid, reinforcing is placed in the vertical cores, and each core is filled with concrete. This technique produces an incredibly strong and solid wall that will last for decades.

A common complaint about concrete block walls is that they look industrial and bleak. Thankfully, options are now available with different colours and styles that mimic natural stone. Block walls can also be rendered and painted, although careful preparation is required.

Given their strength, concrete block walls can easily accommodate a fence, although your fencer will need special core drilling equipment to install fence posts into a block wall.

Core filled concrete block retaining walls are a more expensive option, but they are durable and can be designed to meet engineering standards in a broad range of domestic and commercial construction applications.

Natural stone

Natural stone walls were the first type of retaining wall used thousands of years ago, and they are still used to good effect throughout the world. There are many different types of natural stone walls.

One option is to use large sandstone blocks laid without mortar and held in place by their own weight. These blocks can be expensive, and an excavator is required to move such heavy blocks, so clear site access is necessary and you will have some cleaning up to do once the wall is built.

Another option is to have a stone mason lay mixed field stone, cut sandstone, or other stone in a dry stacked or mortared wall. The options here are endless, determined primarily by your preferred style and the local availability of materials.

The cost of natural stone walls depends on the type of wall, the style and availability of materials, and how much professional assistance is required.

Gabion walls

Gabion walls are made using steel mesh baskets filled with rocks. The baskets are stacked on top of each other, with each basket usually offset slightly back into the slope. Gabion walls can be built by hand, but a mini excavator or bobcat will speed up the process significantly.

In most instances, the gabion baskets are made using galvanised steel, which should stand up to the elements for many decades. The rocks used to fill the baskets can be of any colour or style you prefer, but angular rock is best, as it locks together and prevents movement. Using smooth river rocks is possible, but stronger baskets may be needed to compensate for movement in the rocks.

Over time, the gaps between the rocks in a gabion wall may fill with sediment and soil, providing a home for plants and weeds. In situations where this is undesirable, another retaining option should be considered. Erecting a fence on top of a gabion wall is also challenging.

The cost of gabion walls varies based on size, type of rock used, site access, and other factors.

Drainage

Retaining walls must be properly drained, because a build up of hydrostatic pressure can lead to the failure of a wall. The usual method of draining behind a retaining wall is by using a corrugated drainage pipe (“ag pipe”) in a bed of free draining aggregate rock protected by a layer of filter fabric.

Are you planning a new home on a sloping block? Every site is unique and requires careful consideration to determine which retaining option are most appropriate. Contact us on 1300 056 298 to discuss your options for retaining, designing, and building you new home.

 

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