concrete retaining walls melbourne Archives

A Concrete Retaining wall can be described as a structure that retains the lateral pressure – usually formed due to soil. In simple terms, it is nothing but a wall that holds the soil on one side of it so the desired changes of the ground level elevation can be achieved. A difference of Elevation levels can be seen on the either side of the retaining wall in the illustration. A basement wall, for instance, is a retaining wall.

Concrete retaining wall terminology.

Retaining Walls

Retaining walls can be made up of materials like stone masonry, timber, steel etc. However, the most common material is concrete for retaining walls construction owing to its durability, adaptability and availability. It can be moulded in to any shape and size, can be ordered from concrete plants or made on the site – also called insitu concrete retaining walls.

Commercial retaining wall contractors stripping the formwork off the walls.

Concrete Retaining Walls

Concrete retaining walls can be of different types based on their working methods and type of construction. These include, but are not limited to, Gravity Wall, Cantilever wall, Piling Wall and Anchored Wall. On a smaller scale of retaining walls, which is where smaller lateral loads are applied, block retaining wall and sleeper retaining wall – made of concrete retaining wall blocks and concrete retaining wall sleepers respectively – are also common.

Gravity Retaining Wall

A gravity retaining wall uses its self-weight to counter the lateral loads coming from soil. For this reason, the gravity walls are usually big and therefore heavier as compared to other retaining walls. It can be used for retaining walls as high as 3 metres.

Gravity Retaining Wall with design guidlines.

Construction is pretty simple and can be done with plain concrete only. A proper drainage system should be provided to avoid the lateral pressure caused due to the water in soil.

Commercial Concreting

Although Concrete can be prepared on-site it is better to use the services of Commercial Concreting companies. The concrete provided is designed in a proper manner keeping the right amount of water, cement, fine and coarse aggregate along with any additives, if needed. They provide concrete based on Concrete’s compressive strength which is usually difficult to know when concrete is prepared on-site, unless very strict precautions are taken and a sample has already been checked in the lab.

Generally, the use of Commercial Concreting makes the construction of concrete structures much easier and more reliable.

Cantilever Retaining Wall

A cantilever retaining wall, as the name suggests, is based on cantilever technique where a slab is put underground to support the stem (see illustration).

The cantilever design of retaining wall enables the use of a thinner wall stem. The wall usually uses in-situ reinforced concrete with a toe section and a heel section running under the earth fill. The instillation of the steel reinforcement is critical for this type of wall.

Since the cantilever retaining wall is of a smaller section as compared to gravity walls, reinforcement is provided in normal conditions. Alternatively, Pre-cast concrete and Pre-stressed concrete can also be used for the construction. It can used for retaining walls up to 10 meters of height.

Construction, however, is not as simple as compared to gravity retaining walls. And, a suitable drainage system is provided to avoid to extra lateral pressure that would be caused due to the water available in soil.

Piled Retaining Wall

Piled retaining wall is nothing but reinforced concrete piles driven next to each other in the ground up to a certain depth that is enough to counter the lateral earth pressure. A gravel fill is usually provided for the drainage of water.

Construction site on the edge of a river to protect riverbank collapse with concrete pile foundation and concrete pipeline for water flow.
Retaining wall construction.

Sheet pile retaining walls are also used in a similar manner. These kind of piles can be used for the retaining walls which are up to 6 meters in height. The construction requires heavy machinery for the drilling or driving of piles in the ground. Construction is complex and therefore requires technical knowledge.

Anchored Retaining Wall

Anchored retaining walls are usually provided where a high retaining wall with a thin cross section is required. Wires or deep cable rods are driven deep sideways and then anchored by pouring concrete on to them. These tiebacks or anchors act against the sliding and over-turning of the retaining wall.

Similar to other methods, a drainage system is provided to avoid excess lateral pressure caused due to the water.

An anchored concrete retaining wall.

Gravity Retaining walls, Cantilever Retaining Wall and other described above are the kinds of retaining walls that are supposed to take high lateral loads. These kind of walls usually require an engineer and a technical team to design the retaining walls which would stand against sliding and over-turning failures.

Small retaining walls, usually up to 1.2 meters (4 feet), can be constructed and used on a private property with little expertise. All you’ll need is time and materials to construct these retaining walls. Such small retaining walls are constructed to hold back the garden soil and for other smaller lateral loads.

Concrete Block Retaining walls and Concrete Sleeper Retaining walls are possibly the best choices for such tasks.

Concrete Block Retaining Wall

A concrete block retaining wall is nothing but a set of concrete blocks stacked on top of each other. But, in order to make it a retaining wall that lasts for years and can take loads in a safe manner, some construction techniques should be utilized.

A Concrete Block Retaining wall under construction.

In situ concrete stairs and block walls.

In situ concrete stairs and block retaining walls.

Concrete Block Retaining Wall Construction

First of all, a trench of certain height should be dug and compacted. A layer of crushed stone base, 15 mm 20 mm stones in size, should be provided. This requires less compaction and is good for the drainage.

Concrete blocks, or hollow concrete blocks, are then stacked in the form of a masonry so that all the joints should not come in the same vertical line. This would allow the retaining walls to resist vertical loads as well.

Depending on the concrete blocks you are using, the can come up with butt joints or locking flange to avoid sliding of one block over the other.

The same stones that we used for the stone base can be provided on the back of retaining wall for the drainage of water to avoid extra water pressure.

Concrete Sleeper Retaining Wall

Concrete Sleeper Retaining wall is very similar to concrete block retaining wall. In this retaining wall, concrete sleepers are used instead of concrete blocks. A number of companies provide factory-made sleepers and vertical posts.

An example of a concrete Sleeper Retaining Wall. – Photo Credit – Bunnings.

Concrete Sleeper Retaining Wall Construction

Site should be cleared and levelled. Measurements should be made and auger holes should be dug for a depth of around 1.4 meters for a retaining wall of 1 meter height. A deeper hole would be required in case the retaining wall is higher than 1 meter. The posts should then be concreted into the ground so they are secure. A Concrete of around 20MPa strength would do the job.

Once the concrete is hardened and posts are in their proper place, sleepers should be paced. Seal the back of sleepers with a plastic sheet or with a concrete layer. A perforated drainage pipe should be provided to run along the length of wall. A drainage material such as crushed stone of around 20 mm in size should be provided over the pipe.

After the placement of sleepers, one should wait for around 7 days for the concrete to gain strength after which it is okay to pour the backfill material.

Three Best Types of Concrete Retaining Walls

Since there are different uses for each Concrete Retaining Wall mentioned in this text, it is difficult to choose the best three types. However, if we grade them based on their simplicity of construction, we can list three.

Concrete Block Retaining Wall
Concrete Sleeper Retaining Wall
Concrete Gravity Retaining Wall

Afore-mentioned retaining walls offer simple construction methods and can be constructed without any technical assistance, if their usage is limited to private properties and shorter heights of around 1.2 metres.

Concrete Retaining Walls in Commercial Landscaping

In commercial landscaping there is a need to be able to produce a large quantity of strong retaining walls economically as well as maintaining the required aesthetic qualities. To achieve this In-situ Concrete walls are a good choice for their low maintenance and cost effective installation costs. Concrete block retaining walls are also widely used in commercial landscaping.

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References and Further Reading

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Installation Guide

Trojan Sleeper Installation

In-situ Concrete, also known as insitu concrete, is an outstanding landscaping construction technique. In terms of long terms long term quality, robustness and longevity very few landscape construction techniques come close. To many people in-situ concrete is better know as landscaping architectural concrete and it is the point where landscaping, architecture and civil engineering all meet in landscape design.

For the landscape design of public spaces, architectural concrete or in-situ concrete is a great choice for the severe use of our landscaped parks, gardens or commercial buildings. For these reasons, insitu concrete is now widely used by commercial landscapers in Melbourne.

Some other names for in-situ concrete include “poured in place” concrete or “cast in place” concrete”.

What is in-situ concrete?

In-Situ Concrete cast in place.

In-situ is concrete that as poured into formwork at the building site. When compared to pre-cast concrete, in-situ requires no crane or forklift to get it into position.

In comparison with concrete block and rendered retaining walls, the very low porosity of sealed and vibed in-situ walls will ensure a quality finish for decades of use. With concrete block retaining walls, the blocks themselves will have some porosity which will allow moisture to creep under the rendered surface and may cause it to lift in a few years. In-situ concrete has the colour mixed in and the texture is provided by the formwork but it can be rendered if required.

In-situ Concrete pumped into the formwork through a boom.

In-Situ Concrete – strong connection to footings.

An additional advantage with in-situ is it has a very strong and stable attachment to the very solid footings underneath. This is due to the reinforcement bars coming up from the footings and overlapping with the in-situ concrete steel reinforcement. When compared with concrete block retaining walls, although the steel reinforcement runs up through the centre of the blocks, any voids in the concrete around the reinforcement can lead to corrosion of the reinforcement bars and concrete cancer. The homogeneity of the of the vibed in-situ walls also ensures a good attachment to the steel reinforcing bars and does not suffer the risk of a poor attachment of the core concrete to the inner void of the concrete blocks.

Pre-cast concrete will usually require mechanical fixings to the footing. These fixings can corrode over time or can be ground off by vandals. Being securely attached to the footings is a big advantage of in-situ concrete. Newly constructed public spaces such as parks and gardens will often rely on these quality advantages of in-situ concrete.

Retaining wall footings with steel reinforcement protruding to give the retaining wall a strong connection to the footing. The reinforcement bars must be accurately placed.

Precast concrete versus in-situ in landscaping concrete architecture.

One of the advantages of precast concrete is that it is manufactured under controlled conditions in a factory. This ensures a consistent quality product. There can be a risk however of coloured concrete coming from different batches resulting in slightly different colours close together. The quality advantages of precast concrete can be matched by in-situ concrete with close attention to the details and quality control of the processes.

Quality in-situ concrete retaining walls, stairs and seating.

Commercial Retaining Walls Melbourne

Good quality in-situ concreting requires a lot of attention to the materials and the processes. In-situ concrete wall also need to consider the safety as well as the aesthetics of a public space or residential landscape construction project. A well constructed, quality, curved in-situ concrete wall will achieve both of these aims. For this reason always employ only skilled commercial concrete contractors.

Stripping the formwork off the walls. Extra effort and attention to detail will result in a quality finish on the retaining walls.

Quality In-situ concreting footings.

Concrete sets by a chemical reaction and not by drying. The chemical reaction is ecothermic and water is part of the reaction. If there is a difference in temperature within the concrete or if the water evaporates quickly from the top surface, then cracking can be the result. Concrete footings should not be poured in freezing conditions or below 5 degrees. Fortunately, in Melbourne there are very few days when it is too cold to pour. In hot weather the concrete should not be poured during the hottest part of the day or in extremely hot weather. The top part of the footing can be kept moist with hessian or a light sprinkling of water to prevent the top layer of the footing being weaker.

Concrete reinforcement bars in the footings.

It is essential that the vertical bars coming out of the footing are located accurately. When the in-situ walls are poured, these bars need to be closed to the centre of the retaining wall with good overlap with the wall reinforcement. The concrete reinforcement within the footing should be encased within the concrete as much as possible to minimise the paths for moisture to get into the reinforcement.

Concrete footing design

If designing for in-situ concrete retaining walls, the concrete footing should be a suitable size for the wall, keeping in mind all of the loads on the wall including hydrostatic and mass of the wall. The design of the footings must be to Australian Standards.

Formwork for in-situ concrete retaining walls.

Quality in-situ walls require formwork that is smooth, strong and flexible. Any defect, imperfection of inaccuracy in the formwork will show up in the surface of the wall. The formwork for commercial concrete construction needs to be strong enough to resist the weight of the wet concrete without bulging or deforming. Deflections in timer formwork will show up as ripples in the finished wall. A bulge in the formwork due to the hydrostatic load of the concrete will be a disaster when removing the formwork from the wall. The wall should be designed with a slight draft angle to make the removal easy without causing any damage to the retaining wall.

Strong supports for the formwork keep the wall dimensionally correct.

Accurately positioned formwork under construction.

Pouring the concrete retaining walls. Using the concrete vibrator to get a quality finish and good adhesion to the reinforcement.

Commercial Concreting. Stripping the formwork after the concrete pour.

Stripping the formwork after the concrete pour. The top tip from commercial concreting is that the formwork should be left on as long as possible to prevent the concrete drying excessively during curing. Taking the formwork off too early can result in microcracks in the concrete.

Commercial Concreting Melbourne at our Balwyn North Construction site. Stripping the formwork after the concrete pour

Stripping the formwork after the concrete pour and curing to show a quality result.

In-situ concrete steel reinforcement

The steel reinforcement bars must have a good overlap with the footing reinforcement bars and well encased within the concrete wall. Reinforcement too close to the surface may result in water ingress and concrete cancer.

Pouring the in-situ concrete.

The concrete walls should be poured in one go and definitely from the same batch of concrete. Any interruption of the pour may show up as a line in the finished wall. The freshly poured concrete should be thoroughly vibed to ensure there are no voids within the concrete especially on the outer surfaces or at the interface with the reinforcement.

The steel reinforcement within tin-situ also helps to prevent surface cracking. To minimise the surface cracks, we leave the formwork in place a little longer to keep the moisture in during curing. The concrete should then be given a light sprinkle of water once the formwork is removed as it is still curing. The concrete will not be fully cured for a few weeks so consider this before applying any excessive loads.

Landscape and Concrete Design

Landscape Design with in-situ walls.

In-situ walls give the landscape designer or the landscape architect enormous freedom to design shapes for retaining walls, seating or concrete stairs that would be very difficult, if not impossible, using precast of other landscape construction techniques. In-situ walls can be designed into three dimensional shapes that would be impossible or expensive to do as Pre-cast. Curved retaining walls on an uneven landscape would be very difficult to achieve with any other landscape construction technique. Another advantage is the wide range of colours available.

In-situ concrete curved seating part of our commercial concrete construction Melbourne portfolio.

In-situ concrete curved seating. Shapes like this are difficult with other landscape construction techniques.

Concrete stairs Cast in Place. This process is ideal for intricate or bespoke designs.

Exposed aggregate path and in-situ curved garden retaining wall. Overflowing Australian native grasses in the garden bed soften the look of the concrete. The exposed aggregate path provides an attractive contrast to the wall and plants. It is also a safe non-slip cost-effective solution.

In-situ retaining wall and steps

Curved In-Situ retaining wall prior to the final touch up and surface treatment. The joins in the formwork will be visible until the concrete is ground back to an even join.

As the concrete is pumped in, damage to other landscaping structures can be avoided, making it easier for the landscape project planner to schedule the construction. These are important factors to consider when landscaping Melbourne public spaces.

A very straight In-Situ retaining wall.

Exposed aggregate pathway and in-situ retaining wall prior to filling and grinding by the concrete finisher. There will always be a few small voids to fill, but theses can be minimised with the vibe during the pour.