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Hello @sujoy_kb
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about footing for a raised concrete garden.
Can I please ask for a little bit more information in regards to the materials you'll be using for your project? What are the dimensions of the limestone bricks you are planning to use? How heavy are the limestone bricks (this is important when digging for the foundation). Please confirm the size of the planter 15m long x 0.5m wide and 1m tall are these measurements correct? Due to the size and weight of the planter it is almost like a concrete fence wall. Have you considered making it smaller down to 0.5m tall?
As soon as you provide us with more information, we'll be able to offer recommendations in regards to the foundation.
If you have any other questions we can help with, please let us know.
Eric
Hello @sujoy_kb
To build a stable and durable footing for your revised garden planter measuring 15 metres long, 0.25 metres wide, and 0.5 metres high on Class C sandy soil, I suggest following this step-by-step approach that balances structural integrity with practicality for a DIY project. Since the soil is sandy and thus prone to shifting, it's crucial to establish a well-compacted base and use materials that can accommodate some degree of ground movement without cracking.
Start by marking out the trench for the footing along the entire 15-metre length and around 350 mm wide, this gives a little room on each side of your 250 mm wall. Excavate the trench to a depth of approximately 300 mm. This depth is suitable for a structure of your planter's size on Class C soil and ensures the footing is deep enough to resist minor ground movement. Compact the base of the trench firmly using a hand tamper or mechanical plate compactor to reduce the risk of settling.
Next, lay a 100 mm deep layer of crushed rock or road base at the bottom of the trench and compact it thoroughly. This forms a solid sub-base that helps with drainage and further stabilises the foundation. Then, construct a formwork along the edges of the trench using timber boards to shape the concrete pour. The formwork should match your footing’s final dimensions, typically 300 mm wide and 200 mm deep above the crushed rock, giving a total of about 300 mm depth when combined.
Use a reinforced concrete mix for the footing to prevent cracking and ensure long-term stability. You should incorporate steel reinforcement, specifically, a single N12 or N16 rebar along the length of the trench with small stirrups or corner bars every 600 mm to help resist any soil movement or expansion stresses. Pour standard concrete mix into the formwork, filling to the top, and use a shovel to agitate the mix and remove any air pockets. Level the surface with a screed board and trowel it smooth.
Let the concrete cure for at least 48 hours before removing the formwork, and wait at least a week before starting bricklaying to allow it to reach sufficient strength. For added protection against moisture and expansion, consider placing a damp-proof membrane (such as plastic sheeting) between the footing and the first course of bricks. Also, use a flexible, high-quality mortar mix that includes a plasticiser to allow some movement and reduce cracking over time.
This process ensures that your limestone brick planter has a strong and flexible foundation that can accommodate minor soil shifts, avoiding cracking and providing lasting support for your garden structure.
I also suggest having a look at this discussion - Solved: How to lay bricks for raised flower bed? by @Brunnie_Rich.
If you have any other questions we can help with, please let us know.
Eric
Hello @sujoy_kb
Given the updated size of your planter box 15 metres long, 0.30 metres wide, and 0.90 metres high, I propose consulting a concrete or structural specialist, especially if this is your first time working with reinforced concrete or a structure of this scale. While the construction is still within DIY reach for someone with moderate experience, the height and length do introduce higher loads and stress points that make proper footing and reinforcement important to long-term stability.
A specialist can assess the specific conditions of your site, such as actual soil compaction, drainage patterns, and any nearby structures that might influence ground movement. They can confirm the appropriate depth, width, and reinforcement details for the footing, which is particularly important for sandy Class C soil that may behave unpredictably if not compacted or supported properly. Additionally, a specialist can advise on expansion joints, drainage considerations, and ensure your design complies with any local building codes or restrictions even if council approval isn’t technically required for a planter box.
Plus, engaging a professional for consultation can give you peace of mind that your footing won’t shift, crack, or cause problems over time. It’s a small investment that can prevent costly fixes later on.
If you have any other questions we can help with, please let us know.
Eric
