How to build footing for raised concrete planter?

[sujoy_kb]

It is a 15m x 5m backyard where I want to erect a concrete planter made of limestone bricks with walls on all 4 sides. I want a long narrow planter of 15m x 0.5m x 1m. The base is sandy soil (class C), so please suggest how should I construct the footing as it is a DIY. Please also suggest what material should I use for the footing so that it doesn't crack due to expansion and contraction.

[JacobZ · Accepted solution]

Hi @sujoy_kb,

No, this would not change the fundamental design of your footing. Simply follow the same advice Eric gave above and adjust it to suit the shape of your garden beds.

Jacob

[sujoy_kb]

Hi @JacobZ @EricL ,

How do I take care of water drainage and adding a sprinklers through the foundation slab. I am aware of setting up an irrigation control, but I just need guidance of using it through the concrete planter slab.

[MitchellMc]

Hi @sujoy_kb,

In order to allow your planter to drain, you'll need to create ports at the rear. You can use these ports to route your irrigation pipework too. When creating your formwork for the foundations, you should use blocks of timber to prevent concrete from filling some areas, as per my rendering for you below. Once the formwork is removed, there will be small gaps for drainage and irrigation.

Please let me know if you have any questions.

Mitchell

[sujoy_kb]

Thanks so much @MitchellMc . I really can’t appreciate enough how helpful the above image is!

Just for my curiosity, do you think the below diagram with pipes through the concrete slab help in drainage or will it introduce slab instability?

 

[MitchellMc]

That would be fine, @sujoy_kb, and I do not think it would cause the slab to become unstable. However, the water still needs to flow somewhere freely. If it can't freely flow out, it will remain in the planter soil, which will become sodden. I don't think it's sufficient to have a pipe with the end blocked by the soil underneath the planter.

Mitchell

[sujoy_kb]

Thanks again @MitchellMc . I’m convinced to go ahead with your first suggestion of introducing gaps while pouring the slab. However, the rear side of the planter would face the fence. Does this mean the water coming out of the gaps will flood my neighbours on the other side of the fence?

[MitchellMc]

Flood is not the word I would use, @sujoy_kb. Water will slowly weep out of the back, most of which would likely evaporate rather quickly under warm temperatures. Ideally, no water will come out of the rear ports unless there is an excessive amount in the planter, like when it's pouring with rain.

You could move the ports to the sides or front if you wished. You'd then make a small one at the back to route your irrigation pipe through.

Mitchell

[sujoy_kb]

This totally makes sense. Thanks a lot @MitchellMc 

[sujoy_kb]

Hi @EricL 

Although I had initially planned to use 400 x 200x 100 limestone bricks, could you suggest other alternatives that match my project’s requirements? It’d be great if you could suggest a budget alternative.

[MitchellMc]

In terms of block options @sujoy_kb, there aren’t a huge number of budget alternatives to limestone that will still give a solid, durable footing for your raised planter. A coloured concrete block is usually a more affordable option than natural limestone and can still provide a neat, sturdy finish. It won’t have the same natural texture or variation as limestone, but it will perform well structurally and can be painted or finished if you want to customise the look.

Mitchell