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How to build a large raised garden bed?

Hello,

I am looking to build a raised garden bed that will be quite large.

Dimensions would be 4000 w x 700 d x 800 h.

I am planning on using 200 x 50 Sleeper H4 Treated Pine Sienna MicroPro.

And having an upright at each corner and at the center of the long side. I don't plan to concrete or anchor the uprights at all due to the location, I want to avoid digging into the existing ground.

Does anyone see any issues with this?

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JacobZ

Hi @JesseS6,

The internal volume of soil for a garden bed measuring 4000mm x 700 mm x 800 mm is approximately 2.24 cubic metres.

In terms of weight, wet garden soil typically weighs between 1,700 and 2,000 kilograms per cubic metre depending on moisture content and compaction. Using that range, the soil in this bed would weigh roughly 3,800 to 4,500 kilograms when wet, so in the order of four tonnes. This creates a significant sideways load on the walls, particularly after heavy rain.

From a structural point of view, while the proposed build is possible, without anchored uprights, there is a lot of pressure being applied to those walls. At 800 mm high and 4 metres long, the soil pressure is substantial, and over time the long sides are very likely to bow outward. Timber will flex, fixings can loosen with repeated wet and dry cycles, and once movement starts the soil pressure continues to increase the deformation.

Common failure signs in this type of construction are the long walls bulging between posts, gaps opening between sleepers, and fixings being placed under increasing stress until they begin to pull out. Without some form of restraint, this can eventually lead to partial or complete wall failure and soil loss.

If digging or concreting posts is not an option, additional structural measures are strongly recommended. I would be looking to include at least 3 intermediate uprights per long side, which would increase the number of posts to 10. These additional posts would help to spread that sideways load over more fixings and over shorter sections of the timber.

Ultimately, anchoring in the posts is the best solution, but if this is not possible, then additional posts will increase the longevity of the structure.

Let me know what you think and if you have any further questions, please don't hesitate to ask.

Jacob

JesseS6

Thanks @JacobZ

Would adding a couple of timbers to brace the two longer lengths together around mid height help? (if that makes sence)

see my drawing, my thinking is the timbers at risk would be the middle (2 and 3rd sleeper off the ground), if I brace them with the other side, it should help reduce bowing somewhat.

If I was able to concrete in the uprights, how many would you suggest be used?

JacobZ

Hi @JesseS6,

Yep, that looks like it would make things much more rigid if you couldn't concrete anything in. Having the cross braces would also divide things up, like multiple hulls in a ship, so that the load is isolated into different sections, reducing the force on any one area.

If you could concrete in the uprights, your original design would be fine, you'd just want the uprights to go at least 400mm into the ground and have about 2 bags of concrete per post.

Ultimately, anchoring it would be better long term, but if you made sure to brace things heavily, like in your adjusted design, it should still work out.

Let me know if you have any further questions.

Jacob

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