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Hi all,
I am looking at building a retaining wall in my backyard, and I am wanting to get some advice. For clarification, I have already dug the post holes and my next step would be pouring the concrete and put in the posts.
1. Some of the holes for the posts are directly next to tree stumps. If I fill the holes with concrete, and the tree rots in the future, will the retaining wall move?
2. I intend to use 2.4m sleepers for the retaining wall, supported by galvanised steel posts on both ends. Can a wooden sleeper (200mm x 75mm) be used in the middle of the sleepers to prevent the retaining wall from bowing out? For reference, the holes have been spaced out by 120cm.
3. Some of the post holes have very large rocks at the bottom that cannot be dug out without using heavy-duty tools. If I fill the hole with concrete, will the concrete sit happily above the rock or can it slip and create movement?
4. Is Quick Set concrete any good? I was advised that it would deteriorate after five years. Is there a product that is recommended?
Welcome to the Bunnings Workshop community @KE. It's brilliant to have you join us, and many thanks for your question about creating a retaining wall.
If your post holes are directly next to tree stumps, future movement could occur if the stumps decompose and cause the surrounding soil to shift. To prevent this, it’s important to ensure your posts have adequately sized footings. At least 1/3 of the post should be in the ground, and the footing should be wide enough to provide stability even if the soil around the stump shifts.
For supporting the sleepers, a timber sleeper (200mm x 75mm) can be concreted in as a mid-support to prevent bowing. This will add additional rigidity to your retaining wall and distribute the load more effectively.
If you encounter large rocks at the bottom of the post holes that cannot be removed, the concrete can sit above the rock as long as the posts remain stable and the concrete fully encapsulates the post.
Regarding Quick Set concrete, it can be a convenient option, but standard concrete is a better choice for long-term durability and strength. While I haven’t heard of Quick Set concrete deteriorating within five years, using standard concrete may provide greater peace of mind for a project like this.
Please let me know if you have any questions.
Mitchell
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