Breeze block planter foundations in waterlogged clay corner

You’re probably overcomplicating it slightly, but your thinking is in the right direction. For a raised block planter, especially in a waterlogged clay area, the key priorities are stable bearing ground, drainage, and preventing differential movement — not necessarily creating a fully engineered suspended build-up. A few points from a groundwork perspective: 1. Your proposed shingle layer beneath the footing is the main thing I would reconsider. Footings generally want to bear onto firm, undisturbed subsoil or well-compacted MOT Type 1 — not onto a loose drainage layer. Gravel/shingle is excellent for drainage, but not ideal directly beneath structural footings unless fully designed for that purpose. 2. In your case, once you’ve reached the firmer clay at around 400mm depth, I would normally: - Lay a geotextile directly onto the clay if separation is needed, - Install and compact MOT Type 1, - Then pour the concrete footing directly onto that. 3. The second geotextile above the shingle is probably unnecessary unless you are specifically trying to stop fines migrating into the drainage layer. For a small planter build, one good-quality non-woven membrane is usually enough. 4. Because it’s clay, movement from seasonal moisture changes is more of a concern than outright bearing capacity. Keeping water managed with the French drains you’ve already installed is likely more important long-term than adding multiple layered systems under the footing. 5. Regarding the existing fence post concrete: - If the old concrete is solid and stable, there is usually no issue tying into or casting against it for a non-structural planter. - The main thing is consistency. Problems occur if one section bears on solid old concrete and another on softer ground, which can create uneven settlement. - If possible, I’d try to bridge across the old post footings with a continuous reinforced footing rather than treating them as isolated bearing points. For a breeze block planter, a continuous reinforced concrete strip footing around the planter perimeter on compacted MOT is normally more than sufficient, even on clay, provided drainage around the area is controlled. One other point: Mediterranean planters often end up retaining wet soil internally, so make sure the planter itself has proper drainage outlets and free-draining backfill, otherwise hydrostatic pressure and frost can eventually push blockwork outward.