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Landscape Gardening

Flower raised bed on clay soil

have a problem with heavy clay soil in my garden and want to build some raised flower beds (60 /90cm tall x 300 cm long ) using standard breeze blocks next to the brick boundary wall to aid drainage and help my plants grow better. I've dug 70cm deep trench by 50 cm wide and just need some general advice please on how to stabilise clay soil correctly? Do I need drain pipes/ soakaways/ landscape cloth or thick layer (8 cm deep )of gravel/ subbase/ sharps sand would do the trick followed by 5cm deep layer of cement topped up with double block footings as the base foundation ? Also is the wacker plate necessary to compact the footings for the bed? Many thanks in advance

2 Answers from MyBuilder Landscape Gardeners

make sure when digging the clay it goes deep enough past the clay otherwise it will be to soft to be using any sort of cement, use hardcore atleast 100mm deep compacted using anything to fit in the trench. then lay bed of cement and lay bricks, use spirit level for the correct levels and falls.


Answered 15th Aug 2018

I would stabilize the clay by doing the best to mix in 5 % cement and re back fill it into the hole as it was ...or you could take a chance and just back fill and compact as best you can since it is always difficult to get full compaction with clay. I would then cast a concrete slab ( called a raft foundation) about 200mm thick slightly larger than the actual flower box area say about 200 mm wider all round. If it is 300cm long I would put a joint in the concrete half way to reduce cracking Technically you need steel reinforcement but if its only 500 mm wide I would not bother. You then would build up your walls making sure you have frainage holes every say 500 mm all round at concrete base level. You would need to then add course aggregate to about 100mm deep and cover with geo-fabric to stop the bedding soil from getting into the drainage stone layer. This is a Rolls Royce solution but you can decide how far you want to go .. good luck


Answered 6th Feb 2019

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