Never ending top soil - 230mm deep and counting
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tk onion
- Posts: 8
- Joined: Sun Jun 17, 2012 10:49 pm
- Location: manchester
HI, I'm building an additional section of a patio at my daughters house and have dug down to a depth of 230mm and am still in pretty soft stuff. I note that in the how to section on this forum it advises against laying on top soil but is there a way around this? I had been thinking about going down about 150mm and compacting mot1 for about 75mm and then a mortar bed to lay the 35mm thick Indian stone onto. I'm thinking that if I maybe make a concrete bed of about 150mm and then mortar the slabs on or would 75mm mot1 then 75mm concrete then mortar bed for slabs to finished level be sufficient or do I really need to dig down until all the topsoil is gone? Any advise gratefully accepted, thanks in advance.
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Tony McC
- Site Admin
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- Location: Warrington, People's Republic of South Lancashire
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Get rid of the topsoil!
Topsoil is topsoil because it has a high proportion of organic material which *will* decompose over time - that's how topsoil feeds all the plants and weds and trees and whathaveyou - and with that decomposition comes settlement.
230mm dig depth might seem a lot to you, but it's nothing unusual to a professional paving installer. If it got to be over 400mm, that would be surprising, as, under natural conditions topsoil doesn't achieve such depths unless it is being used for agriculture, so it woiuld suggest someone had been too generous with the backfill.
And 75mm of Type 1 sub-base is not enough. 100mm minimum, but if you have a deeper dig, built it up in layers not exceeding 150mm and thoroughly compact each layer before placing the next.
Topsoil is topsoil because it has a high proportion of organic material which *will* decompose over time - that's how topsoil feeds all the plants and weds and trees and whathaveyou - and with that decomposition comes settlement.
230mm dig depth might seem a lot to you, but it's nothing unusual to a professional paving installer. If it got to be over 400mm, that would be surprising, as, under natural conditions topsoil doesn't achieve such depths unless it is being used for agriculture, so it woiuld suggest someone had been too generous with the backfill.
And 75mm of Type 1 sub-base is not enough. 100mm minimum, but if you have a deeper dig, built it up in layers not exceeding 150mm and thoroughly compact each layer before placing the next.
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