Wrong sub-base... can i get away with this! - 10mm aggregate instead of 20mm limestone

[Canuck]

Hello everyone,

By a collective effort of myself, the sales lady at the local supplier and the contractor, we managed to lay a wrong sub-base on my backyard!

My backyard soil (sub-grade) consists of a very solid patch of clay (I know because I dug a 10 meter trench last year!) we put an average 10cm of a highly compactable aggregate with grains ranging from 7 to 10 mm down to dust. Here in west Canada they call it Crusher Dust. Then compact it with a plate compactor.

It is obvious for me now (after reading many articles) that the industry standard is the 20mm to dust aggregate. My question is, can I get away with the current sub-base I have or I must add more sub-base of 20mm limestone on top of existing stuff? or even worst, Am I going to have to excavate the existing sub-base before laying the new one?

P.S. The plan is to pave the backyard using 10cmx20xm paving bricks.

Many thanks in advance

[r896neo]

You may be ok depending on the amount of dust? Dust is good for binding and compacting but in a large quantity it tends to hold a lot of water. If your sub base has plenty of stone you'll be fine with 10mm to dust but if its largely dust then it may not drain well and may remain a little fluid which would be pretty unstable.

Very hard to give any advice without seeing it in the flesh. Even a photo wouldnt help really. If you try to ram a spade into it, it should disturb the surface but not go into it as such.

A halfway house would be to loosen it back up and add mix in a couple of tons of 20mm stones. This would be very easy and not a big job if there is a digger on site with a toothed bucket.

[Canuck]

Thanks for your quick reply.

Due to reasons such as topography and existing structures around the house, it is not possible to take any kind of machinery to the backyard.

I tried your spade test, and it only went into it for about 2cm.

I have walked on it when it was soaked after few days of straight rain. It did not feel very soft but it definitely does feel more stable when dry.

I also would appreciate if someone tell me what would be a consequence of just paving over this sub-base as it is. what happens in worst case scenario and also realistically?

Thanks again

[r896neo]

Your clay sub base really doesnt help matters. If you had free draining sub soil it might be fine.

The problem is the clay will not drain well meaning that your sub base may be prone to slight water-logging. If your sub base has too much dust it may hold water badly and when a sub base holds water it can behave slightly like jelly under load.

If this was a driveway i'd say no question, you would have to rip it up for the above reason but because its just a patio the load will be negligable.

Are you intending to lay the 20x10cm pavers on a grit sand only bed? i.e flexible block paving?

[Canuck]

I intent to lay about an 3-5 cm of sand that is good for bedding layer (I don't know which sand is the best yet!) on top of the sub-base and lay the concrete pavers on top of sand, as I understand that is how it should be done. Btw, we decided to get slightly larger pavers (mixed sizes up to 20x20cm)

As for your reply about the backyard ground permeability, I don't think it is that bad since I had not noticed a puddle before we put the aggregate, and never felt the compacted crusher dust that soft (like jelly or mud)despite our crazy raining season.

Now, if we consider the foot traffic to be on a low side and the ground's drainage capacity not to be the best but reasonable, would you suggest to re-do the sub-base or leave it?

thanks

[r896neo]

Its honestly so hard to advise without seeing it in the flesh and feeling it.

As long as its not spongy underfoot then it sounds ok.

Perhaps wait for some other opinions for peace of mind.

For sand you want a coarse grit sand that is free draining.

[seanandruby]

Sounds like mot2 which has smaller stones and more fines than mot1. It should be fine for a patio.

[Canuck]

Thank you both. It sounds like, the final verdict is to leave the sub-base as is. However, I welcome more expert input on this.

thanks again.