Excavation depth - down to sub-soil required??

[Euan Cordiner]

Hi there,

My question relates to the depth of excavation required.

I am installing a new patio in my garden where there used to be grass. The patio is for light foot use only.

The area is about 20m2 and I am using something similar to the following cast concrete riven paving stone (http://direct.tesco.com/q/R.210-1252.aspx) 600mm x 600mm x 35mm. My intention is to have the patio at the same elevation that the grass was previously.

I have skimmed off the grass and taken it to the dump. These removed pieces were approximately 60mm – 90mm deep (soil depth).

The person helping me told me that I then needed to dig down to the hard sub-soil layer which was another ~250mm down, i.e. removing ALL of the top soil. We proceeded to do this over half of the area and dumped the top soil at the dump. I continued removing the top soil myself the following day and kept the top soil in a pile elsewhere. About ¼ of the patio site is still to be excavated.

I have now seen on this web-site that only 100mm of sub-base is recommended with no mention of digging down to the sub-soil. I have also looked at how much hardcore I would need to fill the 250mm and was shocked at the volume (and price) required! Approx 10 tonnes would be needed.

My question is therefore – should I continue digging down to the sub-soil and build up ~250mm of hardcore, OR, fill in the hole I have already dug with the remaining pile of top soil and compact and spread to an even level, before applying ~100mm of compacted hardcore? Note that cost is a major factor in this project and I only intend to be in this property for approx 5 years.

Really looking forward to some advice.

Thanks,

Euan

[Pablo]

ideally all topsoil should be removed because it sinks once it's been covered. Putting soil back into the dig will fail because you'll never be able to compact it properly. You're already committed so best just finish what you started and shop around for better stone pricing or collect it yourself.