"falling" sub grade level - Patio groundworks

[apples001]

Gents,
I have started preparing a patio base on an area of 14' long x 38' wide. This is sided by the house wall and 15" sleepers on 3 sides. The problem I have is the sub grade falls away to the slop of the garden - at one end I have dug down 6" and hit the rock, but as I move across the 38' width the subgrade disappears below the soil by at least another 2ft.
Can I just lay this with well compacted Sub-Base, at the deepest end being 15" to the sleeper top edge.

A

[lutonlagerlout]

well obviously rock is a great sub-grade,and i suppose as long as you get 100 mm of compacted type 1 on the soil element it should be ok,needs to be subsoil though and not topsoil
terram will also help
cheers LLL

[Tony McC]

Build up your levels using a reasonable crushed aggregate. Type 1 would be wonderful, but a Type 2 or even a 50mm crusher run woukld be acceptable. The really important thing is to build up in layers, thoroughly compacting each layer in turn before placing the overlying layer(s). If you're relying on a plate compactor, work on layers of around 100mm thickness.

[apples001]

Thanks Guys,

The Soil I was refering to looks like at least 2ft of topsoil, how can I tell? and if it is do I have to take it all out

A

[Tony McC]

Topsoil is defined by its organic content. The easiest way to assess organic content is the old milk-bottle experiment that you probably did when at school. It's explained on my gardening site here.

If there's a lot of the humus/organic material, then it's a topsoil and it will have to come out.