Trench block on uneven chalk

[nordita]

How do I lay trench block on uneven chalk ground?

My plan would be to use a blinding layer of sand but I don't know if that's suitable and if it requires a wacker plate.

Background: I'm building a 4.5 * 3.3m outbuilding. Ideally with a cavity wall of dense and light block. But can scale back to dense block and wood if the foundations aren't sufficient. I've dug a 500mm wide and 500mm deep trench. At one end of the trench the depth is 100mm in a strata of chalk and on the other end it's at least 300mm.

I've got a concrete pump coming but the trench blocks are plan B if they have access issues.

 

[Deluks]

If in doubt, keep digging.

A concrete pump is normally used to overcome access issues, can you not barrow it in?

 

[nordita]

Nah the issue might be parking for the lorry and pump. Even if they parked further from my house the road is congested.

Which is why I'm focusing on trench blocks which I can bring in bit by bit with a van as a backup plan. Specifically how I lay them flush on uneven chalk ground.

 

[Deluks]

You ideally need a level bottom to build off of. Chalk is soft, use a breaker or a digging bar/pickaxe.

If you hit firm chalk at 500mm then consider yourself lucky.

Otherwise you're looking at a stepped footing.

In any case you will need a concrete footing under the blocks, laying directly onto the ground is a bit crap to say the least.

Why not hire a mixer and do the concrete yourself? Sure you'd have to bring in bags of cement and ballast, but then you'd have to do that with the blocks too.

Wheelbarrows.
And a chinese takeaway to thank your mate for helping out.

 

[nordita]

I don't know what counts as firm chalk but the only way to get it out is with a pick. Which isn't a problem. The trench is in chalk all the way around.

Here is a picture. The floor is spoiled by some collapse and my muddy boots.

I had assumed making up concrete to the minimum footer thickness would have be too much to do. But to get up to 150mm thickness is only 1.17m3 which seems reasonable to do with a hired mixer and friend.