Digging up new block paving - Want to add a driveway bollard

[Snowman]

I laid a block paving drive last year and I'm very please with the result, lovely and flat. I now want to add a retractable security bollard, which would mean removing six or so of the blocks and digging a 900mm deep hole to fit the 200mm X 200mm bollard casing.

I'm worried that the immediate area of screed and mot1 will collapse into the hole as I dig (carefully by hand) and I'm going to end up with a complete mess up.

I laid 50mm blocks on 40mm screed on top of about 150mm of compacted Mot1. Under this is clay.

Anyone have any advise or tips?

Thanks very much.

[lemoncurd1702]

A certain amount is inevitable, but if your base is well compacted it will not be a major issue.
Have done this myself and the worst part is digging the hole.
Are you sure it's 900mm deep? I can't remember having to go that far down.