Fixings for bold down fence post bracket

Hello

I'm looking to help my friend who's fence blew down recently.
His 'builder' had screwed some 100mm posts to his three courses of block high wall and fixed three ft of feather edge along the top of the wall.

So of course the wall fell over in the recent wind.
I'm planning to help him out and rather than rebuilding the wall I was thinking of reusing the 100mm posts and cant rails and mounting them in those bolt down post brackets.

The base is asphalt but I'm not sure how thick it is. I've not used the bolt down post brackets before but had assumed they would be ok with shield anchors or similar for mounting. How thick does the asphalt/concrete base need to be?
I can drill an exploratory hole to check. If its too thin I'll have to dig it out and set the posts into concrete but I've no idea what is too thin and what is ok.

Even using tarmac fixings, you won't get a fence to stand up on tarmac alone. If the base is concrete under the tarmac then shield anchors will do the trick but if it's MOT under the tarmac then it's time to start digging!

Supposing it is concrete under the top layer of asphalt, how thick would it need to be?