If they've gone nowhere in 100 years they're not about to, go straight over the top! What you could do is remove a couple of lines of cobbles where the shuttering will go and go 3" deep over the remaining cobbles with your concrete. Unless you are storing elephants, 3" of concrete over a solid base would be plenty.
If they've gone nowhere in 100 years they're not about to, go straight over the top! What you could do is remove a couple of lines of cobbles where the shuttering will go and go 3" deep over the remaining cobbles with your concrete. Unless you are storing elephants, 3" of concrete over a solid base would be plenty.