You shouldn't have told me we have a geologist on tap!
From the paper's historical section it was Professor Herman Merivale that devised the safe working limit to the sea bed for undersea mine workings, and those distances still stand today. [The professor's sons went off to war in 1915 with the 7th battalion Northumberland Fusiliers you may recall - lived Togston Hall]
I see from the paper a borehole was put down at the low water mark on the end of the Bondicarr rocks. That must have been a difficult spot for drilling - wonder how they achieved that?