an “exceptional event” caused by Belfast’s “unique clay geology”

It looks like that suspicion about a connection between the road surface collapse on Cromac Street and work on the Belfast Sewers Project was probably correct. From the BBC report

The collapse of one of Belfast’s busiest roads was caused by an air pocket rising to the surface, an NI Water spokesman has said. Bill Gowdy said the rupture in Cromac Street damaged an old Victorian sewer about four metres under the road. He said the air pocket was probably disturbed a year ago during work on the Belfast Sewers Project. It will take a week to repair the road, and talks about minimising traffic disruption have been taking place.