An enormous sinkhole on a busy road in the Japanese city of Fukuoka has been filled in 48 hours and the road reopened after a week. Meanwhile, in Britain, residents of a terrace in Ripon remain evacuated as the authorities debate a modest – but still terrifying – sinkhole that opened up in a garden a week ago.
How long does it take to fill a sinkhole? And why do the Japanese put us to shame with their efficiency?
Filling a sinkhole is “not rocket science”, according to Tony Bracegirdle, a geotechnical engineer for Geotechnical Consulting Group, who is this week travelling to Ripon to help sort out the problem. “The engineering challenge is to get enough concrete into the hole as quickly as possible and that’s not very challenging at all.”
In Britain, engineers use foam concrete, which is a lightweight concrete containing some air. This flows into a hole more readily than stiffer structural concrete and so fills all the little voids in the sinkhole. This must be done as quickly as possible, says Bracegirdle, because the ground around the hole “tends to relax”, causing cracking and damage to nearby buildings.
When a sinkhole opened up on Fontmell Close, St Albans, over the site of a former chalk mine on 1 October 2015, four local families had still not been returned to their homes the following summer because of a lengthy public safety investigation into the risk of recurrence.
Why is Britain so slow to get back to normal? “It’s not a technical problem, it’s entirely an administrative one,” says Bracegirdle. “It’s a constant squabble about who takes responsibility – local councils are strapped for cash, so they look to pass the buck to insurers. In the UK, water authorities are liable for subsidence caused by leakage from water pipes so they tend to be on the defensive.”
In Britain, sinkholes are usually small – between half the size of a double-decker bus and two buses, although a small sinkhole can still kill someone if they are in the wrong place at the wrong time. In countries such as Japan, where sinkholes are bigger and occur more frequently, local authorities can quickly access a disaster relief fund.
The British response to sinkholes is also slowed by the amount of private housing, meaning that dozens of different insurance companies can be involved in one sinkhole claim.
According to Bracegirdle, local authorities are quick to respond if sinkholes appear reasonably regularly, such as in Ripon, where natural deposits of gypsum dissolve in water – heavy rain or leaking pipes – more quickly than surrounding limestone, causing cavities to appear. But where sinkholes are more unusual, such as in St Albans, the authorities can be more anxious about public safety.
“In Fontmell Close, St Albans, the concrete went in very quickly – full marks to the local authority there,” says Bracegirdle. “But then the arguments started about what would happen next. This is democracy in action – everyone has a say.”
