Bill Grose, tunnelling and geotechnical engineer, explains why if you don't know or understand what you're doing, many things can go wrong.
I’ve been following, with concern, the tunnelling activities of the protesters in Euston Square. The reports, videos and photos show that the protesters have dug what looks to be a 9ft (2.7m) deep shaft, and from that, they say they have about 100ft (30m) of tunnel(s) in an unspecified direction. The tunnels look to be about 1m (3-4ft), square in cross section, with some timbering.
Why we should all be concerned
We know that tunnelling is a risky activity – despite the best endeavours of professional tunnellers all over the world, tunnels still collapse quite regularly, and most of those are during construction. This is why tunnels are best left to qualified engineers, who can assess and deal with the considerable risks of tunnelling, most of which are unseen.
Most tunnel collapses (in the construction industry, rather than amateur tunnels) result from several things going wrong at the same time, and misunderstanding the ground conditions is often a factor.
For example, in 1994, a section of tunnel being constructed at Heathrow Airport collapsed, and misunderstanding the behaviour of the ground was a contributing factor, as it was with the Nicoll Highway, Singapore. In addition, the 1999 collapse of the Hull tunnel was due to non-cohesive soil being washed in.
In the UK, any tunnel under construction would be preceded by an investigation into the ground conditions, including groundwater, and any underground obstructions, including utilities, cables, unexploded ordnance, burial grounds, and so on. There would also be a design for the ground support, and an emergency plan in case things go wrong.
During construction, there would be continuous monitoring for various hazardous gases, and forced ventilation, and anyone entering the tunnels would be trained in safety, rescue, and working in confined spaces.
Hazards and resilience of the makeshift tunnels at Euston
As far as I can see, none of these things have been done at Euston Square, which poses significant and invisible risks to anyone underground, irrespective of whether the tunnel is currently stable.
On the matter of stability, the ground looks to be sand or gravel, or a mix of the two, in other words, a soil with little or no cohesion. One of the protesters said that the soil “comes away easily” – he was happy that the digging was easy, but I would be concerned that the ground is loose and more liable to collapse.
There are also vertical unsupported faces of soil seen in the photos and videos, and these will fail some time in the future. The protesters are using timber sheets in places, and these are unlikely to be strong enough.
Another significant concern is water - any rise in groundwater level, or infiltration from above (rain or burst pipe) could precipitate a collapse or washing-in of the soil.
The ITN News reported that one of the protesters has taken his teenage son into the tunnel. However well-intentioned, this is an irresponsible risk for a parent to take.
Advice to the protestors
My advice to the protesters would be to evacuate the tunnels now, ensure that the area at ground surface above the tunnels is cleared of people, structures and any other valuable items, and hand the tunnels over to professionals who can make them safe.
There must be safer ways to protest than endangering yourselves and others – if anything went wrong in the tunnels it could be very serious indeed.