Meet our giant tunnelling machines
Digging the new tunnels was a 24-hour a
day job, 7 days a week. Crossrail used eight
tunnel boring machines (TBMs) to construct
the new rail tunnels under London. The giant
machines carefully weaved through the capi-
tal’s congested sub-terrain, snaking between
the existing Tube network, sewers, utilities,
and London’s hidden rivers at depths of up
to 40 metres.
Like giant underground factories on rails,
each of the custom made Crossrail tunnelling
machines had an external diameter of 7.1
metres, weighed around 1,000 tonnes and
measure around 150 metre in length – the
equivalent of 14 London buses end-to-end
and a staggering 143 buses in weight.
Each machine has a rotating cutterhead
at the front and a series of trailers behind
housing all the mechanical and electrical
equipment. The TBM is effectively a large
metal cylinder with a rotating cutting head
at the front and conveyor belt at the back to
remove the earth.
At the front of the TBM is a cutting wheel,
which is pressed against the tunnel face by
hydraulic cylinders. Inside the cutting wheel
the disc cutters and scraping tool loosen the
material. The loosened material is removed
from the cutter head via a screw conveyor,
which moves the material through the back
of the TBM and out of the tunnel via a con-
veyor belt.
The tunnel face is continuously monitored
by pressure sensors that check the turning
power of the cutting wheel and the screw
conveyor, keeping track of the material that
has been excavated. The TBM makes use
of the concrete rings using hydraulic rams
which are at the back of the cutter. Once the
machine has moved sufficient distance the
next concrete ring is installed. Crossrail’s
concrete tunnel lining is designed to last 120
years.
An in-built laser guidance system ena-
bled the tunnelling teams to ensure the ma-
chine remains on course, ending up to within
a millimetre of where it needs to be.
During Crossrail’s tunnelling phase, each
TBM was operated by ‘tunnel gangs’ com-
prising of around twenty people – twelve
people on the TBM itself and eight people
working from the rear of the machine to
above ground. The tunnel gangs worked in
12 hour shifts, tunnelling 24 hours per day, 7
days per week.
Six earth pressure balance TBMs were used
to construct around 18 kilometres of twin-