What was there before the Tunnel was built?
Ferry terminals, 250 houses, a church, as well as bowling greens and allotments.
They used a Tunnel Shield invented 139 years before
by Marc Brunel for use in the Thames Tunnel. The Clyde Tunnel version was smaller,
having space for 16 miners instead of 36. Photo:
Side view of Brunel's Shield
1818 -The Circular Tunnel Shield was patented
by Marc Brunel, who got the idea from watching a wood-boring worm while he
was in prison for debt.
1825- Marc Brunel and his son Isambard Kingdom Brunel
began work on the Thames Tunnel in London, using the Tunnel Shield. It was
divided into 36 cells, each holding a miner digging independently. Photo: Brunel's Shield. Thirteen years later...
1843 -The Thames tunnel was opened. This was an unprecedented
success as the Thames River had quicksand under it, and all previous attempts
had failed. They held a candlelit banquet in the tunnel to celebrate. It later became part of the London Underground.
It was cylinder shaped, like a short piece of pipe.
The front end was divided into 16 cells, each cell
had space for one miner digging with a pick axe or pneumatic drill.
The miners stood on platforms, in front of them were
planks of wood holding back the soil. They would unscrew one plank and remove
18 inches (45.7cm) of soil from behind it. Replace that plank further forward,
then unscrew the next plank and do the same. Eventually all the planks had
moved forward by 18 inches; at which point, the whole Tunnel Shield was moved
forward by pistons.
A cast iron hoop was then fitted into the new space at the back of the Shield,
and bolted into place.
As you can imagine, progress was quite slow Typically,
30 feet a week through soft brown clay
15 feet a week through gravel
9 feet a week through Boulder Clay
(see glacier - below)
A modern Tunnel Boring Machine can dig 250 feet (76.2m)
Thousands of years ago, a glacier travelling just south
of the River Clyde cut a huge channel in the earth. It left behind it a trail
of large rocks, gravel and sand.
Clyde Tunnel Engineers knew that the ground under the
Clyde would be very unstable and likely to cave-in...(A bit like digging a
moat around a sandcastle).
A small railway with open wagons took earth back to
the surface, the wagons passed through airlocks to preserve the air pressure
in the tunnel shield.
By injecting cement and clay into the earth as they
This stuck the gravel and sand together so that the
walls didnt collapse.
To stop water coming into the tunnel as they dug, they
increased the air pressure in the digging chamber. If the air pressure inside
the tunnel is higher than the pressure outside of the tunnel, it stops water
and gravel coming in. (graham explains
- air pressure memory)
This technique is not used much today, as working under
pressure is like deep sea diving. The miners had to be decompressed every time
they came out into normal air pressure, to stop them getting the bends. Photo: decompression chambers
Decompression could take over an hour each time. If
it wasnt done properly, the men had to be recompressed to the original
pressure and go through the whole process again (Photo:
Air compression plant)..
It was discovered in the 1960s that working in conditions
more than 2 times normal air pressure could damage your bones, (Bone Necrosis)
because gases in the bloodstream stop oxygen getting to the bones and cause
parts of the bone to die.
During the digging of the Clyde Tunnel 469 workers
had to be recompressed due to the bends 71 cases were serious,
as listed below:
Vertigo, nausea and vomiting
Severe abdominal pains
5 (2 fatal)
Pain in chest
Severe pains in head
This was caused by compressed air finding an escape
route up into the river, it built up an air pocket and then burst out through
the water in a huge fountain. (Graham
explains - explosion memory)
Modern Tunnel Boring Machines (TBMs), like the
ones that dug the Channel tunnel are as long as two football pitches and can
dig 250 feet (76.2m) a day.
One of the TBMs used to dig the Channel Tunnel (from the English side) was
later sold on e-bay for £39,999.
Another was driven off course and concreted into the seabed when it met the
TBM from the French side. It is still there.
It is 762 metres long - approximately 7 football pitches.
It is 21 feet (6.4m) below the river surface, depending on the tide.
In 1964 it was the steepest tunnel ever built. (1 in 16 ratio or 6%). If you
imagine a piece of string looped between your hands, (your hands represent
the tunnel ends) the deeper the loop, the closer your hands are. Engineers
calculated the depth so that the tunnel would connect to existing roads.
(There is a rumour that some cruel parents drive slower
towards the end...)
Because one way traffic has almost double
the flow of two way traffic. It also allows traffic to continue
in one tunnel while the other is closed for maintenance.
Although access is difficult (up a small ladder) it is possible to cross from
one tunnel to the other at the lowest point (the nadir).
The tunnel roof is 16 ft 7(5.05m) from the roadway,
this is only just high enough for large lorries to get through. The road surface
is a W shape ( i.e. higher in the centre and at the edges). This
tilts the lorries slightly so that they fit into the widest parts of the roof.
The tunnel drips because the cast iron bands that form the tunnel arch, contract
in cold weather. This makes a tiny gap, and allows a small amount of water
into the tunnel (more so in the winter and at night). It collects in sumps sat
the lowest point of the tunnels, and is pumped away.
The Clyde Tunnel actually
leaks very little compared to other tunnels. Drips are more visible at the
moment because the old pvc lining has been removed, it will soon be replaced with a new fire-proof lining.
The Portal Buildings at either end of the tunnels were
given a nautical design, as they are near the Clyde Shipyards. They are filled
with 18 huge fans that take away car fumes and pump fresh air in.
The North Portal building holds a control room with
a high tech. camera system, it can sense if traffic has stopped and will sound
an alarm. There are water pumps under both buildings, and one in the middle
of each tunnel. Water and rain is collected in underground chambers and is
then pumped into the main drain system.
If your vehicle breaks down, stay in your car.
The cameras will spot you and tunnel staff will come and help you. The current
charge for this service is £36.