The Crossrail tunnelling is now halfway through. Here's a BBC feature to
mark the occasion:

http://www.bbc.co.uk/news/magazine-23518137

Extract:

"The tunnelling for Crossrail, London's new east-west rail network, is now
in its most intense phase -- and the scale of the engineering challenge is
as jaw-dropping as the cost is eye-watering.

The tourists and shoppers in Red Lion Street in central London can have
little idea that 30m under their feet something is going on, although
people in buildings with deep foundations may feel a curious tingle.

Two tunnel boring machines (TBMs), named Phyllis and Ada, are worming their
way through the earth. Each one is the length of 14 London buses and weighs
1,000 tonnes.

A computer display in Phyllis's control cabin shows the machine is 18mm
below where she should be and 27mm too far to the left. But that's all
within the design's tolerance of 50mm.

"Fifty millimetres is very small in engineering terms," says Andy Alder,
project manager for Crossrail's western tunnels. "When you think that this
machine is 7m in diameter, to steer it within two inches is a pretty
fantastic achievement."

Europe's largest current construction project is now halfway through,
having absorbed over 25 million working hours and produced around eight
miles of tunnels. Last year and this year the project supported the
equivalent of 13,800 full-time jobs throughout the supply chain.

....
....

"I'm an underground kind of person now," says Jim Gagan, lead miner on
Phyllis. "Although it's nice to get out and about during the day, when it's
raining something silly upstairs it doesn't affect us down here - and it's
nice and warm in the winter."

Gagan says that he worked his way up "through shovelling things at the
back". His father was also a tunnel miner, who worked on the Channel
Tunnel.

According to Mike Black, Crossrail's top soil engineer and a Channel Tunnel
veteran, the London project is more complex.

In particular, the nine new stations, some of which require huge
underground excavations, pose the risk of displacing London's soft clay and
sand. As the TBMs move through the ground, the earth at the tunnel face is
kept under pressure - and all the roads and buildings directly above are
monitored around the clock for any movement.

"We have seen movement and we expected to see movement," says Black.
Structurally the buildings are not affected, but Crossrail will have to pay
to redecorate when cosmetic damage happens.

So far, all the movements detected fall within the predictions of 10mm or
less. Crossrail engineers inject grout into the earth underneath buildings
to fill any voids that may have appeared and lift the buildings back into
position.

Subterranean London is riddled with Tube tunnels, secret passages, Roman
ruins, high-voltage cables, long-forgotten aquifers and deep building
foundations. The route squirms past these obstacles, passing just 1.5m over
the Tube's Northern Line.

During the long desk-phase that preceded tunnelling, designers scrutinised
yellowed plans for London buildings.

For some, they had diagrams of what was actually built. For others, they
just had the initial plans. Crossrail's surveyors estimated the size of
foundations by examining buildings.

Then there was the problem of bombs.

"It's estimated that there's about 17,000 tonnes of bombs that were dropped
on London in WWII and the rule of thumb is about 10% failed to explode,"
says Simon Cooke from consultancy 6Alpha. While some bombs were removed,
others were missed or even abandoned because they were too dangerous to
deactivate.

6Alpha studied records kept by wartime ARP wardens, local authorities, and
at times even the Luftwaffe, to identify areas along the route which
presented the biggest unexploded ordnance threat.

Separate contractors were then sent to survey these areas. So far, no
unexploded bombs have been found.

Crossrail's engineers have found a few unpleasant surprises though. It had
been planned to use the Victorian Connaught Tunnel that runs under London's
old docks. But engineers had to scrap their plan to work on the tunnel from
the inside when divers found just 25cm of mud separating the bricks of the
tunnel roof from the Thames.

Instead, Crossrail have built dams to hold back the water from the tunnel
while they take the roof off and widen and deepen it. This technique of
digging from the surface rather than tunnelling laterally is similar to
what was done in the 1870s.

After the tunnelling concludes, some of the tunnel boring machines, which
each cost £10m, will be lifted out in pieces. But where there is no nearby
shaft to the surface that can't happen.

"Then it's not cost effective to dismantle the machine underground and it's
quite risky for the workforce to do that," says Andy Alder. "So in those
cases we would look to bury the TBM and leave it underground forever - we
strip out all the valuable components, but the basic shell of the machine
we'll leave underground."