Engineering Inc. - November/December 2019 - 17

The Alaskan Way
Viaduct Replacement
Program won the
ACEC 2019
EEA Grand
Conceptor
Award for
the year's most
outstanding
engineering
achievement

areas. Moreover, despite a cost of $50 million
or more, they are able to accomplish tasks that
were once impossible.
"They handle intense atmosphere, water,
and ground pressures, they move through
unstable sand and soil, and they operate
without damaging structures above ground.
They are essentially underground submarines," Frank says.
The impact of TBMs has been revolutionary. A quarter century ago, engineers
frequently found themselves limited to
tunneling 50 or 75 feet beneath the surface
when they encountered difficult conditions.
Today, they are able to descend 400 feet or
more and apply materials to reinforce walls as
they move through-even in sandy and wet
terrain. In addition, engineers and construction crews can deploy
specialized robotic boring devices and CCTV systems to build
micro-tunnels where humans cannot go-sometimes in spaces as
small as 2 feet in diameter. These micro-tunnels increasingly house
communications cables and other infrastructure that would be
costly and complicated to place above ground.
These advances in equipment and technology allow engineers
to build tunnels faster, safer, and better than any point in the
past. Although overall costs continue to rise for tunneling projects, and cost overruns are a nagging problem, the equipment
and methods used today are driving significant advances-and,
at least in some cases, curbing even higher costs. Depending on
the type of tunnel, TBM costs can range from approximately
$10,000 to $100,000 or more per linear foot.
"TBM tunneling costs have a very wide range that is dependent on many variables, such as geology, intended use, diameter,
length, logistical constraints, and location," Lawrence says.
Not surprisingly, the materials used to build tunnels have also
advanced. "Today, we have many lightweight but highly effective construction materials at our disposal," says Dave Krywiak,
principal, tunneling and trenchless specialist at Stantec. These
include waterproof linings that construction crews spray on
walls, the use of lightweight metallic fibers that replace steel
rebar and retard fires, and specialized gasket seal technology that
better withstands moisture and pressure. In addition, the use of
LED lighting has improved visibility during construction and,
in the case of transportation tunnels, enhanced visibility for
motorists passing through tunnels after they are opened.
"LED lighting improves visibility and lowers energy and
maintenance costs," says Nicholas DeNichilo, president and
CEO at Mott MacDonald.
DESIGNS ON THE FUTURE

Today's tunnels are nothing short of engineering masterpieces, but
designers, engineers, and technologists are exploring ideas for constructing even more advanced tunnels in the decades ahead.
"Because of the increasing complexity and cost surrounding
the construction of infrastructure on the surface-and the lack
of space to build new roads and infrastructure-many cities are
now considering or planning projects that incorporate tunnel

construction," Lawrence says. "It is now
possible to dig deeper and costs are at a
point where it is an extremely attractive
option for certain situations."
This includes tunneling under mountain ranges and oceans. For instance, when
the Brenner Base Tunnel is completed in
2028, the 39-mile rail tunnel will reach a
maximum depth of 5,200 feet and extend
from near Innsbruck, Austria, to Fortezza,
Italy. The tunnel is expected to reduce
travel time between these two points from
two hours down to less than an hour.
Other proposals include building a transatlantic tunnel from New York City to
London-with vehicles that reach speeds
as great as 5,000 mph using hyperloop or

maglev technologies.
Tunnels present some of the most complex challenges in engineering. But as long as there are obstacles to bypass-mountain
ranges, rivers, urban areas, and more-engineers will continue
designing these remarkable structures.
"We have the ability to go deeper and build tunnels in harsher
environments than ever before," Del Nero says. n

Samuel Greengard is a technology writer based in West Linn, Oregon.

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Engineering Inc. - November/December 2019

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