Engineering Inc. - November/December 2019 - 16

ventilation needs and the vehicles using the
tunnels may have changed," Guptill says.
Understanding the ground conditions
where the tunnel will be built is the starting
point for any project. Engineers typically
collect vertical boring samples at regular intervals-often 1,000 to 2,000 feet
apart-and extrapolate on soil and groundwater conditions using statistical methods
and computer modeling. But this does not
necessarily deliver a complete picture.
"The problem is that there is no way to know exactly what
is there until you get there," says Don Del Nero, vice president
and tunnel and trenchless practice at Stantec. "You assume a
certain advance rate, but sometimes it does not play out." Engineers may also use geospatial imagery and other data to understand earthquake faults, flooding zones, and other risks.
Of course, engineers must pay particularly close attention to
tunnel curvature and grades, and other factors when designing
certain tunnels, particularly those used for high-speed rail. The curvature of a tunnel or the rate of incline or decline not only impacts
operational speeds and safety, but also it can become a problem if
an earthquake occurs and there is fault displacement across rails or
roadways underground.
"You do not want to slow trains down because the track realignment cannot accommodate the design speed, particularly if there
is displacement across a fault, and the tunnel curvature and grade
must be reestablished," Guptill says.

"We have the ability to go deeper
and build tunnels in harsher
environments than ever before."
DON DEL NERO
VICE PRESIDENT
TUNNEL AND TRENCHLESS PRACTICE
STANTEC

TUNNELING IS BORING

If designing a tunnel is challenging, building it is daunting. Today's
TBMs are an example of how far construction has advanced in
recent years. Once operated entirely by humans, these giant pressurized machines-controlled by computers and including as many as
85 rolling steel disc cutters at the front-grind through rock, soil,
and sand at a rate of up to 1,600 tons per hour. Hydraulic cylinders
connected to the drill's spine propel a TBM forward at a rate of
100 feet or more per day-all the while sensing ideal boring speed
and methods. As a TBM worms through a subterranean space,
steel shoes typically push outward and grip the tunnel walls while
retractable legs lift the giant machine slightly off the ground.
As the machine bores and removes debris, additional machines
and workers install materials such as precast concrete linings, to stabilize the tunnel. Today's TBMs and earth balancing machines produce extremely smooth tunnel walls while concurrently reducing
noise and vibration, which makes them highly suitable for urban

STANTEC

Tunnel construction for the Ottawa Combined Sewage Storage Tunnel, part of an overall River Action Plan, is aimed at enhancing the health of the Ottawa River and
protecting water environment for future generations. Stantec provided overall design services on the project leading a multidiscipline team including geotechnical
engineering and materials testing as well as water and wastewater.

ENGINEERING INC.

NOVEMBER / DECEMBER 2019

Engineering Inc. - November/December 2019

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