Each blast burrows forward by approximately 3 m
(10 ft). At the end of each day of blasting, a tunnel
portal will reach 6–8 m (20–26 ft) deeper into the
mountain. Workers remove the rubble left by the
blast and then reinforce the tunnels, primarily with
shotcrete and rock bolts. The heavy work is not lim-
ited to the tunnels. In the project’s first 8 km (5 mi)
alone, construction teams blasted and moved ap-
proximately 2.5 million cubic meters (3.3 million cubic
yards) of rock to create the tunnels and roadways.
Monitoring the tunnels, frequently and at every stage,
is hugely important. At tunnel portals (entry and exit
openings), monitoring is performed at least once a
week; inside the tunnels it takes place every three
days. The team takes cross-section measurements at
5-m (16-ft) intervals.
The highly accurate monitoring data (to mm preci-
sion) is sent to geologists to determine if the portal is
moving or stable. If movement is detected, they need
to determine in which direction it is moving and if it is
necessary to, for example, double drill and inject grout.
Tabatabei’s team employs several Trimble S6 total
stations for this work. According to Tabatabei, “The S6
instrument’s surface-scanning functionality has been
very valuable for monitoring—we can compare the
scan data to the design and determine if we have
an over-break or under-break and if we are right or
wrong. And we can see exactly where we are. We can
get the quantity also.” Inside the tunnels, the teams
conduct surface scans shortly after blasting to give
quick, on-site feedback to the drilling crews. The scan-
ning data also go to the office for detailed analysis
and volume computations.
The Trimble S6 total stations are controlled by Trimble
Access™ field software running on the Trimble TCU
field controller. The teams also rely heavily on Trimble
Access Tunneling and Roading software modules.
High Precision Pays Off in the Middle
In Phase 2 of the project, General Mechanic blasted
600 m (1,970 ft) from each side of the mountain with
the objective of meeting in the middle. When at last
they broke through, it was discovered that the two
portals met with an error of only 2 cm (0.8 in).
“We were really impressed by that,” says Tabatabei.
“The two-centimeter difference between tunnels that
our Trimble instruments gave us was amazing.”