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
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.”

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