Tunnel Project Helps Meet San Antonio Growth

By Jeff Griffin, Senior Editor | March 2009 Vol. 64 No. 3

Juergen Brunswick, lead tunneling foreman for the project, had these comments about the tunneling operation:

"We knew that we were going to be encountering rock with a psi range from 3,500 psi to almost 5,000 psi. We prepared our Lovat M90 tunnel boring machine (TBM) by replacing the 150 horsepower motor with 200 horsepower. This gave us some additional pump capacity for the machine. We also changed our air cooled system to a liquid cooled system. In addition to the added power, an additional benefit was a cooler tunnel for personnel to work in.

"Based on the geotechnical investigation we installed bullet teeth on the cutter and made the first run. We experienced mixed ground conditions that made us rethink the cutter setup and we changed to a combination of bullet teeth and rippers for the rest of the project. This allowed for greater steering control and better production rates in these mixed face conditions."

Pipeline depth of tunnel sections varied from 25 to 45 feet. Carrier pipe was Hobas fiberglass reinforced pipe with 46 psi stiffness. Annular space was grouted with low strength cellular concrete.

Brunswick said ground water was not an issue even though the tunnel was immediately adjacent to the creek.

"We had a closed face setup on the TBM and we installed filter fabric material around the lagging [ring beams] of the tunnel itself,"he explained. "This allowed the ground water to infiltrate the tunnel and run out to the tunnel shaft while filtering the solids. We tunneled uphill to keep the water behind us at all times."

In addition to using the Lovat TBM to tunnel, Deemer said a subcontractor assisted with hand mined tunnels. The primary liner was 96 steel liner plate supplied by American Commercial. The steel liner plate was an acceptable material to use as a primary liner on each of the three road crossings including two Texas Department of Transportation easement crossings.

"This additional manpower assisted in narrowing the lost contract time due to the floods," said Deemer. "The basic tools were a jackhammer, clay spade and a muck bucket. We put the muck bucket on tracks and wheeled it in and out of the hand mined tunnel to the come out pit where it was lifted out of the pit with a crane and emptied. We kept muck buckets in rotation so we never lost time due to dumping.

The average open cut depth for this project was roughly 30 feet.

A Caterpillar 350 excavator equipped with a breaker was used to assist a Komatsu PC1000 excavator for digging the trenches.