Customer Stories

/ Following the dened alignment, TBM operator Reese Tatge drives the TBM through the ground to construct the tunnel. Fitzpatrick designed the control network in the tunnel as a series of braced quadrilaterals roughly 350 feet long. The instruments are mounted in brackets on the walls, with the actual control points marked by spads in the tunnel ceiling. The Towill team has an effective way of ensuring accurate positioning of their instruments. "We put a standard tribrach onto the bracket, and then we use a tribrach adjusting puck that has a level vial," says Fitzpatrick. "We ne level the puck, then place a tribrach with a laser plummet on it. The laser goes up and hits the little dot in the spad. In that way, we make sure the total station is directly under the point." When extending the control, Jones and Gaytan use the Trimble S6 to measure ahead and back to passive prisms at the new and visible existing control points. Fitzpatrick checks the data and runs a rigorous network adjustment, supplementing the total station data with gyro-theodolite measurements to produce nal coordinates on the new points. To steer the machine along the correct horizontal and vertical path, the TBM guidance system uses a Trimble 5600 total station mounted on brackets placed high in the tunnel. The total station measures to a series of active prism targets mounted on the TBM and sends the results via cable to the giant machine's onboard guidance computer. The Towill surveyors keep the instrument as close to the TBM as possible. "We don't let the prisms on the TBM guidance system get more than 300 feet from the 5600," Fitzpatrick says. "Any farther than that, the view of the prisms from the total station can be obstructed. And we don't want to go beyond the length of the guidance communications cable." When the time comes to move the guidance instrument, the team must work quickly. When the TBM stops mining to install a concrete ring, the surveyors remove the Trimble 5600 and install their Trimble S6 into the tribrach. Farther ahead in the tunnel, they install a prism into a new bracket that will serve as the next location for the 5600. The Trimble S6 automatically measures three direct/reverse sets to visible control points and the new bracket. The crew checks the results using the rounds of angles function in Trimble Survey Access, and Jones then enters the coordinates of the new bracket into the TBM guidance system. The process takes about 30 minutes, which is roughly the amount of time the TBM needs to install a concrete ring. As a result, the team can advance the control and guidance without disrupting the normal operation of the TBM. Once the move is completed, Fitzpatrick reviews the measurements and sends adjusted coordinates for the new instrument location to Jones. "Typically the coordinates don't change by more than 3 to 4 millimeters," Fitzpatrick says. "The calculations don't take long, so they have not advanced so many rings

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