Customer Stories

/ Aerial view of the tunnel location using Google Earth. (initially) and Russell Mello (who completed the project) and Chainman Emililano Gaytan, Fitzpatrick is responsible for all surveying activities in the tunnel. One of Fitzpatrick's rst tasks was to verify control supplied by SFPUC's design engineer. He used Trimble R8 GNSS receivers to connect the ends of the project, switching to Trimble S6 1-second total stations as the work approached the shafts and moved underground. At each shaft location, the Towill team used total stations to check the internal consistency of the control and to densify the control around the collar of the shaft. Once a shaft was laid out and excavated, the team brought in control to the concrete collar around the top of the shafts. The Towill team used Trimble DiNi Digital Levels to carry the elevations from the control points to the shaft collar. Both the GNSS and optical systems are controlled with a Trimble TSC2 Controller running Trimble Access software. Moving the control down the shafts called for special procedures. The surveyors installed spherical prisms into three high-precision mounts at the top of the shaft, and used the total stations to establish 3D coordinates for each prism's positions. At the base of the shaft, they bucked in under the prisms. Using a zenith plummet mounted on a translation stage on each tripod, the team established instrument points to high precision. They then placed a total station on each bracket and used the EDM to measure to the prisms above, thereby establishing the elevation of the instrument trunnion axis. Simple leveling with the Trimble DiNi then moved the elevations to spads driven into the shaft walls. (A spad is a at spike installed into the wall or ceiling for use as a survey mark.) As a nal check, the crew measured the distance between the two main points at the top of the shaft, and the two main points transferred to the bottom. The distances differed by just 0.3 millimeters (0.001 foot). "You have this very short line and check angle," Fitzpatrick explains. "If you are very precise, which means you are 2 millimeters or better in all of your points, then you can go ahead and launch the TBM." With the control in place, the TBM started mining in September 2010. At the Menlo Park site, construction crews lower the TBM into the launching shaft in pieces. As it bores its way out of the shaft, additional components are lowered and attached to the TBM. When fully assembled, the machine is roughly 750 feet long. As the TBM advances, it installs precast concrete rings to line the tunnel. Each ring is 5 feet long, and the machine places 14 to 16 rings per 10-hour shift. The TBM's guidance system helps the TBM operator steer the machine by measuring the TBM position based on Towill's network of control points.

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