A utility upgrade project at SeaTac's north terminal required excavations adjacent and below the departure ramp foundation. An AMTS system was selected to monitor for any displacements induced by the excavations.
For the first excavation, AMTS was positioned outside the utility room. For the second excavation, the AMTS was repositioned to a wall inside the utility room. The primary challenge was finding suitable locations for reference prisms. Reference prisms are used to correct for any movement of the AMTS itself and must be located outside the zone of influence.
Since only the front of the utility room was subject to potential displacements, GEO installed reference prisms on the back wall of the room and also at locations across the road, which were visible through the main doorway.
The AMTS delivered accurate measurements 12 times per day for 5 months. The combination of stable reference prisms, short distances to the monitored prisms, and GEO's automated least-squares adjustments provided high confidence in the reliability of the measurements. The measured displacements were very small, leaving the contractor and owner very satisfied with the work.
The AMTS was mounted on a column outside the zone of influence for the first three months of the project.
Prisms circled in green were monitored for displacements. The more distant prism circled in red is one of the reference prisms.
For last two months of the project, the AMTS was mounted inside the utility room to monitor the second excavation. Not all the prisms are visible in the photo. The red circle represents the approximate location of one of the reference prisms.