How do you excavate underneath a 200-year-old building that’s the centerpiece of a World Heritage site? Very, very carefully.
As crews dig under the barrel of the Rotunda to create a large underground mechanical room, their job is made even more delicate by the fact that the Rotunda was built directly on the ground, without a foundation. Along with the digging, workers are adding a 16-foot concrete foundation underneath the east side of the Rotunda to stabilize the building where the new underground room will be.
Modern technology is helping ensure the historic building won’t be damaged during the work. Four laser beam units positioned around the Rotunda are aimed at 130 targets on the walls, measuring the building’s position every four minutes, around the clock. If the Rotunda shifts at all, contractors will know right away. Seven vibration sensors augment the laser system and monitor construction-related shaking of the building.
“If there is a shift by one-quarter of an inch, the work comes to a stop immediately,” says Jody Lahendro (Arch ’82), the supervisory historic preservation architect on the Rotunda project for Facilities Management. “Everybody gets together to find out what happened and, if necessary, we have the material and the equipment stockpiled onsite to stabilize the structure.”