Watching the Palace
Monitoring historic buildings is a crucial aspect of their preservation, particularly when a specific site is in imminent danger -- or perceived to be. In the fall of 2003 the State of New Mexico contracted us to stabilize the north wall of the Palace of the Governors. That wall was the closest to the old administration building, previously the Elk's Club on Lincoln Avenue, which was scheduled for demolition (and is now gone). In the dead of winter we stripped the wall of its hard plaster, documented the newly revealed and complex indications of earlier repair and remodeling campaigns, and prepared the surface for mud plaster to be applied in the spring.
As an amendment to the contract we were asked to design and install a monitoring system that would gather enough data to help assess threats to both the Palace and the History Library which faces Washington Avenue as the administration building and the old armory came down to make room for the new annex.
Over the last twenty years or so we have set up monitoring programs on many, many buildings. We had never been asked to assess a specific threat and to have on hand enough background data or "noise" to be able to make a judgment call on potential damage when the first blow of demolition was struck.
After considerable research and consultation we proposed and installed a series of three seismographs and sixteen crack monitors in strategic locations throughout the Palace/Library complex. Then, weeks before the demolition began, we took daily readings of both sets of indicators to establish the baseline data. Some interesting patterns emerged.
One was that at 6:30 most mornings we got a high frequency vibration and horizontal acceleration that recorded on the seismograph in the Palace security office adjacent to Lincoln Avenue. This happened repeatedly, but no actual movement was registered on the crack monitors in that part of the building. Nor did the other seismographs, both located closer to the Washington Avenue side, register anything notable. When we asked the security personnel if they had noticed anything at that hour, the answer was an immediate, Oh yeah, the street sweeper dumps its load of dust and trash into a truck every morning at about that time. We were indeed collecting data on background noise.
The most serious threat to a masonry structure is a sustained high frequency vibration accompanied by vertical and lateral ground accelerations, or waves. In the case of the Palace, what we were most concerned about was the generation of high frequency vibrations from hydraulic and pneumatic demolition equipment.
There were several events that caused concern -- particularly the use of a jackhammer outside the History Library. Fortunately, the thick old brick walls and deep footings in large part dampened the vibrations. There was no discernable movement.
On the Lincoln Avenue side we had up to half a millimeter vertical displacement in the newly stabilized north wall. This movement occurred not as a result of vibrations, but of ground acceleration when large blocks of masonry fell from the old administration building. That half-millimeter is nothing, really, in a wall that is over 30 inches thick and only eight feet high. The thickness to height ratio is so favorable, almost 3:1, that the wall is exceptionally stable.
I am pleased to report that all of the demolition was completed without damage to any of the surrounding buildings or their contents.