VIBRATING WIRE DISPLACEMENT SENSORS FOR ROCKFALL MONITORING IN CHACO CULTURE NATIONAL HISTORICAL PARK, NEW MEXICO

Fractures in the Cliff House Sandstone threaten rockfall damage to both cultural sites and park facilities at Chaco Culture National Historical Park in northwest New Mexico. Previous efforts to measure fracture change proved to be unsafe and inconsistent. Vibrating wire displacement sensors were installed across fractures in the cliffs over the park’s visitor center, residential housing, and maintenance yard to accurately and safely monitor for rockfall.

The cliffs exposed next to the park facilities consist of the more resistant Cliff House sandstone overlying the less resistant shale of the Menefee Formation. Fractures in the sandstone along the cliffs have produced rockfalls near park housing, leading to the condemnation of four homes. A total of nine sensors were installed across fractures in the cliffs. A temperature probe and sensor were installed in bedrock as a control. Since their installation in June of 2021, rock temperatures recorded have ranged from 39.9°C to 17.1°C . Displacement is measured in sensors every ten minutes. Sensors have recorded a maximum velocity of -0.372 mm/hr.

Displacement measurements show an inverse relationship with rock temperature by decreasing as temperatures rise and increasing as it cools. Displacements that do not return to the baseline over annual cycles would indicate that the rock had permanently moved. Velocities that exceed the normal range of fluctuation would indicate non-thermoelastic processes and a higher chance of rock fall. Negative velocity indicates that the fractured rock is moving in towards the cliff rather than a positive velocity indicating the rock is moving out.

The installation of relatively low-cost vibrating wire displacement sensors eliminates the need for manual measurements, greatly reducing the risk of injury to park staff as well as improving measurement resolution to sub-millimeter accuracy. This is the start of a year-long data collection that will record the seasonal fluctuations of fracture displacements. With the additional data to be added in the following seasons, the park will gain a better understanding of average rock displacement in relation to rock temperature. This will create a baseline range of displacement and velocities that can be used for an early rockfall warning system which could protect cultural resources and human life.