Evaluation of Rock Mass Responses Using High Resolution Water-level Tiltmeter Arrays

External forces act on the surface of the earth and produce deformation across all spatial and temporal scales. This research study focuses on the deformation evaluation of the rock-mass subjected to tidal, earthquake and surface forces. The events are monitored over horizontal distances of over 100 meters with tilt measurement arrays with a resolution of 10-8 radians. These measurements are obtained from hydrostatic leveling system (HLS) arrays that have been installed in the LaFarge mine in North Aurora, IL by Fermilab. Each sensor in the array is equipped with a water-filled reservoir beneath a capacitor. The amount of water in the reservoir is calculated as a function of the measured capacitance. Individual sensors are connected in a closed system via a water and air line. As the host rock expands and contracts sensors are raised relative to another and water is displaced. The water level in each reservoir is sent to a computer in the mine and recorded. In order to measure the tilt of the rock between two points, the difference in water levels between adjacent sensors is computed. The difference between the end sensors is also calculated to determine the larger-scale tilt of the array. The tiltmeters in LaFarge mine are supported by concrete pedestals installed on the floor of the drift. In the Homestake mine the tiltmeters are placed on similar pedestals, as well as platforms made of artificial wood decking. These platforms are fixed to the wall of the drift with a rock bolt. Time and frequency domain analyses were performed on time series ranging from hours to six months to capture relevant time scales including the response to the 2010 Chile Earthquake (hour-long scale), the stages of the moon (month scale), Fox River floods (flooding week long scales and pressure dissipation month-long scales). By monitoring tiltmeter array responses to different forces, we aim at making predictions about the material properties of rock masses.