Hydromechanical Properties at Fermilab

Tiltmeter arrays in the MINOS Near Detector Hall at Fermilab record solid earth tides, large earthquakes, and displacements during a monthly sump pump test. The arrays and sump pit lie in the Galena Wise Lake Formation directly below the floor of the hall, which is approximately 100m underground. Beginning in November 2005 the MINOS-I tiltmeter array was deployed. This array was composed of seven Budker capacitive hydrostatic level sensors (HLS) located in MINOS Hall. Four 30 meter apart HLS ran in the north-south direction and three HLS 8 meters apart ran in the east-west direction. The north-south components of this array were removed in May 2012 due to anticipation of construction of a new cavern for the Off-Axis Neutrino Appearance Experiment (NOvA). A new MINOS-II tiltmeter array was then placed in the fire corridor adjacent to MINOS Hall and began recording in February 2012. The MINOS-II array consists of eight Budker capacitive HLS oriented approximately north-south along with the east-west sensors from the MINOS-I array. Earth-tide data are presented for this new location. We use the sump pump tests recorded by both arrays to characterize the geophysical and hydromechanical behavior of the Wise Lake Formation. There are two sump pumps that alternate to pump the water out of the MINOS Hall sump pit. Once a month a 20 to 30 minute backup sump pump test is run. The tiltmeter array records a response from this test. Five minutes after the test is started tilt is generated, and once the test is complete the tiltmeters slowly equilibrate. Previously, the recorded tilt has been around 30 micrometers depending on the length of the sump pump test. A normal force solution gives expected tilts of less than one micrometer. The difference between these two is too large to be due to a difference in Young's modulus in a lab setting versus a field setting. The tiltmeter array will also record the mechanical unloading effect of the excavation of the NOvA cavern, along with monitoring seasonal changes. Over 1100 cubic meters of rock will be removed during the NOvA cavern construction just off of MINOS Hall. Using a normal force solution, an estimation for tilt to be expected from the excavation of the NOvA cavern is 10 to 20 micrometers.