Subsidence Following Groundwater Drawdown By Excavating of Deep Shafts in Granite in Mizunami, Japan in 2004-2014

Two 500 m deep investigation shafts were excavating in the granite body in Mizunami, central Japanby JAEA (Japan Nuclear Cycle Development Institute) in 2004-2012. Groundwater with volume of 700 m³ was generally pumping a day to prevent the shafts from submerging in 2012 following the excavating. As a result of pumping the groundwater, the ground water level lowered to 60 m in the borehole with the distance of 200 m from the excavating shafts in 2012. Leveling network extending 2 km × 2 km around the shafts was established to detect the vertical deformation around the shafts in 2004, and precise leveling was done every year. An 18 mm ground subsidence was detected in the benchmark close to the shafts for 8 years in 2004-2012, and time series of subsidence at benchmark was consistent with the groundwater drawdown. The groundwater drawdown and ground subsidence were caused by the pumping ground water in excavating shafts.

In 2012, we extended the leveling network to a width of 4 x 5 km around the excavating shafts, and we had precise leveling in 2012, 2013, and 2014 at the network. As results, subsidence of 4mm is detected around the shafts, and subsidence areas are enlarged to the southeast and northwest in 2012-2014.

A depth distribution of granite in the area is estimated from aeromagnetic surveying. As results, Two groundwater veins are suggested in NW to SE and NNE to SSW, which is a good consistent with ground subsidence distribution.

As subsidence is only 2mm/year, we hope to repeat precise leveling for few years, and to make clear the groundwater system around the shafts.