Deformation Monitoring of the Spallation Neutron Source (SNS) Tunnels

The SNS Project is a 1.4 MW accelerator-based neutron source located at Oak Ridge National Laboratory in Oak Ridge, Tennessee. For shielding purposes, 5 meters of native soil backfill has been placed on top of the accelerator tunnel system. This backfill has caused ongoing settlement of the tunnels. The settlement has been monitored by the SNS Survey and Alignment Group in order to determine the patterns of deformation, and to determine when the tunnel system will be stable enough for precise alignment of beam line components. The latest monitoring results indicate that the settlement rate has significantly decreased. This paper introduces the techniques and instrumentation of the monitoring surveys, and describes the types and magnitudes of deformation found.