Geodetic Deformation Measurement and Analysis

ATLAS Experimental Cavern at CERN

Project Summary

Caverns for large physics detectors as the one for the Large Hadron Collider experiments at CERN sit nearly 100 m underground and measure several tens of meters in length, width and height. The deformation of the cavern ground slab over decades has a direct influence on the relative alignment of detectors to the accelerator beamline. The expected long-term movements are larger than the fine adjustment of detectors and accelerators. In this paper, the measured deformations of the ATLAS experiment main cavern floor and lateral walls over more than 15 years have been analysed. The measurement series has been performed in varied time intervals getting down to half a year. The measurement techniques such as polar method (total station and laser tracker) and precise levelling allow to obtain sub-millimetre precision. The measured deformation reaches values up to 4.9 mm for the ground slab and it is significantly (four times) lower compared to the predictions of the civil engineering consultants at the moment of the cavern construction. They reach up to 14.5 mm for the lateral walls. The reference points on the floor show a systematic behaviour that varies depending on the length of the considered period, the coordinate direction and the point location.