The LVDT cell has been developed to be a fast, accurate and reliable stress measurement tool in hard rock conditions. Method is applicable good quality rock where minimum 350 mm long pilot hole can be drilled in solid rock.
Normally LVDT cell is released by overcoring, but so called sidecoring techniques can be used in high stress conditions where coredisking would normally prevent measurement.
For the measurements a quality assurance system has been developed. As a part of this the calibration of the cell is verified before and after every measurement. The drilling induced heat is monitored and taken into account before removing the cell from the installation hole.
Unlike conventional stress measurement probes LVDT cell has a mechanical mounting system, thereby being free from commonly encountered gluing problems in could conditions and long hardening times. The mounting system is tested to be stable and reliable.
For drilling compact drill rig is used which saves time and costs. This enables measurements in such an excellent locations as raise bored shafts. Minimum free space in measurement location is about 1.5 m ie., like raise diameter.
The reliability of the solution has been proven by comparing against know rock stress conditions in ÄSPÖ Hard Rock Laboratory in Sweden. In ÄSPÖ several in situ experiments and tens of stress measurement campaings have been conducted over the years which gives us confidence on the LVDT cell technique.
|Read more about techinque
|Presentation about the technique with case studies from Äspö and ONKALO
|LVDT cell stress measurement description in Posiva-report 2012-43