Valenza Engineering have worked closely with Indodrill to develop a successful installation method for constructing multi-level VWPs in deep, high-temperature rock conditions at a remote mining project in Indonesia.
VWPs measure temperature and pressure data, which depending on the installation method, can be converted to rock mass pore pressure and temperature for a grouted installation, or the height of the water column (piezometric pressure or water level) in an ungrouted installation. High temperature VWPs are supplied by Geokon USA and have a rating of 270°C with several pressure rating options available from 1 to 20 MPa suited to the installation depth and rockmass pressure. The pressure and temperature channels return the respective VW signals through a stainless-steel sheathed cable to a logging device at the surface which is programmed for the required measuring frequency, with calibration applied to provide true pressure, water level and temperature data. The data obtained is used to develop pressure and temperature models used for the prediction of mining conditions such as groundwater inflows, monitor dewatering and depressurisation, and serve as real-time monitoring for groundwater pumping tests.
The method developed for grouted installations accounts for the very difficult ground conditions experienced in the volcanic settings encountered in Indonesian mines, and in particular at the project in question, where temperatures reach over 200°C. A sacrificial casing from used end-of-life drill casing is installed to the final depth with slotted sections place to cover the installation depths of each VWP sensor. Up to 4 sensors have been installed in these drillholes with each VWP placed progressively on a steel tremie line as this is lowered into the cased hole. A predetermined cement-bentonite grout mix suitable for the hole temperature conditions is pumped through the tremie line to completely fill the void between sensors. When cured, this allows each sensor to measure the pressure and temperature at discrete levels and maintains a full hydraulic separation between each.
Over 30 multi-level VWPs have been installed to date in drillholes up to -70° inclination with over a 95% success rate. The network of VWPs provides invaluable baseline rockmass temperature and pressure measurement across the mine footprint to assist mine design and the selection of appropriate mine construction and operation options. This Life of Mine system will be used during mine construction to measure the performance of dewatering and cooling strategies and track progress to achieving suitable operating conditions underground.
Article by Peter Gribbin