Stresses in rock are sometimes sufficiently high to cause rock bursting, spalling, buckling, heaving, or other ground control problems. In such cases, knowledge of the stress is of fundamental importance to the design and construction of structures in rock. Even in cases where the effects of stress are less dramatic, the optimum shape, orientation, and layout of underground structures, as well as the effectiveness and ultimate cost of rock support systems can be significantly impacted by the in situ rock stress. Stress measurements in concrete dams are also a valuable tool for evaluating structural stability and calibrate numerical models.
We measure rock stress via the two most common types of overcoring which utilize either the USBM (United States Bureau of Mine) borehole deformation gauge or the CSIRO (Commonwealth Scientific and Industrial Research Organisation) hollow inclusion cell. Both instruments measure strain relief in the borehole wall of a pilot hole (1.5-inch-diameter) as it is overcored using a larger diameter (6-inch-diameter) bit.
We have completed more than 1,000 overcore stress measurements with our equipment in gold, copper, platinum, molybdenum, trona, oil shale, limestone, and coal mines, and in tunnels, concrete dams, and other civil structures.
Our core drilling rigs are ideal for overcoring up to 80 feet (25 meters) deep with the USBM BDG and HI Cell methods. USBM BDG and HI Cell overcoring is well suited for determining stress in mines, tunnels, and concrete dams. As a general guideline, the pre-mining (native) stress state can be obtained by overcoring at a distance greater than 2.5 times the effective diameter of the entry.
Our smaller drill rigs are portable for difficult access locations and are permissible for use in “gassy” mines. The rigs can be set up on tracks or skids and powered with diesel engine, electric motor, or pneumatic-hydraulic powerpacks. Drilling can be completed from any angle ranging from holes vertical up to vertical down.
The USBM borehole deformation gauge (BDG) is used for 2D-stress measurements in holes oriented at any angle, wet or dry. The BDG measures the orientation and magnitude of the secondary principal strains in the plane normal to the borehole. Each measurement is followed by an on-site measurement of the overcored rock’s Young’s modulus using a portable biaxial pressure chamber. The modulus is used to convert strain to stress. The BDG is constructed of stainless steel and is a reusable tool. Tests are conducted in accordance with ASTM D4623.
The CSIRO Hollow Inclusion Cell (HI Cell) is a single-use (disposable) instrument that is glued in the pilot hole prior to overcoring. The HI Cell is used to measure the full three-dimensional (3D) stress state. Young’s modulus testing is performed using a portable biaxial chamber the same as with the USBM BDG. HI Cells are also used for long-term monitor of stress changes when glued permanently in place. Manufacture details about the HI Cell can be found on the ESS Earth Sciences HI Cell webpage and ESS Stress Measurements in Dams webpage.
Geomechanical or rock mechanics instrumentation is commonly used to assess the stability of surface or underground excavations or concrete structures, or as a means of evaluating design by providing quantitative data which can be compared against modeled or analytical predictions. Instrumentation is also used to diagnose and quantify specific failure mechanisms, all of which can be invaluable for original or remedial design.
DA Smith is experienced with the installation and commissioning of a wide spectrum of instrumentation for underground and surface mines, dams, tunnels, underground laboratories, and remediation sites. We have access to leading instrumentation through our many trusted, world-class instruments vendors. We can work with you to develop the most effective instrumentation program for your project.
Borehole extensometers (MPBXs)
Time domain reflectometry (TDR) cables
Hydraulic roof sagmeters
Borehole pressure cells
Permanent CSIRO HI Cells
Vibrating wire stressmeters
Datalogger systems (manual or telemetric)
Custom scientific instrumentation
Fiber optic cabling
Optical borehole logging