In Situ Stress Measurements

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 borehole deformation gauge or the CSIRO 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 1,500 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, angled, horizontal, to vertical down.