2D and 3D Stress Measurements

DA Smith conducts in situ stress measurements in rock and concrete by both the 2D United States Bureau of Mines (USBM) borehole deformation gage (BDG) method and 3D Commonwealth Scientific and Industrial Research Organisation (CSIRO) hollow inclusion cell (HI Cell) method.  Both methods are used extensively in the civil and mining industries to characterize stress in rock masses and concrete structures as part of engineering studies.

BDG Readouts
Overcoring for Measuring Stress

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 calibrating 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.

Overcore
Overcore Drilling

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.

Overcoring Technical Paper - Walters Dam, North Carolina
USBM Borehole Deformation Gauge

USBM Borehole Deformation Gauge

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.

CSIRO Hollow Inclusion Cell

CSIRO Hollow Inclusion Cell

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 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 on the ESS Earth Sciences HI Cell and ESS Stress Measurements in Dams webpages.