In situ stress refers to the natural state of stress that prevails in the Earth’s crust before any form of engineering intervention. Mining activity in underground workings changes the pre-existing state of stress due to removal of rock mass during the formation of shafts, stopes, haulages, and drifts. The removal of rock mass leads to stress concentration at the faces of underground openings. Excess of local stress over the strength of the rock mass leads to structural instability problems such as rockbursts, wall spalling, floor heaving, and roof collapse. Hence, the estimation of the three-dimensional in situ stress tensor is critical in controlling the ground, scheduling the excavation process, designing of support system, and mine safety.
The total stress underground includes six independent stress elements or three mutually perpendicular principal stresses. The magnitudes and orientations of these stress elements are influenced by natural factors like depth of overburden, tectonic stress, geology, and temperature gradient. Geotechnical engineers use direct testing techniques and indirect diagnostic logging techniques in boreholes to determine the pre-existing state of stress before undertaking any deep excavations.
The overcoring technique is the traditional technique to evaluate the three-dimensional stress field. Specifically, overcoring entails drilling a primary borehole, then drilling a pilot borehole at the bottom of the main borehole, installation of a strain gauge device (e.g., USBM deformation gauge or CSIRO Hollow Inclusion cell) in the borehole, and extension of the borehole such that total stress release is accomplished. The resulting circumferential strains or diametral displacement is evaluated using the principles of elasticity theory to derive the three-dimensional stress tensor. Although overcoring produces detailed information about the magnitude and direction of stresses, it requires physical access to the subsurface, is expensive, and makes measurements in small volumes of rocks.
Another technique is the hydraulic fracturing technique. This technique is widely used where there is no access to the underground environment. In this case, an isolated borehole interval is sealed using straddle packers, after which the pore pressure in the isolated borehole interval is increased until the rocks fracture. The breakdown pressure indicates the initiation of fracturing whereas the shut-in pressure equals the minimum horizontal stress. Hydraulic fracturing produces the average value of stress evaluation in large volumes of rock; however, the traditional test is confined to the two principal stresses in the horizontal plane.
The presence of natural discontinuities or zones with increased porosity complicates the implementation of traditional fracturing because of fluid loss. Technical innovations in this regard involve changing the pumping regime. The injection of fluids at high rates (up to 15 liters per minute) causes high-pressure gradients leading to tensile failure before the fluid is transferred into adjacent joints. On the other hand, pressurization of the formation with high-viscosity fluids reduces fluid losses in porous formations, like sandstone roofs of mines. According to field studies, stress magnitudes resulting from high viscosity fracturing match those obtained from overcoring.
The Hydraulic Tests on Pre-existing Fractures (HTPF) represents another approach, which helps to re-activate existing joint systems rather than create new ones. This involves the mathematical inversion of shut-in pressures in different fracture planes giving the stress field. Furthermore, there is a possibility to use probe microscopes, which do not rely on overcoring. In a plane stress state, the stressed borehole takes an elliptic shape. Probe microscopes measure the displacement and orientation of the ellipse major axis.
Image logging in boreholes is an indirect and fast method for assessing principal stresses in deep boreholes. Higher levels of stress result in compressive failure of the borehole walls, which lead to breakout failures in the borehole walls, parallel to the minimum horizontal stress, whereas drilling-induced tensile fractures (DIF) occur in the direction of maximum horizontal stress. Caliper logging, acoustic television, and resistivity are some of the methods used for identifying these phenomena and for stress analysis in the horizontal plane.
In-situ Stress Measurement is an essential element of rock mechanics in underground mining. Mining activities disturb the natural stress field, thereby creating zones of stress concentration that could be destabilizing to the structure. Overcoring provides a full 3D stress tensor at shallow to medium depths, while Hydraulic Fracturing (HF), Hydraulic Test and Pressure Fracturing (HTPF), and Borehole Breakout provide data on stress state at greater depth or highly anisotropic rock masses. The application of high viscosity fluid fracturing, probe microscopy and numerical back analysis techniques has overcome some technical limitations in measuring stress in poro-elastic and fractured rock media. A combination of these different methods allows mining engineers to build accurate stress databases, determine the orientation of stopes and provide adequate support systems to ensure sustainable ground control.
