It is possible to determine the approximate in situ stress or induced stress at the surface of a rock mass that is undamaged or only lightly damaged by blasting or excavation. Some cases, for example, might be:
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In a tunnel wall for a tunnel advanced by a tunnel-boring machine or through basing with a pre-cut array of holes to reduce blasting damage (decoupling), a pillar that is relatively undamaged …
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An undamaged outcrop such as a glaciated hard-rock surface
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A newly mechanically excavated quarry base
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The bottom of a foundation excavated mechanically
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Other cases where the rock is competent (no good for soft rocks, highly fractured rocks, etc…)
Procedure
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The rock face is prepared, the slot location marked on the rock, and measurement point studs are glued on in an array (10-12 points)
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The distance between points is measured to a precision of 100 th
of a millimeter using a precision Vernier caliper
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The slot is cut with a 5-6 mm wide diamond saw
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Measurements are taken of the displacements that occurred because of stress relief
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A 4-5 mm wide steel flat jack is introduced and generally grouted into place
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The jack is pressurized while taking measurements until the strain is recovered.
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Usually, the strain is ~120% recovered, then backed off to get back to 100% recovery
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The pressure in the flat jack is taken to be the normal stress across the jack slot.
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The stress can be cycled 50% above and below several times to make sure the rock is behaving elastically and that the null measurement (100% recovery) is consistent.
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It is also possible to carefully measure all the displacements at many angles and to use a numerical model to back-calculate the initial stresses. This method can account for shear displacement and for cases where the principal stresses are not necessarily normal to the jack slot.
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3 – 4 m away, another slot can be cut at a different orientation, and the process repeated to get an idea of the other stresses.
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If the case of a tunnel, for example, the data can be used to calculate the initial stresses that existed before the tunnel (as long as the rock is behaving elastically!
Non-linear and plastic deformation invalidates the calculations.)
Adapted from http://www.hydrofrac.com

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Adapted from http://www.hydrofrac.com

Pressurized Slots for Mechanical Properties
The pressurized slot testing provides in situ mechanical properties of the rock. At the selected test locations, the rock surface is instrumented to monitor deformations, and two slots at 90
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to the surface will be cut in the rock to create a "tendon" of rock between them. Flatjacks and bearing plates are placed in the slots, which are instrumented to measure displacement across the slot. Displacements can be measured using transducers, or manual micrometer measurements can be used.
During the initial flatjack pressurization, the pressure at which the deformation of the rock cancels the deformation due to slot cutting can be used to estimate the in situ stress acting normal to the slots. Subsequent loading-unloading cycles provide a pressure/deformation history that is evaluated to determine the rock mass deformation modulus (see above).
Additional information can be obtained by including a borehole in the center of the tendon between the slots and measuring the deformation across the borehole diameter as the slots are loaded. As the pressures in the flatjacks increase, the stress around the borehole will also increase, possibly resulting in failure of the rock. The failure stress of the rock can then be determined from the borehole observations.
Finally, the rock mass strength can be evaluated by increasing the flatjack pressure until gross failure of the tendon of rock between the flatjacks occurs. Because this test is conducted in a complex configuration, numerical modeling of the test is required during both test design and for post-test analyses of the results.
Adapted from http://www.hydrofrac.com