Stress measurement

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Stability is dependent on the strength of the rock mass compared to the active stresses. Determination of the stress field in a mine, and around excavations, is therefore as important as obtaining high quality strength data.

Determination of in situ stresses is very difficult and expensive. For this reason, many projects are carried out in which the stress field has been estimated using compilations of measurement data from nearby or regional mines. If such an approach is taken, it is important to determine the sensitivity of stability to reasonable variations in stress field magnitude and orientation.

Here, the following will be reviewed:

• Stress measurement methods.
• Development of a stress measurement program.
• Stresses in the earth’s crust.
• Stresses in Sudbury Basin - an example of how stress measurement data is analyzed.

The requirements for stress measurement at different stages of mine design are also addressed.

Stress measurement

The most common method of determining stresses is by overcoring of a cell glued onto the walls of a borehole. The cell contains a number of strain gauges which measure deformation during stress relief of the gauge. Stress is computed from the measured strains using the elastic moduli of the rock. In this sense, stresses are not “measured”, only strains are.

Other methods of determining stress include:

• hydraulic fracturing (hydrofracing)
• under-excavation technique
• jacking methods
• borehole breakout method
• various indirect methods such as earthquake focal mechanism analysis, fault slip data analysis

Stress measurement program

Stress is just as important as strength, but generally much more difficult and expensive to quantify.

Typical cost for say 3 measurements (triaxial) in one borehole is approx. US$10,000 (excluding drilling, mobilization).

Almost all measurements are now made using 3D stress cells:

• CSIRO HI cell (continuous measurements)
• CSIR cell (readings before and after overcoring)
• Mechanical USBM gauge now rarely used. It is 2D only, but quick and reliable to use, and complements the 3D measurements in a daily shift cycle of measurement.

Select measurement sites close to area of interest away from excavations (mining induced stresses)in area of simple geology location that does not interfere with mining representative of mining depth range. It may be difficult to satisfy all of these criteria. A sufficient number of tests should be carried out at site to make sure measurements are repeatable. There is usually large scatter of results and it can be difficult to interpret only a few measurements.

Is any form of stress measurement data available locally or from the region?

Note: trends of stress field orientation from the World Stress Map Project and also from Hoek and Brown compilation are not normally reliable except for feasibility stage estimates.

World Stress Map (WSM) data reflects regional trends which may not apply to local mine-scale stress conditions.

H&B values of k ratio show large range in magnitude in depth range of interest to most mines.

Overcoring stress measurement instruments

Hollow inclusion cell

Shown in Figure 1, the HI cell is made of a thin hollow shell with external diameter a few mm less than the diameter of the borehole into which it is installed (EX diam. 38mm). The cell is bonded onto the walls of the borehole with epoxy. The stick on the front of the plunger is cut to a length so that the cell is installed at an exact distance from the end of the hole. The cell has up to 12 strain gauges bonded into the shell oriented in the axial, circumferential and oblique directions. These measure strain during overcoring, which is then converted to stress using the Young’s Modulus of the rock.

 

Figure 1: CSIRO hollow inclusion (HI) stress measurement cell.

The Young’s modulus is determined from a biaxial compression test of the core retrieved from overcoring, shown in Figure 2. The cell applies a known pressure to the outside of the overcore, resulting in deformation of the central hole. The stress cell, either a HI cell or a USBM gauge, is used to measure the borehole wall deformation. Since the borehole deformation is an elastic process and the only unknown in the system is Young’s Modulus, the test enables Young’s Modulus to be determined.

 

Figure 2: Biaxial compression chamber used to apply a known radial pressure on the outside of the recovered overcore

USBM borehole deformation gauge

Although no longer manufactured, this instrument is very useful in situations where only stresses in a plane are required, since it measures deformation radial borehole deformation only. The gauge is mechanically robust, fast to use, and the results are easy to interpret. A photograph of the gauge is shown in Figure 3.

 

Figure 3: USBM borehole deformation guage, disassembled. the radial buttons near the tip of the cell (left) push down on strain gauge instrumented cantilevers. The assembly is watertight.

Unlike the HI cell, the USBM gauge does not require time for glue to set, so it can be installed and overcored immediately. A number of tests per shift are therefore possible. The gauge can also be used in conjunction with an HI cell, as a single HI cell overcoring, USBM gauge overcoring, followed by an additional installation of a HI cell, can be carried out in a single shift.

Applications of the gauge include determination of pillar loads, abutment loads, vertical load.

References

Stresses in Rock by Herget

Amadei, B., and Stephansson, O., 1997. Rock Stress and its Measurement. Chapman & Hall, London.