Overcoring Rock Stress Measurement FAQ's

Is stress measurement testing expensive?

Yes it is. However, the benefits of obtaining good rock stress measurement data can be critical for design purposes.

How long will the job take?

In good rock conditions and no site delays we can drill the hole and conduct 3 tests in about 7 days.

Can things go wrong?

After 30 years experience in overcoring we still come across unexpected problems. Apart from site logistic problems there are many problems that can occur. Core discing, borehole breakout, poor core quality, cell debonding from the pilothole to name a few. We have the experience to decide quickly what to do and what the best approach to still obtain good data in these situations.

Which type of cells do we use for overcoring?

We use the CSIRO HI Cell for overcoring and also generally conduct some USBM tests at a site. In the event the CSIRO cell does not work (very rare) we also have a USBM Cell (2D). The USBM cell does not require gluing having mechanical measuring sensors. To determine the 3D stress field tests need to be conducted in three holes.

How many tests should I do?

Generally 3 good overcore tests (including good rock property data) in 1 borehole are sufficient for mine applications, civil engineering projects may require further testing. Ideally the borehole is approximately perpendicular to the principal stress that is most important for the design. For mine design this may be the maximum principal stress and for civil projects this may be the minimum principal stress. To increase confidence in the site result 2 differently oriented boreholes can be tested with 2 to 3 tests in each hole.

Is drill water temperature control important during overcoring?

Yes most definitely. Ideally the rock temperature at the end of the overcore test should be the same temperature as the start of the overcore test. Very large errors can be introduced to the analysis if this is not done. A possible error in the stress magnitude could be as high as 20MPa! Depending on rock type there can be a 30 microstrain change/°C on the strain gauges due to temperature effects. Even though the HI cell has a thermistor to monitor temperature during the overcore it is not possible to adjust strain gauge data during the test. The core must be left to stabilize and the core temperature returned to within 1°C of the initial ambient rock temperature. If this is not possible the overcore should be left until the core has stabilized wrt to temperature. A final set of readings should be taken then a temperature test conducted on the overcore. Then the final strains can be adjusted for these temperature effects. It is also not possible to use the rock thermal co-efficient expansion of the rock as this does not take into account the epoxy HI cell glued to the rock.

How do we control the drill water temperature during the overcore?

The best way to minimize temperature effects is to have the drill water approximately 2 degrees cooler than the ambient rock temperature. This generally compensates for the heat generated by the drill bit during drilling. To maintain good temperature control during overcoring may require either heating or cooling the drill water. We use a specially developed heat exchange unit to heat the water (sometimes by 20 degrees) and to cool the water we use ice. This temperature controlled water is then pumped up the hole for testing.

Can we combine test results from differently orientated boreholes?

Yes we do this all the time and also combine data from CSIRO HI cells and USBM tests as well. We have special software to do this.

Why are individual test results different?

The overcoring process measures strain not stress. Strains for each individual test are collected from a small rock volume only. Jointed or blocky rock, rock foliation, veining and grain size of the rock can affect the strains measured in individual samples. Individual test results should not be discarded just because they are different unless there is a valid reason.

Which result do I use for modelling?

The best approach is to use all the valid data from all the tests. The overcore strains should be assigned individual rock properties determined for that test. All the overcore strains with their respective rock properties should be combined to derive a site result.

Can you average Principal Stresses?

No you can't average Principal Stresses for individual tests. You can average the Normal and Shear Stress components and then transform these to Principal Stresses.

Why do we have our analysis and report peer reviewed ?

We have our data and report internally checked but we also use AMC Consultants to peer review our work. AMC Consultants would probably have more expertise in stress analysis than any one else in the world.