- April 21, 2017
- Posted by: Ian Gray
- Category: News
The engineering profession needs to measure stress in rock for the purposes of design or for the confirmation of stresses during or following construction.
The question is, what stress matters? In the past the only stress that could be measured was that near an opening and indeed it is frequently the only stress that is really relevant. Its relevance is related to rock strength – has the rock failed, is it about to fail, or is failure a long way off?
The Myth of a Unique Far Field Stress
In the age of numerical modelling the tendency has been to endeavour to measure what is termed far field stress. The supposition being that the far field stress was some unique value and could be incorporated into the model which sufficiently represented the rock mass, so that pre-failure and post failure stresses and associated deformations could be computed.
The reality is that the stress field is frequently complex and there is no such thing as a unique far field stress. The concept of a far field tectonic strain is likely to be more appropriate but there may be complicating factors. For example, movement on a fault invariably de-stresses locally but shifts the load to stress another location. Loading may also be re-established across the fault by continuing tectonic strain. In areas with igneous rock, cooling effects may be important in changing stresses.
A Stress Distribution Model
These variables mean that it is desirable to measure stress in a number of locations and to be able to build up a model of its distribution before undertaking design. Such a model requires a knowledge of the geology and material properties. Once such a model exists it is then possible to embark upon the process of modelling the engineering work in the ground to determine whether the deformations associated with it are acceptable. A good understanding of the various material properties is essential for this. During construction it is generally valuable to monitor the deformations and stress changes occurring to ensure that the ground is behaving as expected. After construction more monitoring may be required to determine if the structure continues to behave as expected.
Fluid Pressure and Effective Stress
How fluid pressures alter the effective stress in the rock also needs to be considered as rock is not a soil, and the fluid pressure cannot be considered to act throughout it. Rock exhibits poroelastic effects where fluid pressure tends to make the rock expand, and de-stress, according to Biot’s coefficients and the rock’s elastic properties. Also to be considered are the effects of fluid pressure acting within the open parts of a fracture surface within the rock mass. In this case the effective stress on the surface may be considered to be the total stress minus the product of the fluid pressure times the fractional surface area over which it acts.
The Measurement of Stress
The measurement of stress in rock is not a simple process as all measurements affect the value of stress being determined. Effective stress, and therefore fluid pressure and Biot’s coefficient, must be considered in these measurements, but is generally ignored. The basic techniques to measure stress are deformation based, failure based and in one case a pressure may be directly related to a stress.
The deformation based measurements are generally stress relief techniques, which include overcoring which may be conducted usefully at surface or down a borehole. In the latter case the tools used are glue-in three dimension devices such as the Leeman cell or the CSIRO hollow inclusion cell. Alternatively there are mechanical devices such as the USBM borehole deformation cell or the Sigra IST tool.
All these deformation devices require that the rock behave in a reasonably elastic manner. An alternative way to measure stress is by hydrofracture, in which the ground is split by fluid pressure. This is practically a biaxial technique which relies on linearly elastic rock properties and some knowledge of Biot’s coefficient for the determination of the major stress. The minor stress may however be determined by the fluid pressure at the closure of the fracture alone. Hydrofracture may also be used to open pre-existing fractures so as to determine the stresses across these. This may be very useful. The failure of a borehole wall by breakout or by the generation of a tensile fracture is an indication of a stress field, but one which lacks any real accuracy in determining the stress magnitude.
An Integrated Programme of Measurement
In designing a programme to determine the stresses in rock, consideration must be given to the likely variability of the stress and the nature of the rock. This will be a function of the lithology and structural geology. In a geologically complex area a mixture of test methods may yield the best result, not least because of the need to contain costs. This might include overcoring, hydrofracturing and the determination of borehole breakout, all examined in the light of the structural geology as revealed by seismic exploration. A single measurement, no matter how high a quality it may appear to be, is unlikely to provide an adequate basis for design, simply because of geological uncertainty and variability. Because of its importance in effective stress fluid pressure measurement is essential, as is a measurement of Biot’s coefficients.
Where a fractured rock mass exists there is a need to understand the normal stress across these fractures, what the fluid pressure is within them, and how much of the fracture area fluid pressure operates on. This is a developing science.
A complete list of Sigra’s Stress Measurement Services can be found under Geomechanics in our Services Menu.