The measurement and interpretation of stress
This paper describes the stress measurements made in the Bowen Basin and Illawarra in the light of geological features. The technique used to gain these measurements is overcoring from surface.
The effects on stress of faults, synclines and anticlines are discussed.
The concept of tectonic strain is presented to provide a more consistent interpretation to stress values in rocks of differing stiffness.
Index
Interpretation Of Stress MeasurementsObservations Of Stress And Tectonic Strain
Application Of The Tectonic Strain Model
STRESS MEASUREMENT
The process of stress measurement can be essentially achieved by two means, overcore or hydrofracture.
Overcoring
Overcoring has been undertaken for many decades. It involves creating a hole, placing some device that measures strains or dimensions in the hole, and then drilling over the top of that hole to relieve the stress and thus cause a dimension change. This dimension change is measured and so is the rock modulus and Poisson’s Ratio. By the use of mathematical formulae it is possible to calculate the magnitude and directions of the stresses existing in the rock.
The system is very reliable but relies on the strain characteristics of the rock being reasonably linear.
The method has until recently been restricted to measurements in short boreholes that are preferably dry. The reasons for this are the need to get information out of the borehole and the use of adhesives to glue strain gauges to the borehole wall. Several overcore devices exist that use the gluing technique.
The most well known of these are the Leeman Cell developed within the CSIR in South Africa and the CSIRO hollow inclusion cell developed in Australia. More recently the Mills cell has been seeing increasing use. All these devices were designed to measure a three dimensional stress field.
The United States Bureau of Mines developed a recoverable overcore cell known as the USBM borehole deformation cell that measured six radial deformations of a borehole which practically converted to three diameteral measurements. This cell could only be used to establish the stress field perpendicular to the borehole.
All these devices relied upon a cable to measure the change in strain with the overcore process.
Hydrofracture
The hydrofracture method of stress measurement involves straddling a section of a borehole with packers and pumping a fluid into the borehole with sufficient pressure to fracture the rock . This is intended to occur in the axis of the borehole. Pressure is permitted to drop off and a crack closure pressure detected. The crack opening pressure is also measured by raising the fluid pressure again until the flow increases indicating crack opening.
The values of stress may be theoretically derived from these pressures and the direction determined by the measurement of crack orientation. The problem with the technique is that the method relies upon scalar measurements to derive a stress tensor. In practical terms the determination of the actual stress values is significantly less certain than those derived from overcoring.
The In Situ Stress Measurement Tool (IST)
The stress measurement technique developed by the author is designed to provide the best possible combination of desirable features. In essence the tool is similar to the United States Bureau of Mines borehole deformation cell in that it is used to measure diameteral change of a pilot hole. The advantages of the tool are that it is smaller, measures six diameters of the pilot hole and is cableless. This means that the use of tool is not restricted by depth. The overcore system is set up to be used as part of a Longyear HQ wireline coring system. It may also be used with NQ wireline operations or with conventional coring.
The process is outlined in Figure 1. It involves pulling the core, then in place of the inner barrel a stump grinding bit is run and used to remove any upstanding core stump. This is withdrawn and a pilot hole drill is used to create a hole 500mm long and 25.5mm in diameter. The pilot hole drill is withdrawn on wireline and the tool lowered into the hole where it locks into place. The rods are pulled back so that the on board orientation tools can detect the location of the tool free from magnetic interference. The core barrel is then pumped into the rods and coring commences.
During the overcoring operations a record of diameteral change is obtained and stored electronically. Once coring has been completed the core containing the tool is pulled and the diameteral measurements and those taken from the accelerometers and magnetometers are downloaded. The core is tested for modulus and Poisson’s Ratio and the results are used with the deformation information to arrive at the biaxial stress field perpendicular to the borehole. Because the orientation of the tool is measured the direction of the principal stresses can be found.
The fact that a continuous trace of diameteral change is recorded enables any invalid traces of diameteral change to be detected. Because of the redundancy in diameteral measurements the six diameter changes may be interpreted in 42 different ways. Thus many cross checks may be applied to the value of stress calculated and a level of error, or certainty, arrived at from the measurements.
The tool has in general performed extremely well giving approximately 75% results out of some 150 measurements to a high level of confidence. Those instances where the tool has not returned a good stress measurement result have generally been due to fractures in the core or in several instances have been brought about by excessive rod slap and hence vibration during coring.
The IST is economic to use. The complete equipment is compact and light (120kg) and thus permits ease of deployment. Measurements can be made in short times.
Stress testing at 350m typically delays the drilling operation by 90 minutes while stress measurements at 90m depth can be undertaken in 30 minutes. The tool has been used to 750m when three measurements were conducted in a 12 hour day.
The tool is also secure, being protected from loss by the ability of the coring method to recover it.
INTERPRETATION OF STRESS MEASUREMENTS
The stress measurements gained from the use of the IST frequently indicated wide variations even in adjacent readings in the same borehole. However the diameteral changes measured during overcoring were not significantly different. The reason for the difference in stress level was the varying stiffness of the strata. This is quite sensible as stiffer rocks can be expected to carry more load when subject to similar horizontal strain as compared to softer materials.
A simple model was therefore derived to examine the strains and stresses in near horizontally layered strata. The model has the following basis:
1) the vertical stress is due to self weight,
2) the horizontal stress is made up of two components: a) due to the effect of self weight acting on a laterally confined segment of strata, and b) due to tectonic strain.
In this context the tectonic strain is that required to load the strata sufficiently to create the stress difference between that measured and that due to self weight.
This model may be thought of as self weight creating a horizontal stress. Added to this the ground has been strained by tectonic events and as a consequence of this stresses have been generated. Tectonic events may create positive or negative tectonic strains.
The use of the term tectonic strain should not necessarily be identified solely with tectonic plate movements. Whilst these do contribute to tectonic strain so to do anticlines, synclines and monoclines. An anticline may be thought of as an upward warping of the crust leading to increased stresses at the base and decreased stress near the surface. Faults are a failures of the rock mass brought about by stress combinations.
The model is simplistic as rock is not elastic, however the model of a tectonic strain has proven extremely useful.
OBSERVATIONS OF STRESS AND TECTONIC STRAIN
The area in the Bowen Basin around the German Creek and Oakey Creek mines has been the subject stress measurement using the IST. In this area the principal tectonic strain has been found to be generally in a North to North Westerly direction. The tectonic strain has been found to vary linearly through the strata, with values increasing with depth. Some notable variations have occurred in these measurements around normal faulting. These areas have shown minor principal stresses perpendicular to the fault as might well be expected.
In BHP Coal’s Illawarra mining leases the concept of tectonic strain applied very well as the rocks were of greatly varying stiffness but the calculated tectonic strain values were extremely even through the sequences tested.
The IST was also used to measure stress in the Cataract Gorge in NSW. The measurements gained there showed a distinct stress concentration perpendicular to the direction of the gorge.
APPLICATION OF THE TECTONIC STRAIN MODEL
It is extremely useful to derive, for an area such as a mine, a relation between tectonic strain, location and depth. This can be achieved through a series of IST measurements. It is then possible to interpolate stresses within this area, provided the rock modulus and Poisson’s ratio are known. By calculating the self weight, imposed stress and the tectonic stress from the tectonic strain the total stress state of the rock can be derived.
An application of this is typically where it is necessary to know the stresses in mine rocks for support design. Here the important number is the ratio of rock strength to stress. The higher the stress and weaker the rock the more likely are roof failure or floor heave problems. By taking core and measuring the modulus and Poisson’s ratio the stress can be calculated. From a strength measurement of the core the ratio of strength to stress can be derived.
The approach suggested provides a basis for dealing with stresses and strains in varying rock types.
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Technical Papers The measurement and interpretation of stress