Measurement of Stresses

8/3/2019 Measurement of Stresses

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International Journal of Rock Mechanics & Mining Sciences 40 (2003) 975989

An overview of rock stress measurement methods3. Ljunggrena,*, Yanting Changa, T. Jansonb, R. Christianssonc

0a SwedPower AB, Sweden

0b Golder Associates AB, Sweden

0c Svensk Karnbr.anslehantering. AB, Sweden

Accepted 20 July 2003

Abstract

This paper presents an overview of methods that have been used to estimate the state of stress in rock masses, with the

emphasis on methods applicable to hard rocks and Scandinavia. Rock stress is a difficult quantity to estimate because therock stress measuring techniques consist of perturbing the rock, measuring displacements or hydraulic parameters, andconverting the measured quantities into rock stresses. There are two main types of method: those that disturb the in situ rockconditions, i.e. by inducing strains, deformations or crack opening pressures; and those that are based on observation of rockbehaviour without any major influence from the measuring method. The most common methods are briefly described includingtheir application areas and limiting factors.r 2003 Elsevier Ltd. All rights reserved.

1. Introduction

This paper focuses on the methods for rock stress measurement with the

emphasis on hard rock. It is known that the reliability of rock stress measurements/

estimations is partially dependent on the measuring technique and equipment, and

partially dependent on the nature of rock masses; a large amount of literature exists

on the subject of rock stress and these factors. One relatively recent compilation of

information is the 1997 book by Amadei and Stephansson [1]. The so-called scale

effects have been studied intensively in recent years and a review of this aspect

and a statistical study have been presented by Ljunggren et al. [2]. However, it is

beyond the scope of this paper to discuss scale effects and other similar issues in

detail; instead, the most common methods are highlighted and discussed in the

context of the rock volume included in the measure-ment. Also included are the

conditions for which the different methods are appropriate. Factors such as, inter

alia, the purpose of the measurements, borehole loca-tions, borehole orientations,

geological circumstances and water conditions will have an impact on the decision

*Corresponding author. Tel.: +46-920-77-335; fax: +46-920-77-375.E-mail address: [email protected] (C. Ljunggren).

1365-1609/$ - see front matter r 2003 Elsevier Ltd. All rights reserved.doi:10.1016/j.ijrmms.2003.07.003

on which method to apply. The paperdiscusses both direct measurementmethods, e.g. hydraulic fracturing andovercoring, as well as indicativemethods, such as core discing. Thepaper distinguishes between methodsapplicable from the ground surfaceand methods to be used when there isunderground access, recognizing thatsome methods can be practised fromboth the ground surface and viaunderground access.

It must be emphasised thatengineering and safety issuesshould govern the rock stressmeasurement process. The rock

stress information may be requiredfor the following engineeringaspects, either directly or as input tonumerical models:

0* long- and short-term stabilityof underground struc-tures(tunnels, caverns, shafts andother openings);

0* determination of excavationmethods (drill-and-blast orTBM and raise-boring);


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0* design of rock support systems;

0* prediction of rock bursts;0* thermo-hydro-mechanical behaviour of the rock;0* design of grout methodology;0* fluid flow and contaminant transport;0* fracturing and fracture propagation.

Rock stress measurements arerequired as input information for theabove engineering issues and design.Hence, for each measurementcampaign, the aim of the


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976 C. Ljunggren et al. / International Journal of Rock Mechanics & Mining Sciences 40 (2003) 975989

measurements must be fully understood in order to developa suitable approach strategy to the estimation programme.The objective in determining rock stresses may in manycases be seen as an interactive process. The type of data

needed may change, depending on at what stage anunderground project is in. Furthermore, preliminarymeasurements may reveal information on the stress statethat further advance of the project should be questioned.

For a given project, stresses can be determined using several(direct and/or indirect) methods at different locations. Thisapproach is recommended since it will provide a measure ofconfidence through considerations of the consistency andreliability of the information. The data obtained from each

method should be analysed separately and checked to see ifthe simplifying assumptions associated with each method are

met. The data from different methods may also be combined inorder to impose more rigorous constraints on the in situ

stresses. The combination of data is also useful when a limitednumber of tests from each method is available. Also, stressmeasurements can be conducted in several stages with one orseveral methods. The idea is to use the best attributes of thedifferent methods for a given project. Combining severalmethods (hybrid measure-ment) based on their respectiveattributes can help in obtaining a more reliable assessment ofthe in situ stresses. A more thorough discussion can be found inBrudy [3].

2. Methods

2.1. Types of stress measurement methods

Methods for the determination of in situ rock stress can beclassified into two main categories. The first consists ofmethods that disturb the in situ rock conditions, i.e. byinducing strains, deformations or

crack opening. The followingmethods may be included in thiscategory:

* hydraulic methods, includinghydraulic fracturingand hydraulictests on pre-existing fractures(HTPF),

0* borehole relief methodsand

0* surface relief methods.

The second consists of methods based on theobservation of rock behaviourwithout any major influence fromthe measuring method. Thefollowing methods belong to this

category:

0* statistics of measureddata (database),

0* core-discing,0* borehole breakouts,0* relief of large rock

volumes (back analysis),0* acoustic methods (Kaiser

effect),0* strain recovery methods,0* geological observational

methods and

0* earthquake focalmechanisms.

The methods may also beclassified by their opera-tionaltype and an indication of the rockvolume involved in their useprovided, see Table 1.

The rock volumes presented in

Table 1 above indicate the typical

volumes involved in a test using the

different methods. When conducting

stress measurements, the procedure

is to conduct a series of tests to

obtain accurate and reliable results ata given location or within a

predetermined depth interval. The

details of the stress measurement

programme will depend on what

questions are to be solved in any

specific project. For example, when

the stress state at a location is to be

determined using the overcoring

technique, it is known from

experience that at least five

successful separate tests should be

obtained close to each other in orderto obtain adequate results on the

stress state at that specific location.

Further measurements, given that the


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geology does not change, may not alter the average results.

Table 1Methods for rock stress measurement classified by operational type (the rock volume involved in each method is alsogiven)

egory MethodRock volume(m3)

Methods performed in boreholes Hydraulic fracturing 0.550

Overcoring 103102

HTPF 110Borehole breakouts 102100

Methods performed using drill cores Strain recovery methods 103

Core-discing 103

Acoustic methods (Kaiser effect) 103Methods performed on rock surfaces Jacking methods 0.52

Surface relief methods 12

Analysis of large-scale geological structures Earthquake focal mechanism 109

Fault slip analysis 108

Other Relief of large rock volumes (back analysis) 102103


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C. Ljunggren et al. / International Journal of Rock Mechanics & Mining Sciences 40 (2003) 975989 977

Fig. 1 showsthe rock volumes that, from experienceon rockstresses and geology, may be involved for some different stressmeasurement situations. The main factors limiting the volumefor which the stress field may be judged to be representative arethe vertical depth variation, the geological boundaries, and thepresence of major faults.

If we study the commercial application of stressmeasurements, two methods dominate the others: hydraulicmethods and borehole relief methods. Although bothmethods have undergone continuous development over theyears, the basic principles have remained unchanged andboth techniques have been practised now for severaldecades.

2.2.1. Classical hydraulic fracturing

The term hydraulic fracturing isused for fluid injection operations insealed-off borehole intervals toinduce and propagate tensilefractures in borehole wall rock. It

was first applied, during the 1940s,in the oil industry to stimulateproductivity from low permeable oil-bearing formations. In the beginningof the 1960s it was proposed toderive the state of stress from suchhydraulic fracturing operations. Theclassical concept for theinterpretation of hydraulic fracturingpressure records was developed byHubbert and Willis in 1957, see, forexample, the 2002 paper byRummel et al. [4].

Several authors, e.g. Bjarnason[5] and Ljunggren [6], havepresented the hydraulic fracturingmethod.

2.2. Hydraulic methods

There exist two stress measurement methods that usehydraulics as an active method to stimulate the rocksurrounding a borehole and hence to determine the stress field.These methods are hydraulic fracturing and HTPF. Bothmethods use the same type of equipment, including straddlepackers, impression packers and high-pressure pumps togenerate high-pressure water during either the formation of newfractures or reopen-ing of pre-existing fractures. Fig. 2 presentsan example of equipment that is used for both hydraulicfracturing and HTPF measurements. The down-hole principleduring testing is shown in Fig. 3.

1 2 3 4

6 5

appr.10

m

Fig. 2. Example of equipment for hydraulicfracturing and HTPF rock stressmeasurements: (1) guidewheel formultihose on adjustable working platform,(2) drum for 1000 m multihose, (3) flowmeter manifold and manifold for control offracturing flow and packer pressure, (4)data registration equipment, signalamplifier, chart recorder and portable PC,(5) high pressure water pump and (6) 400 ldiesel fuel tanks, hydraulic pump and tank.


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Fig. 1. Representative volumes involved in the stress measurement tests.


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Fig. 3. Down-hole principle during (a) hydraulic fracturing and (b) HTPF measurements.

A section, normally less than 1 m in length, of a borehole issealed off with a straddle packer. The sealed-off section isthen slowly pressurised with a fluid, usually water. Thisgenerates tensile stresses at the borehole wall.Pressurisation continues until the bore-hole wall rupturesthrough tensile failure and a hydrofracture is initiated. Fig. 4shows an example of a schematic view of a hydraulicfracturing system.

The fracture plane is normally parallel to the borehole axis,

and two fractures are initiated simultaneously in diametricallyopposite positions on the borehole per-iphery. The

hydrofracture will initiate at the point, and propagate in thedirection, offering the least resistance. The fracture will

therefore develop in a direction perpendicular to the minimum

principal stress. The orientation of the fracture is obtained fromthe fracture traces on the borehole wall. Thus, the orientation ofinitiated fractures coincides with the orientation of the maximumhorizontal stress, in a vertical or sub-vertical hole where it isassumed that one principal stress is parallel to the borehole.The fracture orientation may be determined either by use of animpression packer and a compass or by use of geophysicalmethods such as a formation micro-scanner or a boreholeteleviewer.

In its conventional form, the method is 2D: only themaximum and minimum normal stresses in the planeperpendicular to the borehole axis are established. For avertical borehole, these components are the maximum andminimum horizontal stresses. Since the principal stressdirections in tectonically passive and topographi-cally flatareas are usually close to horizontal and vertical, it can oftenbe assumed that the components measured in a verticalborehole are two of the principal stresses.

Hydraulic fracturing is an efficient method for determining the2D stress field, normally in the horizontal plane, and is thereforesuitable at the early stages of projects when no undergroundaccess exists. Due to its efficiency, it is especiallyadvantageous for

Fig. 4. A schematic view of a hydraulicfracturing system (from Rummel et al. [4]).

measurements at greater depth.The method is also notsignificantly affected by the drillingprocesses. Hydraulic fracturingnormally includes largeequipment, which requires space.Furthermore, the theoreticallimitations normally imply that themeasurements should be done invertical holes. Hence, the methodis most suited for surface

measurements in vertical or sub-


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vertical bore-holes.

The hydraulic fracturing method allows a direct measurement

of the least stress in the planeperpendi-cular to the borehole axis,which is normally the least


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horizontal stress, sh and the accuracy is good (B75%). The

maximum horizontal stress is calculated from equationsincluding a failure criteria and parameters evaluated from thefield pressure data. The accuracy is less good for the maximumhorizontal stress (B710 20% or more). In a study by Rutqvist etal. [7] it is shown that the general theory for calculating the

major horizontal stress from the hydraulic fracturing suffers fromuncertainties in the assumptionsa continuous, linearly elastic,homogenous, and isotropic rock to-gether with the fracturereopening. It is probable that the major horizontal stress,determined from hydraulic fracturing, may be somewhatunderestimated when the major principal stress divided by theminor principal stress is close to, or higher than, a factor of 3.

Classical hydraulic fracturing requires sections in theborehole free from fractures. These sections should be at leasta few meters long so that the induced fractures do not interactwith existing ones. Hydraulic fracturing may be difficult to applywith an acceptable success rate in rock domains with very highstresses, such as when core discing is indicated in the corefrom core drilling. Geological features, such as foliation planesin gneissic rock, may also affect the possibilities of success asthey act as weakness planes and thereby may control thedirection of the initiated fracture.

2.2.2. Hydraulic tests on pre-existing fractures (HTPF)

Cornet and Valette [8] first presented the theoretical basisand practical use for the HTPF method. The method is adevelopment of the hydraulic fracturing technique because ituses the same equipment and is based on measurement of thesame parameters. The HTPF method has been practised for

some 15 years. The method has been applied in four projects inthe Nordic countries, Ljunggren [6,9], Bjarnason and Rail-lard[10], Ljunggren and Raillard [11].

Instead of inducing new fractures in intact rock, the HTPFmethod is based on the re-opening of existing fractures found inthe borehole wall and thereby determining the normal stressacross the fracture plane. Depending on assumptions maderegarding the stress field, the HTPF method allows either a 3Dor 2D determination of the stress state. A 3D determinationrequires a larger number of fractures to be tested.

When conducting HTPF tests, it is of importance that thefracture tested is of a size at which the normal stress can beassumed to be uniform and the geometry of the fracture mustbe planar. The HTPF method relies only on four fieldparameters; test depth, shut-in pressure, dip and strike of thetested fracture. The shut-in pressure is equivalent to the normalstress acting across the fracture plane. Given these parametersfor a sufficiently large number of fractures with different strikeand dips, either the 2D or 3D stress state can be determined.

Theoretically the 2D solutionrequires at least six different

fractures to solve the problem. Inpractise some redundancy,

however, is required. For successfulmea-surements, it is suggested that

at least 1012 isolated, pre-existingfractures with different strikes anddips are found and tested in theborehole wall within the depthinterval of interest. The 3Dalternative of the HTPF methodincludes less assumptions on thestress field but requires a largernumber of fractures to be tested. Inthe 3D alternative the vertical stress

does not have to be a principalstress. Theoretically, 12 unknowns

exist in the system of equations. In

practise, it is suggested that at least1820 successful tests are obtainedto resolve the 3D stress field.

As compared to classicalhydraulic fracturing, the methodhas the advantages of lesslimitations as regards geologicalfeatures. Nor does the methodrequire determination of thetensile strength of the rock and itis independent of pore pressureeffects. As long as a variation in

strike and dip of the existingfractures exists in the rock mass,neither weakness planes such asfoliation planes nor core discingshould cause any problems inobtaining successfulmeasurements. The method ismore time consuming thanhydraulic fractur-ing as the down-hole equipment must bepositioned at the exact location ofeach discrete fracture to betested. This requires good

accuracy in the depth calibration.A drawback, compared tohydraulic frac-turing, is also thatno preliminary results can beobtained until all field-testing hasbeen completed, field dataevaluated and those dataprocessed using computer codes.

2.3. Borehole relief methods

The family of borehole relief

methods can be divided into the


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following sub-groups:

0* overcoring of cells in pilot holes,

0* overcoring of borehole-bottom cells and0* borehole slotting.

2.3.1. Overcoring of measuring cells in pilot-holes

Overcoring based on the principle of overcoring a pilot

hole in which the measuring cell is installed can be dividedinto further groups as follows:

0* soft inclusion cells,

0* deformation metersmeasuring displacementsof the wall duringovercoring and

0* stiff/solid cells.

Stiff/solid cells are moreunusual than the other two groups

and have a general problem withthe difference in materialproperties between the rock andinclusion material [1].


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2.3.2. Soft inclusion cells

The principle of a soft cell is based on the theory of linearelasticity for continuous, homogenous and isotropic rocks. Bymeasuring at least six strain components in different directionson the wall of a borehole, the complete stress tensor at the testlocation can be determined. Theories for stress measurements

in anisotropic rocks have also been developed [12].The most common instruments based on the above

principle are:

0* CSIR cell,

0* CSIRO cell and0* Borre Probe cell.

Common to all these three instruments, and a majoradvantage, is that they allow the 3D state of stress to bedetermined from one single measurement point. The method iswell known and has much testing against, for example,temperature changes and calibration against known boundarystresses. The method is considered to be reliable, givenacceptable field conditions (discussed later). Table 2summarises the characteristics and differences between theinstruments. A description of the CSIR and CSIRO cell may befound in Amadei and Stephansson [1].

The Borre Probe cell includes a built-in datalogger, whichruns on batteries and permits a continuous logging of thestrain gauges before, during and after the overcoringprocess, which enhances the evaluation process. In Fig. 5,the principle of the Borre Probe cell is presented.

The triaxial cells with strain gauges in a pilot-hole are quitesensitive to the isotropy, homogeneity and grain size of the

rock. As a consequence, the results from the triaxial cells oftenshow a certain scatter. It could also be argued that scale is apotential limitation here as small scale variations in the rockmaterial composition may affect the results. Another generallimitation for all three instruments are that relatively longunbroken (40 60 cm) overcores are required and hence similarlengths of the borehole free from fractures. High stresses mayalso put limitations on the method as these may initiate corediscing.

Table 2

Characteristics of the most common soft overcoring cells

2.3.3. Deformation metersmeasuring displacements of thewall during overcoring

The principle of deformation meters

for measuring displacements is the

same as for the soft inclusion cells.

The instrument is installed in a pilot

hole and later overcored. These

instruments measure one or several

change in pilothole diameter during

the process of overcoring, instead of

the strain. Two commercial

instruments of deformation-type

gages are the USBM gage and Sigra

in situ stress tool (IST). The USBM

gauge is extensively used and one of

the most reliable and accurate

instruments for determining in situ

stresses in rock by overcoring [1].

The theory for the USBM is describedin [1] and is in principle the same for

the Sigra IST. Tables 2 and 3

summarises the characteristics and

difference between the two

instruments.

Some of the disadvantages of thegauges are that: it requires anunbroken core of at least 300 mm inlength; the gauges can be damagedif the core breaks; three non-parallelholes are necessary to calculate thein situ stress field; and the gauge

depends on the minerals in contactwith the gauge pistons.

Fig. 5. Principle of soft, 3D pilot holeovercoring measurements:

(1) advance +76 mm main borehole to

measurement depth, (2) drill +36 mm pilot

hole and recover core for appraisal, (3) lower

probe in installation tool down hole, (4) probe

releases from installation tool; gauges

bonded to pilot-hole wall under pressure from

the nose cone,


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(5) raise installation tool; probe bonded in place and (6) overcore the probe and recover to surface in core barrel.

ument No of active gauges Measuring depths Continuous logging Borehole requirements

CSIR cell 12 Normally: 1050 m, modified No 38 mm pilot hole, usually 90 mm

versions: up to 1000 mdrillhole. Modified versions acceptwater

CSIRO cell 9/12 Normally: up to 30 m Yes, via cable 38 mm pilot hole, usually 150 mm drillhole. Problems in waterfilled holes

Borre probe cell 9Practised to 620 m. Testedfor Yes, built in datalogger 36 mm pilot hole, 76 mm drillhole.

1000 m Accepts water-filled holes


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2.3.4. Overcoring of borehole-bottom cells

Methods for overcoring of borehole-bottom cells discussedin this paper include the following:

0* Doorstoppers and

0* spherical or conical strain cells.

The Doorstopper cell, Leeman [13,14], is attached at thepolished flat bottom of a borehole, Fig. 6, while the hemi-spherical or conical strain cell is attached to the hemi-sphericalor conical bottom of the borehole, Fig. 7. Hence, they do notrequire a pilot hole. After the cell has been positioned properlyat the end of the borehole and readings of the strain gaugeshave been performed, the instrument is overcored. Duringovercoring, the changes in strain/deformation are recorded.

Leeman op.cit. developed a cell with strain gauges thatcould be cemented on the bottom of 60 mm

Table 3

Characteristics of two instruments of the deformation-type gauge

boreholes and overcored. The cellis often referred to as CSIR (Councilfor Scientific and IndustryResearch) Doorstopper and hasbeen used for measurements in 60m deep boreholes. The CSIR

Doorstopper is 35 mm in diameterand at the base of the gauge astrain rosette consisting of 3 or 4strain gauges is cemented. The cellis pushed forward by compressedair and glued at the base of a drillhole. Reading of the strain gaugesis taken before and after overcoringof the cell.

A modified doorstopper cellcalled the Deep Door-stopperGauge System (DDGS) has beendeveloped

!

jointly by the Rock MechanicsLaboratory at Ecole Polytechnique in

Montreal! and the Atomic Energy of

Canada. The DDGS was designed to

allow overcoring measurements at

depths as great as 1000 m in sub-

vertical boreholes [16]. The device

utilises an Intelligent Acquisition

Module, a remote battery-powered

data

Instrument No of active gauges Measuring depths Continuous logging Borehole requirements

USBM Normally 3; modified Normally 1050 m; modified No 38 mm pilot hole, usua

versions 4 versions up to 1000 m drillhole. Modified versiwater

Sigra IST3, in two or threelevels Used to 700 m. Designed for Yes, built in datalogger 25 mm pilot hole, 76 m

1500 m Accepts water-filled ho


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Fig. 6. Installation of Doorstopper (after INTERFELS): (a) cored borehole NW=76 mm, (b) borehole bottom flattenedand (c) polished.


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logger that collects and stores strain data during stressmeasurement tests. The principle of the DDGS installa-tionis shown in Fig. 8.

Successful measurements have been performed at theUnderground Research Laboratory (URL), Canada [16]. Themeasurements were made at borehole depthsas great as 518

m (943 m depth from surface), where both hydraulic fracturingand triaxial strain cells were not applicable at depths deeperthan 360 m because of the high stress situation.

Fig. 7. Coordinate system and strains to be measured on a conical bottom

surface (after Obara and Sugawara [15]).

An advantage for theDoorstopper, as well as the conical

or spherical methods, is that they donot require long overcoring lengths,

i.e. only some 5 cm, as compared tothe pilot hole methods (at least 30

cm). As the methods do not requirea pilot hole there are also betterpossibilities for successfulmeasurements in relatively weak orbroken rock, as well as in rocksunder high stresses in which corediscing is common. Compared totriaxial cells, a Dorrstoppermeasurement requires less time,and 23 tests can be conducted per

day. Other advantages, valid onlyfor the modified Doorstopper,

include possibilities for continuous

mon-itoring and the application inwater-filled boreholes.

The disadvantage with thedoorstopper is, however, thatmeasurement at one point onlyenables the stresses in the planeperpendicular to the borehole to bedetermined. Furthermore, the end ofthe borehole must be flat whichrequire polishing of the hole bottom.

For doorstopper cells, solutionsfor anisotropic rocks have also beendeveloped. Corthesy et al. [18,19]developed a mathematical model toaccount for both non-linearity andtransverse isotropy in the analysisof overcoring measurements withthe CSIR Doorstopper.

Doorstopper methods havebeen developed and practised formore than 20 years worldwide.

Using a hemispherical or conicalstrain cell for measuring rockstresses, a borehole is first drilled.Its bottom surface is then reshaped

into a hemispherical or conicalshape using special drill bits.Thereafter, the strain cell is bondedto the rock surface at the bottom ofthe borehole. The latest version ofthe conical strain cell, equipped with16 strain components, has beensuc-cessfully tested by, e.g., Obaraand Sugawara [15].


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Fig. 8. Installation of the DDGS [17]: (1) After flattening and cleaning of the bottom, the instruments are lowered down the holewith the wire line cables. (2) When the DDGS is at the bottom the orientation of the measurement is noted in the orientationdevice and the strain sensor is glued.

(3) The IAM and Doorstopper gauge are removed from the installation equipment. (4) The installation assembly isretrieved with the wire line system. (5) The monitoring and overdrilling start, the strain change in the bottom ismeasured by the time. (6) When overdrilling is completed, the core is taken up and a bi-axial pressure test done toestimate the Youngs modulus.


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Measurements with the conical borehole technique havebeen made mostly in Japan. This technique has been foundto be a useful method for measuring rock stress in a singleborehole and in various rock types.

Like the Doorstopper, a small length of the rock is required forovercoring. For the conical cell, the stress relief is achieved at

an overcoring distance of 70 mm and then the strains remain atconstant values [15]. Hemi-spherical or conical strain cells havemostly been used in Japan and successful applications havebeen reported in the literature (e.g. Matsui et al. [20]).Disadvantages with the conical or hemispherical cell are thatthey require preparation of the borehole bottom, either in theform of a cone or as a sphere. Another limitation is their poorsuccess in water-filled boreholes.

2.3.5. Borehole slotting

The borehole slotter consists of a contact strain sensor,which is mounted against the wall of a large-diameter

borehole, Bock [21]. Thereafter, three slots, 120

apart, arecut into the wall, see Fig. 9. A small, pneumatically drivensaw cuts the slots. Each slot is typically 1.0 mm wide and upto 25 mm deep. Tangential strains induced by release oftangential stresses by the slots are measured on theborehole surface. It is based on the theory of linear elasticbehaviour of the rock and uses the Kirsch solution forstresses and strains around a circular opening.

The most significant advantage with the method is that it doesnot require any overcoring. The method is also quick andbetween 10 and 15 measurements can be made during a dayswork. The instrument is fully recoverable and providescontinuous monitoring of strain as the slot is cut. The method

also contains a number of limitations, such as that the boreholemust be dry, it has only been applied in boreholes at shallowdepths, and the stress parallel to the borehole axis must beknown. The borehole slotter has been designed to work inboreholes with a diameter between 95 and 103 mm. In general,good agreement has been found

Fig. 9. Cross-section of the borehole slotter (INTERFELS).

between stress measurementswith the borehole slotter andmeasurements with othertechniques.

2.3.6. Summaryborehole reliefmethods

There exists a variety of the so-called borehole relief methods.Some of them, however, are stillin the development stage, andhave not become commerciallyavailable. Today the pilot holemethods dominate and are usedon a regular basis in manyunderground projects. As aconsequence, their mainapplication areas and limitationsare fairly well known. Of the other

methods, the doorstopper is thenext most used technique, mainlyto perform stress measurementsin highly stressed rock volumes orwhen the fracturing is too intenseto allow for pilot holemeasurements. The othermethods, although theoreticallyadequate, have only been usedon a few occasions, due to theirlimitations. The conical orhemispherical cell has mainlybeen used in Asia.

2.4. Surface relief methods

This category of methods

measures the rock response to stress

relief (by cutting or drill ing) by

recording the distance between

gauges or pins on a rock surface

before and after the relief. Examples

of the technique are the flat jack

method and the curved jack method.

The category is most suitable for

measurement on tunnel surfaces Forinformation the reader is referred to

Amadei and Stephansson [1] and de

Mello Franco [22].

2.5. Borehole breakouts

Borehole breakout is aphenomenon that occurs whenthe rock is unable to sustain thecompressive stressconcentrations around a borehole,see Fig. 10. This results in


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breakage of the wall on two diametrically opposed zones,called breakout.

Leeman [13] first reported the use of borehole breakouts forthe purpose of rock stress determination. Breakouts were foundto occur along the direction of the least principal stress.Therefore, breakouts are generally used to determine theorientation of in situ stresses.

Attempts can be found in the literature where the authors use the

depth and width of the borehole breakouts in order to estimate themagnitude of the rock stresses (e.g. Haimson and Lee [23]). It has

been found that the shape and depth of breakouts in vertical holes

depend on the magnitude of the major

and the minor horizontal in situ

stresses. This has led several authors

to suggest that the geometry of the

breakouts could be used for

estimating the magnitude of the in situ

stresses. However, this approach

must be used with caution as

breakouts can be enlarged becauseof various other factors, not directly

the stress concentrations. Breakouts

also provide a valuable link between


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Fig. 10. Development of borehole breakouts.

overcoring, hydraulic fracturing and focal mechanism data[24].

To characterize breakouts, logging of the diametrical shape ofthe borehole is required. Such tools can be a caliper like adipmeter, a televiewer, formation micro-scanner or a highresolution TV camera. The use of borehole breakouts as anindicator of the stress field has been used in some projects withdeep drilling such as: the KTB hole in northeastern Bavaria, TeKamp [25], the Cajon Pass hole in the vicinity of the SanAndreas fault in southern California, Shamir et al. [26], and the

borehole for deep earth gas in Precambrian rocks of Sweden,Stephansson et al. [27].

The methods major advantage is that it is quick to use andrequires only measurement of the diametrical changes of theborehole wall to obtain information on the orientation of thestress field. Another advantage is that it may reveal informationvalid at great depth where other methods may be insufficient.The major limitation of the method is that it only works ifbreakouts exist. For example, breakouts do not normally exist inSwedish rocks above 1000 m, so the method cannot be used toobtain information above these depths. Also, the method cannotbe used to determine the magnitudes of the stresses.Anisotropy of the rock may disturb the breakout locations and

thereby also jeopardise the usefulness of the information.

2.6. Core discing

When boreholes are core drilled in highly stressed regions, the

rock core often appears as an assemblage of discs. These discs

sometimes exhibit parallel faces but are often shaped like a horse

saddle. This phenomenon has been called core discing. Attempts

have been made

to analyse this discing process inorder to extract information on thelocal in situ stresses, Hakala [28]and Haimson [29].

Much experimental work has beencarried out at the University ofWisconsin over the last decade in thearea of core discing and boreholebreakouts [29]. The studies show thathigh stresses bring about failure, notonly at the borehole wall (resulting in

breakouts), but also in the base of thecore, giving rise to discing. Coretensile fractures initiated below thecoring-bit extend toward the axis of

the core with slight downward tilt inthe direction of the least horizontal

stress, sh: In the maximum horizontal

stress, sH; direction the same cracks

are practically horizontal. As drillingadvances, these fractures openresulting in saddle-shaped discs

having a trough axis oriented in the sHdirection. This observation reinforces

the idea that discs recovered fromoriented cores could be used as

indicators of the in situ sH orientation.

In addition, the laboratory testssuggest that disc thickness isindicative of the level of theapplied stresses. Carefulmeasurements of core discdimensions show that, for givenmagnitudes of sh and sv; thethickness of discs decreasesmonotonically with increasing s

H;


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Fig. 11. The results indicate that the sH magnitude andorientation could be estimated from the average discthickness and trough axis attitude, provided extractedoriented-core is disced and the relation between thicknessand sH is established. Haimson op.cit., also pointed out that,together with borehole breakouts, core discing can provideupper limits on in situ stress levels and help assess the

maximum horizontal stress fromthe measurements of character-istic dimensions.

Based on numerical analyses,Hakala [28] has suggested amethodology for interpretation ofin situ


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Fig. 11. Example of relation between disc thickness td

(normalised by core

diameter) and sH for given sh

and sv [29].

stresses from core discing. The following minimuminformation is needed for the interpretation:

0* the tensile strength of the rock,0* Poissons ratio of the rock,0* the uniaxial compressive strength of the rock,0* the mean disc spacing,0* the shape of the fracture (morphology) and0* the extent of the fracture in the core.

According to Hakala, op. cit., the confidence of theinterpretation can be increased considerably if the sameinformation can be achieved from both normal coring andovercoring at the same depth level.

In practice, core discing can only be used as an indicatorfor estimation of rock stresses. When core discing occurs,

one can of course also conclude that rock stressconcentrations are higher than the rock strength. Suchinformation, obtained already during the drilling stage, is ofcourse valuable and a guide for further decisions.

2.7. Relief of large rock volumes

During excavation of underground openings, defor-mations/displacements of the rock mass will occur. Thesedisplacements may be measured using instrumen-ted crosssections in the openings, Sakurai and Shimizu [30]. Thedisplacements measured are related to the in situ stresses byuse of numerical methods. More recently, Sakurai andAkutagawa [31] also presented material to account for non-elastic rock behaviour. Methods based on the above principleare often referred to as back-analysis methods.

The advantage of back analysis methods is that they are quick to

use, require limited and simple measure-ment procedure and

include large volumes. Other advantages are that they may be

used to change support systems and excavation schemes on an

almost real time basis. The limitations include that the technique

can only be used on underground openings and only during

the excavation process ofunderground openings. Themethod requires numericalanalyses and does not allowunique solutions.

2.8. Strain recovery methods

Measurement of stress fieldparameters using the method ofstrain recovery is based on theprinciple that a core, after relief fromthe surrounding stress field,expands more in the direction of themaximum stress direction and lessin the direction of the minimumstress. This relaxation consists ofan instantaneous elastic componentand a time-dependent anelastic

component.Methods that utilise the

principle of strain recovery are:

0* anelastic strain recovery(ASR) and

0* differential strain curveanalysis (DSCA)

Both methods are applied ondrill cores from bore-holes.

2.8.1. Anelastic strain recovery (Asr)

When a drill core is removedfrom the rock mass, it tends torelax and expand, due to thatstresses are removed from thecore. Field measurements showthat the opening and propagationof preferential microcracks usuallyaccompanies anelastic strainrecovery. By measuring the strainrecovery, see Fig. 12, the orienta-tion of the principal strains can bedetermined. Thus the orientationof the principal stresses can also

obtained. However, determinationof the magnitude of the stresses ismore difficult and a constitutivemodel for the rock must be used.

The ASR method requires thatthe core is orientated, which may becostly. The ASR method, which isnot commonly used, has mainlybeen tested in very deep


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Fig. 12. Instrument for ASR measurements ofa drill core. Three pairs of radial inductivedisplacement transducers and one axial

transducer are used to measure the anelastic

response of a core sample.


23/29


boreholes where methods such hydraulic fracturing have notbeen possible or too expensive to use.

The following nine parameters could significantly limit theapplication of the ASR method, Teufel [32]:

(1 )temperature variation yielding thermal

strains,

(2 )dehydration of core samples, (3) pore fluid

pressure

diffusion, (4) non-homogeneous recovery deformation,

(5 )rock anisotropy, (6) drilling mudrock

interaction,(7 ) residual strains, (8) core recovery time and(9 ) accuracy of core orientation.

Studies show that the stress orientation obtained with theASR method did not coincide well with that determined byovercoring. The difference was attributed to difficulties inmeasuring small strains, Matsuki and Takeuchi [33].

2.8.2. Differential strain curve analysis (DSCA)

This method is based on the concept that careful monitoring ofthe strain behaviour of a rock specimen upon re-loading can reflect

its past stress history, Amadei and Stephansson [1]. After an

oriented drill core is brought up to the surface, the micro-cracks

have had time to develop and to align themselves in the direction of

the original stresses. By subjecting the core to hydrostatic loading,

the strain due to the closure of the micro-cracks can be obtained by

the core with strain gauges.

To determine the in situ stresses, assumptions are made:

* The principal directions of the in situ stress fieldcoincide withthe principal directions of the strain due to the closure of themicro-cracks.

0* The ratios between the three principal stresses are tobe related to the ratios between the three principalstrains due to the closure of the micro-cracks.

Thus, once one principal stress is known (e.g. assumingthe vertical stress is equal to the weight of the overburdenrock), the other two principal stresses can be determined. Anadvantage with the method is that it may be applied toestimate stresses at large depth as long as it is possible torecover a core. Major limitations include that it includesmeasurements on a micro-scale and involves very smallvolumes. The method is only 2D. Studies have found that thestress orientations determined with the DSCA method

correlated poorly with those determined by the over-coring.

2.9. Geophysical methods

The following geophysical methods for estimation of in situstresses have been reported in the literature:

0* seismic and micro-seismic methods (i.e. Martin et al.[34]),

0* sonic and ultrasonic methods [35],

0* radio-isotope method[36],

0* atomic magnetic method[37],

0* electromagnetic methods[38]) and

0* acoustic methods (Kaisereffects, 1950now).

The techniques listed above have

not gained much popularity inpractice. Some remarks need to be

made, however, about the lastmentioned method (the Kaiser

effect), which has been investigatedover the past 15 years as apotential method to determine insitu rock stresses. Researchoriginally conducted by Kaiser in

1950 on acoustic emission of rockrevealed that when the stress onrock relaxed from a certain leveland then increased, there is asignificant increase in the rate of

acoustic emission as the stressexceeds its previous higher value. It

has been hypothesised that thestress experienced by a rock in situ

could be inferred by monitoring theacoustic emission on core samples

cut from different directions andloaded cyclically in uniaxial

compression in the laboratory. Themaximum stress that a rockspecimen has been subjected tocan be detected by stressing thespecimen to the point where is asubstantial increase in acousticemission (AE).

An extensive review of thedifferent studies conducted on theKaiser effect was conducted byHolcomb in 1993. Despiteencouraging results obtained byseveral authors showing a fairlygood correlation between stressesdetermined with Kaiser effect andwith other methods, the researchcarried out by Holcomb revealedthat using the acoustic emissionemitted during uniaxial compres-sion laboratory tests to infer in situstresses could not be justified.

Utagawa in 1997 concluded thatthe stresses deter-mined by AEwere consistent with the stressesfrom the overcoring method and

hydraulic fracturing method. There


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was also significant correlation between the overburdenpressure and estimated vertical stress from AE. It is not adirectly commercially available method and there still exists adiscussion on its justification as a method to determinestresses.

A method called RACOS is a relative new geophysical method to

determination the in situ stress magnitudes and their orientations

[39]. The principles of the method are the same as the DSCA

except that the compression and shear wave propagations aremeasured instead of the strain. The method is based on the idea

that the re-loading on a sample can reflect its past stress history.

The measurements are made on a total of 5 cubic pieces, with a

length of 25 mm. The pieces are

placed in a pressure cell and

subjected to a defined hydrostatic

loading while the transmission times

for the compres-sion and shear waves

are measured between opposing

block face. The analysis is based on

the change and different in velocity of

the waves during the loading steps.The velocity change depends on the

opening or closure of micro-cracks in

the pieces. The method has


25/29


Table 4Stress measurement methods and key issues related to their applicability

Method 2D/3D Advantages Limitations Suitable for

Overcoring 2D/3DMost developed technique inboth Scattering due to small rock

theory and practice volume. Requires drill rig

Doorstopper 2D Works in jointed and high Only 2D. Requires drill rigstressed rocksHydraulic fracturing 2D Measurements in existing hole. Only 2D. The theoretical

Low scattering in the results. limitations in the evaluation of

Involves a fairly large rock sH: Disturbs water chemistryvolume. Quick

HTPF 2D/3D Measurements in existing hole. Time-consuming. RequiresCan be applied when highstresses

existing fractures in the holewith

exist and overcoring and varying strikes and dipshydraulic fracturing fail

Core discing 2D Existing information, which is Only qualitative estimationobtained already at the drillingstage

Borehole breakouts 2D Existing information obtained at Restricted to information on

an early stage. Relatively quick orientation. Theory needs to befurther developed to infer thestress magnitude

Focal mechanisms 2D For great depths Information only from greatdepths

Kaiser effects 2D/3D Simple measurements Relatively low reliabilityASR/DSCA/ 2D/3D Usable for great depths Complicated measurements onRACOS the micro-scale, sensitive to

several factors

Back calculation 2D Quick and simple. High certaintyTheoretically not uniquesolution

due to large rock volumeAnalysis of geological 2D/3D Low cost Very rough estimation, lowdata reliability

Measurements, depthdown to 1000 mFor weak or high stressed

rocks

Shallow to deepmeasurements. To obtainstress profiles

Since the method is time

consuming, it is of most

interest in situations where

both overcoring and

hydraulic fracturing fail

Estimation of stress at early

stage

Occurs mostly in deepholes

Rough estimations

Estimation of stress stateat great depth

Can only be used during

construction of the rockcavern At early stage ofproject

been applied to sedimentary rocks and metamorphic rock but the results have not beencompared with any established stress measurement method.

2.10. Geological observational methods

From the literature there can be found numerous suggestions for estimating orientationsand/or magni-tudes of the stress field from geological structures, for example faults, dikes,volcanoes, lineaments. A discus-sion of the subject can be found in Amadei andStephansson [1].

It should be pointed out, however, that most geological structures were formed a long timeago and the mechanism controlling the stress field at that time may have changedsignificantly since then. Plate tectonic driving forces may have had a different pattern thantoday, erosion may play a role, and a number of glacial periods have passed in some partsof the world. Hence, caution must be used if stress information is estimated from geologicalstructures.

2.11. Earthquake focal mechanisms

Seismologists have studied the relation between the fault slip occurring during an event andthe state of

stress. Severalattempts have beenmade to correlatestress magnitudes todata fromearthquakes butthese analyses havelimitations. It is,however, possible touse the method to

estimate thedirections of theprincipal stresses. If aseismic acquisitionnetwork exists, themethod allowsmonitoring of eventson a continuous basisif earthquakes occur.As earthquakesnormally occur atgreat depth, themethod only gives


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information on the stress orientations at great depths, which may not be relevant for engineeringat shallow depth, i.e. between 0 and 1000 m.

3. Summary of methods and conclusions

The previous sections have given a brief review of the most common methods todetermine or estimate information about the in situ rock stresses. When studying the stress

measurement methods available, the following can be concluded.

0* A few methods dominate the others. These methods are commercially available; theyhave been applied world wide; many reference data exist; their accuracy given ideal

conditions iswell known;the methodshave beenbenchmarkedagainst eachother; they


27/29


have been practised for many years; and have beendeveloped for the purpose of determining either the 2D or 3Dstress field parameters. The methods in this category arehydraulic fracturing, overcoring, the doorstopper techniqueand back analysis.

0* There exists a number of methods that are based on

the interpretation of geological and/or stress relatedphenomena where no actual measurement of thestress field is performed. Normally these methods areless accurate, as compared to the true measurementmethod mentioned above, but the information may beobtained at a lesser price and they may also serve as acomplement to the true measurement methods. Forcertain conditions, these methods may also be theones to use to obtain stress field information. Typicalmethods that fall into this category are core discing,

borehole breakouts, and analysis of geologicalstructures.

0* A category of methods includes those that do nothave the general advantages of the most commontrue measurement methods but which under certaincircumstances may be of great value. Mostly, thesemethods have not yet found their full commercialplatform and are the ASR method, the DSCAmethod, the evaluation of focal mechanisms andmost of the geophysical methods.

The methods are summarised in Table 4 with focus onsome of the key parameters.

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Measurement of Stresses