insitu stress

8/17/2019 Insitu Stress

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WHAT ARE IN SITU STRESSES?

•Rock at depth is subjected to stresses from weight ofoverlying strata and from locked in stresses of tectoniorigin

• These are called “In situ stresses”


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ESTIMATION OF VERTICAL STRESS

sv = gzwhere s

v

is the vertical stress

g is the unit weight of the overlying rock and

z is the depth below surface

• As an example, consider an element of rock at a depth of 1,000below surface

•

Weight of the vertical column of rock resting on this element isproduct of depth and unit weight of overlying rock mass (typicabout 2.7 tonnes/m3 or 0.027 MN/m3)

• Hence vertical stress on the element is 2,700 tonnes/m2 or 27


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ESTIMATION OF HORIZONTAL STRESS

•

Horizontal stresses acting on an element of rock at a d“z” below the surface are much more difficult to estimthan the vertical stresses

• Normally, the ratio of average horizontal stress to vertstress is denoted by the letter “k” such that:

sh = k sv = k gz


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• Terzaghi and Richart (1952) suggested that the value of “k” isindependent of depth and is given by

k = n (1 − n)

where, n is the Poisson's ratio of rock mass

•

This relationship was widely used in the early days of rock mecbut later it proved to be inaccurate and is seldom used today

• Measurements of horizontal stresses at civil and mining sites athe world show that the ratio k tends to be high at shallow depthat it decreases at depth


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• Sheorey (1994) developed an elasto-static thermal stress modearth. This model considers curvature of the crust and variatioelastic constants, density and thermal expansion coefficients ththe crust and mantle

• Following simplified equation was provided for estimating “k”

k = 0.25 + 7Eh (0.01 + 1/z)

where, E h (GPa) is the average deformation modulus of theupper part of the earth’s crust measured in a horizontal dir

Direction of measurement is important particularly in layersedimentary rocks, in which deformation modulus may besignificantly different in different directions


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•

A plot of this equation is given in next slide for a rangedeformation moduli

The curves relating k with depth below surface z are simthose published by Brown and Hoek (1978), Herget (19and others for measured in situ stresses.

Hence the equation is considered to provide a reasonabbasis for estimating the value of “k”


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ESTIMATION OF HORIZONTAL STRESS ( SHEOREY , 1

k = horizontal stress/ vertical stress

D e p t h b e l o w

s u r f a c e , z ( m )

h = k v

Where,h

is

horizontal s

0 1 2 3 4

0

1000

2000

3000


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ESTIMATION OF HORIZONTAL STRESS (BROWN & HOEK ,


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WHY AND WHEN IS IN SITU STRESS MEASUREMENNECESSARY?

• Measured vertical stresses are often not equal to calculated

overburden pressure• Presence of much higher horizontal stresses at some locations

estimated values

• Two horizontal stresses are seldom equal

• These inconsistencies occur because several factors influence i

stresses, such as, depth, anisotropy, stratification, geologicalstructures and topography

Therefore, where in situ stresses are likely to have a significinfluence on the behaviour of underground openings, it isrecommended that the in situ stresses should be measured


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WORLD STRESS MAP

• The World Stress Map project, completed in July 1992, involved overscientists from 18 countries and was carried out under auspices of tInternational Lithosphere Project (Zoback, 1992)

• The aim of the project was to compile a global database of contempotectonic stress data

• The World Stress Map (WSM) is now maintained and it has been extby Geophysical Institute of Karlsruhe University as a research projecHeidelberg Academy of Sciences and Humanities

• The 2005 version of the map contains approximately 16,000 data sevarious versions of the map for the World, Europe, America, Africa, AAustralia can be downloaded from the Internet

• The WSM is an open-access database that can be accessed at www.wstress-map.org (Reinecker et al, 2005)

WORLD STRESS MAP GIVING ORIENTATION OF

http://www.world-stress-map.org/http://www.world-stress-map.org/http://www.world-stress-map.org/http://www.world-stress-map.org/http://www.world-stress-map.org/


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WORLD STRESS MAP GIVING ORIENTATION OFMAXIMUM HORIZONTAL COMPRESSIVE STRESS


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STRESS MAP OF THE HIMALAYAS GIVING ORIENTATIONS OF MAXIHORIZONTAL COMPRESSIVE STRESS ( WWW.WORLD- STRESS -MAP.O


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DEVELOPING A STRESS MEASURING PROGRAM

• During preliminary design, first rough estimate of verand horizontal in situ stresses can be made using simpequations (sv = gh and sh = k sv)

• If a preliminary analysis of stresses induced around thproposed tunnel shows that these induced stresses arlikely to exceed the strength of the rock, the question

stress measurement must be considered in more deta(Note that for many openings in strong rock at shallowdepth, stress problems may not be significant and theanalysis need not proceed any further)


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DEVELOPING A STRESS MEASURING PROGRAM

•

Next step would be to search the literature to determiwhether the results of in situ stress measurements areavailable for mines or civil engineering project withinradius of 50 km. If yes, these can be used to refine theanalysis.

•

If significant zones of failure are likely to develop aroutunnel, it is justifiable to set up a stress measurementprogramme at site


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CONCEPT OF STRESS


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PRINCIPAL STRESSES IN AN ELEMENT OF ROCK CLTO A HORIZONTAL TUNNEL

Vertical in situ stress, v

H o r i z o n t a l i n s i t u s t r e s sHorizontal in situ stress, h2

Horizontal

tunnel

Induced principal

stresses

h1


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PRINCIPAL STRESS DIRECTIONS IN ROCKSSURROUNDING A TUNNEL

• The three principal stresses are

mutually perpendicular but they

may be inclined to the direction of

the applied in situ stress

• The longer bars in this figure

represent directions of maximum

principal stress , while the shorterbars give the directions of the

minimum principal stress at each

element considered


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CONTOURS OF MAXIMUM AND MINIMUM PRINCIPASTRESS MAGNITUDE

• Redistribution of stresses is

concentrated in the rock

close to tunnel, and

• at a distance of say three

times the radius from

centre of the hole, thedisturbance to the in situ

stress field is negligible.


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NUMERICAL METHODS OF STRESS ANALYSIS

To be covered later in the course

Some examples, however, arepresented here


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COMPARISON OF THREETUNNEL EXCAVATION PROFILES

Floo

stre

on s

larg

mom

Floo

sign

solu

case

Cir

sol

cas


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LARGE UNDERGROUND


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LARGE UNDERGROUNDCAVERNS – COMPARISON OFTHREE LAYOUTS

LARGE UNDERGROUND CAVERNS

DISPLACEMEN


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LARGE UNDERGROUND CAVERNS – DISPLACEMENVECTORS AND DEFORMED EXCAVATION SHAPES

• Smaller of

excavationtowards thand its proin this pro

• This distorreduced by

transformeand by incspacing becaverns asprevious s


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IN SITU STRESS MEASUREMENT METHODS

• Methods that disturb in situ rock conditions

Hydraulic methods (Hydraulic fracturing and HTPF)

Borehole relief methods

Surface relief methods


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IN SITU STRESS MEASUREMENT METHODS

• Methods based on observation of rock behaviour

statistics of measured data (database)

core-discing

borehole breakouts

relief of large rock volumes (back analysis)

acoustic methods (Kaiser effect)

strain recovery methods

geological observational methods and

earthquake focal mechanisms


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HYDRAULIC FRACTURING TEST (HF)

• HF is a borehole field-test method for assessing the state o

situ stress in earth crust.

• This method is also referred to as hydrofracturing, or hydrand sometimes as minifrac

• Successful HF tests result generally in an estimate of statesitu stress (both magnitudes and directions) in the plane

perpendicular to axis of borehole• When both the borehole and the induced HF are nearly ve

the stress component in the direction of hole axis is takenbeing principal and equal to overburden weight


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HYDRAULIC FRACTURING TEST (HF)

• Domain of application of HF method has been extended wHTPF method (Hydraulic Testing of Pre-existing Fractures

• HTPF method provides an evaluation of complete stress te(6 components), independent of borehole orientation andmaterial properties

•

When possible, both methods should be combined for optresults

• HF and/or HTPF are used routinely as part of sitecharacterization of large engineering underground structu


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HYDRAULIC METHODS

• Both Hydraulic Fracturing and HTPF use same kind ofequipment (straddle packers, impression packers andpressure pumps) to generate high-pressure water dureither

formation of new fractures (in Hydraulic fracturing te

reopening of pre-existing fractures (in HTPF)

PRINCIPAL OF HYDRAULIC FRACTURE TEST


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• A section of borehole, norless than 1m in length, is

off with a straddle packer• Sealed-off section is then

pressurised with a fluid, uwater

• This generates tensile strat borehole wall

• Pressurisation continues borehole wall ruptures thtensile failure and ahydrofracture is initiated


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• Fracture plane is normally parallel to the borehole axitwo fractures are initiated simultaneously in diametriopposite positions on borehole periphery

• Hydrofracture will initiate at the point, and propagatedirection offering the least resistance

• The fracture will therefore develop in a directionperpendicular to the minimum principal stress


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• Orientation of the fracture is obtained from

the fracture traces on borehole wall• Thus, the orientation of initiated fractures

coincides with the orientation of maximumhorizontal stress, in a vertical or sub-verticalhole where it is assumed that one principalstress is parallel to the borehole

•

Fracture orientation may be determinedeither by use of an impression packer and acompass or by use of geophysical methodssuch as a formation micro-scanner or aborehole televiewer

Fracture ori

determinati


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TYPICAL PRESSURE-TIME CURVE

Pc = initial

down pres

Pr = Reopepressure

Psi = shut-ipressure

These valuused to calsitu stress

SCHEMATIC VIEW OF A HYDRAULIC FRACTURING


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SCHEMATIC VIEW OF A HYDRAULIC FRACTURINGSYSTEM


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HYDROFRAC SET-UP AND TESTING AT A SITE J&K


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HYDROFRAC SET-UP AND TESTING AT A SITE J&K


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ADVANTAGES & LIMITATIONS OF HYDRAULIC


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ADVANTAGES & LIMITATIONS OF HYDRAULICFRACTURE TEST

• Classical hydraulic fracturing requires sections in the bore

free from fractures. These sections should be at least a fewmeters long so that induced fractures do not interact withexisting ones

• Hydraulic fracturing may be difficult to apply with an accesuccess rate in rock domains with very high stresses, suchwhen core discing is indicated in the core from core drillin

• Geological features, such as foliation planes in gneissic rocalso affect the possibilities of success as they act as weaknplanes and thereby may control the direction of the initiatfracture

ACCURACY OF RESULTS OF HYDRAULIC FRACT


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ACCURACY OF RESULTS OF HYDRAULIC FRACTTEST

• Direct measurement of the least stress in the plane

perpendicular to the borehole axis, which is normallyleast horizontal stress, sh and the accuracy is good (~

• The maximum horizontal stress is calculated from equincluding a failure criteria and parameters evaluated fthe field pressure data. The accuracy is less good for t

maximum horizontal stress (~ +10–20% or more)

HYDRAULIC TEST ON PRE

-

EXISTING FRACTURES


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(HTPF)

• HTPF is a develoof hydraulic fractechnique becau

uses the sameequipment and ion measuremensame parameter

• Instead of inducfractures in intacHTPF method is

on re-opening ofexisting fracturein borehole wallthereby determinormal stress acfracture plane


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ADVANTAGES & LIMITATIONS OF HTPF

• HTPF is more time consuming than hydraulic fracturi

the down-hole equipment must be positioned at the elocation of each discrete fracture to be tested. This reqgood accuracy in the depth calibration

• A drawback, compared to hydraulic fracturing, is also no preliminary results can be obtained until all field-t

has been completed, field data evaluated and those daprocessed using computer


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OVERCORING METHODS

• Tests are conducted by the overcoring of a bore hole g

capable of measuring change in borehole diameter in different

directions as in case of USBM deformation gauge, or

measuring the strain that occur in the walls of a drill when the stresses are relieved by overcoring as in theof CSIRO hollow inclusion cell

• The elastic properties of the rock and strain measuremare combined to calculate the stresses in the planeperpendicular to the axis of the drill hole


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USBM borehole

deformation gauge

CSIRO hollow

inclusion cell

OVERCORING

PROCEDURE


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OVERCORING PROCEDURE


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FURTHER

READING


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FURTHER READING

ISRM

SuggestedMethods forRock StressEstimation –in 4 parts

BOREHOLE

BRAKE

-

OUTS


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CORE DISCING


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insitu stress

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