is stress bad? examples of stress releaseweb.mst.edu/~norbert/ge5471/class notes/05stress.pdfis...

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GE 5471 ROCK ENGINEERING CH 5 – Stress

Dr. Norbert H. Maerz

Missouri University of Science and Technology

(573) 341-6714

[email protected]

Instructional Objectives

1. Explain the consequences of stress release.2. Propose engineering solutions to the problems of stress

release.3. Define stress and strain in 3-dimensions.4. Define principal stresses and special states of stress.5. Calculate vertical stresses based on material properties.6. Explain Fossil stresses.7. Describe how stress is measured using the overcoring, strain

nulling, and hydrofracturing methods, and solve stresses using calculations.

8. Solve using calculations , problems involving unconfined compressive strength, forces and stresses, stress in the earth, stress in underground rooms, and shear stresses.

IS STRESS BAD? Examples of Stress Release

• Earthquakes

• Rockbursts

• Rockfalls

• Pop-ups

• Squeezing

Pop-ups

Photo from the collection of Dr. J. A. Franklin

Stress release

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Stress ReleaseENGINEERING SOLUTIONS TO STRESS (RELEASE) RELATED

PROBLEMS

• Catastrophic– Understand and

manage stress

– Design to remove stress concentrations

– Learn to monitor and predict rockbursts

• Non-catastrophic– Design for movement

What is stress

• Applied force per unit area

• Shear stress

• Normal stress

• (Newton is a unit of force)

2m

N

2m

N

2sec

.mkgmaN

Practical Rock Mechanics (Hencher)

Relationship between normal and shear stress

Practical Rock Mechanics (Hencher)

Stress is 3-D with 9 componentsStress Tensor

zzyzx

yzyyx

xzxyx

zyzxz

yzyxy

xzxyx

Practical Rock Mechanics

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Principal Stresses

• σ1 = maximum principle stress

• σ2 = intermediate principle stress

• σ3 = minimum principle stressz

y

x

00

00

00

Special States of Stress

• Uniaxial Stress

• Biaxial Stress

• Triaxial Stress

• Hydrostatic Stress

Worked Example – Column of 1 m blocks – how high could we stack these?

Worked Example – Column of 1 m blocks – how high could we stack these?

Stress Distribution Around Openings

Practical Rock Mechanics (Hencher) Practical Rock Mechanics

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Stress Distribution Around Openings Stress with Depth

Strain Worked Example – Column of 1 m blocks – what is thestrain?

Horizontal Stresses

• “Virgin” basin

• Tectonism

• Rock anisotropy

• Topography

• “Fossil” Stresses

Fossil Stresses

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Stress with Depth

Methods of Stress Measurement

• Overcoring

• Strain Nulling

• Hydrofracturing

Overcoring:Strain-ReliefMethod

South AfricanDoorstopper

DoorstopperStressMeasurement

Photos from the collection of Dr. J. A. Franklin

CSIR Triaxial Cell

Picture courtesy of Roctest.

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CSIRO Stress Cell


USBM Stress Gauge


LisbonStressMeter


Biaxial Modulus Chamber


Undercoring

https://www.onepetro.org/download/conference-paper/ISRM-EUROCK-2012-009?id=conference-paper%2FISRM-EUROCK-2012-009

Strain Nulling:Flat Jack

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Goodman (Flat) Jack Method FlatjackStress

Measurement


HydrofracturingHydrofracturingStress Measurement



Calculations

• Tensile strength τo =Pf – Pr

• ’min = 3 = Ps – P0

• ’max = 1 = τ + 3Ps – Pf – P0

• ’max = 1 = 3Ps – Pr – P0

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Orientation of Fractures


Strain Relief Methods

• PROS

– Can get all values σ1, σ2, σ3

– Can get the orientation of the principal stresses

– Deep borehole not required

• CONS

– Can’t get too deep into the rock, partially disturbed

– Difficult test, especially in wet and long boreholes

– Need a good stress-strain relationship

Strain Nulling Methods

• PROS

– simple, inexpensive tests– easy accessibility– measures tangential stress

in an opening

• CONS

– measures stress in one direction only per test

– very shallow penetration, consequently operating in “disturbed” stress fields (stress concentration; stress relaxation)

– need numerical solutions to get σ1, σ2, σ3

Hydrofracturing Methods

• PROS

– Good for getting magnitudes of σ1 and σ3

– Deep, deep reach in borehole

– Reaches undisturbed stress fields

• CONS

– No σ2

– Stress orientation somewhat fuzzy.

– Expensive test, especially if you need to bore a deep hole

Slide Show

• Stress manifestation

• Stress measurement

• Stress distribution

Stress Manifestation


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Convergeance


FailedRockbolt


Breakout


Disking


StressRelief

Sliding


Rock Movement


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Convergeance


Popups


Exfoliation Joints

Floor Heave


Stress Distribution


StressDistribution


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Gonzales and Ferrer Gonzales and Ferrer

is stress bad? examples of stress releaseweb.mst.edu/~norbert/ge5471/class notes/05stress.pdfis...

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