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Geology (GLGY 341-UCAL) Midterm Exam

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Question 1

Planes orinated __I__ to the principle stress directions experiance __II__.

A	I. perpendicular II. normal stress and zero shear stress
B	I. perpendicular II. shar stress and zero normal stress
C	I. parallel II. normal stress and zero shear stress
D	I. parallel II. shar stress and zero normal stress

Question 2

What is the failure criterion indicated in A?

A	Von Mises
B	Micro
C	Coulomb
D	Macro
E	Griffith

Question 3

If you are working in an environment with high tensile geologic forces, which of the following failure envelope would you enter into the computer?

A	Von Mises
B	Mohor
C	Griffith
D	Coulomb

Question 4

Which of the following can be classified as a coaxial strain?

A	Pure Shear
B	Sub-simple Shear
C	Simple Shear
D	Rotation
E	Translation

Question 5

What is the difference between magnitude and net of a force? (general physics-structure)

A	Magnitude a measure of force regardless of the direction(net force depends on the direction).
B	Net force a measure of force regardless of the direction(magnitude depends on the direction).
C	Magnitude a measure of force in international standards (net force is often use to simplify force distribution).
D	Net force a measure of force in international standards (Magnitude is often use to simplify force distribution).

Question 6

A thrust fault is a low angle reverse fault with about 20-30 degree in angle between the fault line and the normal.

A	False
B	True

Question 7

Deformation which will not result in physical change in the structural unit in question has underwent a... (there is more than one possible answer, but choose the MOST suitable one)

A	shear deformation.
B	pure shear deformation.
C	translational deformation.
D	rotational deformation.

Question 8

How is stress different from strain?

A	Stress is force per square inch (it can be treated as pressure). Strain is the deformation cause by the applied stress.
B	Stress is when you pull apart rock units and strain is when rock units are pushed against each other.
C	Strain is a force while stress is a the cause of the force that generate strain.
D	Stain is force per square inch (it can be treated as pressure). Stress is the deformation cause by the applied strain.

Question 9

By referring to the following matrix, if F₃₃ is the only (and one and only) one that underwent change, what type of strain is it?

A	Isotropic volume change.
B	Asymmetric strain
C	Anisotropic volume change.
D	Symmetric volume change.
E	Axial strain

Question 9 Explanation: 

aka uniaxial strain

Question 10

According to the following diagram, what do you get if you find the distance between C and D (Note they are pointing to at x-intercepts)?

A	Differential Stress
B	Failure Window
C	Normal Stress
D	Shear Stress
E	Compressional Stress

Question 11

What is the difference between translation and distortion? (or are they the same?)

A	Translation only can occur in brittle materials while deformation can occur in both brittle and ductile materials.
B	Distortion only occur when a body of materials shifted from one position to another.
C	They are the same.
D	Translation require no change in principle elongation axes.

Question 12

Generally what type of stresses would you expect in lithosphere?

A	extensional
B	biaxial
C	compressional
D	deviatoric
E	lithostatic
F	uniaxial

Question 13

What is represented on the x-axis?

A	Differential stress
B	Shear stress
C	Compressional force
D	Normal stress
E	Dynamic stress

Question 14

What is the best indicator of structural deformation history?

A	Undeformed bedding surface
B	Known fossils
C	Mineralogical and chemical composition
D	Deformed bedding surface

Question 15

What type of strains are most common?

A	2D
B	Depend on the environment, all strains are globally occur at the same rate.
C	3D
D	1D

Question 16

What type of geological event the following diagram represents?

A	Earthquake
B	Strike-slip fault
C	Erosion
D	Normal fault
E	Thrust fault

Question 17

Unfortunately as a Geologist, you were asked to determine which mathematical model should be entered into the modeling software(used for plotting the field data). Knowing that you are in an area with extreme tensile pressure, which model do you choose?

A	Griffith model
B	Von Mises model
C	Envelope model
D	Coulomb model

Question 17 Explanation: 

Computers are great tools. I have worked with several Engineering companies and found that it is difficult, if not impossible for a software to predict the best course of action for specific project. This is where professionals like Geologists, Engineers and Mathematicians come in.... No matter how many computers you have or what software you use, you will come across situations where automated systems screwed up calculations due to human errors. For example, if you pick Coulomb model from the software menu, it will come back with a great number, but it will be WRONG. The point of this question is to demonstrate that computers are NOT a replacement for a Geologist!

Warning: I did work for a company where the chief Engineer was fired and Alberta Government charged several people with offenses due to mathematical modeling errors result in an outcrop collapse near a bridge killing a person.

Question 18

Given the following, calculate the deviatoric stress;
σ₁ = 250 MPa
σ₂ = σ₃= 100 MPa

A	175 MPa
B	350 MPa
C	75 MPa
D	150 MPa

Question 18 Explanation: 

(σ₁ - σ₃)/2 = 75 MPa

Question 19

Under high pressure, a quartz vein has dissolved resulting collapse of a fracture. What type of fracture is described here?

A	Mode II
B	Mode I
C	Mode IV
D	Mode III

Question 20

The pulling apart a material parallel (aka same plane) to a fracture is known as a... (also the easiest type to observe in field)

A	Mode IV
B	Mode I
C	Mode III
D	Mode II

Question 21

In this computer model, the most left hand circle (blueish-green) between σ₃ and σ₂ is drawn based on what envelope/principle?

A	Confining
B	Griffith
C	Von Mises
D	Coulomb

Question 22

What is coaxial deformation?

A	It is a type of deformation where the principle axes of the strained body remain in the same orientation.
B	It is a type of deformation where the stress is a consent.
C	It is a type of deformation where the principle axes changes the distribution of force.
D	It is a type of deformation where the principle stress horizontal.
E	It is a type of deformation where the principle axes of strained body changes it's orientation as deformation progress.
F	It is a type of deformation where the volume doesn't change.

Question 23

What is the failure criterion indicated in E?

A	Micro
B	Von Mises
C	Griffith
D	Coulomb
E	Macro

Question 24

In solids it is possible to have variations in the magnitude of the stress field. This is because solids can support stress gradients. This gradient can be best described as...

A	normal stress.
B	shear stresses
C	deviatoric stress.
D	differential stress.

Question 25

Relatively, the ductile zone of a fault is ___ the center of the fault.

A	at the
B	furthest away from
C	closest to
D	90 degree to

Question 26

In this computer model, the differential stress is indicated by....

A	large blue circle.
B	orange circle.
C	the 60-degree angle.
D	the green line between x and y.

Question 27

By referring to the following matrix, lithostatic stress can be defined when...

A	F11, F22 and F33 have changed at the same rate.
B	F11, F21 and F31 have changed at the same rate.
C	F13, F23 and F33 have changed at the same rate.
D	F13, F22 and F31 have changed at the same rate.
E	F12, F22 and F32 have changed at the same rate.

Question 28

Even through the principle stress can be in any direction, due to the strong force of gravity, it is usually oriented vertically to a body.

A	True
B	False

Question 28 Explanation: 

But it still can be at an angle to a plane, a grain, a bed or a feature or a fracture.

Question 29

Given C = 100 MPa and D = 150 MPa (note they are pointing to at x-intercepts), calculate mean stress?

A	250 MPa
B	125 MPa
C	25 MPa
D	150 MPa

Question 29 Explanation: 

C = 100 MPa and D = 150 MPa are σ₁ and σ₃ respectively. Therefore, (σ₁ - σ₃)/2 = 125 MPa

Question 30

What is the fault separation distance between a stratigraphic section?

A	unit separation
B	fault distance
C	stratigraphic separation
D	unit distance
E	fault separation

Question 31

In this computer model, the most right hand circle (orange) between σ₂ and σ₁ is drawn based on what envelope/principle? (choose the best)

A	Von Mises
B	Coulomb
C	Griffith
D	Confining

Question 31 Explanation: 

While more than one answer is possible (Coulomb and Von Mises), they all can be explained by confining principle.

Question 32

The most (ultimate) tensile strength a rock can take is indicated by which letter on the following diagram?

A	C
B	A
C	F
D	D
E	E

Question 33

When dealing with stresses at a point, the maximum stress is in the direction that has the __I__ extension and the maximum strain is in the direction that has the __II__ extension.

A	I. horizontal II. vertical
B	I. largest (most) II. largest (same)
C	I. largest (most) II. least
D	I. least II. largest (most)
E	I. vertical II. horizontal

Question 34

Calculate the lithostatic stress under 10 km of a rock unit composed of olivine which has a density of 3600 kg/m³ (use g = 9.81 m/s²)

A	98.1 Gpa
B	35 Gpa
C	35316 Gpa
D	0.35 Gpa
E	98100 Gpa

Question 34 Explanation: 

Given, stress =
stress = (3600 kg/m³)(9.81 m/s² )(10,000 m)

Question 35

11. What type of transformation is represented by the following diagram? (red dotted line is the original position of the rock mass)

A	Rotation
B	Translation
C	Sub-simple shear
D	Simple shear
E	Pure shear

Question 36

What type of transformation is represented by the following diagram? (red dotted line is the original position of the rock mass)

A	Sub-simple shear
B	Simple shear
C	Pure shear
D	Rotation
E	Translation

Question 37

The rocks along a fault line have moved parallel but opposite to each other. What type of strain would you expect to find along the fault line?

A	Pure shear
B	Super imposed strain
C	Translational deformation
D	Rotational deformation
E	Simple shear

Question 38

Why is the Coulomb failure envelope is not perfect?

A	It predicts unrealistic strength by over predicting the failure envelope in tensile regions.
B	It under-predicts strength in tensile regions.
C	Past practical experiments indicates that it will not comply with 90% of the lab tests.
D	It under-predicts uniaxial compressive strength at higher confining pressures.

Question 39

What type of transformation is represented by the following diagram? (red dotted line is the original position of the rock mass)

A	Simple Shear
B	Translation
C	Sub-simple shear
D	Pure shear
E	Rotation

Question 40

Where on a stereonet a pole to plain will plot?

A	180 degrees on either direction.
B	180 degrees towards the dip.
C	45 degrees away from the dip.
D	90 degrees away from the dip.

Question 41

What type of transformation is represented by the following diagram? (red dotted line is the original position of the rock mass)

A	Simple shear
B	Sub-simple shear
C	Translation
D	Rotation
E	Pure shear

Question 42

Triaxial test is used to determine the deformation of materials, which then is used to determine the strength.

A	False
B	True

Question 43

In the following matrix only one force value (F) has changed. What type of volume change would you expect from the rock body?

A	differential
B	anisotropic
C	deviatoric
D	isotropic

Question 44

What is strike? (Most likely will be in the oral exam part of the lab).

A	It is the azimuthal bearing to an imaginary line measured from the North.
B	It is the azimuthal bearing relative to true north of a vertical line on an inclined planar surface.
C	It is the azimuthal bearing to an imaginary line measured from the true North.
D	It is the azimuthal bearing relative to true north of a horizontal line on an inclined planar surface.
E	It is the line that will generate at exactly 90-degrees of azimuthal bearing to an imaginary line measured from the North.

Question 45

Upon observations, you determined the sediments have few quartz veins. What type of fracture is this?

A	Mode I
B	Mode II
C	Mode IV
D	Mode III

Question 45 Explanation: 

While it could be a combination of any of the fracture modes, the simplest explanation is Mode I. The simplest one is the explanation for most geologic problems.

Question 46

What is the failure criterion indicated in F?

A	Von Mises
B	Coulomb
C	Micro
D	Griffith
E	Macro

Question 47

What is represented on the y-axis?

A	Normal Stress
B	Compressional force
C	Shear stress
D	Differential stress
E	Dynamic stress

Question 48

When will a structure become unstable?

A	2 θ > 45
B	2 θ = 90
C	2 θ < 90
D	2 θ > 90
E	2 θ < 45

Question 49

The following image is a graphical representation of a strain ellipse during deformation. What term would best describe this image?

A	pure shear
B	non-coaxial strain
C	rotation
D	coaxial strain
E	translation

Question 50

In this strain ellipsoid, where is the maximum strain?

A	σ 2 and 3
B	σ 2
C	σ 1
D	σ 1 and σ 2
E	σ 3

Question 51

When using acoustic remote sensing, salts will not appear since salts are great reflectors.

A	False
B	True

Question 51 Explanation: 

Salts will often appear clearly not because they are great reflectors, but because they are poor reflectors surrounded by great reflective materials.

Question 52

Rocks are weaker in tension because of micro defects.

A	False, they are weaker due to macro defects
B	True

Question 53

"A discontinuity with wall-parallel displacement dominated by brittle deformation mechanisms." This is the...

A	traditional definition of a fault.
B	specific definition of a fault.
C	general definition of a fault.
D	dynamic definition of a fault.
E	kinematic definition of a fault.

Question 54

Triaxial test is used to measure ___ of soil types and rock units.

A	history of deformations
B	faulting levels
C	porosity
D	compaction strength
E	strength

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Strong skills in geometry, trigonometry, calculus, spatial visualization and sometimes structural engineering is very helpful in this field of study.

Use of Stereonet

Key terms:

Strike: azimuthal (bearing) relative to true north of a horizontal line on an inclined planar surface

Dip: angle of inclination of the surface of interest in the direction perpendicular to strike

Trend: of a line is the imaginary horizontal line of a vertical plane (similar to strike)

Plunge: angle of inclination in the vertical from the horizontal

Apparent dip: inclination from horizontal of any line on an inclined plane that is not perpendicular to strike

Pitch (rake): angle measured on an inclined plane, between the strike and any other line those lines in the plane. The line of greatest slope in any plane must have a pitch of 90-deg. The pitch of a line on a surface is measured by placing a protractor on the surface of the line

Basic Field Work

Strike and Dip

Best method: using brunton compass

Dumb method

Identifying Unstrain State

With the advancement in computerized databases on geotechnical data, today most companies use software programs to determine the original state of a geological feature. However, all of these programs are based on fundamental principles of structural geology and a geologist has to identify the features first before enter the data in to a computer system.

There are several ways to identify the original state of deposition, which include the following principles or laws:

• Principle (law) of Original Horizontality: due to the force of gravity acting on sediments, layers of sediments are always originally deposited horizontally.
• Principle (law) of Superposition: “Sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest on the top.”
• Principle (law) of cross cutting relationships: an igneous intrusion is always younger than the rock it cuts across.

If a structure do not follow these principles, it can be explained using geological events that effected the structure such as a fluid flow across a crack of a sediment after the sediment was deposited. This is used to compile a list chorological events in order to understand the nature of the sample.

Deformation

The deformation play a major role in how structures are form in nature. By definition, deformation is the transformation of a geological structure from an initial to a final geometrical shape by means of rigid body translation, rigid body rotation, strain and/or volume change. Understanding this definition is critical to further the knowledge in the field of structural geology.

Rotation is caused by the geologic structure rotated in a circle where center moves in a slower speed resulting in a short distance compared to the edges the speed is higher with greater distance.

Translation is caused by shifting the structure from an initial point to a final point. In translation, every single particle of a geologic unit or a structure moves in the same direction for same distance and at the same speed.

Strain and Stress are related to each other, but they are two different concepts. Stress is a force that is applied to a unit area where strain is the deformation of a geologic unit/structure as a result of strain. A geologist cannot observe stress, but rather deduce the characteristics of stress using the strain recorded in rocks and geologic structures. Strain can be classified in to three different categories; pure shear, simple shear, and subsimple shear..

– pure shear is caused by compressional forces into the geologic material from two sides.

– simple shear occurs when a structure is subjected to stresses in opposite direction.

– subsimple shear is complicated because it is caused by a combination of simple shear and pure shear.

Please read more on this at: Shear Strain

Remember that deformation can only be identified using relative theory. In other words, deformation is the difference between known state of a structure to a final state of a structure. Most of the time geologists use known fossils to identify changes.

One dimensional strain involves only either stretching or shortening, where elongation, e is defined as:

e = (l-l_o)/l_o and l is the final length and l_o is the initial length.

The stretching, s of a structure is defined as:

s = 1 + e OR s = l/l_o

Two dimensional strains involves changes in structure in 2D on a plane, where angular shear, shear strain and strain ellipse is described using relative angles. This can be confusing since all angles are given relative to the normal of along a line; usually line A or line B. In angular shear, Ψ represents the angle between two originally perpendicular (the normal line of the undeformed original is perpendicular) lines.

Mohr Circle

Mohr circle or Mohr diagram is named after Christian Otto Mohr is a 2D representation of a 3D stress on a structure on a point, often a crack.

Triaxial Test

Triaxial test is a conventional testing method used in laboratory testing for obtaining shear strength parameters for different soil and rock types under a controlled stresses. The information obtained from such tests are used to determine the history of deformation events and the current strength conditions. The current strength conditions are used for constructing dams, building, roads and such. Not only the Geologists but also Engineers use information obtained from triaxial test.

Here are some videos of real life triaxial tests;