Conjugate Fractures form in Clay

Paul Kelso
Lake Superior State University
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Initial Publication Date: June 22, 2004 | Reviewed: June 13, 2012


Pottery clay deformed by uniaxial compression, with a standard hydraulic rock trimmer, produces conjugate fractures at approximately 30 degrees to sigma 1. This activity allows students to observe fracturing and its orientation relative to sigma 1. It also provides insight into the formation of conjugate fractures and their relationship to Mohr-Coulomb diagrams.

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This conjugate fracture experiment is used in a sophomore level required structural geology course. I usually devote one class period to the activity and associated discussions.

Skills and concepts that students must have mastered

Students must have an introductory understanding of structural concepts such as fracture, stress and strain.

How the activity is situated in the course

I use this as either an introductory, or as a culminating activity for discussions related to Mohr-Coulomb failure or conjugate fracture/fault systems. Thus, the activity can be used as an entry point or as a final exercise for discussions of fractures or at some appropriate point during discussions. I typically undertake this activity prior to a field trip where we observe and measure a conjugate fracture system.


Content/concepts goals for this activity

  1. Use a physical experiment to examine Coulomb failure and its relationship to Mohr-Coulomb diagrams
    Tie together observations and theoretical development of the fracturing of rocks
    Provide students a physical understanding of conjugate fracture/fault systems
  2. Experimentally develop the relationship between the principle stress directions and fracture/fault orientation (strain)
  3. Investigate the theoretical reasons for the observed deformation
  4. Students determine the principle stress directions for this experiment
The following week we measure the orientations of conjugate fractures at an outcrop and then determine the associated principle stress directions

Higher order thinking skills goals for this activity

  1. Student develop hypothesis (for how deformation likely proceeds)
  2. Students interpret the experimental results
  3. Students examine how different variables may influence deformation processes
  4. Students revise their hypothesis and discuss the reasons for the differences between their original and final hypothesis
  5. Students discuss/defend their interpretations citing supporting evidence

Other skills goals for this activity

  1. Observation
  2. Brainstorming

Description of the activity/assignment

Materials Needed:
Pottery clay (enough to make a 10cm cube)
Hydraulic (or screw) rock trimmer
2 - Wood boards ~ 2"x6"x8" - flat surfaces for top and bottom of clay cube
(or could be a metal plate or sturdy plywood)

Physical Model Activity:
Deformation of a clay block using a standard hydraulic (or screw) rock trimmer
(or another uniaxial compression device).
  1. Make a clay cube of about 10cm on a side. Exact size is not important. I usually try to make the clay cubes the day before we are going to do the experiment. It would be better to make a clay cylinder instead of a cube to avoid corner affects. Cubes seem easier to make and so far they have worked fine for me. Place clay cube on a sturdy board. Place a similar board on top of the cube.
  2. Remove splitter points from rock trimmer so there is a flat surface on which to place the boards with the clay cube. If you can't get the splitter points out, design the wooden blocks such that they fit over the points.
  3. Compress the clay block with rock trimmer until it deforms. First it will deform ductilely and than conjugate fractures will develop as deformation progresses.

Organization of Student Activity:
  1. Set up experiment with clay block in the rock trimmer.
  2. Describe experiment to the students, i.e., squeezing the clay block with the rock trimmer.
  3. Students predict what will happen through sketches and written descriptions
    I often do this deformation as a think-pair-share activity
    • A. Students independently sketch the initial experimental set up (think).
      • Students draw a series of 3-5 diagrams showing how they think the block will change as deformation progresses
      • Students must also describe the deformation and explain why they drew their series of diagrams as they did
      • If stress has been discussed in class: have students label the principle stress directions on their diagrams
      • If strain has been discussed in class: have students label the principle strain axes on their diagrams
    • B. Students discuss their diagrams and predictions with a partner. During this discussion they can revise or redraw their diagrams and descriptions as they think is appropriate (pair).
    • C. Class discussion where students predict what they think will happen during deformation and why. Is their a class consensus? Students may again revise or redraw their diagrams and descriptions (share).
  4. If students bring it up, brainstorm variables that might effect how the clay block deforms (water content, type of clay, strain rate, etc). If students don't bring it up, I sometimes have this discussion after deforming the block.
  5. Have a student progressively deform the clay block, stopping several times to examine changes in the block.
    Students can sketch the clay block and describe how it changes at each step.
    BEAUTIFUL conjugate fractures develop for a range of clay conditions.
    The clay cube does not need to be dry for fractures to form.
  6. Students draw deformed clay block in its final state and interpret what actually happened - with appropriate labels and descriptions.
  7. Select students to measure fracture orientations on the sides of the clay block with a protractor.
  8. Discuss why sample fractured as it did.
    • A. Discuss Coulomb failure
    • B. Relate fracture orientations to principle stress directions.
    • C. Relate fractures to a Mohr-Coulomb diagram. We know that ó

Determining whether students have met the goals

The students initial individual, partner and class sketches and descriptions plus their final sketches and interpretations of this experiment may be collected and evaluated.
We also visit an outcrop with great conjugate fractures and students are required to relate the above analog experiment to their field observations to interpret the likely stress orientations in the rock that produced the observed conjugate fractures. The activities related to this outcrop are written up and handed in.

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