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GE 6477 DISCONTINUOUS ROCK6. Review of Rock Mechanics

Dr. Norbert H. Maerz

Missouri University of Science and Technology

(573) 341-6714

norbert@mst.edu

Instructional Objectives

1. Describe how we use rock mechanics to predict the response of the ground to excavation and construction.

2. Explain the relationship between the strength of intact rock and the strength of fractured rock.

3. Evaluate the three failure modes of rock (tensile, compressive, and shear) in terms of their application to rock engineering situations.

4. Interpret the Hudson interaction models in terms of underground design/construction issues.

5. Classify the practical aspects engineering design as they relate to discontinuities.

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Review of Rock Mechanics

• Rock Mechanics of Discontinuities

• Rock Mechanics of Discontinuous Rock

What is Rock Mechanics?

• It is the study of the mechanical behavior of rock and rock masses

– Deformation

– Stability

– Excavation

– Support

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How Do We Use Rock Mechanics

• Determine the mechanical properties of the rock

• Determine the conditions in the ground (stress, water pressure) and predict the changes in theses due to construction

• Predict the response of the rock

• Control the response of the rock

Mechanical Properties

• Strength

• Deformability

• Hydraulic Conductivity

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Index Test: eg. Point Load

From the collection of Dr. J. A. Franklin

Design Test: eg. Field Shear

From the collection of Dr. J. A. Franklin

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Ground Conditions

• Measure in-situ stresses in the ground

• Measure water pressures

From the collection of Dr. J. A. Franklin

Predict the response of the Rock

• Continuum mechanics • Physical Analogs

From the collection of Dr. J. A. Franklin

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Predict the response of the Rock

• Numerical Modeling • Rock Mass Classification

From the collection of Dr. J. A. Franklin

Control the response of the rock

• Final or design element of the process:

– Size, shape, orientation, location of opening

– Excavation method and sequencing

– Short and long term support

– Water control

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Single vs Multiple Joints,FACTS:

1. A single joint is easy to analyze

2. Multiple joints (networks) are very difficult to analyze

3. Real rocks typically have multiple joints

Intact Rock Strength

• Consider a homogeneous isotropic intact rock mass

• The strength of the rock is constant no matter what the orientation of the load

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Single Discontinuity

• Depending on the orientation of the discontinuity with respect to the load, the strength may equal the intact rock strength, or dramatically reduced.

Two Discontinuities

• Wider range of orientations over which the rock mass is weaker.

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Multiple Discontinuities

• Weaker rock mass at all orientations.

Rock Mechanics of Single Discontinuities

• Primarily a shear strength concern.

• Issue of compliance.

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Response of a Discontinuity to Stress

• Tension

• Compression

• Shear

Tension

• No strength

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Compression

• Strain hardening

Shear

• Strain softening

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Shear strength model

Stress Distribution in Jointed Media

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Rock Mechanics of Discontinuous Rock

• Interaction of 4 elements:

• 1) structure

• 2) stress

• 3) water

• 4) construction

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Applications

• Fluid flow

• Slope stability

• Block caving (mining)

• Pre-split blasting

• Mechanical excavation

• Underground support

• Wall Control

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• Fluid flow through fractures

From the collection of Dr. J. A. Franklin

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Slope Stability

From the collection of Dr. J. A. Franklin

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From the collection of Dr. J. A. Franklin

From the collection of Dr. J. A. Franklin

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From the collection of Dr. J. A. Franklin

Block caving

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From the collection of Dr. J. A. Franklin

Pre-split Blasting

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From the collection of Dr. J. A. Franklin

Mechanical Excavation

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From the collection of Dr. J. A. Franklin

Underground support

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From the collection of Dr. J. A. Franklin

Wall control

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