Stereonets Tom Robinson Engineering Geologist London Bridge Associates Ltd

Warwick University M.Sc. Course 2014Geological Investigation & Ground Characterisation.

Stereonets

Tom Robinson Engineering Geologist

London Bridge Associates Ltd.


Contents

1) What are stereographic projections?

2) Plotting stereonets - basics

3) Stereonets in engineering geology

4) Practical Exercise

Warwick University M.Sc. Course 2014Geological Investigation & Ground Characterisation.Warwick University M.Sc. Course 2014Geological Investigation & Ground Characterisation.

Part 1: What are stereographic projections?

1)2)


3D Data in Geology

All problems in geology, by definition, are in 3D. We are interested in what happens at depth, even if the depth is only 10’s of m.

We need a way plotting 3D data onto a 2D plane, i.e. A sheet of paper. A stereonet is one way of doing this.

Stereonets are used by all structural geologists, but have some interesting and unique applications in engineering too.


A note on projectionsThere are two main forms of stereonet used:

•Upper hemisphere or polar net. Use these for linear data.

•Lower hemisphere, or meridional net. Use these for planar and linear data.

A Polar Net

A Meridional Net

The PROJECTION is something different entirely. We will be using equal area projections only. Do not worry about the others!


Great and Small CirclesSmall circle (projection of a curved surface)

Great Circle (projection of a plane)

Equatorial Circle (horizontal plane)

Vertical planes will pass through the origin


Some definitions

Trend ~55°

Any structure in 3D can be split into lines/curves and planes/surfaces. A stereonet can plot both.

A line can be defined by its trend (angle from north) and its plunge (angle from horizontal).

Plunge ~60°

On ANY Stereonet, a line will plot as a point.


A plane can be defined by three properties:

1) Strike (angle from north to a horizontal line on the plane)

2) Dip (angle from the horizontal plane)

3) Dip direction (angle from north that to where the maximum dip is defined).

In engineering, we tend to define a plane by dip and dip direction. Structural geologists use strike and dip. You must be aware of both conventions.

Some definitions


Plotting a simple plane

Strike = 045°

Dip Direction = 135°

Dip = 70°

On ANY stereonet, a plane will plot as a great circle.

The dip direction will always precede the strike by 90°


Plunge ~ 20°

Plotting the pole to a planeIf we have lots of planar data, the stereonet can get very cluttered with great circles.

This is why we plot the pole normal to the plane, which plots as a point on the stereonet.


Trend = 315°


Plotting the pole to a plane

A plane with strike s, dip direction (x = s + 90°), and dip d will define a pole normal to the plane with the following orientation:

Trend of pole (t) = s – 90° (add 360° if answer negative) = x – 180°

Plunge of pole p = 90° - d


Common Geological Structures


If the poles to planes measured in the field look like the points in blue:•Rounded fold with shallow dipping limbs

The poles in red would correspond to an angular fold, with far steeper sides.


Do discontinuity poles exist in any of these zones? Do they daylight?


Friction Cone

Zones of allowable wedge failure

Zone of allowable sliding failure

Zone of allowable toppling failure

Daylight Envelope

Engineering Applications

Existing rock slope


Kinematic AnalysisThe previous slide shows kinematic analysis of a rock slope for sliding, toppling and wedge failure, but it could apply equally to the face or roof of a rock tunnel.

Sliding and wedge failure are important failure mechanisms in rock tunnels, and you should be aware of both.

Kinematic analysis will tell you if the failure mechanism is allowable based on discontinuity data. It will NOT tell you if the slope or roof will or is likely to fail.


Finally...There are computer programmes now that will plot field orientation data for you, and perform the kinematic analysis automatically:

The Rocscience suite above (DIPS) is a very good example, with limit eqm and finite element software too.

But in order to learn, the best way is the old fashioned way...


Thank you for listening

Any further questions?


1. http://maps.unomaha.edu/maher/GPFS/Toadstoolfaultplot.jpg2. http://www.rockware.com/assets/products/165/features/366/1108/rockworks_structural8.jpg3. http://www.rocscience.com/assets/files/uploads/8609.jpg

Figure References

http://maps.unomaha.edu/maher/GPFS/Toadstoolfaultplot.jpg

http://www.rockware.com/assets/products/165/features/366/1108/rockworks_structural8.jpg

http://www.rocscience.com/assets/files/uploads/8609.jpg

Documents

Stereonets Tom Robinson Engineering Geologist London Bridge Associates Ltd