1.5 Deformational Structures Flashcards
What law can be used to explain how rocks are deformed?
Hooke’s law - stress is proportional to strain
What is stress?
The force applied to a rock
What is strain?
The change in shape of a rock
What will the change that will take place in a rock under stress depend on?
- the amount of stress applied to the rock
- the type of stress applied to the rock
- the time over which the stress is applied
- the temperature at which the stress is applied
- the type of rock stress is applied to
What are the 3 main types of stress?
- tensional stress
- compressional stress
- shear stress
What is tensional stress?
Force when the rock is being pulled apart
What is compressional stress?
Force when the rock is being pressed together
What is shear stress?
Force when the rock is being sheared by opposing movements either side
Is elastic deformation common in rocks?
No
When might elastic deformation occur in rocks?
With a rebound effect in earthquakes
What is elastic deformation?
When an object changes shapes and then returns to its original shape
When does brittle deformation occur?
When rock is cold and cracks
What features can brittle deformation form?
Joints and faults
When does plastic deformation occur?
When the temperature of a rock is increased and can bend to form folds
What feature can plastic deformation form?
Folds
What are competent rocks?
Strong rocks
What are incompetent rocks?
Weak rocks
Examples of competent rocks?
Granite, sandstone, limestone
Examples of incompetent rocks?
Shale, mudstone
What are faults?
Fractures in the upper part of the crust, where movement has taken place
What type of deformation are faults a result of?
Brittle
What is the dip of a fault?
The angle of the fault plane measured from the horizontal
What is the throw or downthrow of a fault?
The amount of vertical movement along the fault plane
What is the hanging wall of a fault?
The block positioned over the fault
What is the footwall of a fault?
The block positioned below the fault
What is a normal fault?
When the hanging wall is downthrown
What is a reverse fault?
When the hanging wall is upthrown
What are the types of dip slip faults?
Normal and reverse
What are the types of fault?
Dip slip and strike slip
Where do folds form?
Where rocks are taken deeper into the crust and bent and buckled by compressive forces
What type of deformation is involved when rocks are folded?
Plastic or ductile
What is an antiform?
A fold in the rock that folds upwards (sad face)
What is a synform?
A fold in the rock that folds downwards (happy face)
What will happen to a fold’s axial plane when it is symmetrical?
The axial plane will be vertical
What will happen to a fold’s axial plane when it is asymmetrical?
The axial plane will be tilted at a lower angle
If a symmetrical fold has resulted in an antiform, where will the greatest compressive force be coming from?
The side with the gentlest limb
If a symmetrical fold has resulted in a synform, where will the greatest compressive force be coming from?
The side with the steepest limb
What happens when the forces creating a synform/antiform become greater?
It may lead to one limb being overturned - creating overfold
How is an overfold formed?
When the force on an antiform/synform becomes greater and leads to one limb being overturned`
How can the pattern of beds on the top surface a plan diagram tell you the type of fold?
- narrow single band in middle → antiform
* thick single band in middle → synform
What is a thrust fault?
When some folds are pushed over so much that their axial planes are almost horizontal, and may snap along the axial plane
Why is geotechnical investigation carried out on all major construction sites?
To determine if the foundation material is strong enough for what is being constructed
What should be considered when seeing if a rock is strong enough to be built on?
- ground surface - signs of instability?
- soil - high porosity?
- weathered bedrock
- jointing and faults
What might foundation material be liable to?
- compaction - clay affected particularly
- swelling - expansion of clays and other materials when they absorb water
- rotational slip or sliding - rocks prone to shearing may be affected
Rocks susceptible to compaction, swelling and rotational slip?
- mudstone, siltstone and shale - compaction
- clay - rotational slip
- sliding and rotational slip - glacial till/boulder clay
Ways to make slopes more stable?
- efficient drainage - increase stability
- form loose materials into shallow slopes by planting vegetation
- retaining walls - prevent rock slides
- gabions - prevent rock slides
- rock bolts to tie loose blocks into the firmer rock - prevent rock falls
Ways to make tunnels more stable?
- creating an arch or cylinder shape to tunnel - withstand pressure
- reinforcement - prevent ground swelling from below and closing tunnel
- support sides - e.g. rings, arches and shotcrete to prevent collapse
- good drainage - reduce pressure of water on tunnel sides
What is a good example of how an engineering project was carried out in difficult conditions?
The Channel Tunnel - crossrail in London - clay and chalk
What are the types of dam?
- embankment
- arch
- gravity
- buttress
Where are embankment dams used?
Where foundation is weak, but there are large volumes of material available
How does an embankment dam resist water?
Sheer size of the dam resists pressure of water
Where are arch dams constructed?
In narrow valleys with strong bedrock either side
How is an arch dam kept in position against the valley walls?
Its curved shape
What are gravity dams formed out of?
One giant concrete block
Why is less concrete needed in a buttress dam than a gravity dam?
The concrete slabs are supported by the buttresses (arms projecting out of the dam) leaving a space in the centre
What type of force are normal faults formed by?
Forces of tension
What type of force are reverse faults formed by?
Forces of compression
Why are normal faults more common than reverse faults?
Compression is more likely to cause folding than to produce (reverse) faulting
