Engineering Geology
How Geological Structures Affect Engineering Practice
Direction of Planar Features Water Table Must be Kept Low in a Quarry
• Increased risk of landslide if bedding planes • High water table means high pore pressure
are angled towards a quarry face • Higher pore pressure decreases
• Steeper angle of dip increases the risk of cohesion/friction so the slope can slip
movement
Grade of Weathering Joint Density – number of joints in an area
• Weathered rocks are weaker • More joints increases permeability
• Weight on top of the weak rocks will cause • Water in rocks increases the weathering
it to squash/collapse that occurs, so the rock is weaker
• The slope will collapse due to the inability
of the weaker rocks to hold it back
Joint or Fracture Continuity
• Rocks with joints and fractures have high secondary porosity, leading to high permeability
• Water can move more fluidly/easily through rock, weathering it and making it easier
Rock Head – the boundary between solid rock and overlying unconsolidated sediment
Types of Foundation
CN Tower Toronto
• Built on weak (incompetent) rock – shale
• Slab/raft foundations spreads the 110,000 tonnes of mass across the weak rock
Leaning Tower of Pisa
• Built on very weak, unconsolidated sediment
• To stabilise it engineers:
- 1934 – grout extraction into ground
- 1970 – ground water extraction
- 1993 – 600 tonnes of lead added to north side
- 1995 – increased lead to 900 tonnes
- Cables hold it in place
- Extraction of soil under tower which moved it back to the north slightly
Building/Catastrophic Collapses from Around the World
Istanbul, Turkey
• Steel supports fell due to the weight of the rock pushing against concrete retaining wall
• Rock bolts broke so concrete moved away from the rock behind it
• Rock was fractured/without bedding planes so easily fell
• Ground was saturated with groundwater or from a burst pipe
How Geological Structures Affect Engineering Practice
Direction of Planar Features Water Table Must be Kept Low in a Quarry
• Increased risk of landslide if bedding planes • High water table means high pore pressure
are angled towards a quarry face • Higher pore pressure decreases
• Steeper angle of dip increases the risk of cohesion/friction so the slope can slip
movement
Grade of Weathering Joint Density – number of joints in an area
• Weathered rocks are weaker • More joints increases permeability
• Weight on top of the weak rocks will cause • Water in rocks increases the weathering
it to squash/collapse that occurs, so the rock is weaker
• The slope will collapse due to the inability
of the weaker rocks to hold it back
Joint or Fracture Continuity
• Rocks with joints and fractures have high secondary porosity, leading to high permeability
• Water can move more fluidly/easily through rock, weathering it and making it easier
Rock Head – the boundary between solid rock and overlying unconsolidated sediment
Types of Foundation
CN Tower Toronto
• Built on weak (incompetent) rock – shale
• Slab/raft foundations spreads the 110,000 tonnes of mass across the weak rock
Leaning Tower of Pisa
• Built on very weak, unconsolidated sediment
• To stabilise it engineers:
- 1934 – grout extraction into ground
- 1970 – ground water extraction
- 1993 – 600 tonnes of lead added to north side
- 1995 – increased lead to 900 tonnes
- Cables hold it in place
- Extraction of soil under tower which moved it back to the north slightly
Building/Catastrophic Collapses from Around the World
Istanbul, Turkey
• Steel supports fell due to the weight of the rock pushing against concrete retaining wall
• Rock bolts broke so concrete moved away from the rock behind it
• Rock was fractured/without bedding planes so easily fell
• Ground was saturated with groundwater or from a burst pipe
