tunnels

• Groundwater:

  • Groundwater moves towards areas of low pressure
  • Water may move through sedimentary rocks, faults and joints 
  • Saturation of clays may cause swelling or slumping 
• Rock type:

  • Crystalline rocks are hard, so tunnel needs little support after building 
  • Soft rocks are easy to tunnel though,  need support and water prevention 
  • Weak rocks are prone to collapse, need support and water removal. 
• Faults:

  • Plane of weakness, need support 
  • Potential earthquake risk 
  • Allows water into the tunnel 
• Joints:

  • Areas of weakness 
  • Allows water in 
  • Loose rock may fall into tunnel 
• Dip angle:

  • Horizontal beds are easier - no change in rock type 
  • Dipping beds - rock type changes and more complicated to tunnel through 
• Folding:

  • Changing dip angles 
  • Slippage on fold limbs 
  • A gentle syncline my be used as a path for tunnel 
• Bedding planes:

  • Planes along which slippage can occur from water reducing friction 
• Pressure:

  • Floor of tunnel can be pushed up, reducing tunnel height 
  • Deep tunnelling can release pressure and cause cracking 
  • Jointed rock can be loosened due to lost pressure 
• Drilling and blasting:

  • Circular pattern of shot holes, explosives placed inside to remove rock 
  • Any rock type, good for harder rock and making caverns 
• Tunnel boring machine:

  • Rotating head with discs or chisels that drills a tunnel of a set diameter. 
  • Good in softer rock but can also be used in harder rock 
• For weaker rocks,  shotcrete is used to fill cracks to strengthen rocks. Steel mesh can be used and then sprayed with concrete to deter water. 
• Stronger rocks may need the use of rock bolts or no support. 
• Rock drains can be used to remove water and add strength. 
• Channel tunnel:

  • 50.5 km 
  • 45 m below seabed 
  • 85 % of route is made of one rock called chalk marl 
• Chalk marl:

  • Calcareous mudstone with just under half of clay 
  • Massive - few joints and bedding planes 
  • Impermeable so no risk of flooding 
• Channel tunnel:

  • Tunnels follow a syncline 
• Channel tunnel lining:

  • Reinforced concrete bolted to form a ring 
  • Concrete covered in shotcrete to ensure an impermeable surface 
• Channel tunnel:

  • Spoil created by tunnel used to create a country park in kent.