Dams tunnels reservoirs

• Groundwater

  lf the tunnel is below the water table, then flooding may occur:

  • Water may be free-flowing through unconsolidated sediments.

  • Water flows may occur along joints in limestone.

  • Sandstones can develop high pore fluid pressures.

  • Saturation of clays can lead to mobilisation, movement and failure by slumping.

  • If the water table is affected by tides this would need to be monitored over time so that evacuations of

  workings can be timed properly. There may be a lag between tides and rising groundwater levels depending

  on the permeability of the rock.
• Reservoir


  Body of water behind a dam wall
• Dam


  Structure that holds back water
• Factors determining dam type

  • Profile of the valley
  • Availability of local construction materials
• Masonry/concrete dams


  Dams that need a supply of cement and aggregate to make the concrete
• Arch dam


  • Curved upstream so that the hydrostatic pressure from the water compresses the structure against the sides of the valley and strengthens it
  • Thinner than other types, using less construction material
  • Suitable for narrow gorges with steep sides of strong rock, in remote areas
• Gravity dam


  • Held in place by the force of gravity due to the immense mass of the concrete or masonry
  • Often the best type when built on an impermeable, high load-bearing foundation
  • Sometimes hollow, making them more economical to construct and may be supported on the downstream side by a series of inflexible buttresses
• Arch gravity dam


  • Combines the strength of the arch with the force of the gravity and does not have to be so massive
  • Useful in areas with a high flow of water but limited material for dam building
• Embankment/earth dams


  • Have an impermeable clay or concrete core held in place by piles of rock or earth, sand and clay, with an impervious covering
  • The material binds itself together by friction between the particles so that cement is unnecessary
  • Built in broad, shallow valleys where the mass of the dam is spread over a wide area, so the foundations do not need to be as strong, but they do require very large quantities of fill material
• Geologically stable area for dam site

  • Preferably away from earthquakes, faulting, volcanic eruptions, and previous landslides
• River catchment


  • Sufficient rainfall
  • Underlain by impermeable rocks to promote surface runoff
• If the reservoir is for drinking water, there should be no exposed mineral veins containing toxic elements such as lead or arsenic that could get into the water supply
• Old underground mine workings could cause collapse or leakage
• Rivers should have a low sediment load so the reservoir does not silt up too quickly
• Dam construction


  Construct dam across a river to create a reservoir in the valley behind by storing the water that flows into it naturally
• Stability of the valley

  • Valley sides should be stable so that mass movement is unlikely
• Dip of beds


  • If the dip of the beds on the valley sides is towards the reservoir, then landslips may occur, especially if competent permeable rocks and incompetent impermeable rocks are interbedded
  • Beds that are horizontal or dip upstream provide stable foundations
  • Beds dipping downstream are unstable- there may be leakage and slippage along the bedding planes resulting in collapse of the dam
• Underlying rock types


  • Strong, competent rocks with a high load-bearing strength are needed to support the combined mass of the dam and water, such as crystalline igneous or metamorphic rock or a well-cemented sedimentary rock
  • Clay, mudstone, and shale are weak and incompetent with a low load-bearing strength and will not support the dam
  • Impermeable rocks prevent leakage of water from the reservoir
  • Competent rocks with joints cause leakage problems
• Reservoirs


  • Most reservoirs leak to some extent
• Ground improvement strategies


  Reduce leakage
• Grouting


  Holes are drilled into the rock and liquid cement pumped in. The cement fills pore spaces, joints and fissures, reducing permeability and increasing rock strength.
• Clay or plastic lining/geomembrane

  Prior to filling, the reservoir is lined with an impermeable material such as clay or plastic to prevent leakage of water into the underlying rock. Clay is a good choice because if there is a local supply it will be cheap.
• Cut-off curtain


  An impermeable barrier, usually made of concrete, is constructed as an extension below the dam, preventing leakage especially from dams situated on synclines. It also strengthens the foundations and prevents slippage of beds dipping downstream.
• Civil engineers want soft rock for easy excavation


  But they want hard rock to support it
• As with dam building, the best rock is chosen along the route for the road or railway, and engineering solutions are applied to compensate for the geological shortcomings
• Crystalline igneous and metamorphic hard rocks

  • Very strong so tunnels can sometimes be left unsupported
  • Tunnelling is usually by drilling and blasting, which is slow and expensive
  • The amount of explosives used is carefully calculated, otherwise overbreak or underbreak can occur
  • At depth in hard rock tunnels, the high confining pressure can cause dangerous rock bursts
• Soft rocks


  • Cheap and relatively easy to tunnel through but require lining with concrete or steel ribs
  • A specially designed tunnel boring machine (TBM) can achieve tunnelling rates of 30 metres per day in soft rocks
  • Sandstone, limestone and chalk are all fairly strong and make ideal tunnelling materials
• Weak rock


  • Clay and shale, and unconsolidated materials such as sands and gravels, are prone to collapse and leakage so require support and dewatering techniques
• Lateral variation and changes in rock types

  Makes tunnelling difficult
• Weathering weakens rocks, and variations in compaction and cementation also cause problems
• Porous and permeable rocks allow water seepage into tunnels and the possibility of flooding
• Dip of the strata


  • Flat-lying, competent, uniform strata are best for tunnelling
  • In dipping beds, different rock types may be encountered along the length of the tunnel
  • Slippage along dipping bedding planes may lead to rock falls into the tunnel and problems with water
• Geological structures that create challenges

  • Faults
  • Joints
  • Bedding planes in sedimentary rocks
  • Foliation in metamorphic rocks
  • Folded rock sequences
• Faults


  • Zones of weakness which may have breccia and fault gouge clay along them
  • Zones of permeability that may allow the tunnel to flood
  • Different rock types on either side of the fault
  • In the event of an earthquake, movement may cause the tunnel to collapse
• Joints


  • Zones of weakness and permeability
  • Often more closely spaced than faults
  • Loose blocks of rock between joints may fall out of the tunnel roof
• Bedding planes in sedimentary rocks and foliation in metamorphic rocks


  • Planes of weakness and may allow slippage or leakage of water
• Folded rock sequences


  • Have changing angles of dip, and slippage may occur on the fold limbs
  • If the fold is a gentle syncline, the tunnel can follow the dip of the fold and stay in one bed (Channel Tunnel)
• If the tunnel is below the water table

  Flooding may occur
• Groundwater impacts


  • Water may be free-flowing through unconsolidated sediments
  • Water flows may occur along joints in limestone
  • Sandstones can develop high pore fluid pressures
  • Saturation of clays can lead to mobilisation, movement and failure by slumping
  • If the water table is affected by tides this would need to be monitored over time so that evacuations of workings can be timed properly
  • There may be a lag between tides and rising groundwater levels depending on the permeability of the rock
• Load-bearing strength


  The capacity of rocks to take a load