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Geological Factors affecting Tunnelling :hammer_and_pick: - Coggle Diagram
Geological Factors affecting Tunnelling
:hammer_and_pick:
Crystalline igneous and metamorphic 'hard rock'
Very strong
Tunnels can sometimes be left unsupported
Done by drilling and blasting
Slow
Expensive
Amount of explosives is calculated carefully
Can result in overbreak or underbreak
At depth in hard rock tunnels
High confining pressure
Dangerous rock bursts
Soft Rocks
Cheap
Easy to tunnel through
Requires lining
concrete/steel ribs
Specially designed tunnel boring machine (TBM)
Tunnelling rates of 30 meters in soft rock per day
Sandstone, limestone, chalk
Fairly strong
Ideal tunnelling material
Weak Rock
Clay
Shale
Unconsolidated materials
sands
gravel
Prone to:
Collapse
Support
Leakage
Dewatering
Lateral variation and change in rock types
Makes tunnelling difficult
Rocks may have different strengths
Weathering weakens rock
Variations in compaction and cementation
Cause problems
Porous/Permeable rock
allow water seepage into tunnels
Possible flooding
Attitude to strata
Flat lying, competent, uniform strata
Best for tunnelling
Dipping beds
different rocks may be present
Slippage may lead to rock falls
rock falls into tunnels
Creates problems with water
Geological structures
Create challenges
Faults
Zones of weakness
May have breccia or fault gouge clay
Zones of permeability
Allow the tunnel to flood
May have different rock types on either side of the fault
Events (Earthquakes) may lead to collapse
Joints
Zones of weakness and permeability
More closely spaced than faults
Loose blocks of rock between the joints fall out of the roof
Bedding Planes
Sedimentary rocks
Foliation in metamorphic rocks
Planes of weakness
Allow slippage/leakage of water
Folded rock sequences
changing angles of dip
Slippage may occur
Fold limbs may occur
Gentle syncline
Tunnel can follow the dip of the fold
Stays in one bed
Groundwater
Tunnel below water table 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 pressure
Saturation of clays can lead to mobilisation, movement, failure of slumping
Spoil
Millions of tonnes of material excavated
Can be used to build embankments along the route
Crossrail
Shipped spoil to the Essex coast
Made a salt marsh nature reserve
Wallasey Island
Chalk Marl from the channel tunnel
Landscaped at the foot of the Shakespeare cliff near dover
Ground improvement methods to prevent collapse and flooding of tunnels
:pick:
Prevent collapse
Lining with concrete
Lining with steel ribs
Using rock bolts
To secure loose blocks
Prevent flooding
Grouting surrounding rocks
Using rock drains