tunnels Flashcards
overbreak
too much material removed. gap must be filled
underbreak
not enough material is removed. second phase excavation required
pros and cons of tunnelling into hard rock
con- expensive, working with explosives is dangerous
pro- strong and stable- tunnel unlined
what do you use to tunnel into hard rock?
drilling and explosives
pros and cons of tunnelling into soft rock
pros- tunnel can be machine or hand cut
con- danger of collapse or leakage, will need lining
what can be done to make tunnelling into softer rock safer?
grouting or freezing ground
problems with faults x3
may move and destroy tunnel
bring weak rock against hard rock
breccia or mineralised zone= line of weakness
problems with saturated clays
undergo plastic flow=collapse
problems with joints x2
blocks fall when beds dip into tunnel
leakage- increase permeability
problems with the dip of beds
bedding plane slip- angle very high or vertical= prone to collapse
what are the best rocks for tunnelling?
igneous, metamorphic and sedimentary (hard). Have high bearing strength= unlikely to collapse but difficult to tunnel through
what rocks are problematic for tunnelling? x3
unconsolidated materials e.g. sands lack strength +are full of water= collapse when excavated
thin + alternating sedimentary beds with different hardness, LBS + porosity make planning excavation difficult
beds that vary laterally give unexpected problems
How does groundwater leakage into a tunnel occur? x3
Aquifer excavated. High hydrostatic pressure=rapid flooding
seepage from porous rocks that form sides
leakage down fault lines
when does rock burst and collapse occur?
rock burst- depth and in hard rock
collapse- weal rocks
