As a rule of thumb, remember that the "dumb box principle" is valid also for the construction of underground metro stations: building the retaining walls (piling or slurry wall dug with hydro-mill/clamshell buckets) then digging the station volume remains the cheapest option.
All other factors equal (ie. platform length, soil conditions) the cost of a metro station roughly grows along this sequence:
- at grade
- sub-surface C&C / elevated (quite similar)
- tall elevated
- deep full C&C
- partial cavern +C&C
- lateral C&C + cavern platf
- all mined
- at grade
- sub-surface C&C / elevated (quite similar)
- tall elevated
- deep full C&C
- partial cavern +C&C
- lateral C&C + cavern platf
- all mined
In reality it is a bit more complex than that and the surrounding environment/geometry/interference counts a lot for the choice, but again, as a rule of thumb, the more you want to reduce surface disruption, the more your station will cost.
It's all a cost/disruption tradeoff.
It's all a cost/disruption tradeoff.
For C&C tunnel vs bored, the tradeoff is less intense (the difference in cost is high only between very shallow C&C and the shallowest TBM option), for stations, the tradeoff point can be squeezed much more toward one of the two extremes.
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