I have some experience of tunnelling in S ão Paulo, but I regret that Andrew Mylius's account of the tunnel/shaft collapse (NCE last week) nds fault with the sprayed concrete method of tunnel support, prior to knowledge of the causes of the collapse.
Tunnelling quires continuity between design and construction. Sprayed concrete requires particularly rigorous attention to all intermediate stages of construction, with design analysis providing critical features for investigation and monitoring.
The designer knows the 'ground model' for the design basis and is alert to departures that call for special countermeasures.
Codes of practice give little or no emphasis to the appropriate contractual base to enable such rigorous control.
This omission was a principal factor to the 1994 Heathrow Express collapse, as described by my book Tunnelling: management by design. I will be surprised if this is not a factor in S ão Paulo.
Sir Alan Muir Wood, Franklands, Bere Court Road, Pangbourne, Berks RG8 8JY