The simple criterion for stability of a thin circular arch is often stated as that of uniform loading around the arch directed towards the centre. However, your excellent photograph (News last week) suggests that in this instance the criterion may be over simple.
The development of shear on the extrados of the arch as the result of spreading, compaction and slippage of the backfill over the upper part of the arch could markedly reduce hoop thrust in the vicinity of the crown joint and account for the nature of the failure illustrated.
The external sealing sheets would have a high enough angle of surface friction to permit such shear to develop.
Was nobody monitoring convergences- A little complicated by railway traffic but by no means impossible.
I recall at least two recent papers published in Europe on the loading of canopies (not please tunnels! ) of this nature.
Sir Alan Muir Wood, alanmw@tiscali. co. uk