Your cover photograph of the Gerrards Cross tunnel collapse shows 'hearting' failure of the concrete lining beneath underformed layers of placed fill.
This implies that deformation was taking place at an early stage during the filling process over the tunnel crown. This was presumably not noticed on site.
At the same time, sidefilling seems to have consisted of little more than a tipped slope, a situation which would reduce side support, particularly if surcharge and rolling on the crown was carried out before side support was in place.
One wonders where the resident engineer's role was during these events.
Hamish Orr-Ewing (M), email@example.com