The initial investigations into the cause of collapse focused on the backfilling operation. The tunnel segments allowed for substantial vertical settlement during construction, and it was vital that the material was built up evenly on both sides and over the crown of the arch. Shortly after the incident, however, Gisby said that "something" about the backfilling "is not right." The chief executive of Jackson Civil Engineering, however, said that the process used was "in line with what the design allowed. There was a strict loading regime in place." Geotechnical engineers commenting in the wake of the incident expressed surprise at the difference in fill levels on either side of the arch, and doubts were expressed that the backfilling had been in accordance with the method statement. "The collapse was probably due to too much load on the crown of the arch and not enough fill on the sides," the head of a specialist consulting firm said.
