Ause of the collapse of Heathrow rail tunnel in 1999 and what should have been done to prevent the collapse

In Feb 1999, the Civil Engineering Balfour Beatty was fined a record A?1.2m for the collapse of a tunnel at Heathrow Airport, which put lives at risk and caused the cancellation of flights. Write a 1500 word essay on the cause of the collapse and what should have done to prevent the collapse.