Hundreds of flights were cancelled when an enormous crater appeared in between the airport's two main runways, dragging down car parks and causing buildings to sway during construction of the Heathrow express rail link in September 1994.
Yesterday, the company was fined a record pounds 1.2m for admitting its part in the disaster.
Balfour Beatty pleaded guilty last year to failing to ensure the safety of its employees and the public. The tunnel was empty and no one was killed or injured.
Mr Justice Cresswell told the Old Bailey: "This was one of the worst civil engineering disasters in the United Kingdom in the last quarter of a century.
"The tunnels were being built below part of the world's busiest international airport and there was considerable potential for harm.
"It is a matter of chance whether death or any serious injury resulted from those very serious breaches."
The court had been told that the civil engineering failure could have "unzipped" London Underground's Piccadilly line and crushed people to death and that it was "luck more than judgement" that such a nightmare scenario did not happen.
Balfour Beatty was running a self-certification system, which gave it control of construction and design.
Engineering consultancy Geoconsult GES MBH, experts in the so-called "Austrian Method" of tunnel construction, were dubbed "watchdogs who didn't bark" during the case.
Geoconsult, of Salzburg, monitored the Heathrow Express Rail Link during the four-month period from May 1994 when the tunnel was being built by Balfour Beatty.
The consultancy was responsible for the design and the technical supervision of the tunnel.
A jury convicted the Austrian firm of two charges, brought under the Health and Safety at Work Act 1974, of failing to ensure the safety of employees and members of the public. It was fined pounds 500,000 yesterday.
Geoconsult, which has four directors and 95 staff, denied the charges.
In a statement, Balfour Beatty said it "deeply regretted" the collapse and said working practices across the group had been reviewed after an investigation.
The companies were ordered to pay costs of pounds 100,000 each. Work on the extension to the Jubilee Line had to be halted after it emerged it was using the same "Austrian method" of tunnelling which had led to the collapse.