Based on trial judge Mr Justice Cresswell's summing up.
1992: BAA decides to use NATM after successful 100m trial tunnel is carried out to assess effectiveness of the technique in London Clay.
January 1994: Balfour Beatty wins £60M main civils contract for Heathrow Express tunnels under Heathrow Airport including NATM station under central terminal area.
18 April 1994: Geoconsult on site. Three Balfour Beatty engineers seconded to Geoconsult as NATM engineers.
9 May 1994: Balfour Beatty takes possession of site.
27 May 1994: Station concourse tunnel begins in central terminal area.
June-July 1994: Concourse tunnel passes beneath Camborne House.
14 July 1994: Geoconsult instructs work on downline tunnel to begin.
26 July 1994: 18mm settlement recorded in Camborne House.
29 July 1994: HEX meeting to discuss settlement under Camborne House. Geoconsult's 9mm prediction considered low by Mott MacDonald engineers. Some 29mm recorded.
5 August 1994: Compensation grouting under Camborne House begins.
6 August 1994: Upline tunnel begins.
16 August 1994: Vertical cracks discovered in concourse tunnel lining between chainage 45m and 50m from tunnel access shaft. Geoconsult to instruct on repairs.
18 August 1994: Compensation grouting beneath Camborne House stopped.
23 August 1994: Geoconsult foreman brought on site to supervise repair work.
29 August 1994: Poor invert joint discovered 63m from the entrance shaft.
5 September 1994: BAA HEX team's area manager Jonathan Allen writes to Balfour Beatty expressing reservations about the concourse tunnel's structural integrity.
6 September 1994: HEX team writes to Balfour Beatty expressing its and London Underground's concerns about the tunnel lining. 86mm convergence recorded in station concourse tunnel lining.
9 September 1994: Geoconsult notes that progress in August was hampered by cracking in concourse tunnel.
12 September: HEX team asks Balfour Beatty to clarify role of NATM engineers. Letter not copied to Geoconsult.
13 September 1994: Balfour Beatty tells BAA HEX team that a review of the NATM design has shown the design is adequate.
14 September 1994: HEX team issues System Defect Notice saying there is insufficient data to assess tunnel lining performance.
19 September 1994: Geoconsult fax to Balfour Beatty says no unusual geological conditions. BAA HEX team area manager says 'concourse tunnel not behaving as expected'.
20 September 1994: Down line platform tunnel starts
21 September 1994: Balfour Beatty project director Alan Myers tells BAA that Geoconsult team does not have a quality control role. 'That is down to the construction teams.'
23 September 1994: Work on upline platform tunnel begins.
25 September 1994: Five faces now being excavated; concourse tunnel, downline north and south, and upline north and south.
26 September 1994: Project meeting to discuss settlement in Heathrow's Central Terminal Area. Meeting not attended by Geoconsult. Structural stability of a 39m section of the central concourse tunnel from the access shaft questioned. Tunnel lining convergence in the section between 35m from the shaft and 39m from the shaft above 50mm limit.
8 October1994: Failed escalator box eye discovered. Balfour Beatty to investigate.
10 October1994: Geoconsult team warns that miners are over excavating. Balfour Beatty engineers agree to tighten control on miners.
DAY SHIFT: Repairs start between access shaft and point 39m into the central concourse tunnel. Balfour Beatty foreman told of settlement problems under Camborne House. Cracked invert found in section 15m to 25m from access shaft. Geoconsult senior engineer gives go ahead for full face excavation in concourse tunnel.
NIGHT SHIFT: Miners carry out invert joint repairs in 3m strips. 20m of damaged invert exposed. Miners describe cracking and flaking in the tunnel lining and 'humped profile in the invert'.
20 October 1994:
DAY SHIFT: Invert near tunnel eye found to be too thin. Invert cracking and heaving. Cracks spread through shift. Concrete falling from tunnel lining.
20 October 1994:
Miners repairing concourse tunnel describe movements in tunnel lining and reinforcement mesh distorting and pieces of sprayed concrete falling. Cracks propagating from downline tunnel face. Concrete falling from crown and tunnel sides deform. Cracks in upline central
wall. Tunnels evacuated around 1am. Rate of cracking and spalling increasing. Workers taken to canteen and as they crossed the car park area, movements in the ground felt and cracking noises heard. NATM engineer records failure at 1.30am.
AFTERMATH: Tunnels continue to collapse for several days. Stabilised by pumping foam concrete into tunnels. Area of collapse excavated inside cofferdam.