JACKSON'S £20 million 'tunnelisation' project at Gerrards Cross involves the construction of a 320 m arch over the Chiltern Railway main commuter line, which runs into London's Marylebone station.
The tunnel arch is formed using over 300 precast concrete half sections - each weighing 21.5 tonnes - which link together to form a three-pin arch. Jackson backfilled over this to create a level surface for the new Tesco store.
The collapse happened at the Marsham Lane end of the site, around 200 m away from the store at the Packhorse Road end.
The job slowed due to wet weather last summer and delays in gaining design approval for the Packhorse Road bridge strengthening work.
Gordon Masterton, senior vice-president of the Institution of Civil Engineers, said: 'The critical stage is when you start backfilling around the arch. If you buried an eggshell in the sand you would have to build sand up from the sides rather than put it on the top.
'It is more likely a combination of two or three factors - they will be looking at the soil structuring, design and whether the materials used match the design assumptions.'