The collapse occurred east of the planned Nicoll Highway station in a section of cut and cover tunnel adjoining a large diameter reinforced concrete shaft.
This had been built in preparation for the launch and extraction of a tunnel boring machine, which was to drive a section of tunnel under the Kallang river.
The collapsed cut and cover section was on a curve in an area of reclaimed land, consisting of 6m of made ground over 27m of soft marine clay and 5m of estuarine clay.
It was unusually deep, with a base slab at 33m below ground level. This was to accommodate the LTA's plans to build a future road tunnel above the rail tunnel.
Construction involved the installation of reinforced concrete diaphragm walls and two deep level jet-grout slabs before excavation got under way.
The lower slab formed the tunnel base while the upper one was sacrificial, located 6m above the base slab.
It was installed to provide temporary support to the diaphragm walls.
The crificial slab was to be broken out and replaced with steel struts as excavation advanced.
In all, 10 levels of struts were to be installed between the base slab and the surface.
Level nine struts had been installed and work on level 10 excavation, including breaking out the sacrificial jet grout prop, was under way when the collapse occurred.
Strutting in the area of the collapse was unlike that being used on neighbouring cut and cover tunnel sections where the alignment was straighter.
The inquiry was told NLC was fixing struts directly to the 6m wide diaphragm wall panels over most of the excavation. Struts were at 4m centres.
Struts were connected to diaphragm wall panels symmetrically to prevent uneven forces being applied.
Asymmetrical wall panel loading would have introduced rotational moments that could have undermined the structure's integrity.
The curved tunnel alignment near the TBM launch shaft forced a radial arrangement of struts. This disrupted the symmetry of the strut to panel connection pattern, and would have resulted in uneven loading of diaphragm wall panels.
A ore exible strutting system was therefore designed, with struts connected to horizontal waling beams which spread loads over a larger area.
Strutting forces in the curved section were to be further distributed by I-section spreaders, branching from the strut-ends at 45°.
Struts were supported midspan by king posts connected by longitudinal stringer beams. Struts, waling beams and king posts were 400mm by 400mm I-sections.