Work on contract C924 was being carried out under a design and build contract by a joint venture between Japanese contractor Nishimatsu and Singapore firm Lum Chang.
Client was Singapore's Land Transport Authority (LTA).
At the accident site alongside the Nicoll Highway, the Circle Line excavation passes through some of the most difficult ground in Singapore.
Ground is highly plastic, saturated marine clay.
Engineers familiar with the material described it as behaving like 'toothpaste' when stressed.
Problems stemmed from the non-standard design and construction techniques required to build what would have been the deepest open excavation undertaken in Singapore and from complex contractual relationships which obscured lines of responsibility and communication.
Nishimatsu-Lum Chang hired Maunsell to carry out design of permanent works for the cut and cover Nicoll Highway station box and adjoining tunnels.
Maunsell was also responsible for aspects of the temporary works.
However, much of the temporary works design was handled by subcontractors.
Bachy Soletanche Singapore carried out diaphragm walling for the 33m deep excavation.
Local firm L&M Geotechnic installed the double thickness jet grout base slab and was responsible for soil and diaphragm wall instrumentation.
Singapore's Hiap Shing Construction carried out excavation.
Kori Construction was responsible for strutting the diaphragm walls as excavation advanced.
And Monosys provided strut instrumentation.
The report says that the relationship between client, contractor and subcontractors was complex.
The situation was made worse by the LTA's decision to act as both the qualified person (QP) responsible for supervision and as building control officer.
'NLCJV (Nishimatsu-Lum Chang) is a design and build contractor and therefore has control over worksite risks.
And again, NLCJV has engaged Maunsell as the consultants for the permanent works as well as certain elements of the temporary works, ' the report says.
'This complicated relationship inevitably led to conflicts in the ownership and resolution of problems.
'This critical decision making gap adversely affected judgement calls necessary to deal with the crisis and had caused uncertainty at the worksite.'