The engineer who prepared the plans of the building works for the Pan-Island Expressway (PIE) viaduct that collapsed in July 2017 knew his team of engineers was inexperienced in designing bridges, but failed to give them guidance or instructions.

Indonesian Robert Arianto Tjandra, 46, the qualified person from subcontractor CPG Consultants, also failed to check the design assumptions made for the corbels between the affected piers that collapsed.

Even after he was aware of the errors in the calculations made by the engineering team, he failed to take necessary remedial steps.

It was this reckless act that resulted in the collapse of the PIE viaduct, which killed Chinese worker Chen Yinchuan, 31, and injured 10 others, the court heard.

The 11 workers, who were working on the affected deck slab, fell to the ground from a height of at least 9m.

Arianto Tjandra, a Singapore permanent resident, had faced five charges under the Building Control Act and the Workplace Safety and Health Act - the most among those who were charged.

Yesterday, he pleaded guilty to three of the five charges, with the remaining two charges to be taken into consideration for sentencing.

He admitted to recklessly endangering the safety of others by putting up structural plans without checking on the design assumptions made for the corbels between the affected piers, and after knowing that the wrong design assumptions had been used, failed to carry out the necessary remedial works.

He also pleaded guilty to failing to take reasonable steps and to exercise due diligence to ensure the building works were designed in accordance with the Building Control Act, as well as authorising strengthening works to a permanent corbel without seeking the approval of the Commissioner.

The court heard that Arianto Tjandra led a team of five design engineers who had no or limited experience with bridge design involving the design of the corbels used in the PIE viaduct.

Of the five engineers, Mr Cao Qinghao and Mr William Wang were designing a bridge structure for the first time, while Mr Wang Peng was on his first project that involved designing corbels for supporting loads.

The remaining two engineers had limited experience. Mr Liu Xiayu had designed temporary corbels for only one span in a prior project, while Mr Duong Khanh had designed only one cross head for a two-span bridge.

None of the five design engineers knew how to do the calculations, nor did they make the correct calculations, said the prosecution.

Calling for a sentence of at least 22 months' jail and a fine of at least $10,000, Deputy Public Prosecutor Kristy Tan said "the real sting" was Arianto Tjandra did not take the necessary remedial steps after realising the calculation errors.

"Instead, he told OKP (main contractor Or Kim Peow Contractors) that they could cast 50 per cent of the deck slab," she added.

Five individuals and OKP were charged in May last year for their roles in the accident.

OKP was fined $10,000 for carrying out unauthorised strengthening works on the support structures of the viaduct. It still faces a remaining charge for causing the death of Mr Chen and the injuries of the 10 workers, which it is contesting in a trial.

Engineer Leong Sow Hon, 61, who was appointed by the Land Transport Authority as an accredited checker, was jailed for six months in July for failing to go through the detailed plans and calculations for permanent corbels.

OKP's managing director Or Toh Wat was given a discharge not amounting to an acquittal in July for all his three charges.

The cases of two others - OKP project engineer Wong Kiew Hai, 31, and OKP project director Allen Yee Chee Keong, 49 - are pending.