Structural cracks were found on an uncompleted viaduct near Upper Changi Road about a month before it collapsed, killing one worker and injuring 10 others.

Yesterday, the court heard that cracks were first found on June 16, 2017, before more were discovered on June 30. The Pan-Island Expressway viaduct collapsed on July 14, 2017. The cracks were found on corbels - which are support structures - at Pier 41, and then, on 42. Piers are vertical columns on which the viaduct rests.

By early July, it emerged that the cracks, which were assessed to be structural in nature, were the result of inadequate corbel strength defined during the design stage of the construction.

The design for corbels at eight other piers was also allegedly inadequate, with some at only a quarter of their required strength.

The court heard that Robert Arianto Tjandra of subcontractor CPG Consultants became aware of the errors by early July 2017, but he did not inform Or Kim Peow Contractors (OKP), the main construction company behind the project.

He also did not redesign the corbels but attempted instead to take remedial steps on-site, including strengthening the supporting structures - steps which were illegal and proved futile.

When concrete was poured to cast the span of viaduct between piers 40 and 41 on the early morning of July 14, 2017, the viaduct section collapsed.

Deputy Public Prosecutor Yang Ziliang said in his opening statement that "eight out of the 10 piers with permanent corbels would have developed significant structural cracks leading to sudden brittle failure and collapse when the viaduct was opened to traffic".

The first witness on the stand was engineer Yeung Chun Keung.

Mr Yeung, who was the technical director of OKP during the incident, told the court that the corbels were "underdesigned".

Now retired, the prosecution witness compared the design of the viaduct with another on the Tampines Expressway (TPE) which he had worked on. Although the two viaduct structures were "similar", the difference in corbel design was "very obvious", he said.

When he compared the drawings, he found that for the TPE viaduct, the reinforcement steel bars (rebars) within the corbel were 20mm in diameter and set 150mm apart.

But in the PIE viaduct which collapsed, the rebars were 16mm in diameter and set 200mm apart. This, he said, meant that the PIE corbel design was weaker, noting that it would take less than half its intended load.

Mr Yeung also said the PIE corbels had plinths - slabs on which the viaduct beams rested - while the ones on the TPE did not. This, he said, meant more weight was exerted on smaller areas for the former.

Besides Tjandra, OKP's project director Allen Yee and project engineer Wong Kiew Hai are also on trial over the collapse. Yee and Wong are charged with not stopping work despite being aware of the errors, and for obstructing justice by deleting WhatsApp messages and photos relating to the cracks.

This tranche of the trial is expected to last until Aug 8, while the next one is slated for next month.

OKP was fined $10,000 on Tuesday for carrying out unauthorised strengthening works on the corbels, while accredited checker Leong Sow Hon was sentenced to six months in jail on July 4.

Leong, who was appointed as checker by the Land Transport Authority, is the managing director of Calibre Consulting Singapore.

OKP group managing director Or Toh Wat was given a discharge not amounting to an acquittal on Wednesday for all three charges he faced in relation to the collapse.

The construction company still faces another charge under the Workplace Safety and Health Act.