The issues concern shoddy construction on the north approach tunnel’s stitch joints, missing documents known as RISC forms and the unauthorised change of design at the north and south approach tunnels and the Hung Hom station’s side tracks.

The new reports followed last year’s allegations of defective work on the station’s platforms and walls. The commission concluded in March that those structures were safe despite the shortcomings in the work.

Bar-fixing subcontractor Wing & Kwong Steel Engineering has accused Leighton of being responsible for the defective work at the north approach tunnel, which was found to have a number of unconnected rebar. Leighton was the main contractor on the job.

It claimed Leighton had bought the wrong type of threaded rebar and insisted its staff screw the ill-suited bars into the tapered couplers, resulting in the rebar being left unconnected.

Making his opening submissions on Tuesday, Paul Shieh Wing-Tai, SC for Leighton, admitted there was miscommunication within Leighton as those who knew about the mismatch of materials did not tell their site engineering staff.

“During the inspection process, opportunities for spotting the issues were missed,” he explained.

“From Leighton’s perspective, they were not aware of any issues over the coupler connections during the construction process,” he added.

Shieh argued that it was Wing & Kwong that should be held responsible for the defective work.

“The reason for the actual non-connection, in our submissions, was the rebar fixers’ omission in actually doing the physical work,” he said.

Over 60 per cent of RISC forms were missing at the station’s three locations, but Shieh insisted that the forms were not missing. “They were outstanding,” he said. “Leighton’s engineering staff were too overwhelmed and busy with their workload.”

He said that the lack of the inspection documents did not mean the required inspections had not taken place as there was evidence to show that both Leighton and the MTR staff had made the hold-point inspections, which allowed the next stage of work to proceed.

As to the design change from the use of lapped bars to coupler connections, Shieh said the two construction methods were interchangeable and the modification would not have affected the structural integrity.

However, Philip Boulding, QC, for the MTR Corporation, blamed Leighton for constantly making late submissions of RISC forms due to staff shortages.

It transpired that on many occasions Leighton’s site staff would verbally request the MTR inspectors to conduct the hold-point inspections, after which the latter would give verbal permission for Leighton to proceed with the next stage of work without signing the forms, he said.

Boulding argued that if the MTR Corp had only given the go-ahead after receiving the RISC forms, there would be “significant and unacceptable delays to all of the works”.

Despite the lack of documents, he insisted that the MTR staff did conduct hold-point inspections, which were supported by site diaries, photos and WhatsApp group discussions.

Regarding the shoddy work at the north approach tunnel, he said the MTR believed it was “attributable to the defective workmanship of Leighton and/or its subcontractor Wing & Kwong”.

He said Leighton should have raised the issue of the wrong order with the MTR but it never did.

The commission, chaired by Michael Hartmann, a former non-permanent judge on Hong Kong’s top court, continues on Wednesday.