The Hong Kong-Zhuhai-Macau Bridge is facing more questions over its safety and engineering quality – this time over the protection of its tunnel entrances against the sea.

The concrete tetrapods installed at the edge of the tunnel entrance structures appear to have become dislodged, local media have reported.

Photos circulating online suggest the blocks have shifted from their intended positions, making the 6.7km tunnel under the Pearl River mouth vulnerable to waves and erosion.

The eastern tunnel exit lies just west of the Hong Kong-mainland border, about 4km from Lantau’s north-west coast.

Unlike earlier bridge scandals, which have all been in the Hong Kong segment, this latest engineering flaw falls on the mainland side.

A former senior civil engineer, Ngai Hokyan, says the tetrapods are intended as a buffer against the impact of waves and erosion, but he suspects they have been displaced due to design errors, HK01 reports.

The paper says photos of the breakwater since 2013 show the blocks have been shifting since September 2016.

Ngai says the blocks are meant to be interlocked to diffuse the impact of waves and protect the structure. From the photos, he says some “are obviously scattered.”

He questioned why they remain unrepaired after more than a year. It is “not difficult” to repair, he said.

Another civil engineer, So Yiu-kwan says he believes the concrete defences may have shifted because the reclamation lacks a solid foundation, Apple Daily reported.

The HZMB Authority said in a statement that the tunnels and sea defences had been built in strict accordance with the design, and had been thoroughly inspected and approved, scmp.com reported.

The bridge is officially slated to open in May, some 18 months after the original anticipated opening, a result of a series of setbacks and scandals.

However, that timetable is almost certainly likely to slide to July or later. A report two weeks ago suggested the shifting foundations of the reclaimed land next to the airport is holding up the opening.