Concrete tetrapods that are supposed to protect the edges of two artificial islands that are part of the Hong Kong-Zhuhai-Macau Bridge project have been sinking, collapsing and drifting into a nearby shoal, aerial photos taken by drones have shown, raising concerns about construction flaws as well as the structural integrity of the islands and a vital tunnel running beneath them.
Tetrapods can mitigate erosion caused by weather and impacts from longshore drift, and are used to protect seawalls, breakwaters and artificial islands. Their tetrahedral shape dissipates the force of incoming waves by allowing water to flow around rather than against coastal and maritime structures.
The mega-project has been delayed by several setbacks since construction began in 2011.
Recent photos that have been circulating among drone buffs show that almost the whole section of tetrapods on the western shore of the 100,000-square-meter tunnel-to-bridge artificial island, off Hong Kong’s Lantau Island, has slid down, with numerous individual pieces scattered around the nearby mudflat and in the water as if they had been dispersed by rogue waves, laying bare the artificial island’s concrete base.
The tunnel, island and bridge near Hong Kong shores were built by Chinese contractors. It’s feared that tetrapods on the eastern island in mainland Chinese waters, where the bridge runs underwater into the tunnel, may have also been swept away.
The Hong Kong Highways Department said on Tuesday that the construction and maintenance of the bridge were out of its hands. Meanwhile an official responsible for media relations with the Hong Kong-Zhuhai-Macau Bridge Authority, based in the mainland city of Zhuhai, told Asia Times that the tetrapods in question “were designed to be scattered around.”
A spokesman for the bridge authority said on Wednesday that it still needed time to ascertain what had happened to these tetrapods.
However, engineering experts in Hong Kong have expressed worries over their displacement.
A professor with the City University of Hong Kong’s department of architecture and civil engineering told the Ming Pao daily that the velocity of flow in the waters surrounding these artificial islands might be exceeding prior estimates, and thus the bridge’s maintenance team may have to put in new tetrapods to fill the “void” once existing ones are washed away.
Another engineer said scattered tetrapods, which were interlocked and weighed between 1 and 2 tons each, could be an indication of settlement of the artificial islands themselves. If that is the case, he warned, then the tunnel structure beneath could be at risk of being damaged.
The huge bridge project has been fraught with delays, cost overruns, industrial accidents, deaths of workers and construction flaws since work began in 2011.
Hong Kong is responsible for the construction of link roads, feeder lines and another artificial island housing immigration and customs facilities. Previous scandals have included falsification of concrete test reports as well as the dislodging of cylinders supporting the immigration island.