Ground subsidence rare but citizens should be vigilant and report sightings: Engineer

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Large-scale road subsidence incidents, such as the recent event on Hai Tan Street, are relatively rare, according to Aaron Bok, former president of the Hong Kong Institution of Engineers.
Bok highlighted the unusual nature of the incident during a radio program on Monday morning, emphasizing the importance of public vigilance in reporting any signs of subsidence to the authorities.
Bok's call to action came after the authorities cordoned off a section of Hai Tan Street after a significant portion of the carriageway collapsed into a four-meter-deep hole measuring approximately eight by 12 meters.
The incident prompted swift temporary repairs, allowing the fast lane between Yen Chow Street and Kweilin Street to reopen earlier than anticipated late Sunday evening.
Bok pointed out that over 5,000 to 6,000 kilometers of underground pipelines run beneath the roads in Hong Kong. These pipelines are challenging to monitor on a daily basis and typically undergo inspections every two to three years.
Despite these measures, damage can occur within this period, underscoring the need for the public to report any warning signs, such as slight subsidence, through the 1823 hotline.
The likely cause of the recent subsidence, according to Bok, is attributed to cracks or damage to nearby underground pipes. This damage likely led to underground water washing away sand and mud, resulting in soil erosion. Initially small holes may appear but can gradually expand, causing the road to subside.
Bok also noted that "residual settlement" is common in newly reclaimed areas, which can increase the pressure on underground pipes. Although designs aim to prevent settlement and damage to the pipes, the possibility of related problems in areas of early reclamation has not been ruled out.

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