Earlier on, Hong Kong experienced a series of heavy rainstorms and the Hong Kong Observatory issued landslip warning in the early hours last Saturday (6 June), which remained in force for more than 36 hours, during which 27 reports of landslides were received. The Landslip Investigation Section of the Geotechnical Engineering Office (GEO) under the Civil Engineering and Development Department (CEDD) immediately examined each report and sent its investigation teams of professional and technical staff to inspect the sites and identify the causes of the landslide incidents. This time, I have particularly invited a colleague from the Landslip Investigation Section to introduce their work to us.

A dedicated department to unravel the causes of landslides

A total of 131 landslides were reported in the territory last year, less than the annual average of about 300 reported landslides. However, the public should remain vigilant and should not take it lightly under the threat of extreme weather. Geotechnical Engineer of the CEDD, Mr WAI Cheuk-ting, says that the Landslip Investigation Section was established after a rainstorm in 1994, which caused a serious landslide on a retaining wall of Kwun Lung Lau in Kennedy Town, resulting in five deaths and three injuries. The department later decided to set up a dedicated team to conduct systematic studies of landslides in order to find out the causes and mechanisms of landslide incidents, as well as to make improvement recommendations to prevent similar incidents from happening again.

Thoroughly analysing the causes

Most landslides in Hong Kong are triggered by heavy rainfall, but the causes are varied. According to Mr WAI Cheuk-ting, the most common causes are overly steep slopes, weak geological structures or slopes that are affected by high groundwater levels. The activities of wild pigs had also caused landslide incidents. He compares the identification of causes and locations of landslides to case investigations by the Identification Bureau, which requires careful observation and dispassionate analysis on pieces of evidence left at the scene. For example, they once investigated a Rockfall Incident in which a vehicle was hit. The colleagues inspected the slopes along the road within hundreds of metres from the incident before they identified the location of the slope where it occurred.

Enhancing the slope safety system and engineering techniques

After finding out the causes of the landslides, the Landslip Investigation Section will put forward improvement recommendations in terms of the slope safety system and engineering techniques. For example, in 1997, a major landslide at Ching Cheung Road caused closure of the road for more than three weeks. The Landslip Investigation Section later studied the landslide and recommended the use of soil nailing for slope stabilisation since it is considered to be more reliable than the slope cutting back option. It can also enhance the robustness of slopes and hence reduce the landslide risk. The recommendation has received a positive response from the engineering industry, which has now widely adopted soil nailing to stabilise steep slopes affecting roads or buildings.

Investigation is a race against time

Extreme weather and unpredictable rainstorms brought on by climate change have made the landslide investigation and prevention work more challenging. Mr WAI Cheuk-ting gave the example of an intense rainstorm in May 2016 that resulted in a massive landslide incident on the natural terrain above Sai Wan Road in Sai Kung. The 2 100 cubic metres of landslide debris blocked the access to Sai Wan Village. The slope failure, which was a deep-seated landslide with a depth of about 12 metres, had a severity that is not commonly seen in Hong Kong. At that time, colleagues of the Landslip Investigation Section were faced with two big challenges.

On the first challenge, as the natural terrain was 100 metres above the road, it was both dangerous and difficult to immediately dispatch staff to the scene under the unstable weather. However, investigations of landslides have to compete against time in order to obtain first-hand information because evidence at the scene, such as hydrology, geomorphology and terrain, will change over time. To race against the clock, colleagues adopted innovative technology, including the use of drones to observe and take photos of the post-landslide conditions, and airborne Light Detection and Ranging (LiDAR) and photogrammetry to collect data for the creation of a 3D digital terrain model, which helped them to have a more comprehensive understanding of the causes, scale and mechanism of the landslide and evaluate the risk of reoccurrence.

Steep landslide scars adding to the difficulty of site investigations

The second challenge faced by the section was to conduct post-incident site investigations at the incident location after the weather had become stable. The landslide scars at Sai Wan Road were so steep that it was not feasible to walk uphill along the scars. Fortunately with the help of the Civil Aid Service, they arrived at the scene and completed the task successfully after taking a detour through 300 to 400 metres of rugged terrain. As the incident took place during the summer and the rainy season, travelling uphill and wading waters was also a tremendous physical challenge. At present, flexible barriers have been constructed on the natural terrain with a warning sign set up to remind members of the public to stay away from the slope.

With hilly terrain and heavy rainfall in Hong Kong, it is almost impossible to achieve zero landslide risk particularly when there will be more frequent and intense rainstorms due to climate change. Therefore, members of the public should stay tuned for the weather warnings issued by the Hong Kong Observatory. People should maintain vigilance at all times and stay away from slopes when the rainstorm or landslip warning is in force. Moreover, private slope owners are responsible for regular maintenance of their slopes or retaining walls to keep hazards at bay.