Mass Movement Case Studies

Question 1

CASE STUDY: Hong Kong landslides

Answer 1

• Hong Kong has little flat land and the total land area is small - 63% of land steeper than 15 degrees
• Rapid urban growth led to intense urban development. It involved deforestation, cutting and filling to produce level platforms on which to build roads and buildings - many man-made slopes were formed in the process
• These man-made slopes are susceptible to land sliding during periods of high seasonal rainfall
• Regular occurrence of landslides in Hong Kong over the past century, causing extensive damage and loss of lives
• There were two fatal landslides at Hong Kong: Po Shan Road and Sau Mau Ping. They were both in June 1972, where a total of 138 people were killed

Question 2

CASE STUDY: Attempts to reduce Hong Kong landslides

Answer 2

• In 1980s, it was revealed that the Po Shan hillside was affected by high groundwater levels which were reducing slope stabililty, so 73 long horizontal drainage pipes were installed in the area to lower the groundwater table and to improve the stability of the hillside
• In 2000s, the groundwater levels on the slope can be high during periods of heavy rainfall and the horizontal pipes were becoming less effective. Subsequently, in 2006, the GEO designed a suitable groundwater regulation system to control the groundwater levels.

The system was made up of:

• A pair of drainage tunnels with a network of 172 vertical drainage pipes
• System was equipped with an automatic groundwater monitoring system to control groundwater levels to reduce risk of major landslides
• Almost a thousand pins, ranging in length were installed in the slope
• A total of 120m in length of flexible metal netting was put in place at the toe of the hillside

Question 3

CASE STUDY: Evaluation of attempts to reduce Hong Kong landslides

Answer 3

• Since the Hong Kong government set up the Geotechnical Engineering Office (GEO) in 1977, the upgrading and maintenance work of man-made slopes has improved significantly with a noticeable decrease in the number of landslide fatalities on the island.
• This demonstrates that the government has been working in the right direction for slope safety and landslide prevention