Engineering efforts keep city safe from slipping slopes

Now that we are in mid-September, summer is almost behind us. Unusually, the past few months have seemed a bit cooler than before, mainly because of the many rainy days.

In Hong Kong, the summer season is also the rainy season and while rain helps lower temperatures, it can also bring about landslides in our city, where some 70 per cent of the land is classified as hilly terrain, with thousands of slopes.

We have not heard of any bad landslides over summer, despite the rain. In fact, we have had no disastrous landslides to report in recent years.

Does this mean Hong Kong is now landslide free? Not really. With our hilly topography and annual rainfall of about 3,000mm, hundreds of landslides still occur every year, but mainly in remote and sparsely populated areas on Lantau and in the New Territories.

In more densely populated areas such as Mid-Levels and urban Kowloon, dangerous hillsides, defined as slopes with a significant landslide risk, have been largely reinforced and stabilised.

Credit is due to the Geotechnical Engineering Office of the government, for its tremendous efforts in mitigating landslide hazards over the past decades. This is definitely one of Hong Kong's success stories, and is seen as such globally. Many cities that sit on hilly terrain envy our success in bringing the problem under control.

Step back in time to the twin deadly landslides that hit Sau Mau Ping and Po Shan Road in Mid-Levels and on June 18, 1972. Following these tragedies, the government mounted a very comprehensive prevention programme. In the early days, the key measure was to place a layer of concrete on the slope surface. By and large, this was effective in keeping rainwater out of the slope so that it would not infiltrate the soil.

If the soil is saturated with water, it becomes softened and much less resistant to earth movement. This is why disastrous landslides often occur during or right after rainstorms.

Since the 1970s, laying concrete on the surface of a risky slope, augmented by a subsurface drainage system in the soil, has been common practice for landslide prevention. However, after a while, people started to complain about the ugly sight of concrete stretched across an otherwise green environment.

In the mid-90s, with funding support from the Jockey Club Charities Trust, a research team at the University of Hong Kong was commissioned to study the use of "soil nails" as a means of stabilising slopes.

Driven into the slope in a patterned manner, the nails bind the soil mass together, thus providing the needed effect of reinforcement. Following soil nailing, the surface of the slope can be re-vegetated, with trees and grass, creating a green slope consistent with the surrounding hillside.

The HKU researchers conducted both full-scale field testing and laboratory testing of soil-nailed slopes and proved the effectiveness of the method. Soil nailing is now widely used in Hong Kong, replacing concrete surfacing. It represents a safe and environmentally friendly solution to the landslide problem and a technological innovation for Hong Kong.

Professor Lee Chack-fan is the director of the University of Hong Kong's school of professional and continuing education