The number of landslips affecting roads has been cut in recent years thanks to regular slope maintenance, Highways Department Senior Geotechnical Engineer Andy Wong says.

There was an average of 40 cases a year between 2000 and 2006, compared with 166 from 1984 to 1998. There have been 19 reported so far this year.

Secure slopes: Highways Department Senior Geotechnical Engineer Andy Wong says masonry decoration and greening are some of the measures adopted to enhance slope safety and appearance.

Routine maintenance

Mr Wong told news.gov.hk the department is responsible for maintaining 12,528 roadside slopes, with 80% being sizable ones registered in the Catalogue of Slopes.

Noting mud slips are often related to poorly maintained slope surface covers and drainage provisions, he said the department strives to complete routine maintenance inspection of roadside slopes before the rainy season to ensure they are in good condition.

"We will clear any accumulated debris from drainage channels, repair damaged slope surfaces, remove vegetation that has cracked the slope surface cover, and unblock drainpipe outlets," Mr Wong said.

"Our spending on roadside slope maintenance has been on the rise, to an estimated $82 million in 2007-08, up $19.2% from the $68.8-million spending in 2006-07. The significant increase in expenditure is mainly due to additional costs for maintaining vegetation."

Green slopes

The department also checks the appearance of man-made slopes, making them look as natural as possible and blending them with their surroundings.

"When forming new slopes or upgrading existing slopes, we have to consider a number of factors, including slope stability, maintenance requirements, technical feasibility and public safety. Vegetation will be used as slope surface cover and existing vegetation preserved wherever possible," Mr Wong said.

"The vegetation cover is basically provided by hydro-seeding, with planting of seedlings on top where appropriate. To enhance the appearance of shotcrete slope covers, we will sometimes plant climbers or build planting holes on slope surfaces.

"Other enhancement measures include providing planters at slope toes, decorating slope surface covers through painting or stone-pitching, and using proprietary greening products."

Stressing public safety must always come first, Mr Wong said shotcrete is still necessary in emergency repairs as it is a quick and secure method to remove the immediate danger and to avoid prolonged road closures.

Challenging task

Unlike other slopes, roadside slopes always pose difficulties for maintenance workers.

"If the slope is next to a busy road, we will face traffic constraints and can only carry out maintenance and repairs during off-peak hours. For narrow roads we have to devise temporary traffic arrangements and divert traffic to provide sufficient space for workers. In case the slope is close to a private lot, we have to liaise with the lot owner first before the works start.

"Among the many roadside slopes we manage, those on Repulse Bay Road and Tuen Mun Road are the most challenging, as the former is narrow and poses constraints, and the latter is always busy with traffic and slopes there are very high and steep."

The veteran officer recalled that the fatal landslide on Shum Wan Road in August 1995 was the most striking in his career.

"It was the largest landslide in Hong Kong in the past 20 years. It involved about 26,000 cubic metres of soil and rock debris with a scar of 70-metres high and up to 90-metres wide.

"It severely damaged three shipyards and a factory at the toe of the slope by shoving them into the sea, resulting in two deaths and five injuries. Rescuing, clearance of debris and the subsequent remedial works were the most difficult, as the slope might further cave in and two vehicles were hanging on the edge of the precipice at the collapsed Nam Long Shan Road."

Click here to see more about the mishap.

Future plans

Looking ahead, Mr Wong said apart from routine maintenance and inspection, the department has been upgrading the stability of old and high-priority roadside slopes not covered by the Landslip Preventive Measures Programme.

"Our Enhanced Maintenance Programme aims to improve these slopes to meet safety standards and enhance their appearance.

"We have upgraded 58 slopes in recent years and will boost the stability of 156 slopes - with 117 in the urban area and 39 in the New Territories - from 2007 to 2010."