Policy was misguided

PUBLISHED : Sunday, 30 April, 2000, 12:00am
UPDATED : Sunday, 30 April, 2000, 12:00am

I would like to respond to a number of letters and articles that have appeared in these columns and in the South China Morning Post on the subject of shotcreting of slopes.

Hong Kong is unique among major cities in having such large areas of dense urban development lying on such steep natural slopes. Development over the years has resulted in thousands of man-made slopes and retaining walls being formed.

We carry out two basic types of works on these slopes - a one-off process of upgrading (under a long-term Landslip Preventive Measures, or LPM programme) and regular long-term maintenance. Upgrading involves major engineering works to bring slope stability up to modern standards.

The works usually involve a combination of strengthening/supporting the slope and protecting the finished slope surface against surface erosion and infiltration by rainwater. Under the LPM programme (managed by the Geotechnical Engineering Office), priority is given to upgrading substandard old slopes affecting occupied buildings, schools and hospitals, as well as slopes along busy roads.

Maintenance works by comparison are relatively minor, consisting mainly of clearing surface drainage channels and repairing defective slope surface covers.

It is government policy that vegetated cover should be used wherever practicable and shotcrete should be chosen only where other alternatives are not effective and safety considerations outweigh aesthetics.

Choice of a surface protective cover is a critical aspect in the design of slope works. It is quite safe and easy to plant vegetation on soil slopes inclined up to an angle of about 35 degrees to the horizontal. On steeper soil slopes it is still possible to plant vegetation, but the designer needs to consider carefully whether the higher rates of rainwater infiltration and potential for increased surface erosion with the use of a permeable vegetated cover are acceptable, otherwise an impermeable rigid cover may be required on safety grounds. If the inclination is greater than 55 degrees, it is generally very difficult to make the vegetation stable.

Unfortunately, many of the old slopes surrounding us are so inclined that we are often left with no choice but to use sprayed concrete, or shotcrete, particularly in an emergency situation.

We do admit that there has been an over-reliance on shotcrete in recent years, especially on remote slopes along lightly-used roads, or in country parks. Some of the shotcrete has been poorly constructed and some wrongly applied.

We also know that the problem is mainly on slope maintenance works where the room for vegetation and landscaping measures is much less than that of the larger-scale slope upgrading works.

These are partly due to Hong Kong's congested environment and partly due to our expanded slope maintenance programme in recent years, under which many previously untouched slopes that were originally covered with old (often cracked) chunam and wild vegetation have been chipped off and received surface repair treatment. We are aware of this and have been taking compensatory action. Last year we spent $30 million on providing vegetation, planters, creepers and landscaping measures to our slopes to improve their appearance.

We realise we still need to do much more to ensure that our policy of reducing the use of shotcrete is put into practice and to rectify some shotcrete which has been wrongly applied to flat and small slopes.

We will set up vetting committees in each works department to ensure prior review and endorsement by senior professional officers before shotcrete is permitted to be used in slope works. And we will continue to research new techniques on bio-engineering and landscaping. Whenever practicable, in future maintenance programmes, we will progressively replace shotcreted surfaces with vegetated surface covers.

We are determined to devote a fair portion of our shotcrete budget to make our slopes green and to try our very best to strike the right balance between slope safety and aesthetic quality.

MAK KA-WAI for Secretary for Works