LEGCO PANEL ON PLANNING, LANDS & WORKS
INFORMATION PAPER ON SLOPE SAFETY
INTRODUCTION
1. This is the sixth of a series of reports aimed to inform Members of various slope safety initiatives and obtain Members' views on further improvements. Previous reports were given in July 1995, March 1996, August 1996, May 1997 and March 1998. Members also received a Legislative Council Brief on Upgrading and Maintenance of Man-Made Slopes in July 1998, and had received four reports on clearance of squatters on slope safety grounds in September 1993, December 1994, June 1996 and April 1997.
BACKGROUND
2. The 1998 Policy Objective of "slope Safety for All" states the Government's commitment to meet Hong Kong's need for the highest standards of slope safety. To achieve this, we have identified six Key Result Areas :
- Improve slope safety standards, technology, and administrative and regulatory frameworks.
- Ensure safety standards of new slopes.
- Rectify substandard Government slopes.
- Maintain all Government slopes.
- Ensure that owners take responsibility for slope safety.
- Promote public awareness and response in slope safety through public education, publicity and information services.
PERFORMANCE MEASURES
3. The basic yardstick by which we are measuring our long term performance is global landslide risk in Hong Kong, i.e. total risk to the whole community.
4. In April 1998 the Geotechnical Engineering Office (GEO) completed preliminary inspections of all registered old man-made slopes. Immediate action has been initiated where the preliminary inspections have revealed signs of immediate and obvious danger. The remainder are prioritized for more detailed study. Where found necessary, this is followed by upgrading under the GEO's Landslip Preventive Measures (LPM) Programme for substandard Government slopes, and the issue of Dangerous Hillside Orders (DHOs) for substandard private slopes.
5. By the year 2000, the overall landslide risk from old man-made slopes will have been reduced to 50% of the level which existed in 1977 by the actions taken under the LPM Programme. This does not include the risk reduction achieved by the non-development clearance to date of about 74,000 squatters vulnerable to landslide risk, and by removal or upgrading of substandard old man-made slopes as part of new developments. By the year 2010 the LPM Programme will reduce the risk from old man-made slopes to 25% of the 1977 level. By that time a total of about 4,000 of the highest risk substandard old Government slopes will have been upgraded, and safety screening completed for about 5,000 of the highest risk substandard private slopes. The overall risk will be further reduced below this level by other actions, including upgrading other old slopes as part of Government developments and road improvement projects, private developments, regular and enhanced maintenance of slopes, and further squatter clearance.
6. In practical terms, this means that all "high consequence" slopes (that is, slopes near occupied buildings or heavily trafficked roads) will have been dealt with by 2010, either by full upgrading, road improvement projects, redevelopment or enhanced maintenance measures. Lower consequence slopes (that is, slopes near roads with low to moderate traffic, open space, country parks, etc.) will be dealt with by regular and enhanced maintenance or by redevelopment.
ENHANCING PUBLIC INFORMATION AND ADVISORY SERVICES
7. We completed in September 1998 a new computer-based Catalogue of Slopes covering the whole of Hong Kong. Some 54,000 sizeable man-made slopes are now registered in the Catalogue, which will be continuously updated to include newly formed and modified slopes. The computerized Slope Information System (SIS) containing data on all of the 54,000 registered slopes is now publicly accessible on the Internet and in Building Management Resource Centres.
8. In April 1999 the GEO will create a Community Advisory Unit to provide advice to private owners on the discharge of DHOs and the conduct of slope maintenance.
OLD SLOPES AFFECTING SCHOOLS
9. We have completed stability assessment on 592 old slopes affecting schools, of which 407 have been found to be substandard. Action has been taken on all of these slopes. The service of DHOs has been recommended to Buildings Department on 245 private slopes, and all of the remaining slopes have already been or will soon be upgraded under the LPM Programme. Necessary works on all but 3 of the Government slopes were completed by the end of March 1999. The remaining 3 take longer time to complete, i.e. by the end of 1999, due to complex site constraints and the need to minimize disturbance to the schools concerned.
BURIED WATER-CARRYING SERVICES AFFECTING SLOPES
10. Various works departments and the Housing Department have engaged consultants to carry out systematic inspection and repair of all Government underground drains, sewers and water pipes which may affect the stability of adjacent slopes. The target is to complete the inspection and necessary repair works for underground water-carrying services between 2000 and 2004. The Buildings Department will also step up enforcement of the provisions of the Buildings Ordinance to require owners to inspect and repair private underground services which may affect the stability of adjacent slopes. From 1999 onwards, screening studies of underground services affecting 500 slopes per year will be carried out, and Section 27C Orders will be served requiring investigation and repair of suspected services.
CHECKING OF NEW SLOPES
11. Slopes formed since 1977 are required to be checked by the GEO to current safety standards with respect to the design and supervision of the works. The GEO achieved Certification to ISO 9001 for quality assurance of its checking functions in March 1999.
12. A legislative proposal is in preparation to require that the geotechnical elements of private building works are to be undertaken by Registered Geotechnical Engineers, for which a Geotechnical Engineers" Register will be established by the Building Authority. The proposed duties include preparing and signing of plans for ground investigation and groundwater drainage works in certain designated geotechnically difficult areas, for remedial and monitoring works to discharge DHOs, and for site formation, together with supervision of these works.
SLOPE MAINTENANCE
13. As of March 1999, the maintenance responsibility of over 43,000 man-made features in the Slope Catalogue has been identified under the project Systematic Identification of Maintenance Responsibility of Slopes (SIMAR) being carried out by the Lands Department. The whole project will be completed by the end of 1999.
14. In June 1998 the GEO began auditing the performance of Government departments in their slope maintenance work. The GEO also carries out an annual survey of how well private owners are maintaining certain selected private slopes. The results of these surveys will be used in a thorough review of slope maintenance in 2000, which will include further consideration of the need for introduction of mandatory maintenance of private slopes. Meanwhile, the public education campaign on slope maintenance is continuing, and the results of the annual surveys are being used to improve the effectiveness of the campaign.
SQUATTER CLEARANCE ON SLOPE SAFETY GROUNDS
15. The GEO has been making recommendations for the clearance of squatters on slope safety grounds since the early 1980's under the Housing Department's Non-Development Clearance (NDC) Programme. Initially, clearance recommendations were confined to the urban areas of Hong Kong and Kowloon. Some 58,000 squatters in the urban area have been rehoused.
16. Between the mid 1980's and early 1992 clearance recommendations for squatters in around 70 New Territories (NT) villages were also made. Arising from this work some 6,000 squatters were rehoused. In May 1992, having inspected the more critical areas in the NT, the GEO undertook to re-inspect all of the previously inspected NT villages in a 1-year programme. This gave rise to about 5,200 domestic structures being recommended for clearance on slope safety grounds.
17. This was followed by inspection of new villages under a series of programmes in 1993-97. A series of progress reports were produced for Members. In 1997, as a result of a fatal landslip in Kau Wah Keng, involving an isolated squatter hut detached from main villages, the GEO began a programme of inspecting this category of huts. This was completed in March 1998 and clearance recommendations made to the Housing Department and Lands Department. The GEO has now commenced inspections of villages on the outlying islands.
LANDSLIP WARNING
18. Since 1982, the GEO has been operating an automatic raingauge network in cooperation with the Hong Kong Observatory to provide data for the issue of the Landslip Warning. A project to upgrade the network started in late 1997. The project includes replacement of the existing gauges plus improvements to the hardware and software. The upgraded system will provide faster and more reliable data to decision makers for assessing the need to issue Landslip Warnings, and to personnel involved in landslip emergency duties and landslip-related studies. It is scheduled for operation in May 1999. Forty additional gauges will then be installed to give better coverage of the whole of Hong Kong in the second half of 1999. Complete upgrading of the system will cost about HK$7 million.
19. Two new Announcements of Public Interest (API) on television promulgating personal precautionary measures to be taken during Landslip Warnings were broadcast since June 1998. One API is targeted at squatter residents and the other at the general public. The general feedback on these two APIs is satisfactory. The GEO is also carrying out feasibility study into methods to survey the actual behaviour of the public during Landslip Warnings. This will help us improve our public education strategy so that we can reduce landslide damages through appropriate personal precautionary actions taken by the public. Work is also progressing well on education toolkits for schools on slope safety, for completion before the end of 1999.
TECHNICAL STANDARDS
(i) Prescriptive Measures
20. Prescriptive measures are standard types of slope improvement works prescribed from experience without going through the process of detailed ground investigation, laboratory testing and stability analysis. The use of prescriptive measures can save time, human resources and cost. In October 1996 the GEO published Report No. 56 on the application of prescriptive measures to soil cut slopes. Based on the past successful use of prescriptive measures and on findings of slope and landslide studies, the GEO is now preparing a new guidance document for publication by December 1999 to extend the scope of application of prescriptive measures (mainly in enhanced maintenance of slopes).
(ii) Improved Means of Site Characterization
21. Following on from the Fei Tsui Road and Shum Wan Road landslides of 13 August 1995, a phased programme of studies was initiated to identify locations at slopes where the geological features are similar to those that influenced the Fei Tsui Road and Shum Wan Road landslides. As part of this programme of studies, the GEO is carrying out research on the use of geophysical methods for ground investigation in Hong Kong. The objective of the research is to evaluate various geophysical methods to characterize discontinuities, in particular clay filled joints, within weathered rock in Hong Kong. The research is now focusing on the use of "downhole" geophysical methods which measure physical properties of soil and rock surrounding boreholes, including electrical, electromagnetic, nuclear, acoustic, seismic and optical techniques.
(iii) Highway Slope Manual
22. Recommended standards of good practice for slope engineering in Hong Kong are given in the Geotechnical Manual for Slopes (1984) and the various Geoguides published by the GEO. However, the standards are prepared for slopes in general and are not specifically aimed at highway slopes. A number of major incidents affecting highways have occurred in recent years, causing serious social disruption. For example, the Tuen Mun Highway Rockfall and the Shum Wan Road Landslide in August 1995, and the Ching Cheung Road Landslide in 1997. These have brought to light a number of issues specific to the investigation, design, and construction and maintenance of highway slopes that require attention.
23. A number of major highways projects on steep terrain are being planned or carried out. These projects involve extensive slope works and have the potential to result in an increase in the overall landslip risk. The above underlines the need for a Highway Slope Manual to provide focused guidance on the geotechnical aspects of routing of highways and investigation, design, construction and maintenance of slopes along highways. We aim to publish the Highway Slope Manual in December 2000, after extensive consultation both within and outside the Government. During the course of preparation of the manual, necessary research and development work would be undertaken; in the meantime, the GEO is working closely with the Highways Department to enable the timely adoption of standards of good practice so identified.
LANDSLIPS IN 1998
24. The rainfall in 1998 was somewhat higher than average. About 250 landslips were reported to the GEO. No injuries were reported. Most of the landslips were relatively minor having a failure volume less than 50 m3. But six major landslips occurred which resulted in some extended road closures and evacuations of residents. These incidents are being subjected to independent detailed studies under the GEO's annual consultancy on the more integrated approach for determining the cause of failure and lessons to be learned. Based on a review of the landslides in 1998, an audit report on the GEO's Slope Safety System will be completed by September 1999.
Works Bureau
31 March 1999
