2

Proceedings of The Hong Kong Engineers
Engineering for Public Safety Conference,
HKIE April 1997

Managing Slope Safety in a High Density City
Prone to Landslips

by Dr Andrew Malone
Geotechnical Engineering Office

Synopsis

Hong Kong's terrain and climate make the territory very prone to landslips and significant risk has been created through the post-war urban development of its steep hillsides. However, progressively, the government has introduced slope safety measures over the last 30 years.

Today, Hong Kong has a well developed slope safety system aimed at reducing risk and addressing public attitudes to risk. The system is managed by the Geotechnical Engineering Office. The aims are to be achieved through the setting of safety standards, policing actions, providing public educational and information services and the management of upgrading works programmes for old man-made slopes in private and public ownership.

The purpose of this paper is to outline Hong Kong's slope safety system and evaluate its effectiveness in lay terms.

Figure 1. Landslip fatalities.

Hong Kong's landslip problem

Landslips have been responsible for the death of more than 470 people in Hong Kong since 1948 (Figure 1). Although landslips are common on undeveloped natural hillsides, nearly all of these deaths result from the collapse of man-made slopes, i.e. cut slopes, fill slopes and retaining walls created by the process of hillside development.
There is evidence from analysis of fatal landslips that the origin of a significant portion of Hong Kong's landslip risk lies in the nature of hillside development works in the post-war decades, along with lack of adequate subsequent maintenance (Hong Kong Government, 1972a, 1972b and 1977; Geotechnical Engineering Office 1993a, 1993b, 1994, 1996a and 1996b, Chan et al., 1996).

Creation of a policing body

The two most destructive landslips in the recent history of Hong Kong took place on 18 June 1972, the third day of a severe rainstorm associated with a trough of low pressure. Shortly after 1 pm, a major landslip occurred in the Sau Mau Ping Resettlement Estate in the Kowloon foothills. The failure involved the collapse of the side-slope of a 40 m-high road embankment constructed on sloping ground. The resulting flowslide destroyed many huts in a licensed temporary housing area, killing 71 people and injuring 60 others (Hong Kong Government, 1972a). Hours later, another major landslip occurred, in a private residential district on a steep hillside Lit Po Shan Road in the Mid-levels area of Hong Kong Island. Sixty-seven people were killed and 20 injured when an Occupied 12-storey private apartment building was demolished under the impact of an extremely rapid flowslide (Hong Kong Government 1972b). The landslip, illustrated in Figure 2, was initiated on the hillside above by the collapse of a steep cutting in a works site for a private building.

Figure 2. The landslip at Po Shan Road, Hong Kong Island which occurred on 18 June 1972.

A commission of enquiry, set up amidst the ensuing public outcry, reported in August and by the end of the year a group of civil engineers had been assigned to the building control office to vet the geotechnical aspects of private development submissions.

Four years later another destructive landslip occurred in the Sau Mau Ping Resettlement Estate, on the morning of 25 August 1976 following heavy rainfall associated with a Severe Tropical Storm. At least four landslides took place in the estate resulting from the collapse of the side-slopes of highway embankments formed of earth fill. Three of these turned into flowslides, the most hazardous occurring on the face of a 35 m-high embankment above an occupied public housing block. The debris moved downwards as 'a large sheet' until arrested by the building, the ground floor rooms of which were inundated by fluid mud, trapping many occupants; eighteen people were killed and 24 seriously injured. Subsequent investigation found that the collapse had occurred because the earth fill forming the face of the slope was in a loose condition, having been placed by end-tipping without compaction, contrary to good practice (Hong Kong Government, 1977). The 1976 Sau Mau Ping landslip brought the number of landslip fatalities in a four year period to greater than 175 (Figure 1). Immediately after the landslip the Governor established an Independent Review Panel on Fill Slopes, comprised largely of overseas geotechnical experts, which recommended the creation of a central policing body to regulate the whole process of investigation, design, construction, monitoring and maintenance of slopes in Hong Kong.

The geotechnical control body, created in July 1977, has since evolved in response to experience and through reform initiatives (Malone & Ho, 1994) and today, Hong Kong has a well-developed slope safety regime. The GEO manages the safety regime, which will be referred to as the 'Slope Safety System'. The aims are twofold: to reduce risk and to address public attitudes to risk. Along with GEO as safety manager, the main action parties in respect of slope safety are the private owners and government agencies that are responsible for the construction of slopes and the maintenance of their stability.

The Slope Safety System

To recap, post-war site formation and subsequent maintenance by private owners and Government departments went largely unregulated before the creation of the civil engineering unit in the building control office in 1972 and then the central policing body in 1977. By this date a very large amount of development had already taken place on hillslopes. Some of the developments constructed on hillslopes during the unregulated period turned out to contain design, construction or maintenance defects, when judged on modem standards, causing failure with attendant harm and damage.

Therefore, when the policing body was established it was given two main duties; to establish a control regime for new works on hillslopes, to prevent any increase in risk due to new works, and to be the manager of a slope retrofit programme, under which substandard works of the past would be brought up to modem standards by their owners, with a corresponding reduction in risk. These duties remain the major elements of GEO's work in terms of resources deployed. GEO took on new tasks in the 1980s with the introduction of squatter safety clearances and in the 1990s with its educational initiatives, initially targeted at slope owners to promote good maintenance practice. Today GEO's slope safety functions are fourfold: policing slope safety, setting safety standards and research, carrying out works projects and providing educational and information services. The contribution which each of these components makes towards the two aims of the Slope Safety System (reducing risk and addressing public attitudes) is illustrated in Table 1.

Table 1. The Slope Safety System

Slope Safety System components	contribution by each component
	to reduce landslip risk		to address public attitudes
	hazard	vulnerability
policing
cataloguing and safety screening and statutory repair orders	•
checking new works	•	•
maintenance audit	•
inspecting squatter areas and recommending safety clearance		•
safety standards and research	•	•
works projects
upgrading old Government slopes	•
preventive works for old tunnels	•
education and information
maintenance campaign	•		•
personal precautions campaign		•	•
awareness programme	•	•	•
information services	•	•	•
landslip warning and emergency services	•	•	•
input to land use planning	•	•

Evaluation of the Slope Safety System to date

To evaluate the Slope Safety System to date it is necessary to begin by examining evidence of change in global landslip risk (i.e. landslip risk for the entire territory) in the last 20 years and then to go on to examine, if possible, the efficiency and effectiveness of the component parts of the system.

If landslip risk has been reduced by the Slope Safety System, the trend depicted in Figure 3 ought to be apparent. This trend is based on the premise that risk grew broadly in proportion to population until arrested by the intervention of the Slope Safety System. It is postulated that without the Slope Safety System, landslip risk would have continued to increase with continuing growth in population, the encroachment of development onto steeper terrain, the increase in the number of man-made features and their deterioration due to lack of maintenance.

Figure 3. Hypothetical risk trend.

To measure change in risk with time, quantified risk analyses (QRAs) must be carried out for different times and this work has not yet been done. Evaluation will therefore have to rely for the present on indication of risk rather than calculation of risk by QRA.

A reducing trend with time in the amount of harm and damage occurring annually due to landslips, when normalized for rainfall, would be a prima facie indication of a reduction in risk. Trends in data can be revealed by plotting rolling averages (i.e. moving averages) and this technique is adopted here, using a 15- year datum period after trying several other periods. The plots in Figures 4 and 5 utilize landslip fatality data for the whole territory and rainfall data from the Royal Observatory Tsim Sha Tsui gauge. The past 15-year rolling averages of annual number of landslip fatalities are given in Figure 4. In an attempt to discern trends in rainfall the 15-year rolling averages of the annual number of heavy rainfall events, defined as 24-hour rainfall greater than 175 mm (Figure 5), and the 15-year rolling average annual rainfall are also plotted (Figure 5). The former criterion is chosen because the occurrence of 175 mm of rain in 24 hours at Tsim Sha Tsui is the main Landslip Warning criterion and the data is readily available.

Figure 4. Past 15-year rolling average of annual number of landslip fatalities.

Figure 5. Rainfall trends.

The trend in 15-year rolling annual fatalities resembles that shown in Figure 3 but there is no
corresponding reduction in annual rainfall or the number of heavy rainfall events. This finding may be interpreted as indicating a reduction in risk. Another indication of reduction in risk would be any reducing trend in territorial landslip fatality rate with increasing territorial population. The data are plotted in Figure 6. The trends evident in Figures 4 and 6 provide prima facie evidence of significant risk reduction since the end of the 1970s.

Figure 6. Trend of fatalities with population growth.

In evaluating the efficiency of the Slope Safety System as a whole in terms of outcome, the
fundamental question is 'is Hong Kong getting value for money in terms of cost/benefit?'

Projecting forward the fatality rate trend prior to the 1980s it appears that an annual fatality rate (15-year rolling average) of the order of twenty-five fatalities per year might have been reached by 1996. In fact the actual annual fatality rate in 1996 (15-year rolling average) was of the order of three fatalities per year. Taking these figures, attributing the reduction to the Slope Safety System, making assumptions about the 19-year cost of the Slope Safety System and charging its entire cost to saving life only, it is estimated that up to the end of 1996 each life saved has cost about $20 million. This price would probably be regarded as cost-effective by the stakeholders. Judged on the UK Health and Safety Executive's tolerability rationale (commonly known as the ALARP or 'as low as reasonably practical' rationale) $20 million is higher than but not grossly disproportionate to the values of statistical life assumed in risk assessments for technological hazards in Hong Kong current practice. Therefore, purely on the ALARP rationale, the need is indicated for continuing investment in landslip risk reduction in Hong Kong.

Conclusions

The trends indicated in Figures 4 and 6 provide prima facie evidence of significant risk reduction in the past twenty years which may be attributed to the Slope Safety System introduced progressively since the late 1970s. Crude calculations indicate that the risk reduction effort has been cost-effective and should be continued.

Acknowledgements

The paper is published with the permission of the Director of Civil Engineering of the Hong Kong Government.

References

1. Chan, Y.C., Pun, W.K., Wong, H.N., Li, A.C.O. & Yeo, K.C. (1996) Investigation of some major slope failures between 1992 and 1995. Geotechnical Engineering Office, Hong Kong, 97 p. (GEO Report No. 52).

2. Geotechnical Engineering Office (1993a) Report on the Rainstorm of May 1982, by M.C. Tang (1993), 129 p. plus 1 drg. (Reprinted, 1995).

3. Geotechnical Engineering Office (1993b) Report on the Rainstorm of August 1982, by R.R. Hudson (1993), 93 p. plus 1 drg. (Reprinted, 1995).

4. Geotechnical Engineering Office (1994) Report on the Kwun Lung Lau Landslide of 23 July 1994, Volume 2: Findings of the Landslide Investigation. Geotechnical Engineering Office, Hong Kong, 379 p. (Chinese version, 358 p).

5. Geotechnical Engineering Office (1996a) Report on the Fei Tsui Road Landslide of 13 August 1995, Volume 2: Findings of the Landslide Investigation. Geotechnical Engineering Office, Hong Kong, 68 p. (Chinese version, 64 p).

6. Geotechnical Engineering Office (1996b) Report on the Shum Wan Road Landslide of 13 August 1995, Volume 2: Findings of the Landslide Investigation. Geotechnical Engineering Office, Hong Kong, 51 p. (Chinese version, 49 p).

7. Hong Kong Government (1972a) Interim Report of the Commission of Inquiry into the Rainstorm Disasters, 1972. Hong Kong Government Printer, 22 p.

8. Hong Kong Government (1972b). Final Report of the Commission of Inquiry into the Rainstorm Disasters, 1972. Hong Kong Government Printer, 94 p. (Also published in Chinese, 99 p.)

9. Hong Kong Government (1977). Report on the Slope Failures at Sau Mau Ping, August 1976. Hong Kong Government Printer, 105 p. plus 8 drgs.

10. Malone, A.W. & Ho, K.K.S. (1995). Learning from Landslip Disasters in Hong Kong. Built Environment, 21, no 2/3, 126-144.