Debris flow mitigation works, Foothills Bypass, Tuen Mun

Mark Thorn, Nick Koor

Research output: Chapter in Book/Report/Conference proceedingConference contribution

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Foothills Bypass (now re-named as Lung Fu Road) was officially opened in March 2002. This scheme provides a bypass road along the base of the Castle Peak Foothills to relieve the traffic burden along Lung Mun Road.
The large man-made cut slope immediately above the Castle Peak section of the new road (Tuen Mun Area 19) has experienced continuous displacement for more than 20 years. Channelised debris flows from the hills above the proposed route also present a significant geotechnical hazard, with the largest flow in 1990 involving some 10,000 m3.
In response to these hazards the road was formed on an embankment that acted as a toe weight to stabilise the cut slope, with the road embankment also forming a dam to trap major debris flows from the hills above that might otherwise traverse the road in the future.
Although this arrangement served to address the direct hazards to the road at the design stage, it was considered prudent to construct check dams in the natural valleys above the man-made cut slope and surface drainage system in order to provide a degree of protection to the cut slope from smaller-scale, more frequent debris flow events. It was also anticipated that such an arrangement would encourage deposition of debris in large debris flow events, thereby reducing the potential for damage to the cut slope in such cases. Nevertheless it was accepted that in the event of a major debris flow event, such as that in 1990, the check dams could be irreparably damaged and might need to be replaced.
During construction it was established that rockhead was deeper at the proposed check dam locations than had been anticipated, requiring that piled foundations be introduced. In the light of this finding the opportunity was taken to conduct a cost-benefit assessment in order to determine the way forward. This assessment indicated that the construction of the check dams was not justified economically and, as a result, they were deleted from the works. In their place, less visually-intrusive debris flow deposition basins were constructed in each valley as a prescriptive measure.
This paper broadly describes the approach taken in the cost-benefit assessment, as well as outlining the debris flow mitigation measures that were finally implemented.
Whilst such an approach is not applicable to all cases, for example where there is a clear life-threatening risk, it is recommended that in future cost-benefit assessment form a more routine element of the design of mitigation measures against natural terrain failures.
Original languageEnglish
Title of host publicationNature terrain - a constraint to development: proceedings of the 14 November 2002 conference organised by the Hong Kong Branch of the Institution of Mining and Metallurgy
PublisherInstitution of Mining and Metallurgy
Pages143 - 150
Publication statusPublished - 2002
EventNatural Terrain - A constraint to development? - The Institution of Mining Metallurgy - Hong Kong Branch, Hong Kong, China
Duration: 14 Nov 200214 Nov 2002


ConferenceNatural Terrain - A constraint to development?
CityHong Kong


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