Highway Slope Manual

79
surface water flow paths leading to the failed area and any areas of observed tension cracks
and the natural surface water catchment above them should be considered in determining the
extent of any road closure and in giving other safety recommendations.
An important factor to be considered is the potential for surface water to overflow
from a road surface onto the slopes below. All slope drainage and associated road drainage
provisions in the vicinity of a slope failure should be inspected, and any blockages should be
cleared and damage repaired as early as possible so as to minimise the chance of further slope
failures. Road surface runoff should be diverted away from any open trenches/excavations
along the road. This should be conducted in a manner that can avoid causing slope
instabilities at locations away from the landslide scar.
In many cases, it may not be possible to inspect the failure in detail during the first
visit, e.g. due to the focus on taking immediate actions to safeguard life and property and the
prevailing weather conditions. It may be necessary to carry out subsequent visits to inspect
the slope and the road. Viewing the site from a safe vantage point in daylight or from a
helicopter and examination of available topographical maps, plans and good quality aerial
photographs could help to determine the safe access and the strategy for a detailed inspection
of the failure and the pertinent features along the road.
Repeated minor failures at a slope or at different slopes along a particular road section
should be carefully assessed as these may be indicative of a major failure taking place, e.g.
incidents that took place at Ching Cheung Road (Halcrow, 1998) and Lai Ping Road (Sun &
Campbell, 1999). In this regard, reference may be made to the GEO landslide database
(Chan et al, 1998).
For landslide incidents which have resulted in casualties or serious disruption to traffic,
incidents at slopes which have previously been upgraded or designed to meet current safety
standards, and for incidents at slopes with repeated failures, a detailed investigation/study
should be carried out to establish the probable causes of the failure and identify any lessons
learnt and necessary follow-up actions.
9.2.3 Road Closure
The geotechnical engineer in consultation with the road owner should review and
recommend the extent and duration of any road closure. The risk of further failure,
including failure of the areas in the vicinity of the landslide site, should be assessed and the
extent of the road closure should cover doubtful areas if they are identified. The possibility
of long runout distance/high mobility landslide debris should be considered (see, for example,
the incident that took place at Castle Peak Road in 1997 (Halcrow, 1999a)). Some past
landslide debris mobility data are given in Figure 4.2 (see Section 4.2). The road closure, if
required, should be extended to cover any nearby pedestrian walkways likely to be affected.
Further site inspection and surveillance by the geotechnical engineer, and discussion with the
personnel engaged to investigate the landslide (if any such investigation is carried out) may be
necessary to confirm the assumed causes and trigger of the failure and to ensure that the
extent and duration of road closure are adequate.
The decision as to whether a road should be closed or continued to be closed in the