Highway Slope Manual

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suitable techniques that can allow some of the investigation works to be carried out on or
above the slope, e.g. using lightweight platforms and equipment erected from above, so that
closure of the road is not required at these locations.
Prior to the start of ground investigation works, traffic impact assessments (TIAs) may
have to be carried out where road or pedestrian traffic will be affected. Reference should be
made to Guidance Note No. RD/GN/021 issued by the Highways Department (1995) on this.
Where a TIA is required, adequate time should be allowed for in the ground investigation
programme. Depending on the site conditions, it may take 6 to 8 weeks or more to carry out
the TIA and obtain the necessary permission to commence the ground investigation works on
site.
3.. OTHER CONSIDERATIONS FOR ROAD PROJECTS
3...1 General
Ground investigation works such as drillholes for a road project are often carried out
concurrently at widely spaced locations. In putting together the investigation requirements,
a programme supplemented with a layout plan should be prepared showing the locations and
nature of works that are to be carried out concurrently, so that adequate site supervisory
personnel can be assigned. This should be done taking into account the actual site
conditions ascertained from a walkover survey along the route.
At locations where re-routing and/or ducting of any existing services near highway
slope features are likely to be required, the locations of the services should be confirmed by
inspection pits during ground investigation. Such locations should be surveyed and
documented. This information is important to facilitate both design and construction, in
particular to minimize the need for re-design during the contract.
3...2 Areas with Signs of Distress or with Known or Suspected Ground
Movement/Deformation
In situations where new high cuttings are to be excavated in sloping terrain, whether in
soil or rock, a central issue is whether the natural ground is inherently stable, or whether it
contains relict structures caused by slope movements in the past. Many significant
landslides on roads were “second time slides". Therefore, where a proposed route traverses
areas with signs of distress, or areas with known or suspected past ground
movement/deformation, as identified by aerial photograph interpretation, a detailed study
should be carried out to provide data for the assessment of the stability of the terrain and its
possible impact on the route.
Low altitude aerial photographs and data on past landslides could provide information
for assessing whether there has been progressive development of a ground failure involving
significant ground deformation (Sun & Campbell, 1999). A site walkover survey should be
carried out to collect information on the nature of the distress and/or ground
movement/deformation together with geomorphological and engineering geological mapping.
Where required, conventional surveying should also be carried out to provide ground