Highway Slope Manual

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2.2.2 Planning of Road Improvement Projects
The scope of a road improvement project can vary widely, ranging from upgrading of
a road class (e.g. from a district distributor to a primary distributor), to local improvements of
a road's alignment (e.g. widening). In a road improvement project, existing slopes should be
upgraded to a standard commensurate with the consequence category rating of the improved
road. It may also be beneficial to upgrade adjacent substandard slopes to take advantage of
the economy of scale and closure for road works. Guidelines for including slope upgrading
and landslide mitigation works as part of a road improvement project are given in Table 2.1.
For road improvement projects involving the re-alignment of an existing road to follow a
new route, the guidance given in Section 2.2.1 regarding the carrying out of a GR/GA during
the project planning stage is also relevant. It is particularly important that the scope of the
geotechnical works and hazard mitigation measures is not underestimated for such projects.
As compared with a new highway, planning of a road upgrading or re-alignment
project is generally more constrained by the existing road alignment, structures and land use,
services and utilities and the surrounding topography. The need to maintain traffic flow
during construction is also an important constraint. The selection of appropriate route
alignment and/or design options should take into account these constraints.
2.3 GEOTECHNICAL REVIEW
The objective of a Geotechnical Review (GR) is to identify and assess the impacts of
significant geotechnical hazards and critical geotechnical issues that could influence the
feasibility of a highway project. The geotechnical activities in a GR are illustrated in
Figure 2.2. A model brief for carrying out a GR is given in Appendix B.
In a GR, geotechnical hazards and issues are considered in a general manner as the
time available for the review is usually limited especially where a number of possible route
corridors/alignments are to be examined. The review should be based mainly on available
information collected from a desk study including an aerial photograph interpretation. The
locations of existing slope features and new geotechnical features likely to be formed should
be determined. A walkover survey should also be carried out to confirm the presence, nature
and severity of hazards from the key natural terrain and man-made geotechnical features
identified in the desk study. The potential impacts of the geotechnical hazards along the
possible route corridors/alignments should be assessed. Examples of the geotechnical
hazards that need to be considered are given in Table 2.2. Where natural terrain hazards are
identified, a Natural Terrain Hazard Review (NTHR) should be carried out. Examples of
some common natural terrain landslide hazards posed to a highway are illustrated in Figure 2.3.
Guidance on Natural Terrain Hazard Review is given in GEO Special Project Report
No. SPR 5/2000 (Ng et al, 2000).
Where significant geotechnical hazards are identified, in particular natural terrain
hazards, the GR should include a careful assessment to determine whether an adjustment of a
route corridor/alignment and/or provision of appropriate mitigation measures are warranted.
If alternative route corridors/alignments are to be considered, the assessment should
include, where possible, a ranking of the alternatives taking into account the possible impacts