Highway Slope Manual

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for carrying out a GA is given in Appendix C.
GAs should be conducted to different degrees of detail depending on the scale of the
potential geotechnical risk and the uncertainties involved, and the feasibility of re-aligning the
highway route or adopting an alternative design option later to avoid the risk.
Where natural terrain hazards which could affect a proposed highway project are
identified, a Natural Terrain Hazard Assessment (NTHA) (Ng et al, 2000) should be carried
out and a mitigation strategy developed as part of the GA. In some situations, even minor
earthworks can have a significant impact on the stability of natural terrain. Where the
highway or its works are likely to affect the stability of steep natural terrain, the effects should
also be assessed and any necessary mitigation measures put in place. Particular attention
should be given to assessing whether the natural terrain contains relict structures indicative of
slope movements in the past (see Section 3.4.2). Such structures can have a significant
influence on the stability of the natural terrain particularly if the highway project is likely to
result in changes to ground and/or groundwater conditions.
In assessing the natural terrain hazard mitigation strategy, the provision of buffer zones
or catch ditches along the highway should be considered where space permits. However,
this solution is often not practicable and other mitigation measures such as constructing the
highway on an elevated structure or embankment or provision of defensive works (e.g.
barriers, canopies and rock sheds) may be considered. andslide debris deposition basins,
deflection berms or restraining structures at the mouth of drainage lines should also be
considered where the possibility of channelised debris flows is identified. In some cases,
check dams constructed upstream may be a cost-effective solution for the mitigation of
channelised debris flows. Reference should be made to relevant literature for information on
this subject (e.g. Franks Woods, 1997; o, 2000). The GA should determine the scope of
the mitigation works.
Where a boulder field is identified, there can be great uncertainties in the scope of
works, which needs to be accurately assessed before and during construction. Where new
cuttings are to be made in a corestone bearing rock mass, there can be great uncertainties
associated with the costs and time for excavation. Attention should also be paid to
implementing suitable control and safety precaution during rock excavation (see Section 7.3).
Appropriate contingencies should be allowed for boulder/rock removal and boulder
stabilisation in the works contract in terms of costs and programming.
The formation of high cuts and fill embankments along highways (e.g. 30 m) should
be avoided as far as possible, as these are not only visually obtrusive, they can be difficult and
expensive to maintain. Where any high unsupported cuts cannot be avoided after exploring
the possibility of adjusting the route alignment and adopting design options such as cut and
cover tunnels and elevated structures, design solutions more robust than unsupported cuts
such as soil nailing should be considered.
Where a rock cut is required to be formed or re-profiled, especially a high one, the
design process should commence at the early stage of the project (e.g. at the Feasibility
Study/Preliminary Design stage (see Figure 2.1)). Relevant information should be collected
as part of the Geotechnical Assessment (GA) for the preliminary design of the rock slope
profile including any berms and/or catch ditch needed (see Section 4.4), and of the likely