Highway Slope Manual

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Where a Geotechnical Assessment (GA) is needed (see Sections 2.2.1 and 2.4), further
site investigation should be carried out and be part of the Feasibility Study/Preliminary
Design stage of the highway project. The main aim of such site investigation is to determine
more accurately the nature and severity of all geotechnical hazards affecting the project. It
should include a more detailed aerial photograph interpretation and walkover survey/mapping
along the proposed route to ascertain the nature, location and geometry of the existing and
potential hazard features and any signs of distress or movement of the ground (see
Section 3.4.2).
Simple ground investigation works, such as dynamic probing, trial pits and trenches,
and slope surface stripping, are cost effective for collecting data. Remote sensing techniques
and geophysical methods are also cost-effective for collecting information where the project
study area is large, but the results should be interpreted using logs from ground investigation
drillholes and trial pits, for data control purposes. Examples of geophysical methods include
seismic and resistivity surveys. Reference may be made to the Transportation Research
Board (1996) and World Road Association (PIARC) (1997) for a discussion on the latest
developments in these areas. In some cases, it may be necessary to sink drillholes, carry out
field tests and install suitable instruments for monitoring purposes.
Any ground investigation works required in the GA should be carried out as early as
possible. Such works should be planned to provide sufficient information to formulate the
geological, groundwater and surface water models for the detailed design of the slope features,
geotechnical works and hazard mitigation measures. Data obtained from ground
investigation during detailed design are seldom fully used because they are not available in
time. Therefore, ground investigation should only be carried out in the detailed design stage
where it is essential to fill data gaps.
In the ground investigation, soil and rock samples should be recovered for detailed
examination and checking for adverse features such as weak zones (see Section 3.4.3) and,
where necessary, laboratory testing. Detailed engineering geological mapping of the
geotechnical features along the chosen route should proceed as part of the ground
investigation. It should provide detailed descriptions of the exposed materials, including any
discontinuities, relict structures and their nature, persistence and orientations, at natural terrain
or man-made slope features. Observations of possible adverse topographical features (e.g.
drainage valley with colluvial/alluvial deposit and local depression above a slope), surface
water distribution and seepage points should also be recorded. These observations and other
ground investigation information should be examined together carefully, to develop the
geotechnical hazard models taking into account the hazards that could affect or could be
introduced by the road project during construction and in the long term. The data needs for
resolving key issues in the design of slope features, geotechnical works and hazard mitigation
measures (e.g. stability, land-take, construction, etc.) should be satisfied as far as possible.
At the construction stage, additional ground investigation will be required in areas
where critical information is incomplete for verification of the assumptions made in the
geotechnical design or slope stability assessment, e.g. due to access or time constraints. In
cases where the level of the soil/rock interface is critical, such as in designing new rock slopes,
it is worthwhile to carry out proof drilling at close centres (say 15 to 20 m) to confirm the
rockhead level and the associated weathering profiles prior to cutting the slopes above. This
should be considered if, for example, there are tight land constraints at the crest of the slope,