Highway Slope Manual

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3. SITE INVESTIGATION
3.1 GENERAL
Guidance on the standards of good practice for site investigation is given in
Geoguide 2 (GCO, 1987). Geoguide 3 (GCO, 1988) describes good practice for the
description of rocks and soils for engineering purposes in Hong Kong.
For a road project, site investigation frequently extends beyond the design stage, for
example, to verify critical geotechnical design assumptions during construction or to support
review of performance of slopes during maintenance.
Allowance should be made in the project programme and cost estimates for carrying
out site investigation at different stages. The amount of ground investigation works required
depends on the amount of existing information available, the complexity of the ground and
the scope of works involved in the road project. For even small road projects, the time
required from the resolution of land matters, access and application for permission to carry
out the ground investigation (e.g. road excavation permit) to the completion of ground
investigation reports may range from 4 to 9 months. Extra time will also be needed where a
traffic impact assessment is required (see Section 3.3). The cost of ground investigation
works (including laboratory testing) to provide information for design is normally at least 2%
to 3% of the project estimate. It can be much higher if difficult ground conditions are
encountered or if the project comprises mainly slope works, e.g. road improvement projects.
Sufficient funds should also be provided to engage an adequate number of experienced site
supervisory personnel to supervise the ground investigation works (see Section 3.4.1).
3.2 NEW ROAD PROJECTS
Site investigation for a new road project should be carried out in stages. Information
from the successive stages can be used to adjust project details to account for the ground
conditions encountered and to revise the investigation works and methods to obtain the most
relevant and reliable information. For fast track projects for which there is insufficient time
to carry out multi-stage ground investigations before tendering, and for Design and Build
projects, a comprehensive ground investigation should be carried out to reduce uncertainties
in the design as far as possible.
At the preliminary project feasibility study stage, generally only an indicative route
corridor or possible alignments traversing a broad band of terrain are given. A Geotechnical
Review (GR) is usually required (see Sections 2.2.1 and 2.3). A desk study of the existing
information available is the most cost-effective approach of site investigation at this stage.
The potential uses of different types of information collected for the desk study are briefly
described in Table 3.1. Some of the information available may be of questionable quality
(e.g. very old ground investigation and construction records). Particular care should be
taken to avoid over reliance on such information in drawing critical conclusions. A site
reconnaissance/walkover survey should be carried out at selected locations along the routes
under investigation, to confirm and supplement the information collected during the desk
study. Such reconnaissance work is very valuable. It takes relatively little time to carry out,
and it allows a rapid evaluation of the scale of the hazards for comparison of various route
options.