Highway Slope Manual

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5.2.3 Drainage Improvement Works
For existing roads, critical locations should be identified and any road drainage
improvement works needed should be carried out in a priority order based on the
consequence-to-life and socio-economic consequences should slope failure occur (Figure 5 勾 1) 勾
Where opportunities arise, the necessary improvement works should be carried out as part of a
road improvement (e 勾 g 勾 widening or upgrading) project or a new project nearby (see
Section 2 勾 2 勾 2), or as part of the slope preventive maintenance programme (see Section 8 勾 3) 勾
Any road drainage improvement works required should be carried out to meet the
current standards 勾 The maintenance requirements of the improvement works should be
documented in a maintenance manual 勾 The adequacy of the works should be reviewed by
reference to the maintenance inspections made, including those on the slopes nearby 勾
Further drainage improvement works should be carried out if found necessary 勾
5.3 WATER-CARRYING SERVICES
When designing a new road, opportunity should be taken to locate buried services well
away from the crest of slopes 勾 As a general rule, buried services should not be placed in a
slope nearer to the crest of the slope than a horizontal distance equal to its vertical height 勾
For road or slope improvement works, the same rule should also be applied to re-route the
existing buried services away from the slopes, where practicable 勾 In cases where the siting
of new buried services or the relocation of existing buried services outside the crest area is
impractical, mitigation measures for minimising the impact of water leakage on slope stability
as detailed in the Code of Practice on Inspection & Maintenance of Water Carrying Services
Affecting Slopes (Works Branch, 1996) should be adopted 勾
General guidance on trench excavation on or above highway slopes (e 勾 g 勾 for services
and utilities) and the precautions to be taken for excavation during the wet season are given in
Chapter 9 勾 4 of the Geotechnical Manual for Slopes (GCO, 1984) 勾 Before any trench
excavation at or close to the toe of highway slopes is carried out, an assessment should be
made of the effects of the proposed works on slope stability 勾 Where necessary, the extent of
excavation should be limited and adequate slope supportstabilisation works should be put in
place 勾
Special attention should be given to locations where permeable fill is present beneath
the road surface (e 勾 g 勾 uncompacted fill bodies and granular sub-base) as such material can
convey water over long distances, with the resulting risk of affecting the stability of slopes far
away from the location of the leaking buried services 勾