Highway Slope Manual

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Table 4.5 Recommended Minimum Factors of Safety for the Stability Assessment of/Design of Remedial
or Preventive Works to Existing Highway Slopes
Consequence-to-life Category 1 23
Recommended Minimum Factor of Safety
1.2 1.1 >1.0
for a 10-year Return Period Rainfall
Note: The conditions to be satisfied for a slope to be regarded as an ‘existing slope' for stability assessment and
design purposes are stipulated in GCO (1984). In most instances, it is likely to be difficult to fulfill all
the necessary conditions because of the scarcity and inadequate quality of the available information and
the uncertainty associated with possible adverse changes in the site setting at or in the vicinity of the
slope. For these reasons, the guidance on minimum factors of safety given in Table 4.4 should apply in
general, except in the rare situation where the designer is confident to use this Table after a rigorous
assessment of comprehensive and sufficiently reliable information (see also Works Bureau, 1999b).
4.4 NEW ROCK CUT SLOPES
General guidance on the design of rock cut slopes along highways, including
stabilisation and rockfall control, is given in Federal Highway Administration (1994) and
Transportation Research Board (1996). The design process should include consideration of
stability, rockfall hazards, drainage, aesthetics/landscape aspects, land­take, method of
excavation, construction safety, maintenance requirements, etc., in an integrated manner.
The nature of rock cuts dictates that the design can only be finalised and the extent of
stabilisation works ascertained following detailed mapping after the rock face is excavated.
Notwithstanding this, all available information including ground investigation and geological
mapping data should be used to determine the initial profile of a rock cut. In particular, the
designer should assess at an early stage the need for the provision of berms and/or catch
ditches taking into account all relevant requirements, constraints and available ground
information (see Section 2.4). Table 4.6 summarises the key factors that need to be
considered in the design of new rock slopes along highways, with particular reference to the
provision of berms or catch ditches.
Table 4.6 Key Factors to be Considered in the Design of New Rock Cut Slopes along Highways
Factors
Considerations
Stability • The overall cut slope profile (viz. slope angle and possible use of berms) should be
determined to ensure stability, taking into account geological features and characteristics of
discontinuities in the rock mass, groundwater condition, etc. Major geological features
such as adversely dipping persistent discontinuities, sheeting joints, weak weathered seams,
fault zones, etc., should be given special attention in the investigation and design of rock
cuts.
• The practice of relying on generalised assumptions about strength of discontinuities and
groundwater may not adequately cater for local weak zones in the rock mass. The rock
face should be mapped in detail by experienced personnel. Pitfalls in the practical use of
stereographic projection techniques are highlighted by Hencher (1985).
• Berms should be provided between batters of significantly different gradients associated
with different degrees of weathering.
• Berms are generally not necessary in ‘massive rock' (1) from a stability point of view.