Highway Slope Manual

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Table 4.4 Recommended Minimum Factors of Safety for the Design of New Highway Slopes for a 10-year
Return Period Rainfall
Consequence-to-life Category
Economic Consequence Category
1 2 3
A 1.4 1.4 1.4
B 1.4 1.2 1.2
C 1.4 1.2 > 1.0
Notes: (1) In addition to a minimum factor of safety of 1.4 for groundwater conditions resulting from a 10-year
return period rainfall, a slope of Consequence-to-life Category 1 should have a factor of safety of at
least 1.1 for the predicted worst credible groundwater conditions.
(2) The factors of safety given in this Table are recommended minimum values in general. Designers
should assess if higher factors of safety are warranted in particular situations in respect of loss of life
and economic loss (i.e. considering the cost-benefit in designing a highway slope to a higher factor of
safety). For particularly vulnerable situations (e.g. sizeable slopes affecting heavily-used facilities
with potential major knock-on effects such as landslide debris causing derailment of high-speed
trains), a higher factor of safety will be warranted to ensure a sufficiently low level of risk. In such
cases, the designer may consider it prudent and useful to also undertake supplementary quantitative
risk assessment (Cruden & Fell, 1997) to evaluate the risk and examine the necessary mitigation
works.
(3) The choice of factors of safety against economic loss is a decision that must be made by the owner
upon the advice of the designer. In advising the owners, the designer should decide for himself the
severity of the economic consequence and should balance the potential economic consequence in the
event of a failure against the increased construction costs required to achieve a higher factor of safety.
(4) The reliability and robustness of the slope works should be considered, taking into account the
sensitivity of performance of the slope works to the uncertainties involved.
(5) Adequate detailing and protective measures should be provided in the design (e.g. improved surface
protection systems and drainage provisions, barriers, debris traps, buffer zones, etc.) to reduce the risk
of local failures.
(6) The geotechnical parameters to be used in conjunction with the above minimum factors of safety
should be the selected values determined by the designer based on ‘best estimate' parameters
following the guidance given in Geoguide 1 (GEO, 1993).
(7) Representative geological models and proper geotechnical characterisation are important for
appropriate zoning of the slope-forming material. The relevant design parameters should be
determined taking into account the variability in the ground profile.
(8) Some guidance on the assessment of the design groundwater conditions and the worst credible
groundwater conditions is given in GCO (1984) and GEO (1993) respectively.