Slopes - Geotechnical Investigation and Design Tables - page 173

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This accounts for possible future (minor) changes, either in load on strength
reductions with time due to weathering or strain softening.

Table 14.5 Factors of safety for new slopes (adapted from GEO, 1984).

Economic risk Required factor of safety with loss of life

for a 10 years return period rainfall

Negligible Low High

Negligible 1.1 1.2 1.4 >

Low 1.2 1.3 1.5

High 1.4 1.5 1.6

14.6 Factors of safety for existing slopes
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Existing slopes generally have a lower factor of safety than for new slopes.

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An existing slope has usually experienced some environmental factors and
undergone some equilibration.

Table 14.6 Factors of safety for new slopes (adapted from GEO, 1984).

Risk Required factor of safety with loss of life for a

10 years return period rainfall

Negligible 1.1 >

Low 1.2

High 1.3

14.7 Risk to life
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The risk to life includes both the number of people exposed as well as the length
of time exposed to the hazard.

Table 14.7 Risk to life (adapted from GEO, 1984).

Situation Risk to life

Open farmland Negligible

Country parks, lightly used recreation areas Negligible

Country roads and low traffic intensity B roads Negligible

Storage compounds (non hazardous goods) Negligible

Town squares, sitting out areas, playgrounds and car parks Negligible

High traffic density B roads Low

Public waiting areas (e.g. railway stations, bus stops) Low

Occupied buildings (residential, commercial, industrial and educational) High

All A roads, by- passes and motorways, including associated slip roads, High

petrol stations and service areas
Buildings storing hazardous goods, power stations (all types), nuclear, High

chemical, and biological complexes