Analysis of stability and deformations - Geotechnical Engineering of Dams - page 471

Table 11.6. Baseline recommended minimum acceptable factors of safety and load conditions.

Slope Load condition Reservoir characteristic Minimum factor of safety

Upstream and End of construction Reservoir empty 1.3

downstream
Downstream Steady state seepage Reservoir at normal maximum 1.5

operating level (Full Supply Level)

Downstream Maximum flood Reservoir at maximum flood level 1.5, free draining crest

zones, 1.3 otherwise

Upstream Drawdown Rapid drawdown to critical level 1.3

Note: These factors of safety apply to design of new high consequence of failure dams, on high strength
foundations, with low permeability zones constructed of soil which is not strain weakening, using
reasonably conservative shear strengths and pore pressures developed from extensive geotechnical
investigations of borrow areas, laboratory testing and analysis of the results and using the methods of
analysis detailed above. It is assumed there will be monitoring of deformations by surface settlement
points during construction and during operation of the dams.

Table 11.7. Factors which influence the selection of factor of safety, and their effect on the baseline

minimum factor of safety.

Recommended change to the baseline

Factor Description minimum factor of safety

Existing (vs new) Dam A lower factor of safety may be 0 to 0.1

adopted for an existing dam which is
well monitored and performing well

Soil or weak rock A higher factor of safety 0 to 0.2 for effective stress

foundation may be needed to account for the 0.1 to 0.3 for undrained strength

greater uncertainty of the strength analyses

Strain weakening soils in A higher factor of safety may be 0 to 0.2

the embankment or needed to account for progressive

foundation failure, and greater displacements

if failure occurs

Limited (little or no good A higher factor of safety should be 0.1 to 0.3 for effective stress

quality) strength used to account for the lack analyses, 0.3 to 0.5 for undrained

investigation and testing, of knowledge strength analyses

particularly of soil and
weak rock foundations
Contractive soils in the A higher factor of safety may be 0.1 to 0.3 for undrained strength

embankment or needed to account for the greater

foundation uncertainty in the undrained

strength

Note: These figures are given for general guidance only. Experienced Geotechnical Professionals should
use their own judgement, but note the principles involved in this table.

Table 11.7 details factors which should influence the selection of a minimum acceptable
factor of safety and their extent of influence.
Tables 11.6 and 11.7 are based on the following:

(a) The strengths and pore pressures used in the analysis should be selected reasonably
conservatively, e.g. use lower quartile shear strengths; high k H /k V in compacted earth-
fill. This is always a matter of judgement and the authors' recent experience is that
practitioners are sometimes overly conservative in selection of strengths, e.g. using