SLIDING FACTOR, FSS
FSSis expressed as a function of the resistance to simple sliding over the
plane considered. It is assumed that resistance is purely frictional, and no
shear strength or cohesion can be mobilized. FSScan then be defined as the
ratio of the summation of all horizontal load components, ∑H, to the sum-
mation of all vertical loads, ∑V, on the plane considered, i.e. for a horizon-
tal plane:
FSS∑H/∑V. (3.20)
If the plane is inclined at a small angle , the foregoing expression is modi-
fied to
FSS. (3.21)
Angleis defined as positive if sliding operates in an uphill sense. The
foundation interface is frequently excavated to give a small positive
overall inclination and so raise FSS.
In assessing FSS, ∑H and∑V are the respective maximum and
minimum values appropriate to the loading condition under review, i.e.
∑Vis determined allowing for the effect of uplift.
Applied to well-constructed mass concrete, FSSon a horizontal plane
should not be permitted to exceed 0.75 for the specified normal load com-
bination.FSSmay be permitted to rise to 0.9 under the extreme load com-
bination. Similar maxima for FSSare applied to possible sliding at the base
interface on a sound, clean and irregular rock surface, or to sliding on
planes within a competent foundation. Planes of low shear resistance will
require a significant reduction in the permissible maxima, e.g. FSSmay be
limited to 0.50 or less on some limestones, schists, laminated shales and
similar low-strength foundations.
∑H/∑V tan
1 ( H/ V)tan
138 CONCRETE DAM ENGINEERING
Table 3.5 Examples of shear strength degradation
Rock type and condition Cohesion, Friction,
c (MN m^2 ) tan
Gneiss ‘A’: sound 1.0 1.7
jointed–decomposed 0.4 0.5
Granite ‘B’: sound 1.0 1.8
weathered–disintegrated 0.1 0.8
Shale ‘C’: dry 0.2 0.4
saturated 0 0.2
