Environmental Engineering Reference
In-Depth Information
Figure 16.4.
Assumptions for analysis of potential failure surfaces in the foundation (BC Hydro, 1995).
(b) No resistance to loading from the foundation exists beyond a line oriented downward
at 45° from the dam's toe;
(c) Uplift pressures are either estimated as outlined in Section 16.4, or consistent with
observed piezometric conditions, provided instrumentation is adequate and drainage
is well maintained. Bounding pressures are:
(i) Reservoir pressure at the intersection of the potential failure surface and the surface
extending downward from the heel of the dam;
(ii) Pressure due to tailwater or due to a piezometric surface at the elevation of the
ground surface downstream of the dam at the intersection of the potential failure
surface and the 45° line from the dam's toe.
The potential for artesian pressures in the vicinity of the dam's toe due to causes
such as limited drainage, anistropic permeability or lateral groundwater flows,
should be evaluated.
(d) There is no tensile strength on the potential failure surface or the surface extending
downward from the heel of the dam.
Rigid body analysis of static stability on a plane in the foundation consists of:
(a) Vector summation of all forces including uplift acting on the foundation and dam
above the assumed failure surface into force components normal and parallel to the
failure surface;
(b) Determination of the resultant force location on the assumed failure surface from the
summed moments due to all forces acting on the foundation and dam above the sur-
face and the normal component of the resultant force;
(c) Calculation of stresses normal to the assumed failure surface from the normal compo-
nent of the resultant force and its position on the surface, assuming a linear variation
of stress;
(d) Adjustment of the assumed uplift distribution to reservoir pressure where tensile nor-
mal stresses greater than the available tensile strength are found and iterative recalcu-
lation of the resultant force and the normal stresses until the assumed and calculated
amount of the surface in compression correspond. For failure in the foundation, the
rock will have zero tensile strength;
(e) Calculation of the stability indices using the resultant force components. These are
the sliding factor (calculated using peak and residual strengths in the concrete or
