Environmental Engineering Reference
In-Depth Information
critical failure surfaces are in the rock, as they usually will be, there will be little benefit
and the effect of the contact drains should be ignored.
16.7
SILT LOAD
Sediment in the reservoir which deposits against the upstream face of the dam exerts a
pressure on the dam and must be allowed for in the analysis. For small structures with a
heavy sediment bed load this may be substantial since with time sediment will partly or
completely fill the storage up to spillway crest level.
The load applied by the “silt” should be estimated using Coulomb earth pressure
theory, assuming the at-rest, or Ko, condition applies. The sediment is likely to be
loose, so in the absence of specific data, properties consistent with loose material should
be used.
For preliminary assessments only, the following properties could be used to assess the
pressures:
(a) For weirs and small structures in fast flowing rivers and streams where the sediment
consists mainly of sand and gravel, adopt a saturated unit weight of 20 kN/m
3
, an
effective friction angle of 30° and effective cohesion zero;
(b) For structures where sediment is likely to be fine-grained silt and sand, adopt a
saturated unit weight of 17 kN/m
3
, an effective friction angle of 25°, and effective
cohesion zero.
In the calculation of the silt pressure, if silt is submerged, the saturated unit weight of
the soil should be reduced by the unit weight of water to determine the buoyant weight.
When assessing dams under earthquake loading, the possibility of liquefaction of the
“silt” should be assessed.
If this is likely, the silt loads will be greatly increased because the strength of the soil is
reduced and the load will approach a Ko
1.0 condition.
16.8
ICE LOAD
Where the reservoir may freeze, the ice will exert additional forces on the dam. FERC
(2000) and US Corps of Engineers (1982) give guidance on how to estimate the forces.
16.9
THE DESIGN AND ANALYSIS OF GRAVITY DAMS FOR
EARTHQUAKE LOADING
16.9.1
Introduction
This section deals with the earthquake loading of gravity dams. It concentrates on the
more general aspects of the analysis rather than discussing the details of the methods and
the analytical tools. Background data on these aspects, particularly of the finite element
methods that are so popular today, can be found in ANCOLD (1998), FERC (2000) and
in the manuals produced by companies that have developed the finite element packages.
Relevant design guidelines, such as ANCOLD (1998) and FERC (2000), should be used
to define the dam's design earthquakes (MDE and DBE in Chapter 12), permissible
stresses and so on. Attention is also drawn to the earlier parts of this chapter for design
strength parameters.
