Environmental Engineering Reference
In-Depth Information
narrow valleys (width/height ratio less than about 3.0), beneficial three-dimensional
effects could be present. If beneficial three-dimensional effects are demonstrable, the
stated sliding factors may understate the stability of the dam.
The minimum acceptable Sliding and Strength Factors in
Tables 16.1
and
16.2
for the
post-earthquake conditions are not intended for long term application. Thus, provisions
should be made to inspect the dam promptly after a significant earthquake, to monitor its
behaviour and to make any necessary repairs within a reasonable period of time or to
clean blocked drains. The reservoir could be operated temporarily, if required, at a
reduced level until repairs are made and/or safety of the dam is confirmed by analysis.
16.3
STRENGTH AND COMPRESSIBILITY OF ROCK FOUNDATIONS
16.3.1
Some general principles
The strength and compressibility of rock masses in the foundations of concrete dams can
be assessed systematically by well established engineering geological and rock mechanics
principles.
Too often, in the authors' experience, strength and compressibility are taken from
guidelines. These, such as ANCOLD (1991), are often not conservative and potentially
quite misleading. They may include large effective cohesion components to the strength
which are simply not available in many situations and may, quite wrongly, suggest that the
rock may reliably have some tensile strength.
It is therefore quite essential, even for relatively small structures, such as spillway crests,
or gated spillway structures to engage the services of a geotechnical professional who has
a knowledge of rock mechanics to assess the strength and, for larger projects, the com-
pressibility of the rock foundations.
This assessment should include:
(a) Mapping of rock exposure, inspection of photographs taken during construction (for
existing dams) and logging of drill core to determine the stratigraphy, bedding orienta-
tion, joint orientation, continuity and nature of the joint surfaces and to locate and delin-
eate shears, faults, bedding surface shears and other features important to the strength;
(b) Carry out laboratory testing on representative samples of the rock to determine the
unconfined compressive strength and basic friction angle (
b
) of the bedding surfaces
and joints.
In preliminary assessments, or for some smaller dams, the basic friction angle may be
assessed from published information, in which case conservative values should be
adopted;
(c) Develop a geotechnical model of the foundation (i.e. plan, factual and interpretive sec-
tions) to define any possible kinematically feasible potential failure surfaces within the
rock and/or on the foundation-rock contact.
There will be variations in these conditions across the dam and it may be necessary to
analyse the stability of several cross sections.
16.3.2
Assessment of rock shear strength
16.3.2.1
General requirements
The shear strength of the rock depends on the nature of the potential failure surface and
the rock it passes through. The following approach to assessing the strength is based on
BC Hydro (1995) and consultation with colleagues with a knowledge of rock mechanics.
