Environmental Engineering Reference
In-Depth Information
to how continuous they are through the core of the dam. If they are through the whole core,
they are of greater concern than if they only persist for say 5% of the width of the core.
17.8
FOUNDATION PREPARATION FOR CONCRETE GRAVITY DAMS ON
ROCK FOUNDATIONS
17.8.1
The general requirements
17.8.1.1
Foundation strength
The primary critical issue is whether there are continuous, or near continuous, weak,
unfavourably oriented discontinuities in the foundation e.g.:
- bedding surfaces;
- bedding surface shears;
- stress relief (sheet) joints;
- faults and shears.
The geotechnical investigations must assess the foundation for such features and either
the dam is designed to found on them or excavation goes below the feature.
The secondary but important issue is that the foundation must be cleaned of loose and
loosened rock, soil, water and other matter, prior to placing the concrete so that there is a
good bond between the foundation and concrete.
17.8.1.2
Foundation modulus
The requirement is that the rock mass has a high enough modulus so deflections under the
loads of the dam are not excessive. The rock mass modulus is related to the rock sub-
stance strength, but in particular to the nature of the discontinuities, including spacing,
opening, degree of infilling and orientation relative to the load direction.
Consolidation grouting (see Chapter 18) may be used to grout the joints and other frac-
tures “to increase the rock mass modulus” if it is not economic to remove this material.
The grouting is likely to make the rock mass more an elastic medium, than to necessarily
increase the modulus.
17.8.1.3
Erodibility and uplift pressure
The seepage gradients beneath concrete dams, particularly arch dams, can be high, so it is
necessary to carefully consider the erodibility of the joints, particularly if the joints are infilled
with clay. Foundations would usually be excavated to non-erodible rock, and the rock
grouted with consolidation (and curtain) grouting to reduce the flow of water through the
rock. Borehole drains are provided to intercept the seepage water and reduce the uplift pres-
sures under the dam and its foundation. The design of these drains is discussed in Chapter 16.
17.8.2
Excavation to expose a suitable rock foundation
The upper surface of rock which can be treated economically to meet these requirements
is predicted from the results of the site exploration, as presented in the geotechnical
model. The predicted surface is shown on drawings by means of cross sections and/or
contours. Invariably, some excavation is required to remove overlying materials which
may include the following:
- Soils - colluvial, alluvial and residual;
-
Insitu
rock mass, the substance and defects of which are too weak and compressible,
either inherently or as a result of weathering;
