Concrete gravity dams and their foundations - Geotechnical Engineering of Dams - page 663

16

Concrete gravity dams and their foundations

16.1 OUTLINE OF THIS CHAPTER

This chapter presents the basics of the analysis of the stability of concrete gravity dams on
rock foundations using two dimensional rigid body analysis methods. It is based largely
on four texts.

Australian National Committee on Large Dams, “ Guidelines on Design Criteria for
Concrete Gravity Dams ” (ANCOLD, 1991).
Canadian Dam Safety Association, “ Dam Safety Guidelines ” (CDSA, 1999).
BC Hydro “ Guidelines for the Assessment of Rock Foundations of Existing Concrete
Gravity Dams ” (BC Hydro, 1995).
Federal Energy Regulatory Commission, “ Engineering Guidelines for the Evaluation of
Hydropower Projects, Draft Chapter 111, Gravity Dams ” (FERC, 2000).

The ANCOLD Guidelines are written to use the limit state design method. In practice
this has proven to cause some difficulties, particularly when considering existing dams
which may be just satisfactory or have marginal stability, so there is most emphasis on
CDSA (1999) which uses a more conventional approach.
This Chapter deals mainly with the normal operating and flood load cases, with a brief
outline of analysis of the earthquake loading case.
As detailed in Douglas et al. (1998, 1999), and recognized in CDSA (1999) and BC
Hydro (1995), the most common mode of failure is for sliding or overturning in the foun-
dation, so there is a discussion here on how to estimate the shear strength of the rock in
the dam foundation. There is also a discussion on the estimation of uplift pressures and
the shear and tensile strength of the concrete in the dam.

16.2 ANALYSIS OF THE STABILITY FOR NORMAL OPERATING AND FLOOD

LOADS

16.2.1 Design loads

The following design loads are usually considered in the analysis of stability. Most are
shown in Figure 16.1 .
Dead loads ( D ), comprising the weight of the concrete plus the weight of spillway
bridges, gates, piers and attached intake structures.
Water loads ( H, H F ), consisting of:

(a) Horizontal load on the upstream face;
(b) Horizontal load from the tailwater;
(c) Vertical loads on the upstream face, if it is sloping, or behind crest gates;
(d) Pressures from the water flowing over a spillway section. These will also have a verti-
cal component.