Geotechnical questions associated with various geological environments - Geotechnical Engineering of Dams - page 123

The authors note that the individual conclusions of James and Kirkpatrick (1980) and
James (1981) assume:

(a) the relatively simple joint and “particulate” models show on Figure 3.31 ;
(b) the largest joint apertures for limestone, on Table 3.5 , and
(c) the limiting seepage velocity of 3 10 4 m/s for pure water through particulate or

fragmented limestone, on Table 3.6 .

With regard to (a) and (b) we note that at sites in faulted and weathered carbonate
rocks the geological situation as exposed in detail at the excavated foundation surface and
in tunnels, shafts or boreholes below it, can be very complex. In addition to features of the
types on Figure 3.31, any or all of the following have been observed, often at great depths
below ground surface:

- Open cavities or tunnels of all sizes, regular or irregular shapes, connected or disconnected;
- Cavities as above, partly or wholly filled with soils of variable erodibility;
- Beds or masses of variably weathered and erodible carbonate rock (see Section 3.7.1.2 ) , and
- Beds or masses of weathered and erodible non-carbonate rock.

Achieving a satisfactory grout curtain can be extremely difficult or impossible in a
foundation containing such features. During dam operation removal of soil or weathered
rock by erosion could provide a flow path wide enough to cause ongoing solution of adja-
cent limestone. This type of process may have occurred at Hales Bar, Lar and Attaturk
Dams (see Section 3.7.2.1 and Table 3.3 ) .
With regard to (c), the maximum leakage rates shown on Table 3.3 suggest that rates
much higher than 3 10 4 m/s are/were being tolerated, at least locally, through the

foundations of some of these dams. Such local flows may have high velocities and could
be turbulent. It is noted also that Bozovic et al. (1981) and Riemer et al. (1997) fore-
shadow the possibility of future increases in leakages at Keban and Attaturk Dams.
However, as discussed previously in Section 3.7.2, many dams have been built in
Category O limestone sites, with cement grouting for seepage control and have performed
well for many years without excessive or increasing seepage. The authors accept that a
good grouting program should be adequate to control seepage and prevent ongoing solu-
tion at many dams built on such carbonate rocks. However, it is recommended that those
responsible for building any dam on these rocks should:

- Pay special attention to the design of monitoring systems (see Chapter 20) and
- Make adequate allowance (e.g. access and ability to lower storage levels) for additional
grouting programs which might be required after first filling or later during the lives of
the dams.

3.7.6.2 Category Y carbonate rocks
The resistance of Category Y carbonate rocks to dissolution in fresh water has not been
researched to the extent of the work done on Category O rocks by James and Kirkpatrick
(1980) and James (1981). However, the following reasons suggest strongly that their
potential for continuing dissolution, if present in a dam foundation, would be higher than
that for Category O rocks:

- Their lower strengths and higher porosities;
- The field and laboratory evidence of their rapid solution and redeposition, presented in
Section 3.7.7.2, and
- The experiences at Kopili, Montejagne, Perdikas and May dams ( Table 3.4 ).