Key geological issues - Geotechnical Engineering of Dams - page 68

which have been studied and monitored are likely to be slow moving because they are
active or reactivated slides with little likelihood of brittle failure.

2.12 WATERTIGHTNESS OF STORAGES

Absolute watertightness is unlikely to be achieved in most natural storages. However it is
usually desirable that the rates of any leakage are minimized, for any of the following reasons:

- The cost of water lost can be unacceptably large e.g. in arid areas, or in offstream stor-

ages fed by pumping;

- In some situations leakage may cause raising of water tables, development of swamps,

or flooding, in areas adjacent or downstream.

In a few cases leakage has caused instability of slopes on the outer edge of the storage
rim (see Section 2.11 ).The watertightness of a storage basin will depend upon

- The permeability of the underlying rock or soil mass;

- The permeability of the rock or soil mass surrounding it;

- The groundwater situations in those masses, and

- The lengths and gradients of potential leakage paths.

Masses of soluble rocks (carbonates and evaporites, see Sections 3.7 and 3.8) are often
cavernous and have permeabilities many orders higher than those of non-soluble rocks.
This discussion on storages will therefore consider non-soluble rocks separately from
those on soluble rocks.

2.12.1 Models for watertightness of storages in many areas of non-soluble rocks

Most non-soluble rock substances are effectively impervious, and rock masses formed by
them are only permeable if sufficient numbers of interconnected open joints are present.
As pointed out in Section 2.5.2, joints generally become more tightly closed with increas-
ing depth, so most masses of non-soluble rock will become less permeable with increasing
depth. Also, their effective porosities (or storage capacities) decrease with increasing depth
and are usually very low (i.e. 10 3 to 10 6 ). The diagrams on Figure 2.45 show the effects
of creating storages in such areas of non-soluble rock, in five relatively common topo-
graphic and groundwater situations. In the discussion of each it has been assumed that:

(i) The rock becomes less permeable with increasing depth;
(ii) Across the floor of the storage area, the water table lies close to the ground surface;
(iii) The water tables shown represent end of dry season conditions;
(iv) Groundwater pressures measured below the water table as shown will all confirm it
as the piezometric surface, and
(v) Flow beneath the high ground or ridges forming the rims will be essentially laterally to
or from the storage, unlike the isotropic material case, proposed by Hubbert (1940).

The perimeter of any particular storage area may include sections topographically sim-
ilar to any or all of Models a/b, c/d or e.
Model (a). In this case the rainfall is high and the storage is in an entrenched valley
underlain by rock and surrounded by wide areas of high ground underlain by rock. The
rock in the valley floor may be partly or wholly concealed by soils of alluvial or glacial
origin. Beneath the high ground, the water table rises above the proposed FSL. There is no