Key geological issues - Geotechnical Engineering of Dams - page 72

and graded towards the river, so the issue is essentially one of the limiting seepage under
and around the ends of the embankment as discussed in Chapter 10. Some storages have
to rely on liner systems for watertightness if the underlying soils are very permeable, deep
and/or the water table is low.

2.12.5 Assessment of watertightness

During the planning stages of the project (Stages 1 to 3 on Table 4.3) the following ques-
tions should be addressed and answered:

1. Will the proposed storage area be essentially watertight?
2. If not, where are the areas of potential leakage?
3. What is the estimated total leakage rate?
4. What are the consequences of this leakage?
5. What, if any, treatment is needed?

The amount and types of investigation needed to answer these questions will depend
largely on whether the storage area is formed in essentially non-soluble rocks or largely
on soluble rocks, or soils. This “overall geological situation” will usually be evident from
examination of existing regional geological maps. If these are not available, new mapping
will be needed. It is important also that the existing maps or new mapping provide an
understanding of the geological (and modern) history of the area.

2.12.5.1 Storages in non-soluble rock areas - assessment of watertightness
It is usually possible to get preliminary answers to Questions 1 to 3 using existing contour
plans, regional geological maps, air photos and climatic and groundwater records. The
perimeter of the proposed storage area is divided into a number of watertightness regimes,
based on differences in topography and assumed geology and groundwater situations. A
judgement on the probable watertightness of each regime is made by comparison with the
models on Figure 2.41 . Regimes which appear to fit Models (a), (b) and (c) are assumed
to be watertight. For regimes which appear to fit Models (d) or (e), ranges of possible leak-
age rates through each can be obtained by simple calculations using wide ranges of possi-
ble rock mass permeabilities and assumed flow paths, cross sectional areas and gradients.
Construction of realistic flow nets is unlikely to be possible, or necessary.
If the geological and modern history suggests that high permeability features as listed in
Section 2.12.3 may occur, but none are shown on the geological maps, they can be looked
for on the air photos. They are often indicated by areas of anomalous vegetation. If such a
feature is recognised and confirmed by ground inspection, ranges of possible leakage rates
through it can be found be similar simple calculations.
The results of the above preliminary assessment are considered in relation to the conse-
quences of leakage, which will usually be known. If the largest calculated leakage rates are
shown to be of no consequence and the geological and groundwater data known to be
reliable, then the watertightness studies could be considered complete, with no further
action needed.
If some of the calculated leakages are seen to be significant, and there are doubts about
the reliability or sufficiency of the geological or groundwater data, further field and office
studies would be required. The field studies may include any or all of the following:

- Further regional geological mapping;

- Plotting the positions and levels of springs or seepages, and measuring flow rates;

- Plotting the positions and levels of existing water wells or bores and recording of water

levels in them;