Environmental Engineering Reference
In-Depth Information
-
Detailed studies of any recognized high permeability features, and of parts of the stor-
age rim which are suspect for leakage because of, for example, their lack of rock expo-
sures, low and narrow shapes, suspected low water tables, and steep potential leakage
gradients. Such studies have typically included detailed surface mapping, excavation
and mapping of trenches, and drilling and permeability testing of cored boreholes. The
positions of water tables may be judged during the progress of drilling, using the DVD
method (see Chapter 5), but would have to be confirmed by piezometers.
The results of the above, presented in plans, sections and diagrams, would be used for
new calculations of potential leakage rates. It is unlikely that the structure-related perme-
ability of any jointed rock zones would be sufficiently well known to justify more than the
simplest flow nets.
In cases where the calculated rates are deemed to be unacceptable, consideration might
be given to alternative storage sites, or to measures such as blanketing or construction of
a grout curtain or slurry trench (see
Sections 2.12.6
and 10.4). Further investigation
would be needed before adoption of any of these.
2.12.5.2
Storages in soluble rock areas - assessment of watertightness
From the discussion in Section 2.12.2 it is clear that the flow paths developed in a soluble
rock area may have developed as results of:
-
Past, and in some cases presently active, tectonic processes;
-
Dissolution and erosion by surface and underground waters, at times when the geo-
logical model and topography were quite different to those of today, and
-
Dissolution and erosion by surface and underground waters, in the present day geology
and topography.
The keys to watertightness assessment therefore lie in:
-
Knowing the regional topography and geological picture;
-
Understanding the geological history, and
-
Understanding the flow pattern of the present day surface and underground waters,
and their relationship to the geological structure and history.
Accordingly, the initial activities would usually include:
-
Using existing contour and regional geological plans (supplemented where necessary
by new mapping) to compile cross sections and/or diagrams providing a 3-dimensional
picture of the geological structure beneath and surrounding the storage area;
-
Compiling a table and diagrams explaining the assessed geological history, and
-
Hydrological studies, including recording wet and dry season flow rates of surface
streams and springs, water levels in wells and boreholes.
The likely overall watertightness, and likely areas and models for leakage, would be
assessed from the results of the above. Consideration of the range of models described by
Ruquing (1981) and of experiences at the dams listed in Tables 3.3 and 3.4 should be
helpful. When likely areas of leakage are recognised, they can be investigated further
using any of the following methods:
-
Detailed geological surface mapping, plus trenching or test pitting;
-
Mapping of caves;
-
Geophysical methods, which might include microgravity, resistivity, or ground probing
radar (from the surface or between boreholes);
