Environmental Engineering Reference
In-Depth Information
Figure 3.25.
Karst landscape in New Zealand. The outcrops of limestone are up to 1.5 m high and sep-
arated by soil-covered holes and slots up to 1 m across.
3.7.1.1
Rock masses composed of dense, fine grained rock substances comprising
more than 90% of carbonate (usually Category O)
When fresh and intact these rocks usually have very low porosities and their substance
permeabilities are effectively zero. Groundwater flow is confined to joints or other defects
and these are widened by solution to form slots, shafts, tunnels and cavities of all shapes
and sizes. Important features are:
- The upper surface of rock, both in outcrops and below ground, usually consists of pin-
nacles of fresh rock separated by deep slots or cavities, which may be empty or soil-
filled (Figures 3.25 and
3.26
);
- No weathered rock substance is present, either at the ground surface or at depth. This is
because dissolution of the relatively pure limestone substance leaves less than 10% of insol-
ubles, which form residual soils at or near the ground surface and infill some cavities below
(
Figure 3.27
)
. The residual soils are commonly fissured and rich in clay and iron oxides;
- Dam construction experience has shown that cavities capable of accepting or transmit-
ting large inflows of water can be met at any depth down to 300 m below ground sur-
face (see
Section 3.7.2.1
);
- Sinkholes, usually shaft-like cavities, are often exposed at the ground surface and the
formation of new sinkholes can be initiated by natural or man-made activities.
Figure 3.27 represents the uppermost 10 m or so of a karst profile beneath a small area,
showing relatively small cavities and solution-widened defects, which would be connected
to larger cavities at greater depths. It also illustrates some of the mechanisms by which
sinkholes have been formed.
Feature (a) is a sinkhole formed directly by the collapse of the roof of a cavity.
Feature (b) can also be referred to as a sinkhole. It is a gentle, soil-covered depres-
sion, roughly circular in plan view. Exploration shows it to be underlain by residual
