Environmental Engineering Reference
In-Depth Information
were located by sonar survey. Uromeihy (2000) showed that 8 large sinkholes within 6 km
upstream from the dam lie close to the trace of a known fault. The water table near the dam
rose by 80-100 m. New springs appeared downstream and flow from the original spring
increased to 9.9 m 3 /sec. The total flow rate from all springs indicated that leakage from the
reservoir was about 16 m 3 /sec. This was more than the average inflow at the site and the high-
est reservoir level reached was 23 m below Full Supply Level. After studies and analyses of the
leakage, site exploration, grouting and other remedial works were commenced in 1983.
These involved 55,000 m of drilling and 100,000 tonnes of injected materials. The largest
cavern located and treated was 200 m below the river bed, and was 27 m high and 68 m wide.
Smaller cavities were located down to 430 m below the river bed (Djalaly, 1988). The treat-
ment works did not lower the leakage rate and were stopped in 1990.
In 1991 site studies started again, but with a new emphasis, as indicated in the follow-
ing words by Salambier et al. (1998) “ The key to understanding the leakage conditions
and consequently proposing the most appropriate measures was geological modelling
based on extensive investigation and comprehension of the geological history of the site”.
The scope of the new studies is summarized by Salambier et al. (1998). They included
geological surface mapping, interpretation of air photographs and satellite imagery, an
extensive geophysical program and drilling 10 holes totalling 3,750 m. The deepest drill
hole was 650 m.
This work resulted in improved geological and hydraulic models from which Salambier
et al. (1998) showed that the extent of permeable ground was much greater than previ-
ously assumed and that there was a serious problem with potential sinkhole formation
(see Section 3.7.3).
They also predicted the extent of ground requiring treatment to solve the leakage and
associated problems and were able to delineate this in plan view and on sections. In the
bedrock beneath the dam and its right abutment the treatment area continued to 600 m
below river bed level as shown on Figure 3.29 . Under the storage area the treatment area
was shallower, but extended more than 5 km upstream from the dam.
Salambier et al. (1998) concluded that “successful plugging and grouting treatment
cannot be guaranteed” and that the cost could not be estimated “within a reasonable
range of certainty”. They recommended that, as an alternative, blanketing of the ground
surface be considered - by shotcrete and poured concrete on the limestone and by
geomembrane, over the alluvial/lake deposits. Clay was considered feasible, but too
scarce near the site. Reimer (personal communication) advises that as at mid-2003 no
work has been done.
3.7.3
Potential for sinkholes to develop beneath a dam, reservoir or associated works
Sinkholes which develop naturally ( Figure 3.27 ) are present in most areas underlain by
carbonate rocks. In many such areas collapse of the ground surface to form a new sink-
hole is a relatively common occurrence. Some of these collapses may happen naturally but
more often they are induced or, accelerated by man's activities, as seen at Lar Dam.
Possible mechanisms for sinkhole formation include the following:
1. Dewatering . This can cause loss of buoyant support of the rock or soil forming the roof
of a cavity or, by steepening the hydraulic gradient, cause erosion and collapse of over-
lying soils into the cavity. It may also cause collapse due to drying out, causing shrink-
age and ravelling failure of overlying soils.
2. Inundation . This can cause previously dry soil to lose apparent cohesion and collapse
or erode into a cavity or can steepen the gradient in previously wet soils, causing them
to erode into a cavity or into joints widened by solution.
3. Vibrations from machinery or blasting.
Search WWH ::




Custom Search