Environmental Engineering Reference
In-Depth Information
Table 3.6.
Solution of particulate mineral deposits (James and Kirkpatrick, 1980).
Mineral
Limiting seepage velocity (m/s)
Length of solution zone (m)
10
6
Gypsum
1.4
0.04
10
6
Anhydrite
1.6
0.09
10
9
Halite
6.0
0.002
10
4
Limestone (Category O)
3.0
2.8
Note: Rate of movement of solution zone is 0.1 m/year. Mineral particles diameter 50 mm; pure
water.
shape and sizes of the particles and the proportion of soluble minerals present, are of sec-
ondary importance. They show further that the downstream migration rate of the solu-
tion zone is governed largely by the seepage velocity and by the solubility, the proportion
of soluble mineral present being less critical.
Table 6 of James and Kirkpatrick (1980), reproduced here as Table 3.6, shows the cal-
culated seepage velocities which would be required to cause 0.1 m/year downstream
migration of solution zones in a range of such “particulate mineral deposits”. Based on
their opinion that the required seepage rate for “particulate” limestone (3.0
10
4
m/s) is
so high that it would be “rarely tolerated in dam foundations” they conclude that control of
seepage by a good grouting program or other means should prevent dangerous progressive
solution of “particulate” forms of dense carbonate rock.
James (1981) determined the solution parameters for 10 carbonate rocks of different com-
positions - 8 limestones, 1 calcitic dolomite and 1 probable chalk. They ranged in age from
Carboniferous to Cretaceous. Using finely crushed samples and pure water, he found their sol-
ubilities to be virtually the same as that of “pure calcium carbonate” (presumably calcite) and
that differences in their solution rates were too small to be significant in engineering design.
James (1981) showed for one sample of limestone, that small amounts of carbon diox-
ide dissolved in the water lowered the solution rate by about a factor of 10, but caused
large increases in the solubility. Under “Engineering Considerations” he confirmed the
conclusions and recommendations of James and Kirkpatrick (1980), but recommended
that the following were also needed when assessing the potential for progressive solution
of carbonate rock in a dam foundation:
- Chemical analyses of the appropriate reservoir, river or groundwaters, and
- Laboratory tests to determine the solubility of the carbonate rock in these waters.
3.7.6
Discussion - potential for continuing dissolution of carbonate rocks in
foundations
3.7.6.1
Category O carbonate rocks
A conclusion which might be drawn from the work of James and Kirkpatrick (1980) and
James (1981) as discussed in Sections 3.7.4 and 3.7.5, is that dangerous ongoing solution
of old, dense carbonate rocks in a dam foundation can always be avoided by a thoroughly
planned and executed grouting program using Portland cement. However, early in their
paper, under “Modelling of ground conditions”, James and Kirkpatrick include the fol-
lowing warnings against such a general conclusion.
“The conditions analysed below are idealized and do not take account of complex vari-
ations which occur in the ground”
and
“Eventually it must be a matter for the engineer to judge what reliance to place on site
investigation data and other engineering geology considerations, and thence to decide
what factors of safety to employ”.
