Environmental Engineering Reference
In-Depth Information
Figure 3.30.
Lar Dam, flow net in the embankment and foundation (Salambier et al., 1998).
3.7.4
Potential for continuing dissolution of jointed carbonate rock in dam
foundations
The following sections relating to dissolution processes and effects rely heavily on theo-
retical and experimental material described by James and Lupton (1978, 1985), James
(1981) and James and Kirkpatrick (1980).
Most of the material in those papers is included and expanded upon in James (1992), a
comprehensive book covering a wide range of aspects ranging from the fundamentals of
solubility to the engineering behaviour of all soluble rocks, i.e. carbonates and evaporites.
Also included are useful case histories of dams and other structures in which solution
effects were investigated.
James and Lupton (1978) discuss the principles governing the dissolution of minerals,
develop mathematical models which predict how the minerals gypsum and anhydrite dis-
solve in the ground and describe laboratory tests which confirm the validity of the pre-
dictions. Water flow through both jointed rock and porous granular material are
considered. They show that the rate at which the surface of a mineral or rock retreats
depends primarily upon two properties of the mineral, namely:
- the solubility (c) of the mineral, which is the amount which can be dissolved in a given
quantity of solvent, at equilibrium, and
- the rate of solution of the mineral, which is the speed at which it reaches the equilib-
rium concentration.
The solution rate constant (K) is further dependent on the flow velocity and tempera-
ture of the solvent and concentrations of other dissolved salts.
James and Lupton (1978) point out that for water flowing through a joint in an effectively
impervious but soluble rock substance, widening of the joint by solution of the rock walls will
occur only until the water becomes saturated with the soluble material. Thus a solution zone
(
Figure 3.31a
)
is initiated near the fresh water interface and migrates progressively down-
stream. For water flow through soluble rock substance with intergranular (fabric) perme-
ability a comparable solution zone is formed and migrates downstream (Figure 3.31b).
James and Kirkpatrick (1980) use the methods and models of James and Lupton (1978)
to derive conclusions on foundation treatments needed to prevent dangerous progressive
dissolution when dams are built on rocks containing gypsum, anhydrite, halite and lime-
stone. They use the following solution rate equations to predict the ways in which the
materials dissolve:
For gypsum, halite and limestone
dM
dt
(3.1)
KA(c
c)
s
