Environmental Engineering Reference
In-Depth Information
-Presence of sinkholes, exposed or concealed?
- Composition and pH of the groundwater and reservoir water?
-Presence, amounts and distribution of any sulphide minerals?
- Potential for dangerous ongoing solution in the dam foundation?
- Suitability for use for embankment materials?
- Suitability for use in concrete and pavements?
- Alkali-carbonate reaction?
- Chert present: Alkali-silica reaction?
- Shaley (argillaceous) rocks: Durability?
- Unstable slopes, where interbeds of mudrocks are present?
3.8
EVAPORITES
The common evaporites, gypsum (CaSO
4
.2H
2
O) and halite (NaCl) are formed in arid
areas by evaporation from inland seas, from inland and coastal salt lakes and tidal flats
(Stewart, 1963; Murray, 1964). They can occur as individual beds in sedimentary rock
sequences, often in association with or interbedded with carbonates. They occur also as
matrix, cement, nodules, veins or joint fillings, in mudrocks or sandstones. Anhydrite
(CaSO
4
) is a less common evaporite. Most anhydrite is believed to be formed from gyp-
sum, when it is buried at depths greater than 150 m and its chemically attached water is
removed due to overburden pressure and heat. Anhydrite also occurs as nodules or infill-
ing cavities in limestone and dolomite (Murray 1964; Brune, 1965).
The evaporites are much more soluble than the carbonates and only outcrop in arid
regions. Sequences containing evaporites show a wide range of solution effects related to
present and/or past groundwater levels. Brune (1965), Bell (1983b, 1993), Cooper
(1988), Thompson et al. (1998), Hawkins and Pinches (1987), James (1992, 1997),
Hawkins (1998) and Yilmaz (2001) describe such effects and related ground engineering
problems. These include:
- karst topography with extensive systems of caves, tunnels, depressions, chimneys and
sinkholes;
- ongoing subsidence, on small and large scales; very slow or sudden collapse;
- leakage through or into solution cavities;
-ground weakening and/or increasing permeability due to ongoing solution;
- heaving ground, due to growth of gypsum crystals;
- explosive heaving/uplifting of ground due to the hydration of anhydrite to become gyp-
sum. Brune (1965) describes how creek channels in Texas were cracked open and
uplifted several metres along distances of up to 300 m during such explosions.
3.8.1
Performance of dams built on rocks containing evaporites
The authors know of no dam built at a site containing thick beds of halite. Many dams
and reservoirs have been built successfully on sites containing gypsum. A few have later
suffered some degree of distress, caused by ongoing dissolution of this material from their
foundations. James and Kirkpatrick (1980) refer to Poechos Dam in Peru, founded on
clay shales with gypsum, noting that seepage from its right “dyke” was saturated in cal-
cium sulphate and also contained 3.5% of sodium chloride. Notable embankment dam
examples in the USA include McMillan and Cavalry Creek Dams (Brune, 1965), Tiber
Dam (Jabara and Wagner, 1969). San Fernando Dam (concrete gravity) was affected by
rapid solution of gypsum cement from very weak conglomerate (Ransome, 1928; Jansen,
1980).
