Environmental Engineering Reference
In-Depth Information
Table 7.5.
Clay mineral identification from the soil profile (Ingles and Metcalf, 1972).
Inferences from the profile
Observation
Dominant clay component
Mottled clays, red-orange-white mottle
Kaolinites
Mottled clays, yellow-orange-gray mottle
Montmorillonites
Medium to dark gray and black clays
Montmorillonites
Brown and red-brown clays
Appreciable illite, some montmorillonite
White and light gray clays
Kaolinites and bauxites
Discrete microparticles of high light reflectance (micas)
Micaceous soils
Discrete microcrystals, easily crushed
Gypsum-rich soils, or (rarer) zeolites
Soft nodules, acid-soluble, disseminated
Carbonates
Hard nodules, red-brown
Ironstones, laterite
Extensive cracking, wide, deep and closely spaced at
Calcium-rich illites and montmorillonites
5 to 6 cm or less
Up to intervals of 30 cm and more
Illites
Open-textured friable loamy soils with appreciable
Usually associated with carbonate,
clay content
allophane or kaoline, but never
montmorillonite and seldom illite
Open-textured friable loamy soils with appreciable clay
Organic soils, peats
content, black
Open-textured friable loamy soils of low clay content
Carbonate, silts and sands
Wormy appearance on exposed pre-existing weathered
Montmorillonites, plus soil salinity
profile
Relatively thin, strongly bleached horizon near the soil
Above the bleach, fine silt: below the
surface (up to 60 cm from the top)
bleach, dispersive clay. Probably a seasonal
perched water table at the bleach level
- The presence of soil structure, e.g. root holes, fissures or cracks, in the soil in the dam
foundation, and with no adequate cutoff or erosion control measures;
-Erosion of dispersive or erodible embankment soils into open fractures in the rock in
the sides of the cutoff trench;
-
Poor cleanup of loose soil, grass etc from the cutoff foundation prior to placing earthfill;
-
Cracking due to differential settlement or desiccation;
-
Rapid filling of storages, giving insufficient time for cracks in the soil in the embankment
to be sealed by the soil swelling or being wetted. The cracks may be due to desiccation
during or after construction, differential settlement or hydraulic fracturing.
As indicated in ICOLD (1990) the majority of piping failure in dams constructed of dis-
persive soil occur on first filling, including cases when the reservoir has been raised after
being at a given elevation for a period of time. The reasons for this include the presence
of cracks and high permeability zones being present and “found out” on first filling, and
the cracks not having time to close as the soil around swells on saturation.
Wan and Fell (2002, 2004) have also shown that the rate of erosion of saturated clay
soils is lower than partially saturated.
Piping failure can also be caused by introduction of water with low ionic concentration
(i.e. “fresh” water with a low salt content) into soils which were naturally saline, e.g. as
shown in
Figure 7.17
. Ingles and Wood (1964) describe such a case. ICOLD (1990) indi-
cates that virtually all failures occur in the embankment, not through soil in the founda-
tion. However the authors have observed failures through the foundation in small farm
dams. Examples of piping failure of dams constructed using dispersive soils are given in
Aitchison, Ingles and Wood (1963), Aitchison and Wood (1965), Cole and Lewis (1960),
Sherard and Decker (1977), Phillips (1977), Cole (1977), Rallings (1960), Wagener et al.
(1981), Bell and Maud (1994).
