Environmental Engineering Reference
In-Depth Information
Table 7.3.
Swelling index values for several minerals (Olson and Mesri, 1970).
Pore fluid, adsorbed cations
electrolyte concentration,
Void ratio at effective
in gram equivalent
consolidation
Swelling
Mineral
weights per litre
pressure of 100 psf
index
Kaolinite
Water, sodium, 1
0.95
0.08
10
4
Water, sodium, 1
1.05
0.08
Water, calcium, 1
0.94
0.07
10
4
Water, calcium, 1
0.98
0.07
Ethyl alcohol
1.10
0.06
Carbon tetrachloride
1.10
0.05
Dry air
1.36
0.04
Illite
Water, sodium, 1
1.77
0.37
10
3
Water, sodium, 1
2.50
0.65
Water, calcium, 1
1.51
0.28
10
3
Water, calcium, 1
1.59
0.31
Ethyl alcohol
1.48
0.19
Carbon tetrachloride
1.14
0.04
Dry air
1.46
0.04
10
1
Smectite (montmorillonite)
Water, sodium, 1
5.40
1.53
10
4
Water, sodium, 5
11.15
3.60
Water, calcium, 1
1.84
0.26
10
3
Water, calcium, 1
2.18
0.34
Ethyl alcohol
1.49
0.10
Carbon tetrachloride
1.21
0.03
Muscovite
Water
2.19
0.42
Carbon tetrachloride
1.98
0.35
Dry air
2.29
0.41
Sand
0.01-0.03
The dispersivity depends also on the pore water chemistry since, as discussed above,
this affects the diffuse double layer geometry and electrical charge. In particular low elec-
trolyte (pore water) salt concentrations lead to a large diffuse double layer and greater dis-
persivity. Hence percolation of a saline soil with fresh water can lead to dispersion.
Cation exchange of say Ca
for Na
leads to a smaller diffuse double layer, and lower
dispersivity. Hence addition of lime (CaO or Ca(OH)
2
), or gypsum (CaSO
4
) leads to cation
exchange and reduced dispersivity.
There have been many failures of dams constructed of dispersive clays. Mitchell (1993)
indicates these have been mostly low to medium plasticity (CL and CL-CH) clays that
contained montmorillonite. Bell and Maud (1994) concur with this view. The authors'
experience is similar, that is the most susceptible soils are not those with a high plasticity,
but those with limited clay size fractions sufficient only to give low to medium plasticity.
Sherard et al. (1976a, b) however tested some soils as dispersive in the pinhole test with
% passing 0.005 mm greater than 50%. The better performance of the higher plasticity
clays probably relates to greater resistance to erosion and greater likelihood for cracks to
close as the soil swells.
Sherard et al. (1976a, b) indicate that based on their tests, soils with less than 10% finer
than 0.005 mm may not have enough clay to support dispersive piping.
7.5.2
Shrink and swell characteristics
The shrink and swell characteristics of a soil can be related to clay mineralogy. Table 7.3
shows swelling index (de/d(log p
), where e
void ratio, p
effective stress on the soil)
