Geotechnical Engineering

DHARM

SOIL MOISTURE–PERMEABILITY AND CAPILLARITY 141

Table 5.2 Typical ranges of capillary rise in soils (Mc Carthy, 1977)

Soil Designation Approximate Capillary height in mm

Fine Gravel 20 – 100

Coarse sand 150

Fine sand 300 – 1000

Silt 1000 – 10000

Clay 10000 – 30000

Temperature plays an important role in the capillary rise in soil. At lower temperature
capillary rise is more and vice versa. Capillary flow may also be induced from a warm zone
towards a cold zone.
It is to be noted that the negative pressures in the pore water in the capillary zone
transfers a compressive stress of equal magnitude on to the mineral skeleton of the soil. Thus,
the maximum increase in interangular pressure in the capillary zone is given by :
σc = hc. γw ...(Eq. 5.39)
This is also loosely referred to as the ‘capillary pressure’ in the soils. This leads to shrink-
age effects in fine-grained soils such as a clay. Representative values of capillary pressures are
given in Table 5.3 :

Table 5.3 Representative values of capillary pressures (Mc Carthy, 1977)

Soil Capillary pressure–kN/m^2

Silt 10 to 100

Clay 100 to 300

5.9.3 Time Rate of Capillary Rise

In cases where a fill or an embankment is placed for highways, buildings or other purposes,

the time necessary for the capillary rise to gain maximum height requires consideration. On

the basis of typical sizes of voids, clay and fine silt will have a significant capillary rise. How-

ever, the time required for the rise to occur may be so great that other influences, such as

evaporation and change in ground water level, may also play their part.

The “Capillary Conductivity” or “Capillary permeability” is the property which indi-

cates the rate of capillary rise. The factors known to effect the capillary permeability of a soil

are size of voids, water content and temperature of the soil. This property is quantitatively

greater for higher water contents and lower temperatures. The relative rates of capillary con-

ductivity are similar to the comparative values for Darcy’s permeability—that is, more for

coarse soils and low for silts and clays. Absolute values of capillary conductivity are not avail-

able.

5.9.4 Suspended Capillaries

Percolating surface water due to rain or pore water resulting from a formerly higher water

table can be held in a suspended state in the soil voids because of the surface tension phenomenon