DHARMSOIL MOISTURE–PERMEABILITY AND CAPILLARITY 143
At level PP:
Total stress σ = (hc + hs) γsat
Neutral stress u = hsγw
Effective stress σ = σ – u = hc γsat + hsγsat – hsγw = hcγsat + hsγ′
At level QQ:
Total stress σ = hc. γsat
Neutral stress u = zero
Effective stress σ = σ – u = hc. γsat
This is because the capillary phenomenon increases the effective or intergranular stress
by a magnitude equal to the negative pore pressure hc. γw at the top of the capillary fringe, the
pore pressure being zero at the bottom of the capillary fringe.
This is interesting because the effective stress increases from hc. γ′ to hc. γsat at the
bottom of the capillary zone, when the saturation is by capillarity and not by submergence.
At level RR:
Effective stress σ = capillary pressure = hcγw
The effective stress diagram is shown in Fig. 5.17.
The effect of a capillary fringe of height hc is analogous to that of a surcharge hc. γw
placed on the saturated soil mass.
At depth h below the surface (h < hc) :
Effective stress σ = hγ′ + hc γw
This may be shown as follows :
Total stress σ = h. γsat
Neutral stress, u = – (Pressure due to weight of water hanging below that level)
= – (hc – h)γw
∴ Effective stress σ = σ – u
= hγsat + hcγw – hγw
= hγ′ + hcγw
When h = hc, this becomes:
σ = hcγ′ + hcγw = hcγsat, as earlier.
The surface tension phenomenon also contributes to the strength of the soil mass in
partially saturated coarse-grained soils. The moisture will be in the shape of wedges at grain
contacts while the central portion of the void is filled with air. Thus, an air-water interface is
formed. The surface tension in this meniscus imposes a compressive force on the soil grains,
increasing the friction between the grains and consequently the shear strength (more of this
will be seen in Chapter 8). This strength gain in partially saturated granular soils due to
surface tension is termed ‘Apparent Cohesion’ (Terzaghi). This gain can be significant in some
situations. This apparent cohesion disappears on full saturation and hence cannot always be
relied upon.
5.9.7 Horizontal Capillarity Test
The ‘Horizontal capillary test’, also known as the ‘Permeability–capillary test’, is based on the
determination of the rate of horizontal capillary saturation of a dry soil sample subjected to a
hydraulic head from one end. The set-up for this test is shown in Fig. 5.18.