The Concept of Effective Stress in Unsaturated Soils - Multiscale Geomechanics: From Soil to Engineering Projects - page 158

In a representative elementary volume (REV), the “capillary” stress in a
direction is expressed as

Σ F

i

σ ' = [6.11]

cap

S

REV

where

F Σ is the sum of the capillary forces acting in this direction and REV

the

S

cross-section of the REV normal to that direction.

Figure 6.3. Two spherical particles in contact with a water meniscus

Figure 6.4. Different types of regular arrangements of spherical particles