"Symbols and definitions used in geotechnical engineering" is quoted from "Lexicon in 8 languages" (the fifth edition) published in 1981 by International Society for Soil Mechanics and Geotechnical Engineering.
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2. Stress and strain
| Symbol | Dimension | Unit | Measured quantity | Description |
| u | ML-1T-2 | kPa | Pore pressure | Stress (above atmospheric pressure) in the water in the voids of a fully saturated soil |
| \( u_{w} \) | ML-1T-2 | kPa | Pore water pressure | Stress in the water in the voids of a partially saturated soil |
| \( u_{a} \) | ML-1T-2 | kPa | Pore air pressure | Stress in the air in the voids of a partially saturated soil |
| \( \sigma \) | ML-1T-2 | kPa | Total normal stress | Stress (above atmospheric pressure) acting perpendicularly to a given plane |
| \( \sigma^{_{´}} \) | ML-1T-2 | kPa | Effective normal stress | Normal stress transmitted by intergranular contacts (\(\large\sigma^{_{´}}=\sigma-u\;\) for saturated soils) Note: \(\large\overline{\sigma}\) should be avoided |
| \( \tau \) | ML-1T-2 | kPa | Shear stress | Stress acting tangentially to a given plane |
| \( \sigma_{1} \) | ML-1T-2 | kPa | Major principal stress | Maximum stress action on one of the three orthogonal planes where shear stresses are equal to zero |
| \( \sigma_{2} \) | ML-1T-2 | kPa | Intermediate principal stress | Intermediate stress action on one of the three orthogonal planes where shear stresses are equal to zero |
| \( \sigma_{3}\) | ML-1T-2 | kPa | Minor principal stress | Minimum stress action on one of the three orthogonal planes where shear stresses are equal to zero |
| \( \sigma_{oct} \) | ML-1T-2 | kPa | Average stress or octahedral normal stress | \(\large\sigma_{oct}=\frac{1}{3} \left (\sigma_{1}+\sigma_{2}+\sigma_{3} \right) \) |
| \( \tau_{oct} \) | ML-1T-2 | kPa | Octahedral shear stress | \(\large\tau _{oct}=\frac{1}{3}\sqrt{\left ( \sigma_{1}-\sigma_{2} \right )^2+\left ( \sigma_{2}-\sigma_{3} \right )^2+\left ( \sigma_{3}-\sigma_{1} \right )^2} \) |
| \( \epsilon \) | - | 1, % | Linear strain | Change in length per unit length in a given direction |
| \( \gamma \) | - | 1, % | Shear strain | Change of the angle between two planes originally perpendicular to each other (expressed in radian) |
| \( \epsilon_{1} \) | - | 1, % | Major principal strain | Maximum strain corresponding to one of the three orthogonal directions the shear strains of which are equal to zero |
| \( \epsilon_{2} \) | - | 1, % | Intermediate principal strain | Intermediate strain corresponding to one of the three orthogonal directions the shear strains of which are equal to zero |
| \( \epsilon_{3} \) | - | 1, % | Minor principal strain | Minimum strain corresponding to one of the three orthogonal directions the shear strains of which are equal to zero |
| \( \dot{\epsilon} \) | T-1 | s-1 | Linear strain rate | Rate of change of \(\epsilon\) |
| \( \dot{\gamma} \) | T-1 | s-1 | Shear strain rate | Rate of change of \(\gamma\) |
| \( \nu \) | - | 1 | Poisson ratio (\(\mu\) is also used) |
Ratio between linear strain changes perpendicular to and in the direction of a given uniaxial stress change |
| E | ML-1T-2 | kPa | Modulus of linear deformation | Ratio between a given normal stress change and the linear strain change in the same direction (all other stresses being constant) |
| G | ML-1T-2 | kPa | Modulus of shear deformation | Ratio between a given shear stress change and the corresponding shear strain change (all other stresses being constant) |
| K | ML-1T-2 | kPa | Modulus of compressibility | Ratio between an isotropic stress change and the corresponding volume change per unit volume |
| \( \mu \) | - | 1 | Coefficient of friction | Maximum ratio between shear and normal stress at point of contact between two solid bodies |
| \( \eta \) | ML-1T-1 | kPa.s | Viscosity | Shear stress required to maintain a unit difference in velocity between two parallel layers of a fluid a unit distance apart |