There is however one large difference: using the External Stress, you can also prescribe a stress contribution that only acts as a load on the structure but is not added into the stress tensor. The typical case is when there is a pore pressure in a porous material, a common case in geotechnical engineering. The stress carried by the solid material, excluding the pore pressure, is often called the effective stress. The load from the pore pressure helps to balance the external loads, while not contributing to the stress tensor of the solid. The contribution to the virtual work of the external stress is then

The External Stress subnode can be added to Linear Elastic Material, Nonlinear Elastic Materials, Elastoplastic Soil Models, Shape Memory Alloy, or Hyperelastic Material Models.

The vertical stress, σv, also called overburden pressure, lithotastic pressure, or confining pressure, represents the stress in a point given by the weight of the overlaying material.

The elevation D is the distance of a point in the soil to the top boundary, ρ is the density and g the acceleration of gravity. This analytical expression for the vertical stress is derived for a slap of soil of infinite lateral extension.

The lateral or horizontal stress σv is normally given as a factor or the vertical stress

The factor k0, called the coefficient of lateral earth pressure, is normally computed from the angle of internal friction, from the Poisson’s ratio, or more complex formulas.

When the z-axis represents the vertical coordinate, the in situ stress tensor is written as