Initial Conditions

Initial conditions should represent the state immediately before the modeled loading sequence. For geomechanical models, this commonly includes gravity, initial stress, pore pressure, and an equilibrium stage.

Applying gravity

The command model gravity defines the gravity vector or its magnitude. A full gravity vector may be supplied, but if only one number is given, then gravity is assumed to act in the -y direction in 2D and in the -z direction in 3D.

model gravity 9.81

Density must be assigned to all participating material points or source zones.

Gravity from a zone-derived initial state

When the model is created with zones, initialize or solve the zone stress state before importing material points when that state is intended to be inherited. The command zone initialize-stresses will work out the initial vertical stresses based on gravity, depth, and material density. The horizontal stresses can be given by supplying a ratio of horizontal to vertical stress. By default, the ratio is 1.

model gravity 9.81
zone initialize-stresses
model solve
model save 'zone-gravity'

model domain extent ...
mpoint node spacing [node_spacing]
mpoint import from-zones
model save 'mpoint-gravity'

Inspect the imported stress field and confirm the total/effective stress and pore-pressure conventions for coupled analyses.

Gravity for directly generated material points

Directly generated material points normally begin without a zone-derived stress field. Apply gravity and allow the model to establish equilibrium. For a static initialization, local nodal damping may be used through mpoint node damping.

model gravity 9.81
model history mechanical ratio-average
model solve elastic
model save 'gravity-elastic'

The elastic solve is useful for establishing a preliminary self-weight stress state without plastic yielding. Restore the intended plastic constitutive behavior or continue with the assigned plastic model as required by the model purpose. A second equilibrium stage may be required after plasticity is activated.

Evaluating equilibrium

Do not rely on a single scalar criterion. Review:

• average or maximum mechanical force ratio;

• nodal and material-point velocity histories;

• displacement increments over successive cycle windows;

• kinetic energy relative to strain or potential energy when available;

• stress profiles compared with expected self-weight trends;

• reactions at constrained boundaries.

Save the accepted initial state before excavation, loading, strength reduction, or conversion stages.