Steps

A step is one stage of the analysis. A geotechnical model is built up in stages — the initial stress state, then excavation, construction or loading — and each of those is a step.

Steps run in the order they appear in the tree. Each one starts from the state the previous one left behind, so the sequence matters: the stress at the end of a geostatic step is what the next step begins from.

Each step is a named block, and a model may hold as many as the analysis needs.

Step settings

Three settings sit at the top of every step.

	Setting	Choices
	Active	Yes or No. A step set to No is skipped, which is useful for trying a sequence without deleting anything
	Analysis type	Static, Geostatic, Transient, Dynamic or Reduction
	Updated Lagrangian	Yes or No. Accounts for the change of geometry under large deformation

Analysis type

This is the most consequential choice in a step, because it decides which equations are solved and what appears in the branches below.

Type	Solves
Geostatic	The initial equilibrium under self-weight and surface loads
Static	A problem in which time carries no physical meaning and is used to define load increments
Transient	Consolidation and flow, where time is physical but inertia is neglected
Dynamic	Inertia included, for earthquakes, vibration and impact
Reduction	Strength reduction, to find a factor of safety

Geostatic is normally the first step of a geotechnical analysis. Its purpose is that the applied loads and the prescribed initial stresses balance exactly, so the model settles into equilibrium without deforming. A geostatic step that produces visible deformation is a sign that the initial stresses and the self-weight do not agree.

It is otherwise the same as the static procedure, except that it is calculated as a single increment: neither a step time nor an incremental solution is used.

Static treats time as a load fraction rather than a duration, so a step time of 1 corresponds to full application of the load.

Reduction lowers the strength of the materials by a factor until the system fails, and the factor at failure is the factor of safety. Its parameters are on the time integration page.

The procedures are described under Step definition in the numgeo reference manual.

What a step contains

	Branch	Holds
	Solution settings	Solver, iteration limits, convergence controls and accelerators
	Time integration	Time step sizes, step duration, integration scheme and factor of safety
	Boundary conditions	Prescribed values at the boundaries of the model
	Loads	Applied loads
	Outputs	The results written for the step
	Reset, reaction forces, constraints and model change	What carries over from the previous step, and what changes in this one

A first step is usually geostatic: it establishes the initial stress under self-weight, with a reset afterwards so that any deformation it produces does not carry into the stages that follow.

The tree is where a step is created and where its conditions and output are defined. The step manager is the faster way to work with the sequence itself — reordering, activating and deactivating, and editing the time settings of each step.