In many situations, the Initial Phase in PLAXIS is sufficient for establishing the initial stress state. However, for some particular or complex initial situations, it may be required to use multiples phases to achieve the current accurate stress field.

This article provides some general considerations for setting up particular or complex initial conditions when needed.

In general, it is recommended to start any project with a realistic initial stress state, which means modelling how the project site has reached its current state. In other words, the project should begin with the correct effective stress, pore pressure, and pre-consolidation stress to ensure equilibrium conditions are satisfied.

To achieve this, it is suggested to model the initial project conditions in the same chronological order as they occurred in reality. Begin by defining the Initial Phase and using the K0 procedure or Gravity loading calculation options. Next, you can add any additional phases, gradually incorporating different stages or loads until the current state is reached.

It is recommended to consider undrained behaviour and consolidation if necessary, and to use realistic construction times, especially when creep is important.

In the first phase of the current project, do not forget to reset displacements to zero and possibly also small-strain stiffness and/or state variables (for advanced models) so that displacements that took place during the ‘setting up’ of initial stresses is not accumulated in the Total displacement results.

The Image 1 below serves as an explanatory example. By following this approach, the calculations will start with a realistic initial effective stress field and pore pressure.

Image 1: Example defining initial stresses with load history.

 

Additionally, it is always good practice to check the correctness of the effective initial stress state by defining a relevant vertical cross-section through the model in PLAXIS Output.

In Images 2 and 3 below, you will find an example demonstrating how to check the effective initial stress ratio and the pre-consolidation stress state in a simple case. This example was created using the Hardening Soil model, with k₀=0.7 and a pre-consolidation stress state (OCR) of 2.

 

Image 2: Example for checking the stress state.

 

Image 3: Stress variation with depth along cross-section A-A’.

Note: The maximum major effective principal stress σ'_1,max is the maximum value of the major effective principal stress in the stress history up to the current results for Hardening Soil model.