Problem description

In PLAXIS 2D AE, 2D 2015 and later and PLAXIS 3D (2011 and later), you can specify a contraction value to model a contraction of (2D) line elements and (3D) surface elements. To specify this contraction, you define a contraction value Cref as a strain value in percentage. However, it is not entirely clear what value you should specify: the actual contraction value that will be applied on the 2D line or 3D surface during the calculation will be half of the specified value for Cref.

Volume loss

When defining circular (TBM) tunnels, this contraction feature can be used to simulate “volume loss“. In the manual it is stated that in order to model this volume loss, you would need to set the contraction value Cref to half the value of the volume loss as a percentage of the tunnel volume.

However, the actual volume loss that will be applied in the calculation will now be equal to the specified contraction value Cref:
    volume loss = Cref

Solution

The contraction is defined as:

• when specifying the contraction for a line (2D) or surface (3D) to model an axial strain, please multiply the intended line contraction (2D) and intended surface contraction (3D) Cref with a factor of 2:
      Cref =  2 * axial contraction
• when modelling volume loss for a circular tunnel, specify Cref as the volume loss as a percentage of the tunnel’s volume:
      Cref = volume loss

The PLAXIS 2D 2016 and PLAXIS 3D AE manuals have been improved to state the above clearly.