Analysis Tips

Below are some tips that may help you in your own geotechnical finite element analyses.
If you are still stuck, contact us for some ideas or try the webpages of our software providers Plaxis and Crisp, or those listed on the Geotechnical Directory.

Soil models: to predict the settlement of buildings caused by nearby tunnelling or excavations, use a soil model with non-linear stiffness because a linear stiffness model will significantly over-predict these settlements.

Meshing: use very small elements where there are rapid changes in direction of principal stress, such as at the edges of footings. Away from areas of rapid stress change, soil elements can be made much bigger with little loss of model precision.

Modelling 3D structures in 2D: 2D analysis is significantly more straightforward than 3D, but structures such as ground anchors, piles, nails and tie rods can only be approximated in 2D plane strain analyses. Remember that structures such as the anchored contiguous piled wall shown here, when modelled in 2D plane strain are really equivalent to uniform plates of the same material with properties modified to take account of the altered geometry. Bear this in mind when defining the geometrical and material properties of the structure and when interpreting the results of the analysis. Isolated structures, such as bearing piles, cannot be modelled realistically in a 2D plane strain analysis – consider using a 2D axi-symmetric analysis.

Contiguous piled wall with ground anchor support

Equivalent plane strain representation

Modelling ground anchor pre-stress:

	1. Install the fixed length of the ground anchor and apply the horizontal and vertical components of the pre-stress force to the node at the head and to appropriate nodes on the fixed length.		2. Install the free length of the anchor connecting it between the anchor head and the fixed length.
	3. In subsequent construction stages the load in the anchor will be the pre-stress plus any load indicated in the free length element in the analysis output.		4. If the anchor is later removed from the analysis, the pre-stress will also have to be removed by applying opposite forces to those applied in order to impose the pre-stress.