How do you model mechanically stabilised soil in FEA?

Previously on our last blog...

As we saw in our previous blog, “How do you model geogrids in FEA”, it is incorrect to represent the geogrid only as a discrete tensile element as this completely ignores the beneficial effect that the geogrid has on the strength of adjacent soil. We refer to this strengthening effect as mechanical stabilisation and modelling it is the focus of this blog. The first thing to clarify is: What exactly is mechanical stabilisation and how does it work?

What is mechanical stabilisation?

When a soil shears under load, the particles at the shear plane are forced to rotate. Rounded particles can do this more easily than angular particles in a compacted soil, hence soils with angular particles have a greater resistance to shear: a higher shear strength.

When granular soil is placed and compacted over a geogrid, the soil particles interlock with the geogrid and are confined by the geogrid ribs. This restricts rotation of the soil particles, locally increasing shear strength of the soil. The soil particles above and below this confined layer interlock with the fully confined particles and are themselves restricted from rotating. This creates a zone of stabilisation above and below the geogrid. Of course, as we move further away from the geogrid the stabilisation effect reduces. This results in a zone of stabilisation extending above and below the geogrid with maximum soil strength increase close to the geogrid, reducing to zero increase some distance away from the geogrid.

The effect of mechanical stabilisation can be measured in large scale triaxial testing. Granular soils with and without a single layer of stabilising geogrid, exhibit both higher shear strength and greater ductility in the mechanically stabilised samples.

In this episode of "Ask Andrew", Andrew Lees tells us how to model mechanically stabilised soil in FEA (Finite Element Analysis)

How do we model mechanical stabilisation?

The only way to correctly model mechanical stabilisation in FEA, is to model a stabilised soil layer with a strength transition, from increased soil strength in contact with the geogrid, reducing down to nil effect some distance from the geogrid.

To do this it is necessary to know the strength increase in various stress states and the depth of the zone of influence of the geogrid. We also need to know the specific grade of geogrid and soil type being considered.

The Tensar Stabilised Soil Model

Tensar has developed the Tensar Stabilised Soil Model to accurately model the effect of Tensar geogrids in mechanically stabilised layers. The model has been fully calibrated using data obtained from a very large number of large scale triaxial tests using different Tensar geogrids and a wide range of soil types. The model has then been further validated by full scale load testing. The Tensar Stabilised Soil Model is available for use in most of the leading FEA programs.

Why does the Tensar Stabilised Soil Model use a non-linear strength envelope?

For those keen to understand more about the increase in strength arising from mechanical stabilisation, we can examine the strength envelope of soils represented in principal stress space, where major principal stress is plotted against minor principal stress.

A frictional failure would be represented by a straight line passing through the origin, where the angle of the line depends on the friction angle of the soil. This line is referred to as the failure envelope. In typical non-stabilised granular soils, there is slight non-linearity close to the origin which is caused by the need for the soil to dilate.

Angular soil particles are more resistant to rotation and hence require greater dilation. In the case of the mechanically stabilised soil, there is greater non-linearity due to the enhanced interlock restricting particle rotation, plus a greater strength at all levels of confining stress. The area between the two failure envelopes represents the increased strength provided by mechanical stabilisation of the soil.

Further information

For further information about FEA use in geotechnics, there are several texts and areas of study available.

One key resource is the practical guide to Geotechnical Finite Element Analysis, available here.