Modelling Pile and Stone Column Installation in PLAXIS

It is well know that pile and stone column installation causes large strain and deformations to occur in the soil. Large deformations can change the properties of the soil in the zone immediately around the pile or stone column (SC). This can result in a strength reduction at the interface or in some cases (for clays) a strength increase.

Although the 'Geo-Install' workshops have shown significant advances towards modelling installation, many of these methods are currently not available in commercial packages.

Cavity Expansion

A common method used to model both stone column and pile installation is cavity expansion. This is the process of expanding a cylindrical cavity of zero thickness in a radial direction until the radius of the cavity equals the radius of the pile or SC. This is an effective method for modelling the excess pore pressures, in particular, generated during pile/SC installation.

Interface Strength Reduction Factor

PLAXIS allows for an input of an 'interface strength reduction factor' in order to take into account a reduction in soil strength arising from installation. Typical values range between a value of 0.6 to 1.0 depending on the particular soil type. This strength reduction factor can only be applied when using interface elements, however, and thus may not be applicable to SCs. During the installation of SCs, the stone and soil being treated become interlocked. Thus modelling SCs using interface elements may over-predict punching of the stone columns in the soil.

In PLAXIS 3D, the user can select between volume piles and embedded piles to model a pile foundation. Embedded piles comprise of beam elements and have special interfaces. Thus strength reduction at the pile-soil interface can not be modelled using embedded piles.The differences between these two types of pile will be covered in another post in more detail. 

Most frequently used soil models

For my PhD research on pile group foundations, I am using the advanced nonlinear Hardening Soil model in PLAXIS 3D Foundation. Although the HS model captures the nonlinear (more realistic) behaviour of soil, the more basic soil models idealising the soil as a linear elastic medium are most often used in industry.
Idealising the soil as a linear elasic medium has the effect of over-predicting pile/soil settlement thus leading to an increased factor of safety in the design. Is this extra (and often unnecessary) factor of safety necessary when the engineering profession is constantly striving for more efficient structures particularly in the current economic climate?
Vote on the poll on the right hand side and see what are the most popular soil models today.
You can also post your comments below or suggestions to soil models that should be included in the list