With this record you can model frictional slip of soil material and alike granular materials on other materials like concrete, steel, etc.
This is done by reducing the friction angle phi and diletance angle phiflow
and cohesion 
of the granular material with a factor (2./3.).
The reduction is done for records group_materi_plasti_mohrcoul and
group_materi_plasti_druckprag, if specified.
The index specifies the element_group of the granular material, see element_group.
With group_0 , group_1 etc. you can specify the groups of the concrete material, steel material etc. The reduction of the friction angle and diletance angle will only be applied to the granular elements (of element_group) which are a direct neighbour of an element which has one of the groups group_0 , group_1 etc.
Please realise that this method only works well if the finite elements are not too large.
See also group_materi_plasti_boundary_factor.