The pre-mining stress state is specified as a linear variation with depth in either Cartesian or principal components.
The variation of each of the Cartesian components is described as a linear variation with depth as (for example for the σxx component)
σxx + Δσxx (z - Datum)
To determine appropriate values for each of these components the user must first select a depth
(z = Datum)
where the stresses have known values. These later values are the
σxx σyy σzz τxy τyz τxz
constant values.
The variation of each component with depth can then be specified as
Δσxx Δσyy Δσzz Δτxy Δτyz Δτxz
variation values. If you have set up your coordinate system with z positive up, then the variations should be specified as negative quantities so that the stresses increase with depth.
For example if you know that the σxx component has a value of
30 MPa at z = -1000m
and varies as (z is positive up)
0.027 MPa/m
you would specify
Datum = -1000
σxx = 30
Δσxx = -0.027