Finite Element Method (FEM) - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 233

Three-dimensional isoparametric elements with 8 to 21 nodes that are described in de-
tail in Zienkiewicz (1977), Wittke (1990) and Wittke (2000b) have been used by WBI
for stability analyses for more than three decades. These elements allow the modeling
of jointed rock according to the homogeneous model. For this purpose, it is assumed
that any point in an element has deformability and strength parameters (Fig. 10.6, left)
or permeability parameters (Fig. 10.6, right) of both the intact rock and the discontinu-
ities. Also interfaces such as master joints or faults can be modeled by thin isoparametric
elements (Fig. 10.7).
Figure 10.6 Three-dimensional element modeling of homogeneously jointed rock
Figure 10.7 Thin three-dimensional element modeling an interface (e.g. master joint, fault)
WBI generally uses elements with 20 nodes for three-dimensional computations. This is
particularly important when a curved geometrical structure such as a tunnel lining has
to be modeled. Such types of elements are also required when using highly nonlinear
stress-strain laws, such as for rock salt and swelling rock (Chapters 5 and 8, respectively).