Technical Review of the Lined Rock Cavern Concept and Design ...

Technical Review of the Lined Rock Cavern (LRC) Concept and Design Methodology 30
Figure 4-19 illustrates the results of the UDEC model for the horizontal section in terms
of cavern deformation, fracture shear displacement, and fracture separation. Although
the in-situ horizontal stresses and material properties are isotropic, a non-uniform cavern
displacement indicates anisotropic conditions. The apparent effect of anisotropy is a result
of the fracture system and, more importantly, the fracture orientation — thus, it is an
effect of material anisotropy. The direction of the largest wall displacement is indicated
in Figure 4-19; it falls along the line of most wedge formation and fracture separation.
Note that the purpose of these UDEC models was not to predict accurate deformations of
the cavern, but to illustrate qualitatively the richness of the rock mass response embedded
in these models. At any given LRC site, rock wedges are likely to appear, and fracture
separation will occur during cavern pressurization. Discontinuum analysis is an approach
that considers the local aspect of these issues in the context of predicting the steel liner
strain.
4.3 Concrete-Liner Fracture Analysis
The development of cracks in the reinforced concrete liner can be expected as a result of
local deformations in the adjacent rock mass during cavern pressurization. An accurate
evaluation of the size of these cracks is important to the prediction of the associated local
strain in the steel liner. In this context, the review documents present, in some detail, an
approach for analyzing induced fractures in the concrete liner. The approach uses a finite
element model to predict the local crack development in the concrete as induced by
the separation of a vertical fracture in the rock mass. The model characterizes the concrete
as a Mohr-Coulomb material with a tension cut-off (i.e., finite tensile strength).
Unless the tensile strength in each element of this model reflects the critical fracture
toughness (or critical energy-release rate) of the reinforced concrete, the solution becomes
mesh-dependent (Detournay et al., 2001). This can affect both the predicted crack
separation and distribution.
In lieu of a more sophisticated fracture propagation analysis, the finite element approach
described in the review documents should use a tensile strength that reflects the material
toughness of the concrete liner. An example of the derivation of this tensile strength is
provided in Appendix B.