Design Methods - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 334

Unlike the design methods described below, this method does not need to define a rock
mass classification scheme. A classification is only introduced in tunneling. Here, the
excavation method and the amount of support are classified into so-called “excavation
classes” that are specified according to the expected rock mass conditions (DIN 18312
2002). The latter are described by so-called “homogeneous areas” - rock mass units
with a more or less uniform behavior in terms of the homogeneous model.

12.3 Design Methods Based on the Assessment of the Rock Mass Behavior

In tunneling and mining the rock mass classification on the basis of rock mass behavior
has a more than 200 year long tradition, mainly in Austria (Bierbaumer 1913, Rabcewicz
1944, Terzaghi 1946, Stini 1950, Rabcewicz 1957, Lauffer 1958, Pacher et al. 1974, Lauffer
1988).
The guidelines of the Austrian Geomechanical Society (ÖGG 2001) led to the prepara-
tion of the Austrian standard ÖNORM B2203-1 (2001) that is based on so-called “rock
mass behavior types”. A rock mass behavior type describes the mechanical and hydraulic
behavior of the rock mass, including influencing factors such as groundwater conditions,
in-situ stress state and orientations of discontinuity sets with respect to an underground
structure. Also size, shape and location of the structure are incorporated in the definition
of project-specific behavior types. These are defined as the rock mass response to the ex-
cavated underground structure without any support measures. In the Austrian standard
ÖNORM B2203-1 (2001) 11 main rock mass behavior types are defined with the possi-
bility of further subdivision representing a classification scheme. Each rock mass unit is
then assigned to one or a combination of these behavior types. Examples and a detailed
description of these behavior types can be found, for example, in Goricki (2007).
Based on the rock mass behavior types the support measures are specified. The rock-
structure interaction, which is referred to as “system behavior”, is derived from the
results of numerical and analytical analyses or from experience gained from other pro-
jects. The evaluated system behavior for each rock mass unit then leads to design re-
quirements which depend on the corresponding behavior type.
The design method used in Switzerland, instead of rock mass behavior types, introduces
potential hazard scenarios in order to obtain a basis for the assessment of rock mass behav-
ior (SIA199 1998). They include water inflows, gas explosions and possible failure modes.
Rock mass units with the same or similar potential hazard scenarios are classified with
respect to their level of risk, including a specification of the required support measures.

12.4 Design Based on Classification Systems

In principle, all classification systems follow the procedure outlined in Fig. 12.2.
Rock mechanical parameters, in-situ stresses, groundwater conditions and so on,
evaluated from geotechnical investigations are described in a simplifying manner
by coefficients. Tables, formulas, diagrams and combinations of them are used to
obtain ratings for these coefficients. By means of an empirical function, the so-
called “rock mass rating index” is calculated from these coefficients. On the basis