Engineering geology aspects for design and classification - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 41

A rock mass with coarse jointing is referred to as coarsely jointed, one with cracks as
finely jointed . If the thickness of a bed or stratum is greater than 30 cm, then it is called
coarsely bedded, thicknesses of 2 to 30 cm are finely bedded and thicknesses of less than
2 cm are schistose or slaty [34] .
A fault is a sheared sequence of strata. The shear zone is normally extended and the bed-
ding in the fault is fully destroyed (Fig. 2-5).
The components of the rock can be bonded together by substances (lime, silica), surface
tension of the pore water (clay) or friction and bracing forces. This bonding can also be
significantly disturbed by tectonic effects, but also particularly by weathering (bursting
effect of ice, erosion by water and wind).
2.4.2 Classification of the rock mass
2.4.2.1 General
One of the important tasks in the design, tendering, construction and invoicing of tunnels
is to classify the rock mass. The essential objective is to classify the rock mass along the
tunnel alignment according to its properties and its behaviour with regard to the excava-
tion and support of the cavity. In addition to the preparation of tenders, this provides the
basis for the selection of a construction process, the specification of the required support
measures and estimation. During the construction phase, the classification has to be con-
tinuously adapted to the actually encountered rock mass to serve as a basis for invoicing.
The behaviour of the rock mass as the tunnel is excavated is determined by the interacting
factors of geology, geomechanics and tunnel construction. This complexity justifies the
demand that tunnel engineers should possess interdisciplinary knowledge, particularly of
geotechnics.
2.4.2.2 Basic system of classification
Until the start of the 1950s, classification of the rock mass for a tunnel was still based on
a predominantly geological and purely qualitative description. This was assigned in rela-
tion to the possible methods of support, which were decided according to the experience
of the specialists involved in the project. Only the combination of rock mechanics and
construction technology aspects with the geological description to give a partially quan-
titative evaluation led to a practically oriented reference to the planned structure. More
recent classifications were based on the work of Terzaghi [241] and Stini [234], who were
the first to propose a classification according to the behaviour of the rock mass (Table
2-8). Lauffer [121] in 1958 selected the stand-up time and the free span length as criteria
for his generally accepted and applied classification process. The system of rock mass
properties related to technical working methods developed by Müller, Rabcewicz, Pacher
and others [159, 175] together with the introduction of shotcrete in 1957, and later revised
many times [compare 130], divides the rock mass classes according to the composition of
the rock mass and its behaviour during the creation of a cavity. Seeber attempted, in his
proposed classification based on Mohr, to emphasise the differentiation of squeezing rock
mass behaviour from brittle behaviour [219].