Civil Engineering Reference
In-Depth Information
2.4.4 Ground (rock mass) classification
In his book 'Engineering Rock Mass Classifications', Bieniawski (1989)
states 'Rock mass classifications are not meant to be taken as a substitute
for engineering design. They should be applied intelligently and used in
conjunction with observational methods and analytical studies to formulate
an overall design rationale with the design objectives and site geology.'
The objectives of rock mass classifications are therefore to (after
Bieniawski, 1989):
•
identify the most significant parameters influencing the behaviour of a
rock mass;
•
divide a particular rock mass formation into groups of behaviour, that
is, rock mass classes of varying quality;
•
provide a basis for understanding the characteristics of each rock mass
class;
•
relate the experience of rock conditions at one site to the conditions
and experience encountered at others;
•
derive quantitative data and guidelines for engineering design;
•
provide a common basis for communication between engineers and
geologists.
An early classification system for soft ground is the Tunnelman's ground
classification as shown in Table 2.15 and provides information on the likely
tunnel working conditions and some idea of the types of soils in which
these conditions might occur.
For harder ground, a number of classification systems have been devel-
oped. Three of these classification systems are briefly described in this topic:
Rock Quality Designation (RQD), which is one of the simpler classification
methods and is described in section 2.4.4.1, the Rock Mass Rating (RMR)
system and the Rock Mass Quality Rating (Q-method) (sections 2.4.4.2
and 2.4.4.3 respectively). Further details are given in Appendix A. The
reader is encouraged, however, to read the original, and subsequent, publi-
cations by the relevant authors of these systems in order to fully appreciate
their usefulness and limitations.
2.4.4.1 Rock Quality Designation
The Rock Quality Designation index was developed by Deere in 1967
(Deere
et al.
1967, Deere 1989) to provide a quantitative assessment of
ground quality from drill cores. RQD was developed for assessing rock
and can be misleading in soft ground. RQD is defined as the total length
of 'solid' core pieces each greater than 100 mm between natural (not drill-
induced) discontinuities expressed as a percentage of the total length of
each core run, measured along the core axis. A solid core is defined as a
core with at least one full diameter (but not necessarily a full circumference)
