The development of empirical support design methods for the stability of underground project aim to include the experience gained from similar projects, in similar circumstances, in its construction. The main characteristic of these methods is the breakdown of a wider range of rock mass quality by matching each of these categories with various support measures. Moreover, in those support measures, the stages of excavation, and the presence of underground water, as well as stresses developed, where taken into account.
In order to estimate the geomechanical parameters of the rock mass, and the appropriate support measures, rock mass classification was performed following the Geological Strength Index method (GSI after Hoek - Marinos), Rock Mass Rating or Geomechanics classification (RMR after Bieniawski) and the Rock Tunnelling Quality Index (Q after Barton - NGI).
These classification systems help in grouping the rock mass into sections with similar geomechanical behavior based on qualitative and quantitative data measured on site. The analysis of these data leads to different classifications of each of the selected rock mass section, making possible to calibrate the final rock mass quality and the acquisition of the limits of variation of geomechanical parameters
Bieniawski 1989 Method (Rock Mass Rating)
Bieniawski (1976) published the details of a rock mass classification called the Geomechanics Classification or the Rock Mass Rating (RMR) system. Over the years, this system has been successively refined as more case records have been examined, and the user should be aware that Bieniawski has made significant changes in the ratings assigned to different parameters. The discussion which follows is based upon the 1989 version of the classification (Bieniawski, 1989). Both this version and the 1976 version deal with estimating the strength of rock masses and the evaluation of the necessary support measures. One of the main advantages of the RMR classification seam is the easy of data collection needed to classify the rock mass. The following six parameters are used to classify a rock mass using the RMR system:
o Uniaxial compressive strength of rock material
o Rock Quality Designation (RQD)
o Spacing of discontinuities
o Condition of discontinuities
o Groundwater conditions
o Orientation of discontinuities
By applying this classification system, the rock mass is divided into a number of structural regions and each region is classified separately. The boundaries of the structural regions usually coincide with a major structural feature such as a fault or with a change in rock type. In some cases, significant changes in discontinuity spacing or characteristics, within the same rock type, may necessitate the division of the rock mass into a number of small structural regions.
The Rock Mass Rating system is presented in Fig B1, giving the ratings for each of the six parameters listed above. These ratings are summed to give a value of RMR. It should be noted though, that when applying and evaluating RMR a range of values must be introduced rather than a single RMR value, corresponding to the expected range of the typical rock mass parameters (upper and lower bound values principle).
Fig. Β1: Rock Mass Rating System (After Bieniawski 1989)
Using the basic RMR value, rock mass can classified in the following five categories (Fig. Β2). For each one of the above categories Bieniawski proposed a range of values for friction angle and cohesion, as well as the average stand up time where unsupported excavation can withstand (Fig. Β3).
Fig. Β2: Rock mass classifications, shear strength estimation and average stand - up time for unsupported excavation after Bieniawski (1989)
Fig. Β3: Relationship between effective span and stand - up time for each of the rockmass classification categories after Bieniawski (1974)
NGI Method 1974 (Q Rock Tunneling Quality Index)
By evaluating a large number of case histories of underground excavations, Barton et al (1974) of the Norwegian Geotechnical Institute proposed a Tunnelling Quality Index (Q) for the determination of rock mass characteristics and tunnel support requirements. The numerical value of the index Q varies on a logarithmic scale from 0.001 to a maximum of 1,000 and is defined by:
where:
RQD is the Rock Quality Designation
Jn is the joint set number, ranging 0.5 – 20
Jr is the joint roughness number, ranging 0.5 – 4
Ja is the joint alteration number, ranging 0.75 – 20
Jw is the joint water reduction factor, ranging 0.05 – 1
SRF is the stress reduction factor, ranging 0.5 – 400
In explaining the meaning of the parameters used to determine the value of Q, Barton et al (1974) offer the following comments:
o The first quotient (RQD/Jn), representing the structure of the rock mass, is a crude measure of the block or particle size.
o The second quotient (Jr/Ja) represents the roughness and frictional characteristics of the joint walls or filling materials.
o The third quotient (Jw/SRF) consists of two stress parameters describing the 'active stress'.
It appears that rock tunnelling quality Q can now be considered to be a function of only three parameters which are crude measures of:
1. Block size (RQD/Jn)
2. Inter-block shear strength (Jr/ Ja)
3. Active stress (Jw/SRF)
The source of identification of the necessary factor needed to evaluated Q is one field surveys. Moreover, in the extreme case where no borehole data are available, the range of RQD values can be estimated using the relationship introduced by Hudson - Priest (’79) and ISRM (’78), taking into account the mean discontinuity span (x) and Jv (sum of discontinuities per cubic meter of rock).
Hudson - Priest (’79) where λ=1/x
ISRM (’78)
Q values ranging from 0.001 to 1000 (Table C1) classifing the rock mass into several support categories (Fig. C1) after Barton & Grimstad (1994).
Q | Rock mass quality type |
0.001 ÷ 0.01 | Exceptionally poor |
0.01 ÷ 0.1 | Extremely poor |
0.1 ÷ 1 | Very poor |
1 ÷ 4 | Poor |
4 ÷ 10 | Fair |
10 ÷ 40 | Good |
40 ÷ 100 | Very good |
100 ÷ 400 | Extremely good |
400 ÷ 1000 | Exceptionally good |
Table C1: Rock mass quality types after Q
Fig. C1: Estimated support categories based on the tunnelling quality index Q (after Grimstad and Barton, 1993, reproduced from Palmstrom and Broch, 2006)
