Engineering classification of rock masses for the design of tunnel support

Abstract

have obviously put a great deal of effort into developing their classification system which differs considerably in detail if not in concept from those described by Abel 1, Wykeham et al 2 and B i eni a w s ki a, 4, 5. Unfortunately the system proposed by B a r t o net al. seems overcomplicated. As the authors themselves point out there are over 300,000 different geological combinations that can be represented. This number of possibilities is simply not "workable". The fact that this number is based on the analysis of 200 case records (commendable though that is) appears to be extrapolating to the extreme.These classification systems have definite limitations and it cannot possibly be expected that they will allow for all geological possibilities and stress conditions. The system described by B i eni a w ski a, 4, 5 recognizes this fact and is intended primarily for cases where tunnel wall displacements are the result only of rock mass loosening. Cases involving swelling rock/ gouge or extensive compressive failure of the rock mass (socalled "true" rock pressure) are specifically excluded. However, Barton et al. have attempted to cover all these possibilities. This appears to be asking too much of a classification system especially when the rock mass rating is related directly to tunnel support pressure. Particularly in the case of "true" rock pressure, many factors (e. g. excavation method, type and sequence of primary support installation), which cannot be accounted for in a classification system, will affect the load carried by the primary support G.Other drawbacks in the system proposed by B art o net al. appear to be:1. The only allowance for the strength of the rock material is in the SRF factor for "competent rock". For other categories of rock no account is taken of the material strenght. The term "competent rock" is itself not clearly defined.2. No allowance is made for the orientation of joint sets. This is unrealistic as it is well known v that the orientation of discontinuities relative to the tunnel axis can have an important influence on roof loosening.3. The system is based fundamentally on RQD which is both directionally dependent and rather sensitive to the skill of the driller.4. The system involves 9 rock mass classes and 38 categories for permanent support. These numbers are simply too large to be generally acceptable in practice. Similarly numbers such as 0.0015 or 0.018 for rock mass quality are likely to be viewed with some suspicion.

Stope Stability Assessment and Effect of Horizontal to Vertical Stress Ratio on the Yielding and Relaxation Zones Around Underground Open Stopes Using Empirical and Finite Element Methods

“…Where Q' is the modified tunnelling quality index introduced by (Barton et al, 1974); A is the stress factor, B is the joint orientation factor, and C is the gravity adjustment factor. HR, or S, accounts for the influence of the shape and size of the stope surface and is calculated using equation (2).…”

Reliability-based Support Optimization of Rockbolt Reinforcement around Tunnels in Rock Masses

“…Traditional empirical and deterministic design approaches do not include uncertainty in tunnel support design [1][2][3], but tend to be based on trial-anderror processes that consider safety and cost [4][5][6]. Reliabilitybased optimization (RBO) makes provision for the uncertainty of structures by adding probabilistic constraints.…”

Engineering Geological Assessment of Diversion Tunnel of Bakhtiari Damsite (Biggest Two-Arch Concrete Dam in Southern Iran)

“…Rock-mass quality (Q) system that was developed by Barton et al [1], mainly for tunneling has been proposed. In this classification system, the Q values of the rock masses along the tunnels are evaluated based on six parameters, and the required support system is specified for each rock mass [10,17].…”

“…Based on RQD values five definite rock-mass classes are described (Table 4) [1]. RQD values in all zones have been calculated and their descriptions are presented in Table 5.…”

“…The relevant ESR value for the diversion tunnel was taken as 1.6, according to Barton et al [1]. Excavation diameter of the upper diversion tunnels is 13.7 m. Having De and Qvalues for different types of rock masses along the diversion tunnels, the required rock support measures for the different rock masses were estimated based on the Q-support chart proposed by Grimstad and Barton [10] as shown in Figure 3.…”