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Introduction
Rock mass classification schemes have been developed to assist in (primarily) the collection of rock into common or similar groups. The first truly organized system was proposed by Dr. Karl Terzaghi (1946) and has been followed by a number of schemes proposed by others. Terzaghi's system was mainly qualitative and others are more quantitative in nature. The following subsections explain three systems and show how they can be used to begin to develop and apply numerical ratings to the selection of rock tunnel support and lining. This section discusses various rock mass classification systems mainly used for rock tunnel design and construction projects.
Terzaghi's Classification
Today rock tunnels are usually designed considering the interaction between rock and ground, i.e., the redistribution of stresses into the rock by forming the rock arch. However, the concept of loads still exists and may be applied early in a design to "get a handle" on the support requirement. The concept is to provide support for a height of rock (rock load) that tends to drop out of the roof of the tunnel (Terzaghi, 1946). Terzaghi's qualitative descriptions of rock classes are summarized in Table 1.
RQD
In 1966 Deere and Miller developed the Rock Quality Designation index (RQD) to provide a systematic method of describing rock mass quality from the results of drill core logs. Deere described the RQD as the length (as a percentage of total core length) of intact and sound core pieces that are 4 inches (10 cm) or more in length. Several proposed methods of using the RQD for design of rock tunnels have been developed. However, the major use of the RQD in modern tunnel design is as a major factor in the Q or RMR rock mass classification systems described in the following sub-sections. Readers are referred to Subsurface Investigation Manual (FHWA, 2002) for more details.
Q System
On the basis of an evaluation of a large number of case histories of underground excavations, Barton et al. (1974) of the Norwegian Geotechnical Institute proposed a Tunneling Quality Index (Q) for the determination of rock mass characteristics and tunnel support requirements. According to its developer: "The traditional application of the six-parameter Q-value in rock engineering is for selecting suitable combinations of shotcrete and rock bolts for rock mass reinforcement, and mainly for civil engineering projects". The numerical value of the index Q varies on a logarithmic scale from 0.001 to a maximum of 1,000 and is estimated from the following expression (Barton, 2002):
Where RQD is Rock Quality Designation, Jn is joint set number, Jr is joint roughness number, Ja is joint alteration number, Jw is joint water reduction factor, and SRF is stress reduction factor. It should be noted that RQD/Jn is a measure of block size, Jr/Ja is a measure of joint frictional strength, and Jw/SRF is a measure of joint stress.
Table 2 gives the classification of individual parameters used to obtain the Tunneling Quality Index Q for a rock mass. It is to be noted that Barton has incorporated evaluation of more than 1,000 tunnels in developing the Q system.
Table 2 (Continued) Classification of Individual Parameters for Q System (after Barton et al, 1974)
Evaluation of these Q-parameters and the use of Table 2 can be illustrated considering a reach of tunnel with the following properties:
With the parameters established, Q is calculated:
For most other types of ground behavior in tunnels, the Q-system, like most other empirical (classification) methods has limitations. The Q support chart gives an indication of the support to be applied, and it should be tempered by sound and practical engineering judgment" (Palmstream and Broch, 2006). The Q-system was developed from over 1000 tunnel projects, most of which are in Scandinavia and all of which were excavated by drill and blast methods. When excavation is by TBM there is considerably less disturbance to the rock than there is with drill and blast. Based upon study of a much smaller data base (Barton, 1991) it is recommended that the Q for TBM excavation be increased by a factor of 2 for Qs between 4 and 30.
Rock Mass Rating (RMR) System
Z.T. Bieniawski (1989) has developed the Rock Mass Rating (RMR) system somewhat along the same lines as the Q system. The RMR uses six parameters, as follows:
- Uniaxial compressive strength of rock
- RQD
- Spacing of discontinuities
- Condition of discontinuities
- Groundwater condition
- Orientation of discontinuities
The ratings for each of these parameters are obtained from Table 3. The sum of the six parameters becomes the basic RMR value as demonstrated in the following example. Table 6-9 presents how the RMR can be applied to determining support requirements for a tunnel with a 33 ft (10 m) width span.
Determination of the RMR value using Table 3 can be demonstrated in the following example:
Bieniawski, Barton and others have suggested various correlations between RMR and other parameters. For the purpose of this manual, the most applicable correlation between Q and RMR is given in:
Estimation of Rock Mass Deformation Modulus Using Rock Mass Classification
The in situ deformation modulus of a rock mass is an essential parameter for design, analysis and interpretation of monitored data in any rock tunnel project. Evaluation of the stress and deformation behavior of a jointed rock mass requires that the modulus and strength of intact rock be reduced to account for the presence of discontinuities such as joints, bedding, and foliation planes within the rock mass. Since the in situ deformation modulus of a rock mass is extremely difficult and expensive to measure, engineers tend to estimate it by indirect methods. Several attempts have been made to develop relationships for estimating rock mass deformation modulus using rock mass classifications.
The modulus reduction method using RQD requires the measurement of the intact rock modulus from laboratory tests on intact rock samples and subsequent reduction of the laboratory value incorporating the in-situ rockmass value. The reduction in modulus values is accomplished through a widely used correlation of RQD (Rock Quality Designation) with a modulus reduction ratio, EM/EL, where EL represents the laboratory modulus determined from small intact rock samples and EM represents the rock mass modulus, as shown in Figure 4. This approach is infrequently used directly in modern tunnel final design projects. However, it is still considered to be a good tool for scoping calculations and to validate the results obtained from direct measurement or other methods.
Figure 4 Correlation between RQD and Modulus Ratio (Bieniawski, 1984)
Based on the back analyses of a number of case histories, several methods have been propounded to evaluate the in situ rock mass deformation modulus based on rock mass classification. The methods are summarized in Table 4.
* GSI represents Geological Strength Index. The value of GSI ranges from 10, for extremely poor rock mass, to 100 for intact rock. (GSI = RMR76 = RMR89 - 5 = 9LogeQ + 44)
** D is a factor which depends upon the degree of disturbance due to blast damage and stress relaxation. It varies from 0 for undisturbed in situ rock masses to 1 for very disturbed rock masses.
Source : http://www.fhwa.dot.gov
** D is a factor which depends upon the degree of disturbance due to blast damage and stress relaxation. It varies from 0 for undisturbed in situ rock masses to 1 for very disturbed rock masses.
Source : http://www.fhwa.dot.gov
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