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© JCRM All rights reserved.

Volume 11, Number 1, October 2015, pp.17-20

[Summary]

Rock Mass Quality Rating (RMQR) for Rock Engineering

Ömer AYDAN*, Naohiko TOKASHIKI* & Reşat ULUSAY**

* Member of ISRM: Dept. of Civil Eng. & Architecture, Faulty of Engineering, University of the Ryukyus, Okinawa 903-0213 Japan ** Member of ISRM: Dept. of Geological Engineering, Faulty of Engineering, Hacettepe University, 06800 Beytepe, Ankara, Turkey

Received 05 10 2015; accepted 06 10 2015

ABSTRACT

A new rock classification named as Rock Mass Quality Rating (RMQR) proposed by the authors (Aydan et al. 2013). This new rock classification quantify the state of rock mass and possible geo-mechanical properties of rock masses can be estimated using the classification system together with intrinsic geo-mechanical properties of intact rock. This system eliminates some shortcoming of previous systems. It is correlated with existing quantitative rock classification systems as well as qualitative rock classification systems used in Japan. The fundamental parameters of this system are explained and the correlations with quantitative and qualitative systems are presented. Further applications of this new system are pointed out.

Keywords: RMQR, rock classification, geo-mechanical properties, Japanese rock classification systems,

1. INTRODUCTION

The qualitative description of rock masses by means of classification systems and subsequent correlations to establish engineering quantities or design parameters have become one of the most challenging topics in rock engineering. Many rock mass classification systems have been proposed for rock masses with the consideration of a particular rock structure and/or specific purposes. The common purpose of these systems was to quantify rock mass characteristics previously based on qualitative geological descriptions. They were originally developed for assisting with rock engineering design of tunnels or dam foundations. However, many available rock classification systems have some repetitions such as RQD and discontinuity spacing resulting in essence doubles the influence of the spacing of discontinuities on the final rating. In addition, although the effect of water particularly on water-sensitive rocks plays an important role in decrease of their geo-mechanical properties, this effect is not adequately considered in the existing rock mass classification systems. Therefore, direct utilization of these systems, in their original form for characterization of complex rock mass conditions, is not always possible. This is probably one of the reasons why rock engineers continue to develop new systems or modify and extend the current ones.

In this summary, the fundamental elements of a new rock mass rating system designated as Rock Mass Quality Rating (RMQR) proposed recently by the authors (Aydan et al. 2013) are explained and its correlations with quantitative and qualitative rock classification systems are presented and

brieflydiscussed. Furthermore, the possible extensions ofthis system to applications in geo-engineering are pointed out.

2. ROCK MASS QUALITY RATING (RMQR)

It is well known that rock masses have discontinuities of various scale associated with the formation in their geologic past. The authors particularly prefer to use the term “discontinuity” instead of “joint” as it covers all types of interruptions of structural integrity of rock masses. The most commonly used factors in engineering description of rock masses are the condition and geometrical characteristics of discontinuities. Therefore, the parameters associated with discontinuities could be the discontinuity set number (DSN), discontinuity spacing (DS) and discontinuity condition (DC) The intact rock bounded by discontinuities may be subjected to weathering or alteration. The weathering of rocks results from the physical and/or chemical actions of atmospheric conditions and causes the weakening of bonds and decomposition of constituting minerals into clayey materials. The alteration process is due to percolating hydrothermal fluids in rock mass and it may act on rock mass in a positive or negative way. As the intact rock is one of the important elements influencing the mechanical response of rock masses, weathering and/or the negative action of hydrothermal alteration may be accounted as the degradation degree (DD) of intact rock.

Groundwater (GW) is also an important parameter affecting the mechanical response of rock masses. There are also cases, that some rocks may absorb groundwater electrically or chemically, resulting in the drastic reduction of material properties and/or swelling. In addition to seepage

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18 Ö. AYDAN et al. / International Journal of the JCRM vol.11 (2015) pp.17-20

condition of groundwater (GWSC), the water absorption characteristics of rocks (GWAC) should also be taken into account.

RMQR has six basic parameters, which provides rating of each parameter, and ranges between 0 and 100 (Table 1). If detailed surveys on the conditions of discontinuities are carried out, a more detailed rating is necessary for characterization of rock discontinuities. For the evaluation of discontinuity condition from detailed surveys, Table 2 is recommended together with roughness concept of surface profiles adapted by ISRM (2007).

Table 1. Classification parameters and their ratings for Rock Mass Quality Rating (RMQR)*.

Table 2. Ratings for sub-parameters of discontinuity condition excluding None and Healed or intermittent classes.

3. CORRELATIONS AMONG RMQR AND ROCK MASS CLASSIFICATION SYSTEMS OF JAPAN

RMQR could be related to the well-known quantitative two rockmass rating systems, RMR (Bieniawski, 1989) and Q-system (Barton et al., 1974), through some relations given in Figure 1, which shows the correlations between RMQR, RMR and Q-value.

)

(

100

RMQR

A

RMQR

RMQR

RMR

+

=

β

or

)

100

(

1

.

1

100

RMR

RMR

RMR

RMQR

+

=

(1) 50 ) log( 7 . 16 + = Q RMQR or

Q

=

10

0.06RMQR−3 (2) The value of parameter

β

is 0.8 and the value of parameter

A ranges between 90 and 100. Figure 1 shows the correlations between RMQR, RMR and Q-value together with data from various projects in Japan. It should be noted that the value of RMR is generally higher than the value of RMQR.

Figure 1. The relations between (a) RMQR and RMR, and (b) RMQR and Q-value based on data from Japan. There are rock classifications proposed for the preliminary assessment and design of support systems for underground caverns and tunnels (Saito, 1992). The rock classification of the Central Research Institute of Electric Power Industry (CRIEPI) is known as “DENKEN” classification in Japan and it is used for underground caverns and dam-sites (Saito, 1992). Table 3 correlates the rock classes of RMQR with those of DENKEN classification system.

Table 3. Rock classes of RMQR rock classification system and its relation to DENKEN classifications

As for tunneling, there are two rock classifications (Saito, 1992). The rock classification of the Former Japan Roadway Authority, recently named as NEXCO is known as DOROKODAN classification and it is widely used in roadway tunnels. The rock classes of the RMQR rock classification system are associated with the rock classification of DOROKODAN or NEXCO (JRoC) by Aydan et al. (2016)as given in Table 4. Former Japan State Railways known as KYU-KOKUTETSU also proposed a rock classification system for railway tunnels. The privatised Japan State Railways, which is now named as Japan Railways (JR), utilizes the KYU-KOKUTETSU or JraC rock classification for tunnel design. An attempt is done to correlate the rock classes of the JRaC (KYU-KOKUTETSU)

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Ö. AYDAN et al. / International Journal of the JCRM vol.11 (2015) pp.17-20 19

to RMQR as given in Table 5 with the consideration of the previous study by Saito (1992) the correlations among Japan rock classifications systems. However, it should be noted that this system based on very few parameters such as rock type, p-wave velocity and competency factor.

Table 4. Interrelations between RMQR and NEXCO (JRoC) Rock Classes

Table 5. Interrelations between RMQR and rock classes of JRaC (KYU-KOKUTETSU)

4. ESTIMATION OF ROCK MASS PROPERTIES FROM RMQR

Aydan et al. (2013) provided relations for six different mechanical properties of rock mass using the relation proposed by Aydan and Kawamoto (2000). Aydan et al. (2013) replaced RMR by RMQR, and it is given in the following form for any mechanical properties of rock mass in terms of those of intact rock:

) 100 ( ) ( 0 100 0 RMQR RMQR RMQR − + − − = β α α α α (3)

Where

α

0and

α

100 are the values of the function at RMQR = 0 and RMQR = 100 of normalized property

α

and

β

is a constant to be determined by using a minimization procedure for experimental values of given physical or mechanical properties. The authors proposed some values for these empirical constants with the consideration of in-situ experiments carried out in Japan as given in Table 5. When a representative value of RMQR is determined for a given site, the geomechanical properties of rock mass can be obtained using Eq. (3), together with the values of constants given in Table 5 and the values of intact rock for a desired property.

The empirical relations for normalized properties presented in the previous section are compared with the experimental results from in-situ tests carried out at various large projects (underground power houses, dams, nuclear power plants and underground crude oil and gas storage caverns) in Japan. Figure 2 compares the experimental results for elastic modulus and Poisson’s ratio of rock mass. The experimental results on normalized elastic modulus of rock mass are closely represented by Eq. (3) together the values given in Table 5 and they are clustered around the curve with

the value of coefficient

β

as 6.

It should be noted that experiments on the Poisson’s ratio of rock masses are quite rare. In this particular comparison, Poisson’s ratio of rock mass in tunnels through squeezing rocks correlated with RMQR. The data for RMQR value less than 50 are mainly from those of rock masses exhibiting squeezing behaviour. The measured data is well enveloped by the empirical relation with the values of coefficient

β

ranging between 0.1 and 3. The authors suggest that the values of

100 0

α

and

β

should be 2.5, 1.0 and 1, respectively as given in Table 3.

Table 5. Values of

α

0

100and

β

for various properties of rock mass

Figure 2. Comparison of experimental data for (a) deformation modulus and (b) Poisson’s ratio of rock mass with estimations from Eq. 3.

Figure 3(a) compares experimental results with empirical relations for normalized uniaxial compressive strength (UCS) and tensile strength of rock masses by those of intact rock. The UCSs of rock masses plotted in this figure are mostly obtained using rock shear test together with Mohr-Coulomb failure criterion. The experimental results generally confirm the empirical relation given in Eq. (3).

Figure 3. Comparison of experimental data for (a) uniaxial compression and (b) tensile strengths of rock masses with estimations from Eq. (3).

In literature, there is almost no in-situ experimental procedure or experimental results for the tensile strength of rock mass to the knowledge of the authors. The authors (Aydan et al., 2013) utilized back-analysed data on the stable

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20 Ö. AYDAN et al. / International Journal of the JCRM vol.11 (2015) pp.17-20

and unstable (failed) cliffs along seashores of major islands of Ryukyu Archipelago using a theory based on the cantilever theory and fitted the inferred tensile strength of the rock mass normalized by that of intact rock using Eq. (3). The results are plotted in Figure 3b by varying the value of empirical constant

β

between 5 and 7. It is found that the value of empirical constant

β

could be designated as 6 in view of inferred tensile strength of rock mass.

The authors again utilize Eq. (3) together with the values of parameters given in Table 5 for comparing with experimental results as shown in Figure 4. The data used in this comparison are directly from rock shear tests carried out on rock masses in Japan. The experimental results generally confirm Eq. (3).

Figure 4. Comparison of (a) cohesion and (b) friction angle of rock mass with estimations from Eq. (3).

5. APPLICATION OF RMQR TO ROCK SUPPORT DESIGN FOR UNDERGROUND OPENINGS

The design of support systems of large underground openings and tunnels in rock engineering is of great importance, as these structures are required to be stable during their service lifetime. Rock discontinuities may cause structurally controlled or local instability modes while inward displacement of rock mass may be due to elasto-plastic or elasto-visco plastic behavior induced by in-situ stresses. Therefore, the main purpose of the design of support systems must be well established with due considerations of these situations.

Aydan (2016) established several interrelations for the dimensions of support members and related size parameters of the large underground openings and tunnels with the consideration of structurally controlled and stress induced instability modes as given in Table 6. It may also be used for preliminary support design when surrounding rock mass is subjected to even stress-induced yielding.

Table 6. Empirical relations between rock mass quality rate (RMQR) and the dimensions of support members normalized by arch span (La) or sidewall height (Hs) (* for large underground caverns)

Aydan and Ulusay (2013) proposed Table 7 for the empirical design of support systems for tunnels, which may be subjected to even stress-induced failure modes such as squeezing and rock bursting, respectively. In case of tunnels, when RMQR<20, UCS of intact rock is less than 20 MPa and overburden is greater than 100 m, squeezing problems may be encountered. Under such circumstances, forepoles, face bolting and shotcreting may be required.

Table 7. Support systems for tunnels (D or B, 10 m span).

6. OTHER REMARKS

RMQR value of rock mass is considered to be a universal state parameter and it is obtained from the summation of the rating of six parameters. It is a scalar value (a zero rank tensor). However, its tensorial utilization of rank one or higher evaluations may be necessary, depending upon the nature of the conditions of physical phenomenon associated with rock mass and geo-engineering structures. In such cases, the rating of six parameters may be varied, for example, by introducing some weighting functions and/or orientations.

REFERENCES

Aydan, Ö., 2016. The state of art on large cavern design for underground powerhouses and long-term issues. The second Volume of Encyclopedia on Renewable Energy, John Wiley and Sons (in press).

Aydan, Ö. and Kawamoto, T., 2000, Assessing mechanical properties of rock masses by RMR rock classification method, Proc.of GeoEng 2000 Symposium, Sydney, Paper No. OA0926. Aydan, Ö., and Ulusay, R. 2013, Application of RMQR Classification

System to Rock-Support Design for Underground Caverns and Tunnels. Proc. of the 3rd Int. Symp. on Underground Excavations for Transportation, İstanbul, 387-398.

Aydan, Ö., Ulusay, R. and Tokashiki, N., 2013, A new Rock Mass Quality Rating System: Rock Mass Quality Rating (RMQR) and its application to the estimation of geomechanical characteristics of rock masses, Rock Mech. and Rock Eng., 47:1255-1276. Aydan, Ö., R. Ulusay, N. Tokashiki, M. Imazu, (2016b). Application

of Rock Mass Quality Rating (RMQR) to design of support systems for tunnels and underground caverns. ITA WTC 2016 Congress and 41st General Assembly, San Francisco, USA. Barton, N., Lien, R. and Lunde, I., 1974, Engineering classification

of rock masses for the design of tunnel supports, Rock Mech., 6 (4), 189-239.

Bieniawski, Z.T., 1989. Engineering Rock Mass Classifications. John Wiley & Sons, New York.

ISRM, 2007, The complete ISRM suggested methods for rock characterization, testing and monitoring: 1974–2006. In: Ulusay R, Hudson JA (eds), Suggested methods prepared by the Commission on Testing Methods, ISRM, Compilation arranged by the ISRM Turkish National Group, Kozan Ofset,

Saito, K. (1992). Study on rock mass classification method. CRIEPI Report, U91059, 36p (in Japanese).

Table 3. Rock classes of RMQR rock classification system  and its relation to DENKEN classifications
Figure  2.  Comparison  of  experimental  data  for  (a)  deformation modulus and (b) Poisson’s ratio of rock  mass with estimations from Eq
Table 6. Empirical relations between rock mass quality rate  (RMQR) and the dimensions of support members  normalized by arch span (L a ) or sidewall height (H s )  (* for large underground caverns)

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