Development of a new method for RMR and Q classification method to optimize support system in tunneling

Abstract

Rock mass classification system is very suitable for various engineering design and stability analysis. JH classification method is confirmed by Japan Highway Public Corporation that this method can figure out either strength or deformability of rock mass, further appropriating the amount of rock bolts, thickness of shotcrete, and size of pitch of steel ribs just after the blasting procedure. Based on these advantages of JH method, in this study, according to data of five deep and long tunnels in Iran, two equations for estimating the value of JH method from Q and RMR classification systems were developed. These equations as a new method were able to optimize the support system for Q and RMR classification systems. From JH classification and its application in these case studies, it is pointed out that the JH method for the design of support systems in underground working is more reliable than the Q and RMR classification systems.

Keywords

JH classification / Q and RMR classification / new method

Introduction

Due to the uncertainty of geomechanical parameters affecting underground construction, the empirical design method is still widely used in engineering activities. The most important points in the design of support systems for underground structures can be referred to initial studies to determine the type of rock and the rock mass. In the 1970s, various methods were proposed for designing of support systems in underground spaces by rock mass classification systems, that the main objective of rock mass classification was to provide the quantitative data for engineering purposes that could have an important role in the design of support systems. The earliest accessible rock classification system was a qualitative one proposed by Agricola [ 1] in his famous De Re Metallica, which was published in Latin 1 year after his death. In Book V of this famous manuscript, he classified ores and surrounding rocks as “crumbling”,“hard”,“harder”, and “hardest”; in addition, he gave short descriptions to each class. Also, the rock load theory published by Terzaghi [ 2]. Moreover, many systems have been developed to improve methods for designing structural support systems, such as, the RQD system [ 3], RMR system [ 4– 6], the Norwegian geotechnical institute Q-system (Q) [ 7– 10], the rock quality designation (RSR) system [ 11], the geological strength index (GSI) [ 12], the rock mass index (RMi) [ 13, 14], and the new Austrian tunneling method (NATM) [ 15].

The most commonly used rock mass classification systems to assess underground support system are the RMR system, the RMi system, the NGI-index (Q-system), and the GSI as well as the NATM. Akagi et al. [ 16] presented a new method (JH) for the classification of rock mass which this method proposes the support patterns. The JH classification is the rock mass rating system which relies primarily upon the following four general observation data related to the rock mass strength: compressive strength, weathering, spacing of joints and condition of joints. JH rock mass classification system is based on new face observation items determining the tunnel support. The main objective of this paper is develop of two equations by RMR and Q classification systems for estimating of JH value as a new method to optimize support system in tunneling.

Some case studies under study

All data have been collected from five tunnels, including Golab, Behesht Abad, DashteZahab, Alborz and, Sabzkooh tunnels in Iran, that the general specifications of five tunnels and geological properties of rock mass are given in Table 1. These data are included three types conditions of rock mass like fair (41<RMR<60), poor rock (21<RMR <40) and Good rock (61–80). In other words, the rock mass of Golab main tunnel, Alborz tunnel and DashteZahab is classified as poor class, the rock mass of Golab access tunnel and Sabzkooh tunnel is classified as fair and Behesht Abad tunnel has good rock condition. Also, Geomechanical properties of rock masses are given in Table 2.

Rock mass classification in tunnels

Applying one classification method alone is not enough because all tunnels are formed through a variety of different tectonic conditions like many joints and so on; Thus, in this study, the rock mass of tunnels were calculated by RMR, Q and JH classification systems.

Rock mass classification by RMR method

Bieniawski [ 4] published a detailed rock mass classification called the geomechanics classification or RMR system for rock masses. The system was originally developed for the calculation of rock load and tunnel support selection. Significant changes had been made over the years with revisions in 1984 and 1989 [ 5, 6]. RMR makes use of six parameters, which are readily determined in the field, that are uniaxial compressive strength of the intact rock, RQD, spacing of discontinuities, condition of discontinuities, ground water conditions and orientation of discontinuities. The rating is an outcome of classification of each parameter. RMR value used to find rock mass classes from very good rock to very poor rock. In spite of all its advantages, RMR geomechanical classification ignores several important factors including: the degree of roughness discontinuities and the role of joint filler material. These factors influence the quality of rock mass; especially, in the case of swelling rocks, they have a special significance. Rock mass classes rating with descriptions are given in Table 3. Also, RMR rating for the each parameter is summarized in Table 4.

Rock mass classification by JH method

Akagi et al. [ 16] presented a new method for the classification of rock masses, which is used for tunnels, employs some rock parameters to classify rock mass.

In JH method rocks are divided into four classes according to compressive strength in the fresh state and the modes of subsequent weathering and deterioration, and investigated the degrees of contribution of observation items in each class. For simplicity, the rock groups are hereafter referred to by the rock group names shown in Table 5. Also, Rock mass was classified from the total points of tunnel face observation based on chart as presented in Fig. 1.

JH method can suggest support systems such as bolts, steel rib, and Shotcrete. The support patterns are shown in Fig. 2 and Table 6. As can be seen from Fig. 2 and Table 6, six support patterns has been designed for this classification system. Figure 2 shows the support pattern (light=>heavy) on the horizontal axis and evaluation (good=>poor) on the vertical axis rises to the right. Also, descriptions of all classes classified by JH method are given in Table 7. JH Rating system was obtained from the study of 6101 tunnel sections constructed by Japan Highway Corporation. It should be noted that the range of JH system is from 0 to 100.

JH classification system uses a number of criteria, including type of rock, joint condition, joint spacing, weathering alteration, rock strength and joint orientations that with different rates for any rock groups classifies rock masses. In JH method, if tunnel face is composed of several types of rocks, the classification will more reasonable by rating different sections of tunnel face.

Akagi et al. [ 16] suggested Eq. (1) to calculate final rate of JH classification system.

(1) \left[\mathit{R}\mathit{a}\mathit{t}\mathit{i}\mathit{n}\mathit{g}\right]=\left(\mathit{l}\mathit{e}\mathit{f}\mathit{t}+\mathrm{2}*\mathit{c}\mathit{r}\mathit{o}\mathit{w}\mathit{n}+\mathit{r}\mathit{i}\mathit{g}\mathit{h}\mathit{t}\right)/\mathrm{4}.

Another advantage of this method is that it assigns greater weight to strength parameters of rock rather than joint parameters. JH values were calculated for different tunnel sections are given in Table 8. In some cases, because tunnel face has been formed from multiple rock types, rock mass classification with this system is much better than other methods.

Rock mass classification by Q method

The traditional application of the six parameters to determine the Q-value in rock engineering is to select suitable combinations of shotcrete and rock bolts for rock mass reinforcement and support. This value is specifically used for the permanent “lining” estimation for tunnels or caverns in rock that are part of civil engineering projects [ 10]. These parameters are estimated from the following expression (Eq. (2)):

(2) \mathit{Q}={\displaystyle \frac{\mathit{R}\mathit{Q}\mathit{D}}{{\mathit{J}}_{\mathrm{n}}}}\times {\displaystyle \frac{{\mathit{J}}_{\mathrm{r}}}{{\mathit{J}}_{\mathrm{a}}}}\times {\displaystyle \frac{{\mathit{J}}_{\mathrm{w}}}{\mathit{S}\mathit{R}\mathit{F}}}.

Q system, due to providing enough data for proper and accurate assessment of factors that can affect the stability of underground structures, is noteworthy. Also, in this study, rock masses of all tunnels were classified by Q method, that the results are given in Table 9.

The relationships between the various classification systems

Rock formations in tunnels were classified by various engineering classification methods such as RMR, Q and JH. Their results are given in Table 9. In this section, the results of various analyses for each method are compared with each other. Finally, the results are presented by statistical analysis.

Relationship between RMR and JH

Because the support system proposed by JH method is more detailed and optimal, and it also offers stiffness ratio by Hokuriku Method, it is necessary to obtain an equation between JH, Q and RMR. This method evaluates the limit support stiffness as sum of thickness of shotcrete, number of rock bolts and the size of steel ribs. According to studies, at various sections, Eq. (3) is defined between RMR and JH that the correlation coefficient (R²) and standard error are 0.8822 and 4.7410, respectively.

(3) \mathit{J}\mathit{H}=77.087 \ln \left(\mathit{R}\mathit{M}\mathit{R}\right)-231.9,

where, JH and RMR are rock mass classification systems. The graph of this section is shown in Fig. 3. In this figure, the vertical axis is JH value and the horizontal axis is the RMR.

Relationship between Q and JH

Due to the use of Q system in most underground projects, the logical relation between JH system and Q was obtained. Equation (4) shows the relationship between these two systems.

(4) \mathit{J}\mathit{H}=0.6179{\mathit{Q}}^{\mathrm{2}}+\mathrm{9.4133}\mathit{Q}+48.876,

where, JH and Q are rock mass classification system and tunneling quality index classification system, respectively. The graph of these sections is shown in Fig. 4, too. The correlation coefficient (R²) was 0.698 and the standard error was 7.590.

In Fig. 4, the vertical and the horizontal axis are JH and Q, respectively.

Discussion

Rock mass classification methods enable designers to have better understanding of the impact of various geological parameters on the behavior of rock mass. In this study, three classification methods, RMR, Q and JH, were used in various tunnel sections. In these sections, the range of RMR was 34 to 65 and Q was 0.0125 to 10.3; in addition, the range of JH was 45 to 81. By the sufficient number of tunnel sections under study, RMR and Q methods were compared with JH.

After comparing the results of JH by using RMR and Q methods, it was determined that the equation of RMR (Eq. (3)) had a good agreement with JH.

In this section, for example the support system of Golab main tunnel (Section (2)) has been evaluated. As shown in Table 10, JH support system has good accordance with proposed support system of RMR in comparison with Q system. As can be seen in Table 10, the JH value for Golab main Tunnel Section (2) is 53 and in accordance to JH rock mass classes (Table 7) this value is in fair to poor rock condition. Also, this rock mass condition was observed in this tunnel. Moreover, because of the condition of rock mass in this tunnel, the excavation length in field was selected 1-1.5 m. On the other hand, in accordance with Table 10, excavation length for JH is 1.2 m. Then, JH system has better excavation length than other systems. Furthermore, JH system suggested a stiffness ratio that the other systems have not this factor. From data processing and application of JH method, it can be pointed out that the results of JH method are conservative in hard rocks and their application could be more suitable in weak rocks. Among the advantages of JH method, one can refer to the use of steel ribs with shotcrete as a support system. In tunnel faces passing from multiple types of rocks this method has a better performance and this subject is not found in any of the rock mass classification systems.

Based on what was mentioned, these equations (Eq. (3) and Eq. (4)),which the values of RMR and Q were converted to the value of JH, as a new method were able to optimize the support system for Q and RMR classification systems by use of JH support system.

Conclusions

In this study, according to data obtained from five deep and long tunnels in Iran, two equations for estimating of JH were developed. Also, it was determined that the equation of RMR (Eq. (3)) has a good agreement with JH classification method. From JH method, optimum support systems for underground structures can be evaluated. JH method where the tunnel passes several rock types is very useful and therefore, proposes a suitable and efficient support system in comparison with other methods. Based on these advantages of JH method, these equations as a new method were able to optimize the support system for Q and RMR classification systems. In other words, by use of new equations, the values of RMR and Q can be converted to JH; therefore, the suitable support system can be obtained from JH method that this support system is more suitable than RMR and Q support system.

References

[1] Agricola G, Metallica D R. Translated from the first Latin edition of 1556 by H C Hoover and L H Hoover, 1950. New York: Dover Publications, 1556

[2] Terzaghi K. Rock Defects and Loads on Tunnel Support. In: Proctor R V, White T, eds. Rock tunneling with steel supports. Youngstown, Ohio: Commercial Shearing & Stamping Co, 1946, 15–99

[3] Deere D U. Technical description of rock cores for engineer purposes. Rock Mechanics and Engineering Geology, 1964, 1(1): 17–22

[4] Bieniawski Z T. Engineering classifications of jointed rock masses. Transactions. South African Institution of Civil Engineer, 1973, 15: 335–345

[5] Bieniawski Z T. Rock Mechanics Design in Mining and Tunnelling. Rotterdam, A.A. Balkema, 1984

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

[7] Barton N, Lien R, Lunde J. Engineering classification of rock masses for the design of tunnel support. Rock Mechanics, 1974, 6(4): 189–236

[8] Barton N. Geotechnical Design. World Tunnelling, 1991, 406–410

[9] Barton N. The Influence of Joint Properties in Modelling Jointed Rock Masses. In: Proceedings of the 8th Congress of ISRM. Tokyo, Rotterdam: A.A. Balkema, 1995

[10] Barton N. Some new Q-value correlations to assist site characteristics and tunnel design. International Journal of Rock Mechanics and Mining Sciences, 2002, 39(2): 185–216

[11] Wickham G E, Tiedemann H R, Skinner E H. Support determination based on geological predictions. In: Proceedings of the 1st North American Rapid Excavation and Tunnelling Conference. AIME, New York, 1972, 43–64

[12] Hoek E, Brown E T. Practical estimates of rock mass strength. International Journal of Rock Mechanics and Mining Sciences, 1997, 34(8): 1165–1186

[13] Palmström A. Combining the RMR, Q and RMi classification systems. Tunnelling and Underground Space Technology, 2009, 24(4): 491–492

[14] Stille H, Palmström A. Classification as a tool in rock engineering. Tunnelling and Underground Space Technology, 2003, 18(4): 331–345

[15] Dhawan A K, Joshi A B. The Basic Approach to New Austrian Tunnelling Method, Symposium on Tunnelling, 52nd Board Session, CBIP. New Delhi, 1982, 183–196

[16] Akagi W, Ito T, Shiroma H, Sano A, Shinji M, Nishi T, Nakagawa K. A proposal of new rock mass classification for tunnelling. In: Kimura M, Tateyama K, Adachi T, eds. Proceedings of the International Symposium on Modern Tunnelling Science and Technology. Kyoto, Japan: Swets & Zeitlinger, 2001, 371–371

RIGHTS & PERMISSIONS

Higher Education Press and Springer-Verlag Berlin Heidelberg