Chapter 6 - RMR [PDF]

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CHAPTER- 6



ROCK MASS RATING (RMR) "Effectiveness o f knowledge through research (E) is E = mc2, 9 where m is mass o f knowledge and c is communication of knowledge by publications" Z. T. Bieniawski



6.1



Introduction



The geomechanics classification or the rock mass rating (RMR) system was initially developed at the South African Council of Scientific and Industrial Research (CSIR) by Bieniawski (1973) on the basis of his experiences in shallow tunnels in sedimentary rocks (Kaiser et al., 1986). Since then the classification has undergone several significant changes: in 1974 - reduction of classification parameters from 8 to 6; in 1975 - adjustment of ratings and reduction of recommended support requirements; in 1976 - modification of class boundaries to even multiples of 20; in 1979 - adoption of ISRM (1978) rock mass description, etc. It is, therefore, important to state which version is used when RMR-values are quoted. The geomechanics classification reported in Bieniawski (1984) is referred in this book. To apply the geomechanics classification system, a given site should be divided into a number of geological structural units in such a way that each type of rock mass is represented by a separate geological structural unit. The following six parameters are determined for each structural unit: (i) (ii) (iii) (iv)



(v) (vi)



uniaxial compressive strength of intact rock material, rock quality designation RQD, joint or discontinuity spacing, joint condition, ground water condition, and joint orientation.



6.2



Collection of Field Data



The rating of six parameters of the RMR system are given in Tables 6.1 to 6.6. For eliminating doubts due to subjective judgements, the rating for different parameters should be given a range in preference to a single value. These six parameters are discussed in the following paragraphs.



34



Rock mass rating (RMR)



6.2.1 Uniaxial Compressive Strength of lntact Rock Material (qc) The strength of the intact rock material should be obtained from rock cores in accordance with site conditions. The ratings based on uniaxial compressive strength (which is preferred) and point load strength are both given in Table 6.1. TABLE 6.1 STRENGTH OF INTACT ROCK MATERIAL(BIENIAWSKI, 1979) Qualitative Description Exceptionally strong Very strong Strong Average Weak



Compressive Strength (MPa) > 250



Point Load Srength (MPa) 8



Rating 15



100 -250 50 - 100 25 - 50 10-25



4-8 12 2-4 7 1-2 4 use of uniaxial compressive strength is preferred Very weak 2-10 1 -doExtremely weak 1- 2 -do0 Note: At compressive strength less than 0.6 MPa, many rock material would be regarded as soil



6.2.2 Rock Quality Designation (RQD) Rock quality designation (RQD) should be determined as discussed in Chapter 4. The details of rating are given in Table 6.2. TABLE 6.2 ROCK QUALITY DESIGNATION RQD (BIENIAWSKI, 1979) Qualitative Description Excellent Good Fair Poor Very poor



RQD 90-100 7s-90 50-75 25-50 < 25



Rating 20 17 13 8 3



6.2.3 Spacing of Discontinuities The term discontinuity covers joints, beddings or foliations, shear zones, minor faults, or other surfaces of weakness. The linear distance between two adjacent discontinuities should be measured for all sets of discontinuities and the rating should be obtained from Table 6.3 for the most critical dicontinuity.



35



Rock Mass Classification. ,4 Practical Approach in Ci~'il Engineering



TABLE 6.3 SPACING OF DISCONTINUITIES(BIENIAWSKI, 1979) Description Spacing (m) Rating >2 20 Very wide Wide 0.6-2 15 Moderate 0.2 - 0.6 10 Close 0.06- 0.2 8 Very close < 0.06 5 Note: If more than one discontinuity sets are present and the spacing of dicontinuities of each set varies, consider the set with lowest rating



6.2.4



Condition o f Discontinuities



This parameter includes roughness of discontinuity surfaces, their separation, length or continuity, weathering of the wall rock or the planes of weakness, and infilling (gouge) material. The details of rating are given in Table 6.4. TABLE 6.4 CONDITION OF DISCONTINUITIES(BIENIAWSKI, 1979) Description Very rough and unweathered, wall rock tight and discontinuous, no separation Rough and slightly weathered, wall rock surface separation < 1mm Slightly rough and moderately to highly weathered, wall rock surface separation < l m m Slickensided wall rock surface or 1-5ram thick gouge or 1-5ram wide continuous discontinuity 5mm thick soft gouge, 5mm wide continuous discontitnuity



6.2.5



Rating 30 25 20 10



Ground Water Condition



In the case of tunnels, the rate of inflow of ground water in litres per minute per 10m length of the tunnel should be determined, or a general condition can be described as completely dry, damp, wet, dripping, and flowing. If actual water pressure data are available, these should be stated and expressed in terms of the ratio of the seepage water pressure to the major principal stress. The ratings as per the water condition are shown in Table 6.5. Ratings of the above five parameters (Tables 6.1 to 6.5) are added to obtain what is called the basic rock mass rating RMRbasi c.



36



Rock mass rating (RMR)



6.2.6



Orientation o f Discontinuities



Orientation of discontinuities means the strike and dip of discontinuities. The strike should be recorded with reference to magnetic north. The dip angle is the angle between the horizontal and the discontinuity plane taken in a direction ill which the plane dips. The value of the dip and the strike should be recorded as shown in Table 6.6. In addition, the orientation of tunnel axis or slope face or foundation alignment should also be recorded. The influence of the strike and the dip of the discontinuities is considered with respect to the direction of tunnel drivage or slope face orientation or foundation alignment. To facilitate a decision whether the strike and the dip are favourable or not, reference should be made to Tables 6.7 and 6.8 which provide a quantitative assessment of critical joint orientation effect with respect to tunnels and dams foundations respectively. Once the ratings for the effect of the critical discontinuity is known, as shown in Table 6.9 an arithmetic sum of the joint adjustment rating and the RMRbasi c is obtained. This number is called the final rock mass rating RMR.



TABLE 6.5 GROUND WATER CONDITION (BIENIAWSKI, 1979) 125



0-0.1



0.1-0.2



0.2-0.5



>0.5



completely dry



damp



wet



dripping



flowing



15



10



7



Inflow per 10m tunnel length (litre/min.) Joint water pressure / major principal stress General description



none



Rating



TABLE 6.6 ORIENTATION OF DISCONTINUITIES



A~



Orientation of tunnel/slope/foundation axis ..................................



B.



Orientation of discontinuities:



set- 1 set- 2 set- 3



Average strike .................... (from .......... to .......... ) Average strike .................... (from .......... to .......... ) Average strike .................... (from .......... to .......... )



Dip .............. Dip .............. Dip ..............



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Rock Mass Classification A Practical Approach in Ci~'il Engineering



T A B L E 6.7 ASSESSMENT OF JOINT ORIENTATION EFFECT ON TUNNELS (DIPS ARE APPARENT DIPS ALONG TUNNEL .AXIS) (BIENIAWSKI, ] 989) Strike Perpendicular to Tunnel Axis Drive with dip Dip 45 ~ - 90 ~



Dip 200-45 ~



Very favourable



Favourable



Drive against dip Dip 45~ ~ Dip 20 ~ 45 ~ Fair Unfavourable



Strike Parallel to Tunnel Axis



Irrespective of Strike



Dip 20o-45 ~



Dip 0 ~ - 20 ~ "



Fair



Dip 45 ~ 90 ~ Very unfavourable



Fair



T A B L E 6.8 ASSESSMENT OF JOINT ORIENTATION EFFECT ON STABILITY OF DAM FOUNDATION Dip 10 o - 30 ~



Dip 0 ~ - 10 ~



Very favourable



Dip Direction Upstream Downstream Unfavourable Fair



Dip 30 ~ - 60 ~



Dip 60 ~ - 90 ~



Favourable



Very unfavourable



T A B L E 6.9 ADJUSTMENT FOR JOINT ORIENTATION (BIENIAWSKI, 1979) Joint Orientation A s s e s s m e n t for Tunnels Raft foundation



Very Favourable 0 0



Favour -able -2 -2



Fair -5 -7



Unfavourable -10 - 15



Very Unfavourable -12 -25



Slopes*



0



-5



-25



-50



-60



* It is recommended to see slope mass rating (SMR)in Chapter 17



6.3



Estimation of Rock Mass Rating (RMR)



The rock mass rating should be determined as an algebraic sum o f ratings for all the parameters given in Table 6.1 to 6.5 and Table 6.9 after adjustments for orientation o f discontinuities given in Table 6.7 and 6.8. The sum o f ratings for four parameters (Table 6.2 to 6.5) is called R o c k Condition Rating (RCR) which discounts the effect o f compressive strength o f intact rock material and orientation o f joints (Goel et al., 1996). H e a v y blasting creates new fractures. Experience suggests that 10 points should be added to get R M R for undisurbed rock m a s s e s in situations where T B M s or road headers are used for tunnel excavation 3 to 5 points m a y be added depending upon the quality o f the controlled blasting. On the basis o f R M R values for a given engineering structure, the rock mass is classified in five classes n a m e d as very good ( R M R 100-81), good (80-61), fair (60-41), poor (40-21) and very poor (0.4



6 months for 8m span 0.3-0.4



1 week for 5 m span



10 hrs for 2.5 m span



100 MPa). E d --



2 RMR - 100,



(6.2)



GPa (applicable for RMR > 50)



Serafim and Pereira (1983) suggested the following correlation Ed =



10 (RMR-i~176 ,



GPa (applicable for RMR < 50 also)



(6.3a)



These correlations are shown in Figure 6.3. Here qc means average uniaxial crushing strength of the intact rock material in MPa. Hoek and Brown (1997) suggested a correction in Eqn. 6.3a (also see Chapter 25),



Ed



10 (RMR-10)'40



=



] 80-O ca m



o O



60v n



"lJ



...,



O



"'"



:J



O



9--



E i-



if)



GPa,



qc
50m), and short-term roof support pressure in MPa.



Bieniawski (1989) provided guidelines for selection of tunnel supports (Table 6.11). This is applicable to tunnels excavated with conventional drilling and blasting method. These guidelines depend upon the factors like depth below surface (to take care of overburden pressure or the insitu stress), tunnel size and shape and method of excavation. The support measures in Table 6.11 are the permanent and not the temporary or primary supports.



6.5



Inter-relation Between R M R and Q



An inter-relation was proposed between the RMR and the Q (Bieniawski, 1976) based on 111 case histories. The correlation is RMR = 91nQ



+



44



(6.7)



The correlation in Eqn. 6.7 is quite popular despite a low reliability. A more realistic approach for inter-relation between RMR and Q is proposed by Goel et al. (1996) as presented in Chapter 9.



6.6



Precautions



It must be ensured that double accounting for a parameter should not be done in the analysis of rock structures and estimating rating of a rock mass. For example, if pore water pressure is



44



Rock mass rating (RMR)



T A B L E 6.11 GUIDELINES FOR EXCAVATION AND SUPPORT OF ROCK TUNNELS IN .ACCORDANCE WITH THE ROCK MASS RATING SYSTEM (BIENIAWSKI, 1989) Rock Class



Mass



Excavation



Very good rock RMR=81-100 Good rock RMR = 61-80



Full face. 3 m advance



Fair rock RMR = 41-60



Heading and bench. 1.5 3m advance in heading. Commence support after each blast. Complete support 10m from face Top heading and bench. 1.0-1.5 m advance in top heading. Install support concurrently with excavation 10m from face Multiple drifts 0.5 - 1.5 m advance in top heading. Install support concurrently with excavation. Shotcrete as soon as possible after blasting



Poor rock RMR = 21-40



Very poor rock RMR