ELSEVIER GEO-ENGINEERING BOOK SERIES VOLUME 5 Part 2 pps

Terzaghi’s rock load theory 33 Table 3.5 Recommendations of Singh et al. (1995) on support pressure for rock tunnels and caverns. Terzaghi’s classification Classification of Singh et al. (1995) Recommended support pressure (MPa) Category Rock condition Rock load factor H p Category Rock condition p v p h Remarks I. Hard and intact 0 I. Hard and intact 0 0 – II. Hard stratified or schistose 0to0.5B II. Hard stratified or schistose 0.04–0.07 0 – III. Massive, moderately jointed 0 to 0.25B III. Massive, moderately jointed 0.0–0.04 0 – IV. Moderately blocky seamy and jointed 0.25B to 0.35 (B + H t ) IV. Moderately blocky seamy very jointed 0.04–0.1 0–0.2 p v Inverts may be required V. Very blocky and seamy, shattered arched 0.35 to 1.1 (B +H t ) V. Very blocky and seamy, shattered highly jointed, thin shear zone or fault 0.1–0.2 0–0.5 p v Inverts may be required, arched roof preferred VI. Completely crushed but chemically intact 1.1 (B +H t ) VI. Completely crushed but chemically unaltered, thick shear and fault zone 0.2–0.3 0.3–1.0 p v Inverts essential, arched roof essential continued 34 Tunnelling in weak rocks Table 3.5—Continued Terzaghi’s classification Classification of Singh et al. (1995) Recommended support pressure (MPa) Category Rock condition Rock load factor H p Category Rock condition p v p h Remarks VII. Squeezing rock at moderate depth 1.1 to 2.1 (B +H t ) VII. Squeezing rock condition VIIA. Mild squeezing (u a /a upto 3%) 0.3–0.4 Depends on primary stress values, p h may exceed p v Inverts essential. In excavation flexible support preferred. Circular section with struts recommended. VIIB. Moderate squeezing (u a /a = 3to5%) 0.4–0.6 As above As above VIII. Squeezing rock at great depth 2.1 to 4.5 (B +H t ) VIIC. High squeezing (u a /a>5%) 6.0–1.4 As above As above IX. Swelling rock upto 80 m VIII. Swelling rock VIIIA. Mild swelling 0.3–0.8 Depends on type and content of swelling clays, p h may exceed p v Inverts essential in excavation, arched roof essential. VIIIB. Moderate swelling 0.8–1.4 As above As above VIIIC. High swelling 1.4–2.0 As above As above Notations: p v =Vertical support pressure; p h =horizontal support pressure; B =width or span of opening; H t =height of opening; u a =radial tunnel closure; a =B/2; thin shear zone =upto 2 m thick. Terzaghi’s rock load theory 35 by 5.5 m. The estimated roof support pressures from Table 3.5 were found comparable with the measured values irrespective of the opening size and the rock conditions (Singh et al., 1995). They have further cautioned that the support pressure is likely to increase directly with the excavation width for tunnel sections through slickensided shear zones, thick clay-filled fault gouges, weak clay shales and running or flowing ground conditions where interlocking of blocks is likely to be missing or where joint strength is lost and rock wedges are allowed to fall due to excessive roof convergence on account of delayed supports beyond stand-up time. It may be noted that wider tunnels require reduced spacing of bolts or steel-arches and thicker linings since rock loads increase directly with the excavation width even if the support pressure does not increase with the tunnel size. REFERENCES Barton, N., Lien, R. and Lunde, J. (1974). Engineering classificaion of Rock Masses for the Design of Tunnel Support. NGI Publication No.106, Oslo, 48. Brekke, T. L. (1968). Blocky and seamy rock in tunnelling. Bull. Assoc. Eng. Geol., 5(1), 1-12. Cecil, O. S. (1970). Correlation of Rock Bolts – Shotcrete Support and Rock Quality Parameters in Scandinavian Tunnels. PhD thesis, University of Illinois, Urbana, 414. Deere, D. U., Peck, R. B., Parker, H., Monsees, J. E. and Schmidt, B. (1970). Design of tunnel support systems. High Res. Rec., 339, 26-33. Goel, R. K., Jethwa, J. L. and Dhar, B. B. (1996). Effect of tunnel size on support pressure. Technical Note. Int. J. Rock Mech. and Min. Sci. & Geomech. Abstr., Pergamon, 33(7), 749-755. Rose, D. (1982). Revising Terzaghi’s tunnel rock load coefficients. Proc. 23rd U.S.Sym. Rock Mech., AIME, New York, 953-960. Singh, Bhawani, Jethwa, J. L. and Dube, A. K. (1995). A classification system for support pressure in tunnels and caverns. J. Rock Mech. & Tunnelling Technology, India, 1(1), January, 13-24. Sinha, R. S. (1989). Underground Structures – Design and Instrumentation. Elsevier Science, U.K., 480. Terzaghi, K. (1946). Rock defects and loads on tunnel support. Introduction to Rock Tunnelling with Steel Supports. Eds: R. V. Proctor and T. L. White, Commercial Sheering & Stamping Co., Youngstown, Ohio, U.S.A., 271. Verman, M. K. (1993). Rock Mass – Tunnel Support Interaction Analysis. PhD thesis, IIT Roorkee, Roorkee, India, 258. This Page is Intentionally Left Blank 4 Rock mass rating (RMR) “Effectiveness of knowledge through research (E) is E = mc 2 ; where m is mass of knowledge and c is communication of knowledge by publications.” Z. T. Bieniawski 4.1 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 evo- lutions: 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 by 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 repre- sented by a separate geological structural unit. The following six parameters (representing causative factors) are determined for each of the structural unit: (i) Uniaxial compressive strength of intact rock material, (ii) Rock quality designation RQD, (iii) Joint or discontinuity spacing, (iv) Joint condition, (v) Ground water condition and (vi) Joint orientation. Tunnelling in Weak Rocks B. Singh and R. K. Goel © 2006. Elsevier Ltd 38 Tunnelling in weak rocks 4.2 COLLECTION OF FIELD DATA The rating of six parameters of the RMR system are given in Tables 4.1 to 4.6. For reducing doubts due to subjective judgments, 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. The beginners do not get the feeling of the value of RMR or Q, etc. at a location and they get confused on transition from one category to another (Tables 4.4 and 4.5). In fact, approximate average RMR is good enough. 4.2.1 Uniaxial compressive strength of intact rock material (q c ) The strength of the intact rock material should be obtained from rock cores in accordance with site conditions. The ratings based on both uniaxial compressive strength (UCS) (which is preferred) and point load strength are given in Table 4.1. UCS may also be obtained from the point load strength index tests on rock lumps at the natural moisture content. Please see Table 5.12 also for UCS. 4.2.2 Rock quality designation (RQD) Rock quality designation (RQD) should be determined from rock cores or volumetric joint count (Singh & Goel, 1999). RQD is percentage of rock cores (equal to or more than 10 cm) in one meter of drill run. The details of rating are given in Table 4.2 (see also Section 5.1.1). The fresh broken cores are fitted together and counted as one piece. Table 4.1 Strength of intact rock material (Bieniawski, 1979, 1984). Qualitative description Compressive strength (MPa) Point load strength (MPa) Rating Exceptionally strong >250 8 15 Very strong 100–250 4–8 12 Strong 50–100 2–4 7 Average 25–50 1–2 4 Weak 5–25 Use of uniaxial compressive strength is preferred 2 Very weak 1–5 As above 1 Extremely weak <1 As above 0 Note: At compressive strength less than 0.6MPa, many rock materials would be regarded as soil. Rock mass rating (RMR) 39 Table 4.2 Rock quality designation, RQD (Bieniawski, 1979). Qualitative description RQD Rating Excellent 90–100 20 Good 75–90 17 Fair 50–75 13 Poor 25–50 8 Very poor < 25 3 4.2.3 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 4.3 for the most critically oriented discontinuity. 4.2.4 Condition of 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 4.4. The joint set which is oriented unfavorably with respect to a structure (tunnel or cavern) should be considered as in Section 4.2.3. 4.2.5 Ground water condition In the case of tunnels, the rate of inflow of ground water in liters per minute per 10 m length of the tunnel should be determined, or a general condition may be described as completely Table 4.3 Spacing of discontinuities (Bieniawski, 1979). Description Spacing (m) Rating Very wide > 2 20 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 discontinuities of each set varies, consider the unfavorably oriented set with lowest rating. 40 Tunnelling in weak rocks Table 4.4 Condition of discontinuities (Bieniawski, 1979). Description Joint separation (mm) Rating Very rough and unweathered, wall rock tight and discontinuous, no separation 030 Rough and slightly weathered, wall rock surface separation <1 mm <1 25 Slightly rough and moderately to highly weathered, wall rock surface separation <1 mm <1 20 Slickensided wall rock surface or 1–5 mm thick gouge or 1–5 mm wide continuous discontinuity 1–5 10 5 mm thick soft gouge, 5 mm wide continuous discontitnuity >5 0 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 4.5. Ratings of the above five parameters (Tables 4.1 to 4.5) are added to obtain what is called the basic rock mass rating, RMR basic . 4.2.6 Orientation of 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 in which the plane dips. The value of the dip and the strike should be recorded as shown in Table 4.6. 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 facili- tate a decision whether or not the strike and the dip are favorable, reference should be made Table 4.5 Ground water condition (Bieniawski, 1979). Inflow per 10 m tunnel length (liter/min.) None <10 10–25 25–125 >125 Ratio of Joint water pressure to major principal stress 0 0–0.1 0.1–0.2 0.2–0.5 >0.5 General description Completely dry Damp Wet Dripping Flowing Rating 15 10 7 4 0 Rock mass rating (RMR) 41 Table 4.6 Orientation of discontinuities. A. Orientation of tunnel/slope/foundation axis B. Orientation of discontinuities: Set - 1 Average strike (from to ) Dip/Dip direction Set - 2 Average strike (from to ) Dip/Dip direction Set - 3 Average strike (from to ) Dip/Dip direction to Tables 4.7 and 4.8 which provide a quantitative assessment of critical joint orientation effect with respect to tunnel and dam foundations, respectively. Once the ratings for the effect of the critical discontinuity is known, as shown in Table 4.9 an arithmetic sum of the joint adjustment rating and the RMR basic is obtained. This number is called the final rock mass rating (RMR). It should be kept in mind that the effect of orientation in rough-dilatant joint is not so important in the case of tunnels according to Table 4.9. That is why orientation of joints is ignored in the Q-system of Norwegian Geotechnical Institute (NGI) (Chapter 5). The effect of orientation of joints is more important for rafts. It is most important obviously in rock slopes for which slope mass rating (SMR) is recommended. 4.3 ESTIMATION OF ROCK MASS RATING The rock mass rating should be determined as an algebraic sum of ratings for all the parameters given in Tables 4.1 to 4.5 and 4.9 after adjustments for orientation of Table 4.7 Assessment of joint orientation effect on tunnels (dips are apparent dips along tunnel axis) (Bieniawski, 1984). Strike perpendicular to tunnel axis Strike parallel to tunnel axis Irrespective of strike Drive with dip Drive against dip Dip 45 ◦ –90 ◦ Dip 20 ◦ –45 ◦ Dip 45 ◦ –90 ◦ Dip 20 ◦ –45 ◦ Dip 20 ◦ –45 ◦ Dip 45 ◦ –90 ◦ Dip 0 ◦ –20 ◦ Very favorable Favorable Fair Unfavorable Fair Very unfavorable Fair Table 4.8 Assessment of joint orientation effect on stability of dam foundation. Dip 10 ◦ –30 ◦ Dip direction Dip 0 ◦ –10 ◦ Upstream Downstream Dip 30 ◦ –60 ◦ Dip 60 ◦ –90 ◦ Very favorable Unfavorable Fair Favorable Very unfavorable 42 Tunnelling in weak rocks Table 4.9 Adjustment for joint orientation (Bieniawski, 1979). Joint orientation assessment for Very favorable Favorable Fair Unfavorable Very unfavorable Tunnels 0 −2 −5 −10 −12 Raft foundation 0 −2 −7 −15 −25 Slopes* 0 −5 −25 −50 −60 * It is recommended to use slope mass rating (SMR) (Singh & Goel, 1999). discontinuities given in Tables 4.7 and 4.8. The sum of ratings for four parameters (Tables 4.2 to 4.5) is called rock condition rating (RCR) which discounts the effect of compressive strength of intact rock material and orientation of joints (Goel et al., 1996). Heavy blasting creates new fractures. Experience suggests that 10 points should be added to get RMR for undisturbed rock masses in situations where TBMs or road headers are used for tunnel excavation and 3 to 5 points may be added depending upon the quality of the controlled blasting. On the basis of RMR values for a given engineering structure, the rock mass is classi- fied into five classes, namely very good (RMR 100–81), good (80–61), fair (60–41), poor (40–21) and very poor (<20) as shown in Table 4.10. In case of wider tunnels and caverns, RMR may be somewhat less than obtained from drifts. As in drifts, one may miss intrusions of weaker rocks and joint sets having Table 4.10 Design parameters and engineering properties of rock mass (Bieniawski, 1979). S. No Parameter/properties of rock mass Rock mass rating (Rock class) 100–81 (I) 80–61 (II) 60–41 (III) 40–21 (IV) <20 (V) 1. Classification of rock mass Very good Good Fair Poor Very poor 2. Average stand-up time 10 years for 15 m span 6 months for 8 m span 1 week for 5 m span 10 h for 2.5 m span 30 min. for 1 m span 3. Cohesion of rock mass (MPa)* > 0.4 0.3–0.4 0.2–0.3 0.1–0.2 < 0.1 4. Angle of internal friction of rock mass >45 ◦ 35 ◦ –45 ◦ 25 ◦ –35 ◦ 15 ◦ –25 ◦ <15 ◦ 5. Allowable bearing pressure (T/m 2 ) 600–440 440–280 280–135 135–45 45–30 * These values are applicable to slopes only in saturated and weathered rock mass. Note: During earthquake loading, the above values of allowable bearing pressure may be increased by 50 percent in view of rheological behavior of rock masses. [...]... deformations in massive rock σθ /qc > 20 0 20 0–10 10 5 5 10 20 (d) Swelling rock; chemical swelling... 4.0 3.0 2. 0 1 .5 1 .5 1.0 0 .5 Jr 4.0 3.0 2. 0 1 .5 1 .5 1.0 0 .5 Jr 1.0 Jr 1.0 tan−1 (Jr /Ja ) (Thin coatings) Ja = 0. 75 1.0 2. 0 ◦ ◦ 79 76 63◦ ◦ ◦ 76 72 56 ◦ ◦ ◦ 69 63 45 ◦ ◦ 63 56 37◦ 63◦ 56 ◦ 37◦ ◦ ◦ 53 45 27 ◦ ◦ ◦ 34 27 14◦ (Thin filling) Ja = 4.0 6 8 ◦ ◦ 45 34 27 ◦ ◦ ◦ 37 27 21 ◦ ◦ ◦ 27 18 14◦ 21 ◦ 14◦ 11◦ ◦ ◦ 21 14 11◦ ◦ ◦ 14 9 .5 7.1◦ ◦ ◦ 7 4.7 3.6◦ (Thick fillings) Ja = 5 6 8 11.3◦ 9 .5 7.1◦ Ja = 13 16 20 4.4◦... condition) 0. 75 25 35 25 30 1.0 2. 0 20 – 25 8–16 3.0 4.0 25 30 16 24 4.0 6.0 12 16 8.0 6– 12 8– 12 6 24 6, 8 or 8– 12 5 – 6 24 10, 13 or 13 20 case of clay-filled joints The value of Jw should correspond to the future ground water condition where seepage erosion or leaching of chemical can alter permeability of rock mass significantly 5. 1 .5 Stress reduction factor (SRF) The parameter SRF (Table 5. 6) is a measure... 0 .2 Swell 0.33 Faults Stress/Strength 20 10 20 10 10 5 10 20 5 0 .5 2 .5 15 5 15 5 7 .5 2 .5 5 10 2 1 Joint Alteration Number (Least Favorable), Ja Shear Thick Fills Heat 1.0 0.1 Stress Reduction Factor, SRF 15 Active 20 Soil Fresh Weathering Grade As Per ISRM W VI Fig 5. 1 Data sheet for recording Q parameters (Barton, 1993) V IV III II I Rock mass quality Q 61 Table 5. 8 Weighted average method of obtaining... Ext Good Good 04 02 Support Pressure, kg/cm sq 10 5. 0 2. 0 1.0 Jr = 0 .5 0 .5 Jr = 1.0 Jr = 1 .5 Jr = Jr = 2. 0 Jr = 2 .5 3 Jr = 0 Jr = 4.0 5. 0 0 .2 0.1 0. 05 0. 02 0.01 0.001 0 02 004 0.01 0. 02 0.04 0.1 0 .2 0.4 1 2 4 10 20 40 100 20 0 400 1000 Rock Mass Quality (Q) Fig 5. 2 Correlation between support pressure and rock mass quality Q (Barton et al., 1974) ph = (0 .2/ Jr ) Q−1/3 w where pv = ultimate roof support... 0.1–0. 25 1 0.66 0. 25 1.0 0 .5 0. 25 1.0 0.33 >1.0 0 .2 0.1 >1.0 0.1–0. 05 Notes: (i) Factors C to F are crude estimates Increase Jw if drainage measures are installed (ii) Special problems caused by ice formation are not considered (iii) For general characterization of rock masses distant from excavation influences, the use of Jw = 1.0, 0.66, 0 .5, 0.33, etc as depth increases from 0 5, 5 25 , 25 25 0 to > 25 0 ... SRF increase from 2 .5 to 5 for such cases (see H) (iv) Cases L, M and N are usually most relevant for support design of deep tunnel excavation in hard massive rock masses, with RQD/Jn ratios from about 50 20 0 (v) For general characterization of rock masses distant from excavation influences, the use of SRF = 5, 2 .5, 1.0 and 0 .5 is recommended as depth increases from 0 5, 5 25 , 25 25 0 , > 25 0 m This will... Range): 0 10 20 30 40 50 Three Earth Four 60 70 80 90 One Two Q (Mean): 100 None High Pressure Wet Dry 0 .5 0.66 1.0 Stress Joint Set Number, Jn Joint Water Pressure, Jw Block Exc Inflow 12 6 4 Planar 3 2 1 Undulating 0. 05 0 .5 Disc Strength Joint Roughness Number (Least Favorable), Jr Fills 9 0 .5 1.0 1 .5 1 .5 2. 0 Thin Fills 3.0 4.0 Coat Unfill 20 13 12 10 8 6 5 12 8 6 4 4 3 2 1 0. 75 Squeeze 0 .2 Swell 0.33... Factor Qw 5. 0Q 2. 5Q 1.0Q Barton et al (1974) further suggested that if the number of joint sets is less than three, equations (5. 2) and (5. 3) are expressed as equations (5. 4a) and (5. 4b), respectively pv = ph = 1 /2 0 .2 · Jn · Q−1/3 3 · Jr 1 /2 0 .2 · Jn · Q1/3 w 3 · Jr (5. 4a) (5. 4b) They felt that the short-term support pressure can be obtained after substituting 5Q in place of Q in equation (5. 2) Thus,... (10%) 25 12 1 .5 4 0.66 7 .5 Most typical value (60%) 65 9 3 2 1 5 Maximum value (30%) 85 – 4 1 1 2 .5 Weighted average 67 9. 42 2. 05 1.9 0.966 4 .5 observed parameter, i.e., 10% poorest, 60% most typical, 30% best or maximum value, since the weighted average from all the histograms masks the extreme values For example, the values of Q parameters collected at a location are shown in the following Table 5. 8 . 0 .2 0.3 0.1–0 .2 < 0.1 4. Angle of internal friction of rock mass > 45 ◦ 35 ◦ – 45 ◦ 25 ◦ – 35 ◦ 15 ◦ – 25 ◦ < 15 ◦ 5. Allowable bearing pressure (T/m 2 ) 600–440 440 28 0 28 0–1 35 1 35 45 45 30 *. Rating Exceptionally strong > 25 0 8 15 Very strong 100– 25 0 4–8 12 Strong 50 –100 2 4 7 Average 25 50 1 2 4 Weak 5 25 Use of uniaxial compressive strength is preferred 2 Very weak 1 5 As above 1 Extremely. favorable Favorable Fair Unfavorable Very unfavorable Tunnels 0 2 5 −10 − 12 Raft foundation 0 2 −7 − 15 − 25 Slopes* 0 5 − 25 50 −60 * It is recommended to use slope mass rating (SMR) (Singh