ELSEVIER GEO-ENGINEERING BOOK SERIES VOLUME 5 Part 4 doc

Norwegian method of tunnelling 135 Table 10.1 Essential features of NMT (Barton et al., 1992). S.No. Features 1. Areas of usual application: Jointed rock giving overbreak, harder end of scale (q c = 3 to 300 MPa) Clay bearing zones, stress slabbing (Q = 0.001 to 10 or more) 2. Usual methods of excavation: Drill and blast, hard rock TBM, hand excavation in clay zones 3. Temporary rock reinforcement and permanent tunnel support may be any of the following: CCA, S(fr) + RRS + B, B + S(fr), B + S, B, S(fr), S, sb, (NONE) * Temporary reinforcement forms part of permanent support * Mesh reinforced shotcrete not used * Dry process shotcrete not used * Steel sets or lattice girders not used, RRS and S(fr) are used in clay zones and in weak, squeezing rock masses * Contractor chooses temporary support * Owner/consultant chooses permanent support * Final concrete lining are less frequently used; i.e., B +S(fr) is usually the final support 4. Rock mass characterization for: * Predicting rock mass quality * Predicting support needs * Updating both during tunnelling (monitoring in critical cases only) 5. The NMT gives low costs and * Rapid advance rates in drill and blast tunnels * Improved safety * Improved environment Notations: CCA = cast concrete arches; S(fr) = steel fiber reinforced shotcrete; RRS = reinforced ribs of shotcrete; B =systematic bolting; S = conventional shotcrete; sb = spot bolts; NONE =no support needed. 10.3 DESIGN OF SUPPORTS The Q-value is related to the tunnel support requirements with the equivalent dimensions of the excavation. The relationship between Q and the equivalent dimension of an exca- vation determines the appropriate support measures as depicted in Fig. 10.1. Barton et al. (1974) have identified 38 support categories (Fig. 10.1) and specified permanent sup- ports for these categories. The bolt length l, which is not specified in the support details, 136 Tunnelling in weak rocks can be determined in terms of excavation width, B in meters using the following equations of Barton et al. (1974). l = 2 +(0.15 B/ESR), m for pre-tensioned rock bolts in roof (10.1) l = 2 +(0.15 H /ESR), m for pre-tensioned rock bolts in walls of height (H ) (10.2) and l = 0.40 B/ESR, m for the untensioned rock anchors in roof (10.3) l = 0.35 H/ESR, m for the untensioned rock anchors in walls (10.4) Table 10.2 (Barton et al., 1974) suggests the type of bolt, its spacing and the thickness of conventional shotcrete for a given rock mass quality Q, equivalent span B/ESR, RQD/J n and J r /J a values. For design of wall support system of a cavern, Q should be replaced by Q w . In case of shaft, Q w may be used for designing the support system for equivalent span (or diameter or size of shaft/ESR) and corresponding bolt length from equations (10.1) or (10.3) (Barton, 2001). Many supplementary notes are given at the end of Table 10.2. Other practical recommendations on shotcrete are compiled in Table 10.3. It should be realized that shotcrete lining of adequate thickness and quality is a long- term support system. This is true for rail tunnels also. It must be ensured that there is a good bond between shotcrete and rock surface. Tensile bending stresses are not found to occur even in the irregular shotcrete lining in the roof due to a good bond between shotcrete and the rock mass in an arched-roof opening. Rock bolts help in better bonding. Similarly, contact grouting is essential behind the concrete lining to develop a good bond between the lining and rock mass to arrest its bending. However, bending stresses may develop in lining within the faults. Rock has ego (Extraordinary GeologicalOccurrence) problems. As such, wherecracks appear in the shotcrete lining, more layers of shotcrete should be sprayed. The opening should also be monitored with the help of borehole extensometers at such locations par- ticularly in the squeezing ground. If necessary, expert tunnel engineers should be invited to identify and solve construction problems. At this point in time, NTM does not suggest the tunnel instrumentation in hard rocks, unlike NATM. In the over-stressed brittle hard rocks, rock anchors should be installed to make the reinforced rock arch a ductile arch. Thus, a mode of failure is designed to be ductile from the brittle failure. Hence, failure would be slow giving enough time for local strengthening (or retrofitting) of the existing support system. 10.4 DESIGN OF STEEL FIBER REINFORCED SHOTCRETE Wet process SFRS has the following advantages (Barton et al., 1992). (i) high application-capacity rate upto 25 m 3 per hour, (ii) efficient reinforcement, Table 10.2 Recommended support based upon NGI rock mass quality Q (Hoek & Brown, 1980). Rock mass quality Q or Q w or Q av or Q seismic Equivalent dimension (Span/ESR) Block size (RQD/J n ) Inter-block strength (J r /J n ) Approx. support pressure (p roof ), MPa Spot reinforcement with untensioned grouted dowels Untensioned grouted dowels on grid pattern Tensioned rock bolts on grid with spacing Chainlink mesh anchored to bolts at intermediate points Shotcrete applied directly to rock, thickness indicated Shotcrete reinforced with weld-mesh, thickness indicated Unreinforced cast concrete arch, thickness indicated Steel reinforced cast concrete arch, thickness indicated Notes by Barton et al. (1974) Notes by Hoek and Brown (1980) 1000–400 20–100 <0.001 ≤ 1a 400–100 12–88 0.005 ≤ 1a 100–40 8.5–19 >20 0.025 ≤ 1a 100–40 8.5–19 >20 0.025 2.5–3 m 100–40 14–30 >30 0.025 2–3 m 100–40 14–30 >30 0.025 1.5–2 m ≤ b 100–40 23–72 >30 0.025 2–3 m 100–40 23–72 >30 0.025 1.5–2 m ≤ b 40–10 5–14 >10 >1.5 0.05 ≤ 2 40–10 5–14 >10 <1.5 0.05 1.5–2 m 2 40–10 5–14 >10 >1.5 0.05 1.5–2 m 2 40–10 5–14 >10 <1.5 0.05 1.5–2 m 20–30 mm 2 40–10 15–23 >10 0.05 1.5–2 m ≤ 2,3 b 40–10 15–23 >10 0.05 1.5–2 m 50–100 mm 2,3 c 40–10 9–15 0.05 1.5–2 m ≤ 2,4 b 40–10 15–40 >10 0.05 1.5–2 m ≤ 2,3,5 b 40–10 15–40 >10 0.05 1.5–2 m 50–100 mm 2,3,5 c Continued Table 10.2—Continued Rock mass quality Q or Q w or Q av or Q seismic Equivalent dimension (Span/ESR) Block size (RQD/J n ) Inter-block strength (J r /J n ) Approx. support pressure (p roof ), MPa Spot reinforcement with untensioned grouted dowels Untensioned grouted dowels on grid pattern Tensioned rock bolts on grid with spacing Chainlink mesh anchored to bolts at intermediate points Shotcrete applied directly to rock, thickness indicated Shotcrete reinforced with weld-mesh, thickness indicated Unreinforced cast concrete arch, thickness indicated Steel reinforced cast concrete arch, thickness indicated Notes by Barton et al. (1974) Notes by Hoek and Brown (1980) 40–10 30–65 >15 0.05 1.5–2 m ≤ 2,6,7,13 b 40–10 30–65 >15 0.05 1.5–2 m 50–100 mm 2,6,7,13 c 10–4 3.5–9 >30 0.10 ≤ 2a 10–4 3.5–9 >10<30 0.10 1–1.5 m 2 10–4 6–9 <10 0.10 1–1.5 m 20–30 mm 2 10–4 <6 <10 0.10 20–30 mm 2 10–4 10–15 >5 0.10 1–1.5 m ≤ 2,4 a 10–4 7–10 >5 0.10 1–1.5 m ≤ 2a 10–4 10–15 <5 0.10 1–1.5 m 20–30 mm 2,4 10–4 7–10 <5 0.10 1–1.5 m 20–30 mm 2 10–4 20–29 0.10 1–2 m 100–150 mm 2,3,5 c 10–4 12–20 0.10 1–1.5 m 50–100 mm 2,3 c 10–4 35–52 0.10 1–2 m 200–250 mm 2,6,7,13 c 10–4 24–35 0.10 1–2 m 100–200 mm 2,3,5,13 c 4–1 2.1–6.5 >12.5 <0.75 0.15 1 m 20–30 mm 2 4–1 2.1–6.5 >12.5 <0.75 0.15 20–30 mm 2 4–1 2.1–6.5 <0.75 0.15 1 m 2 4–1 4.5–11.5 >10 <30>1 0.15 1 m ≤ 2a 4–1 4.5–11.5 <10 >1 0.15 25–75 mm 2 4–1 4.5–11.5 <30 <1 0.15 1 m 25–50 mm 2 c 4–1 4.5–11.5 >30 0.15 1 m 2 4–1 15–24 0.15 1–1.5 m 100–150 mm 2,3,5,8 c 4–1 8–15 0.15 1–1.5 m 50–100 mm 2 c 4–1 30–46 0.15 1–1.5 m 150–300 mm 2,6,7,13 c 4–1 18–30 0.15 1–1.5 m 100–150 mm 2,3,5 c 1–0.4 1.5–4.2 >10 >0.5 0.225 1 m ≤ 2d 1–0.4 1.5–4.2 <10 >0.5 0.225 1 m 50 mm 2 c 1–0.4 1.5–4.2 <0.5 0.225 1 m 50 mm 2 c 1–0.4 3.2–7.5 0.225 1 m 50–75 mm 14,11,12 c 1–0.4 3.2–7.5 0.225 1 m 25–50 mm 2,10 1–0.4 12–18 0.225 1 m 75–100 mm 2,10 c 1–0.4 6–12 0.225 1 m 50–75 mm 2,10 c 1–0.4 12–18 0.225 1 m 200–400 mm 14,11,12 c 1–0.4 6–12 0.225 1 m 100–200 mm 14,11,12 c 1–0.4 30–38 0.225 1 m 300–400 mm 2,5,6,10,13 c,f 1–0.4 20–30 0.225 1 m 200–300 mm 2,3,5,10,13 c 1–0.4 15–20 0.225 1 m 150–200 mm 1,3,10,13 c 1–0.4 15–38 0.225 1 m 300 mm–1 m 5,9,10,12,13 0.4–0.1 1–3.1 >5 >0.25 0.3 1 m 20–30 mm 0.4–0.1 1–3.1 <5 >0.25 0.3 1 m 50 mm c 0.4–0.1 1–3.1 <0.25 0.3 1 m 50 mm c 0.4–0.1 2.2–6 >5 0.3 1 m 25–50 mm 10 c 0.4–0.1 2.2–6 <5 0.3 50–75 mm 10 c 0.4–0.1 2.2–6 0.3 1 m 50–75 mm 9,11,12 c 0.4–0.1 4–14.5 >4 0.3 1 m 50–125 mm 10 c 0.4–0.1 4–14.5 <4>1.5 0.3 75–250 mm 10 c 0.4–0.1 4–14.5 <1.5 0.3 1 m 200–400 mm 10,12 c 0.4–0.1 4–14.5 0.3 1 m 300–500 mm 9,11,12 0.4–0.1 20–34 0.3 1 m 400–600 mm 3,5,10,12,13 f 0.4–0.1 11–20 0.3 1 m 200–400 mm 4,5,10,12,13 c 0.4–0.1 11–34 0.3 1 m 400 mm–1.2 m 5,9,11,12,13 Continued Table 10.2—Continued Rock mass quality Q or Q w or Q av or Q seismic Equivalent dimension (Span/ESR) Block size (RQD/J n ) Inter-block strength (J r /J n ) Approx. support pressure (p roof ), MPa Spot reinforcement with untensioned grouted dowels Untensioned grouted dowels on grid pattern Tensioned rock bolts on grid with spacing Chainlink mesh anchored to bolts at intermediate points Shotcrete applied directly to rock, thickness indicated Shotcrete reinforced with weld-mesh, thickness indicated Unreinforced cast concrete arch, thickness indicated Steel reinforced cast concrete arch, thickness indicated Notes by Barton et al. (1974) Notes by Hoek and Brown (1980) 0.1–0.01 1–3.9 >2 0.6 1 m 25–50 mm 10 c 0.1–0.01 1–3.9 <2 0.6 50–100 mm 10 c 0.1–0.01 1–3.9 0.6 75–150 mm 9,11 c 0.1–0.01 2–11 >2 >0.25 0.6 1 m 50–75 mm 10 c 0.1–0.01 2–11 <0.25 0.6 150–250 mm 10 c 0.1–0.01 2–11 0.6 1 m 200–600 mm 9,11,12 0.1–0.01 15–28 0.6 1 m 300–1 m 3,10,12,13 c,f 0.1–0.01 15–28 0.6 1 m 600 mm–2 m 3,9,11,12,13 0.1–0.01 6.5–15 0.6 1 m 200–750 mm 4,10,12,13 c,f 0.1–0.01 6.5–15 0.6 1 m 400 mm–1.5 m 3,9,11,12,13 0.01–0.001 1–2 1.2 100–200 mm 10 c 0.01–0.001 1–2 1.2 0.5–1 m 100–200 mm 9,11,12 c 0.01–0.001 1–6.5 1.2 200–600 mm 10 c,f 0.01–0.001 1–6.5 1.2 0.5–1 m 200–600 mm 9,11,12 c,f 0.01–0.001 10–20 1.2 1–3 m 10–14 0.01–0.001 10–20 1.2 1 m 1–3 m 3,9,11,12,14 0.01–0.001 4–10 1.2 700 mm–2 m 10,14 c,f 0.01–0.001 4–10 1.2 1 m 700 mm–2 m 4,9,10,11,14 c,f Norwegian method of tunnelling 141 Table 10.3 Summary of recommended shotcrete applications in tunnelling, for different rock mass conditions. Rock mass description Rock mass behavior Support requirement Shotcrete application Massive metamorphic or igneous rock. Low stress conditions No spalling, slabbing or failure None None Massive sedimentary rock. Low stress conditions Surfaces of some shales, siltstones, or claystones may slake as a result of moisture content change Sealing surface to prevent slaking Apply 25 mm thickness of plain shotcrete to permanent surfaces as soon as possible after excavation. Repair shotcrete damage due to blasting Massive rock with single wide fault or shear zone Fault gouge may be weak and erodible and may cause stability problems in adjacent jointed rock Provision of support and surface sealing in vicinity or weak fault or shear zone Remove weak material to a depth equal to width of fault or shear zone and grout rebar into adjacent sound rock. Weld mesh can be used if required to provide temporary rockfall support. Fill void with plain shotcrete. Extend steel fiber reinforced shotcrete laterally for at least width or gouge zone Massive metamorphic or igneous rock. High stress conditions Surface slabbing, spalling and possible rockburst damage Retention of broken rock and control of rock mass dilation Apply 50 mm shotcrete over weld mesh anchored behind bolt faceplates, or apply 50 mm of steel fiber reinforced shotcrete on rock and install rock bolts with faceplates; then apply second 25 mm shotcrete layer Extend shotcrete application down sidewalls where required Continued 142 Tunnelling in weak rocks Table 10.3—Continued Rock mass description Rock mass behavior Support requirement Shotcrete application Massive sedimentary rock. High stress conditions Surface slabbing, spalling and possible squeezing in shales and soft rocks Retention of broken rock and control of squeezing Apply 75 mm layer of fiber reinforced shotcrete directly on clean rock. Rock bolts or dowels are also needed for additional support Metamorphic or igneous rock with a few widely spaced joints. Low stress conditions Potential for wedges or blocks to fall or slide due to gravity loading. Provision of support in addition to that available from rock bolts or cables Apply 50 mm of steel fiber reinforced shotcrete to rock surfaces on which joint traces are exposed Sedimentary rock with a few widely spaced bedding planes and joints, low stress conditions Potential for wedges or blocks to fall or slide due to gravity loading. Bedding plane exposures may deteriorate in time Provision of support in addition to that available from rock bolts or cables. Sealing or weak bedding plane exposures Apply 50 mm of steel fibre reinforced shotcrete on rock surface on which discontinuity traces are exposed, with particular attention to bedding plane traces Jointed metamorphic or igneous rock. High stress conditions Combined structural and stress controlled failures around opening boundary Retention of broken rock and control of rock mass dilation Apply 75 mm plain shotcrete over weld mesh anchored behind bolt faceplates or apply 75 mm of steel fiber reinforced shotcrete on rock, install rock bolts with faceplates and then apply second 25 mm shotcrete layer Thicker shotcrete layers may be required at high stress concentrations Bedded and jointed weak sedimentary rock. High stress conditions Slabbing, spalling and possibly squeezing Control of rock mass failure and squeezing Apply 75 mm of steel fiber reinforced shotcrete to clean rock surfaces as soon as possible, install rock bolts, with faceplates, through shotcrete, apply second 75 mm shotcrete layer Norwegian method of tunnelling 143 Highly jointed metamorphic or igneous rock. Low stress conditions Revelling or small wedges and blocks defined by intersecting joints Prevention of progressive ravelling Apply 50 mm of steel fiber reinforced shotcrete on clean rock surface in roof of excavation Rock bolts or dowels may be needed for additional support for large blocks Highly jointed and bedded sedimentary rock. Low stress conditions Bed separation in wide span excavations and revelling or bedding traces in inclined faces Control of bed separation and ravelling Rock bolts or dowels required to control bed separation Apply 75 mm of fiber reinforced shotcrete to bedding plane traces before bolting Heavily jointed igneous or metamorphic rock, conglomerates or cemented rock fill. High stress conditions Squeezing and “plastic” flow of rock mass around opening Control of rock mass failure and dilation Apply 100 mm of steel fiber reinforced shotcrete as soon as possible and install rock bolts, with faceplates, through shotcrete. Apply additional 50 mm of shotcrete if required. Extend support down sidewall if necessary Heavily jointed sedimentary rock with clay coated surfaces. High stress conditions Squeezing and “plastic” flow of rock mass around opening. Clay rich rocks may swell Control of rock mass failure and dilation Apply 50 mm of steel fiber reinforced shotcrete as soon as possible, install lattice girders or light steel sets, with invert struts where required, then more steel fiber reinforced shotcrete to cover sets or girders. Forepoling or spiling may be required to stabilize face ahead of excavation Mild rockburst conditions in massive rock subjected to high stress conditions Spalling, slabbing and mild rockbursts Retention of broken rock and control of failure propagation Apply 50 to 100 mm of shotcrete over mesh or cable lacing which is firmly attached to the rock surface by means of yielding rock bolts or cablebolts 144 Tunnelling in weak rocks (iii) lesser rebound in the range of 5–10% which is lower than that in the dry process, (iv) uniform and high quality SFRS, (v) less dust than in dry process, (vi) no mesh is needed and so no air gaps behind shotcrete, (vii) low permeability due to low water–cement ratio, (viii) no corrosion of short-stainless steel fibers and (ix) cost-effective in long tunnels or large caverns. However, technology calls for skilled workers, engineering geologists and rock engineers. It can be noted that compression structures have longer life than the tension structures. The analysis shows that the shotcrete with good bond with the homogeneous rock mass is likely to be in compression in the tunnels with arched roof. Thus structures may have long life upto 60 years in dry rock masses. Since the early 1980s, wet mix steel fiber reinforced shotcrete (SFRS) together with rock bolts have been the main components of a permanent rock support in underground openings in Norway. Based on the experience, Grimstad and Barton (1993) suggested a different support design chart using the SFRS on the basis of 1260 case records as shown in Fig. 10.2. This chart is recommended for tunnelling in poor rock conditions and moderate squeezing ground conditions also. Shear zones are encountered in the underground openings specially in the tectonically disturbed geological conditions. The average value of rock mass quality Q av is estimated as suggested by Bhasin et al. (1995) in Section 28.7. This value is then used in Table 10.2 and Fig. 10.2 for designing the support system in the neighborhood of shear zones. In fact, the rock masses are classified into various grades I, II, III, etc. at the tunnel projects. The drawings of temporary and or permanent support systems are prepared for all grades in advance of tunnelling. This is called flexible and robust planning strategy. Thus, all that is needed is on-the-spot decision of choice of the support system according to actual tunnelling conditions. Supplementary notes by Barton et al. (1974): (i) The type of support used in extremely good and exceptionally good rock will depend upon the blasting technique. Smooth wall blasting and thorough scaling- down may remove the need for support. Rough wall blasting may result in the need for a quick single application of shotcrete, especially where the excavation height exceeds 25 m. (ii) For cases of heavy rock bursting or “popping,” tensioned bolts with enlarged bearing plates often used, with spacing of about 1 m (occasionally 0.8 m). Final support is installed when “popping” activity ceases. (iii) Several bolt lengths often used in same excavation, i.e., 3, 5 and 7 m. (iv) Several bolt lengths often used in same excavation, i.e., 2, 3 and 4 m. [...]... 100 Exceptionally poor 50 Very poor Extremely poor ing in ac olt sp B a d are 1.2m 1.5m 1.7m V good Good Ext good Exc good 2.5m 2.1m crete shot Fair Poor 2.3m 11.0 1.3m 7.0 1m 5. 0 8) 9) 6) 7) 5) 10 1 2.7m 4c m 1 m 2c 5c m 25 m 5c cm 1m 0.01 0.1 otc sh 1.6m un in ing ac 1.3m sp lt Bo 10.0 1.0 ed ret 2.0m 2 1 0.001 1) 2) 3) 4) 3.5m 9c m Equivalent Dimension = 20 5 20.0 a are 3.0 2 .4 Bolt Length in m for... formations (Table 11.8) 5 Depth of a Round, m 4 3 2 1 0 0 10 20 Cross-sectional Area of Drift, 30 m2 Fig 11.9 Depth of a blast round vs tunnel size in Germany (Ziegler, 19 85) 40 Blasting for tunnels and roadways 169 Table 11.8 Number of holes per round (Whittaker & Frith, 1990) Tunnel cross section (m2 ) Number of holes per round Weak Strong 10 25 50 23–27 45 50 75 85 36 50 60–70 95 110 11.8.3 Explosives... overburden f = 1 + ( 45 0 −320)/800 = 1.16 The correction for tunnel closure f ′ = 1.0 Short-term support pressure in roof from equation (5. 6) is (f ′′ = 1) = (0.2/1 .5) (5 × 8)−1/3 1.16 = 0.0 45 MPa Short-term wall support pressure is = (0.2/1 .5) (5 × 2 .5 × 8)−1/3 1.16 = 0.033 MPa (practically negligible) Ultimate support pressure in roof from equation (5. 10) is given by proof = (0.2/1 .5) (8)−1/3 1.16 = 0.077... parallel cut blasting in different tunnels (Chakraborty, 2002) S No Formation qc (MPa) 1 Coal 23 2 3 Rock 1 Rock 2 29.9 18 46 RQD Closely jointed, 10 (approx) 36 40 . 75 91 .4 Vp (m/s) Void by relief holes in percent of cut area Pull (%) 9 54 6–7 0.88 2800 2910–7690 8.7 10 0. 75 0. 75 156 Tunnelling in weak rocks that in all the cases, the rate of reduction in both the specific charge and the specific drilling... 1 45 1 .5 100 1000 Jr Jw RQD Rock Mass Quality Q = _ × _ × _ J J n a SRF REINFORCEMENT CATEGORIES 1) 2) 3) 4) Unsupported Spot bolting, sb Systematic bolting, B Systematic bolting (and unreinforced 4 to10cm, B(+S)shotcrete, 5) Fiber reinforced shotcrete and bolting, 5 to 9cm, S(fr)+B 6) Fiber reinforced shotcrete and bolting, 9 to 12cm, S(fr)+B 7) Fiber reinforced shotcrete and bolting, 12 to 15cm,... equation (5. 10) is given by proof = (0.2/1 .5) (8)−1/3 1.16 = 0.077 MPa Ultimate wall support pressure is (see Section 5. 6) given by pwall = (0.2/1 .5) (2 .5 × 8)−1/3 1.16 = 0. 057 MPa The modulus of deformation of the rock mass is given by equation (5. 13), Ed = (8)0.36 ( 45 0 )0.2 = 7.0 GPa 150 Tunnelling in weak rocks The ESR is 1.0 for important structures Fig 10.2 gives the following support system in the... causing parting between two different rock units or lithological boundaries between similar rock types Engineers International Inc modified basic RMR (MBR) considering blast-induced damage adjustments, as shown in Table 11 .4, were suggested for planning of caving mine drift supports (Bieniawski, 19 84) Chapter 4 in this book defines basic RMR Table 11 .4 Blast damage adjustments in MBR (After Bieniawski, 19 84) ... consideration 1 Specific Charge, kg/m3 5 2 3 4 3 2 d = 51 mm 1 0 d = 38mm 0 20 40 60 80 100 Tunnel Area, m2 1 – Steeply reducing zone, 2 – Moderately reducing zone and 3 – Mildly reducing zone Fig 11.7 Specific charge vs tunnel area (Olofsson, 1988) [d = blast hole diameter] Blasting for tunnels and roadways 161 1 Specific Drilling, m/m3 8 2 3 6 4 d = 38mm 2 d = 51 mm 0 0 20 40 60 80 100 Tunnel Area, m2 (1... The joint roughness number Jr is 1 .5 and the joint alteration number Ja is 1.0 for critically oriented joint in the underground machine hall The width of the cavern is 25 m, height is 50 m and the roof is arched The overburden is 45 0 m Suggest a design of support system The average rock mass quality is (6×10)1/2 = 8 (approx) The overburden above the crown is less than 350 (8)1/3 = 700 m Hence the rock... charge for breaking the rock against a free face in kg/m3 , 1 .5 1.2 if Protodyakonov Index is 20– 15 1.2–1.0 if Protodyakonov Index is 15 10 1.0–0.8 if Protodyakonov Index is 10–8 0.8–0.6 if Protodyakonov Index is 8 4 (11.1) Blasting for tunnels and roadways 163 = = st = = = = f = = A = swr = def = = = = 0.6–0.2 if Protodyakonov Index is 4 2 0. 15 if Protodyakonov Index is . 4 .5 11 .5 <10 >1 0. 15 25 75 mm 2 4 1 4 .5 11 .5 <30 <1 0. 15 1 m 25 50 mm 2 c 4 1 4 .5 11 .5 >30 0. 15 1 m 2 4 1 15 24 0. 15 1–1 .5 m 100– 150 mm 2,3 ,5, 8 c 4 1 8– 15 0. 15 1–1 .5 m 50 –100 mm 2 c 4 1. m 25 50 mm 10 c 0 .4 0.1 2.2–6 < ;5 0.3 50 – 75 mm 10 c 0 .4 0.1 2.2–6 0.3 1 m 50 – 75 mm 9,11,12 c 0 .4 0.1 4 14 .5 > ;4 0.3 1 m 50 –1 25 mm 10 c 0 .4 0.1 4 14 .5 < ;4& gt;1 .5 0.3 75 250 mm 10 c 0 .4 0.1. >1 .5 0. 05 1 .5 2 m 2 40 –10 5 14 >10 <1 .5 0. 05 1 .5 2 m 20–30 mm 2 40 –10 15 23 >10 0. 05 1 .5 2 m ≤ 2,3 b 40 –10 15 23 >10 0. 05 1 .5 2 m 50 –100 mm 2,3 c 40 –10 9– 15 0. 05 1 .5 2 m ≤ 2 ,4 b 40 –10