Rock mass classification and support/slope design in metalliferous mines

/ Government of India /Ministry of Labour & Employment /Directorate General of Mines safety BRIC DGMS Circular No.1Eof 2026 To Dhanbad, dated ag/ 08 / 2026 All the Owners, Agents and Managers of Metalliferous Mines Subject: Guidelines for use of Rock Mass Classifications and Support/Slope Design for Improving Stability of Workings in Metalliferous Mines.

Sir, Stability of workings is essential for economic and safe exploitation of mineral from mines. The geotechnical assessment, safe support/slope design and monitoring of stability of workings is one of the major challenges in Indian Metalliferous Mines. The stability analysis of mine workings, support/slope design and their monitoring is not yet an integral part of mine planning. In Indian mining conditions, support/slope design guidelines are yet to be formulated for different types of mining practices for maintaining safety and productivity.

However, there is a lot of diversity amongst different mines in understanding and practicing for Rock Mass Classification systems and Support/Slope Designs. Therefore, it was felt necessary to evolve a broad standardization/ guideline for use of Rock Mass Classifications and Support/Slope Design for Improving Stability of Workings in Metalliferous Mines.

In view of the above, a workshop on the subject "Rock Mass Classifications and Support Design for Improving Stability of Workings in Metal Mines" was jointly organised by DGMS and IIT Kharagpur on 28th November 2025 in Jaipur, Rajasthan to deliberate and review the present status of use of different types of Rock Mass Classification systems and Support/Slope Designs. In this workshop standard guidelines are finalized for use of Rock Mass Classifications and Support/Slope Design methodologies for Improving Stability of Workings in Metalliferous Mines. This workshop was attended by about 80 officials and experts from regulatory authorities, academic institutions, research organizations, and various mining industries like DGMS, IITs, NITS, NIRM, CIMFR, M/s HZL, M/s HCL,M/s MOIL, M/s UCIL, M/s IMFA, M/s OMC, M/s Birla Corporation Ltd., M/S Jindal Saw Ltd., M/s JK Cement, M/s JSW Cement, M/s Wonder Cement Ltd., M/s Ultratech Cement, M/s Shree Cement Ltd., M/s Mangalam Cement Ltd., M/s Nuvoco Vistas Corporation Limited, M/s RK Marble Limited, M/s Shree Cement Limited, M/s UMDS Pvt. Ltd., etc., in which 10 technical presentations were made by different experts giving valuable inputs on the subject matter.

Based on various inputs drawn from the detailed deliberations and discussions made in the workshop, the following guidelines are being made for use of Rock Mass Classifications and Support/Slope Design for Improving Stability of Workings in Metalliferous Mines.

1. GSI and Q values shall be calculated by practicing geologists and mining engineers at multiple locations and levels based on the complexity of the geology and geotechnical domain within underground mines. Structured training shall be provided to these personnel for the correct application of these systems.

2. It is time for the Indian metal mining sector to adopt Q and GSI as standard systems for rock mass classification and support design. RMR shall gradually be replaced by Q or GSI, particularly in underground mines and shall be incorporated in technical and scientific study reports, A step-by-step support design procedure using the Q system is provided in Appendix A, and the application of GSI in bolt design and numerical modelling is outlined in Appendix B.

3. The NGI support design chart shall be used for determining support requirements in drives, drifts, cross-cuts, and other underground excavations. Installed support systems may initially be benchmarked against NGI recommendations, and if any discrepancies found, it shall be scientifically investigated and rectified.

4. In slope stability analysis, overall slope stability shall be assessed using rock mass properties-such as cohesion and friction angle-adjusted through GSI values measured at different slope locations depending on the complexity of the geology and geotechnical domain. Q-slope method or Slope Mass Rating (SMR) shall be used to estimate rock mass quality and bench slope angles to validate the slope stability analysis. A step-by-step methodology is provided in Appendix C for Q-slope and in Appendix D for SMR.

5. Scientific organisations, academic institutions, and mining companies shall adopt Q- or GSI-based approaches for adjusting rock mass properties to replicate field conditions in numerical models for bringing uniformity in stability assessment and generate data to develop support/slope design charts for site specific conditions.

6. Periodical/ real-time support performance monitoring like load—displacement, pillar stresses and roof displacements is crucial for understanding rock mass behaviour and validating numerical model outcomes. It shall be carried out at various locations near the active stopes, junctions, geologically disturbed areas and a Trigger Action Response Plan (TARP) shall be developed based on numerical and monitoring data.

7. Slope bench movement shall be monitored regularly or in real time to generate heat maps of displacements and validate the slope stability analysis. Zones showing major displacement ("hot zones") shall be identified and TARP shall be developed based on numerical and monitoring data.

8. Mine management shall establish a dedicated Geotechnical Cell, comprising trained geologists and mining engineers with expertise in rock mechanics, ground control, and support/slope design. This team shall be regularly kept updated in the application of GSI, Q, Q-slope and SMR methods, stability analysis of stopes/slopes and monitoring to develop robust mine/support / slope design.

9. In the unorganized sector, small mine owners often lack access to advanced technology, skilled manpower, and proper safety systems. They shall seek technical guidance from nearby mines in the organised sector or any other experts in the concerned field.

All the Owners, Agents and Managers are advised to prepare Strata/Ground Control Management Plan keeping in view of the above-mentioned guidelines.

Ujj I Tah Directorate General of Mines Safety Chief Inspector-cum-Facilitator of Mines Enclosure to DGMS (Tech)/S&T/Circular No.

APPENDIX-A:

UNDERGROUND SUPPORT DESIGN BASED ON Q Step I: Estimate Q values at multiple locations in the area

a) Estimation of parameters based on the tables given in the references (NGI Publication , Hoek et a/., 19982, Edelbro, 20033, Deb and Verma, 20254). The parameters are i) RQD = Joint set number, iii)Jr = Joint roughness number, — Rock Quality Designation, ii) Jn — Joint alteration number, v) Joint water reduction factor and vi) SRF Stress

iv) Ja ¯ Reduction Factor. Refer Appendix E for the description and numerical values of each parameter.

RQD Jr

b) The value of Q Ja SRF Step 2: Adjustment of Span or Height based on Excavation Support Ratio (ESR)

a) Selection of Excavation Support Ratio (ESR) based on excavation category (Table 1).

Span or height in m

b) The Equivalent Dimension De = ESR A C D F G Table l. ESR values for different excavation categories (NGIpublication, 2025)

Type underground facility Temporary mine openings, etc.

Vertical shafts*: i) circular sections

ii) rectangular/square sections * Dependant of purpose. May be lower than given values.

Permanent mine openings, water tunnels for hydro power (exclude high pressure penstocks), water supply tunnels, pilot tunnels, drifts and headings for large openings.

Minor road and railway tunnels, surge chambers, access tunnels, sewage tunnels, etc.

Power houses, storage rooms, water treatment plants, major road and railway tunnels, civil defence chambers, portals, intersections, etc.

Underground nuclear power stations, railways stations, sports and public facilitates, factories, etc.

Very important caverns and underground openings with a long lifetime, 100 years, or without access for maintenance.

ESR a-5 2,5 Using The Q-System, Rock Mass Classifications and Support Design, NGI, 2025 Hoek E., P.K. Kaiser and W.E. Bawden, 1998, " Support of Underground Excavations in Hard Rock", A.A. Balkema 3 Cartrin Edelbro, 2003, Rock Mass Strength: A Review, Lulea University of Technology, Sweden 4 Deb and Verma, 2025t "Fundamental and Applications of Rock Mechanics", PHI Publisher Step 3: Determination of the length ofa bolt (L)(Reference: Barton et.al., 19745)

2 +0.15B ESR Where, B is the width of the excavation in m.

* Select the excavation category and the corresponding Excavation Support Ratio (ESR)

based on Table l.

Step 4: Determination of the maximum unsupported span (S), m (Reference: Barton et.al., 1974)

S = 2 X ESR X QO•4 Step 5: The maximum permanent roof pressure (proof) in kg/cm2(Reference: Barton et.al., 1974)

2 X (2-1/3 X rcor ¯ Step 6: Estimation of support requirement (NGIpuhlication, 2025)

Based on the estimated Q value and De, the support requirement is derived from the chart in Figure 1.

Step 7: Adjustment of Q Value for Wall Support The support requirements for the wall are not as rigorous as the roof support. The equivalent dimension is calculated based on wall height. Then, the Q value is adjusted based on Table 2 and the Step-2 to Step-6 are followed.

Table 2. Adjustment of Q value for wall support (NGIpublication, 2025)

Dimensioning of wall support In rock masses of good quality For rock masses of poor-fair quality For rock masses of poor quality Watl height > span width Q>IO Multiply Q-values by a factor of 5 Multiply Q-values by a factor of 2.5. In cases of high rock stresses, use the actual Q-value Use actual Q-value Applies for all Use actual Q-value Q-vatues N. Barton, R. Lien, and J. Lunde, 1974, "Engineering Classification of Rock Masses for the Design of Tunnel Support", Rock Mechanics 6, 189--236 by Springer-Verlag 1974 ROCK MASS QUALITY AND ROCK SUPPORT Exceptionally Extremely Very Poor Fair Good I ExtremeW IExcep good good : good poor o.ooa 0.01 poor boli 0.04 0.1 , 2.4 ROD J, Rock mass quality SRF Support categories O Unsupported or spot bolthg Spot bottir,g, SB Systorrotk: bolting. fibre reinforced vroyed concroto. cm BWSfr @ Fibre tehforcod sprayed corcrete ond botthg 6-9 crn Sfr G) Fbre rehtorced sprayed corcrete crud bodhg W12 cro Str (E700)+B (6) Fbre rehforced sprayed comrete and bolthg 12-15 cm reinforced ribs of concrete md bolting str (E700)+RRS I (D Fibro rehforced sprayed corcroto cm rehforcod ribs of sprayed concrete md bolting. str (j cast concrete CCA or str (El OOO).RRS 1114B O Specd evahJ0tion Bolts spachg is molnly based on Ø20 rnrn E Erorgv ob601btion in fibro rentorcod sptc.veci corctoto ESR Support Ratio Areas with dashed lines have no empirical dota RRS - spochg toloted to Q•vduo Si30/6 ø16- ø20 (span 10m)

0@/6+2 20m)

Si35/6 Ø16•20 (span 045/6+2 ø1&20 (spon 10m)

055/6+4 Ø20 (X)m 20m)

055/6+4 ø20 (span to m)

Spockol ovdcntbn 20 m)

SOO/6 = Shgle layer of 6 rebars.

30 cm thickness of sprayed concrete D Doublo layer of retxys Ø16 Rebor diarneter is 16 mm c/c = BS spacing centre - centre Figure I . Support chart for the selection of support requirements based on Qand De(NGI publication, 2025)

APPENDIX-B:

APPLICATIONS OF GSI IN BOLT DESIGN AND NUMERICAL ANALYSIS Step 1: Estimate GSI at Multiple Locations in the Study Area (Reference: V. Marinos P.

Marinas E. Hoek, 2005)

a) Conduct a detailed field survey of rock faces.

b) Estimate the maximum and the minimum GSI at the study area

c) GSI is estimated based on the structure of the rock mass and surface conditions.

d) The GSI chart, as shown in Figure IA, is to be used for the determination of GSI.

e) Ifthe surface is wet, Figure 1B must be used to modify the GSI value GEOLOGICAL STRENGTH INDEX FOR JOINTED ROCKS (Hoek and Marinos. 2000)

From the lithology, structure and surface conditions of the discontinuities. estimate the average value ot G Sl. DO not try to be too precise. Quoting a range from 33 to 37 is more realistic than stating that GSI 35. Note that the table does not apply to structurally controlted failures.

Where weak planar structural planes are present in an unfavourable orientation With respect to the excavation face. these wilt dominate tho rock mass behaviour.

The shear strength of surfaces in rocks that are prone to deterioration as a result Of changes in moisture content will be reduced is water is present. When working With rocks in the fair to very poor categories. a shift to the right may be made for wet conditions. Water pressure is dealt with by effective stress analysis.

s STRUCTURE INTACT OR MASSIVE - intact rock specimens or rnassive in situ rock With few widely spaced discontinuities BLOCKY - interlocked un- disturbed rock mass consisting of cubical blocks formed by three intersecting discontinuity sets VERY BLOCKY- interlocked.

partially disturbed rnass with multi-faceted angular blocks formed by 4 or more joint Sets BLOCKY,'DISTURBEO'SEAMY

- folded with angular blocks formed by many intersecting discontinuity sets. Persistence of bedding planes or schistosity DISINTEGRATED - poorly inter- locked, heavily broken rock mass with mixture Of angular and rounded rock pieces LAMINATED,'SHEARED - Lack Of blockiness due to close spacing Of weak schistosity or shear planes DECREASING SURFACE QUALITY Figure IA. Classification of rock mass based on interlocking and joint conditions (V. Marinos P.

Marinos E. Iloe/c, 20056)

6 V. Marinos P. Marinos E. Hoek, 2005, "The geological strength index: applications and limitations", Bull Eng Geol Environ (2005) 64: 55-65 DOI 10.10071<0064-004-0270-5 GEOLOGICAL STRENGTH INDEX FOR JOINTED ROCKS (Hoek and Marinos, 2000)

From the lithology. structure and surface conditions ot the discontinuities, estimate tha average value of GSI. Do not try to be too precise, Quoting a range from 33 to 37 is more realistic than stating that GSI 35. Note that the table does not apply to structurally controlled failures.

Where weak planar structural planes are present in an unfavourable orientation with respect to the excavation face, these will dominate the rock mass behaviour.

The shear strength of surfaces in rocks that are prone to deterioration as a result Of changes in moisture content will be reduced is water is present. men working with rocks in the fair to very poor categories, a shift to the right may be made for wet conditions. Water pressure is dealt with by effective stress analysis.

STRUCTURE INTACT OR MASSIVE - intact rock specimens or massive in situ rock with few widely spaced discontinuities BLOCKY - well interlocked un- disturbed rock mass consisting of cubical blocks formed by three intersecting discontinuity sets VERY BLOCKY- interlocked, partially disturbed mass with multi-faceted angular blocks formed by 4 or more joint sets BLOCKY/DISTURBED/SEAMY

- folded with angular blocks formed by many intersecting discontinuity sets. Persistence of bedding planes or schistosity DISINTEGRATED - poorly inter- locked, heavily broken rock mass with mixture of angular and rounded rock pieces LAMINATED/SHEARED - Lack of blockiness due to close spacing of weak schistosity or shear planes c DECREASING SURFACE QUALITY Figure I B. A shift towards the right due to wet condition (V. Marinos P. Marinos E. Hoek, 2005)

Step 2: Estimation of Bolt Load and Spacing Step 2.1: For determination of the Length of a Bolt (L), refer to step 3 of Appendix A.

Step 2.2: Determination of Rock Load Height (Ili) (Bieniawski, 19897)

Rock load height in m for the excavation width, R is given by (100 — h 100 )

Ensure that L is at least h: + 0.5 m for proper anchorage. (Note: In the above reference, equation contains RMR. Here, it is replaced by GS/).

Step 2.3: Determination of Rock Load (RL) (Reference: Deb and Verma, 2025)

The number of rock bolts, B as the width of the excavation, and s X s as the square spacing is Rock load per bolt, in kN, is estimated for y,.ock., as the unit weight of a rock in kN/m3aé RL — S2 X X Y rocg Step 2.4: Determination of the Allowable Bolt Load (BL) (Reference: Deb and Verma, 2025)

The allowable bolt load, in klV, for as the yield strength of the bolt rod in kPa, d as the diameter in m, and SF, as the required safety factor, is estimated as rrd2 Where f is the bolt efficiency factor, typically taken between 0.7 and 0.9.

Step 2.5: Determination of Bolt Spacing (Reference: Deb and Verma, 2025)

From the above two equations, the spacing of the bolt is determined as f X X ITå2 4hi X x SF Bieniawski Z.T., 1989, "Engineering Rock Mass Classification: A Complete Manual for Engineers and Geologists in Mining, Civil and Petroleum Engineering, Wiley, New York Step 3: Application of GSI in Numerical Analysis Step 3.1: Determine intact rock properties such as bulk density, compressive and tensile strength, Hoek-Brown parameter, mr, cohesion, friction angle, elastic modulus and Poisson's ratio in the laboratory.

Step 3.2: Adjust the strength, cohesion, friction angle and elastic modulus based on GSI value, mi and blast disturbance factor D, using the procedure outlined in references Hoek et al., 20028 and Deb and Verma, 20259 (page numbers: 72-73 and 265-266).

Step 3.3: In the numerical analysis, apply the adjusted rock mass properties for determining displacements and stresses.

Step 3.4: Perform the analysis using elasto-plastic material models.

Hoek E. , Torres C.C. and Corkum B, 2002, "Hoek and Brown failure criterion", 501 ed. North American Rock Mechanics Sym. And 17th Tunnelling Association of Canada Conf, 267-271 Deb and Verma, 2025, "Fundamental and Applications of Rock Mechanics", PHI Publisher APPENDIX-C:

SLOPE STABILITY BASED ON Q-SLOPE AND NUMERICAL MODELLING Step 1: Estimate Q-Slope values at Multiple Locations in a Bench

i) For each slope bench, estimate Qslope based on six parameters as given below.

ii) Estimation of parameters based on the tables given in the references (Bar and Barton, 202510). The parameters are i) RQD — Rock Quality Designation, ii) Jn — Joint set number, iii)Jr = Joint roughness number, iv) Ja = Joint alteration number, v) J vice Environmental and geological condition number and vi) SRF — Maximum of three stope strength reduction factors a, b, and c, (SRFa: Physical condition number, SRFb'. Stress and strength number and SRFc: Major discontinuity number). Refer Appendix E for the description and numerical value of each parameter.

;wtce

iii) The value of Qstope SRFsIopg

iv) Inter-block shear strength: least favorable or average shear strength in the case of wedges is to be adjusted by the discontinuity orientation factor (O-factor). Refer to Table 1 for the estimation of the O-factor.

Table l. Description of O-factor (Bar and Barton, 2()25)

O-factor description Very favourably oriented Quite favourable Unfavourable Very unfavourable Set A 2.0 1.0 0.75 0.50 Causing failure if unsupported 0.25 Set B (whet? applicable)

1.5 1.0 0.9 0.8 0.5 Step 2: Determination of Bench Slope Angle (Bar and Barton, 2025)

Apply the following equation for the estimation of the bench slope angle in degrees 20 + 65 Q-Slope: Rock Slope Engineering 10 Years on, Rock Mechanics and Rock Engineering (2025) 58: 11453—11474, https://doi.org/10.1007/s00603-024-04064-5 Step 3: Estimation of thc safety factor of the overall slope (Bar and Barton, 2025)

Develop a numerical model of the overall slope using the estimated bench slope angle and the bench geometries, such as width, height and material properties. Friction angle of slope may also be estimated as follows:

p tan¯ Determine the safety factor of the overall slope based on the developed numerical model.

APPENDIX-D:

SLOPE MASS RATING (SMR)

The 'Slope Mass Rating' (SMR) is obtained from RMR, estimated according to Bieniawski's 1979 RMR chart, and then four additional factors are used for the adjustment II 12 Three factors depend on the joint-slope relationships, and the fourth factor is added based on the method of excavation. SMR is calculated as SMR = RMR + (Fl x F2 x F3) + F4 The RMR is computed by adding rating values for five parameters: (i) strength of intact rock (ii)

RQD (iii) spacing of discontinuities (iv) condition of discontinuities and (v) water inflow through discontinuities (estimated in the worst possible conditions). RMR has a range between 0 and 100. Then the adjustment is applied as given below.

Step-I: The factorF1 depends on parallelism between joints and slope face strikes. Its range is from 1.00 (when both are near parallel) to 0.15 (when the angle between them is more than 300)

(Table l). Alternatively, Fl can be computed as sin A)2 Fl - Where, angle,4 denotes the angle between the strikes of the slope face and the joint.

Table 1. Factors Fl,F2 and F3 prr Case lßJl Fs Very Favorable Fair 20-100 0.70 30-350 0.70 > 1200 -25 Unfavorable Very favorable > 300

- 18001 0.15 < 200 0.15 > 100 < 1100 30-20' 0.40 20-300 0.40 10-00 110-1200 10-50 0.85 35-450 0.85 00 to 100 -50 unfavorable 1.00 1.00 P. planc failure; T, toppling raiture; joint dip direction; u. slope dip direction; ly, , jomt dip: slope dip M. Romana, R. Tomås and J. B. Serån, 1985, "Slope Mass Rating (SMR) Geomechanics Classification:

Thirty Years Review" 12M. R. Romana, 1993, "A Geomechanical Classification for Slopes: Slope Mass Rating' , COMPREHENSIVE ROCK ENGINEERING Principles, Practice & Projects, Chief Editor: J.A. Hudson, Volume 3 Step-2: The factor, F2 refers to joint dip angle in the planar mode of failure. Its value varies from 1.00 (for joints dipping more than 450) to 0.15 (for joints dipping less than 200).

Alternatively, F2 can be approximated as F2 = tan2ßj where Dj denotes the joint dip angle. For the toppling mode of failure F2 remains 1.00.

Step-3: The factor, F 3 reflects the relationship between the angle of slope face and joint dip. The values of F3 are shown in Table l. Conditions are fair when slope face and joints are parallel.

When the slope dips 100 more than joints, very unfavourable conditions OCCUr.

Step-4: The adjustment factor for the method of excavation (Factor F4) has been fixed empirically as shown in Table 2.Details of this factor are available in the literature mentioned above.

Method Natural Slope Table 2. Factor F4 Smooth Presplitting blasting +10 Blasting or mechanical o Deficient blasting -8 Step-5: Table 3 provides classes and conditions of slope based on SMR value. Figure I shows tentative support requirements (Romana, 1985).

Table 3. Description of SMR classes Classes SMR Description Stability Failures 0-20 Very bad Completely unstable Big planar soil-like or IV Bad Unstable Planar or wedges 0.6 41-60 Normal 61-80 Good big Partially stable Stable Some joints or Some blocks many wedges Failure probability 0.9 0.4 0.2 81-100 Very good Completely stable None Page II of 18 SMR Drainage Concrete Reinforcement Protection So support o 15 20 eexcavatron Waits Surface drainage Deep drainage Scaling None Shotcrete Dental concrete Ribs and/or beams Toe wall Bolts Anchors Toe ditch Toe or slope fences Nets Figure l. Slope support guidelines based on SMR (Romana, 1985).

APPENDIX-E Tables of Parameters for Q and Q-Slope Table 1: The RQD Classification RQD (0/0)

Rock Quality Very poor 25-50 Poor 50-75 Fair Table2: The Joint Set Number (In)

B c D E H Class Massive, no or few joints One joint set One joint set + random joints Two joint set Two joint set + random joint Three joint set Three joint set + random joints Four or more joint sets Crushed rock Good 0.5-1

(i) For tunnel intersections: 3 x Jn

(ii) For portals: 2 X Jn Table 3: The Joint Roughness Number Class A B C D E F

(a) Rock-wall contact, and (b) rock —wall before IO cm shear Discontinuous joints Rough, undulating Smooth, undulating Slicken sided, undulating Rough, planar Smooth, planar 90-100 Very good 1.5 1.5 G H Slicken sided, planar

(c) No rock-wall contact when sheared Zone containing clay minerals thick enough to prevent rock-wall contact Sandy, gravely or crushed zone thick enough to prevent rock-wall contact I. Add I if the mean spacing of the relevant joint set is greater than 3m.

0.5

2. = 0.5 can be used for planar, slicken sided joints having lineations, provided that the lineations are oriented for minimum strength.

Table 4: The Joint Alteration Number (J a)

B c D E G H KLM N OPR Class

(a) Rock-wall contact (no mineral fillings)

Tightly healed, hard, non-softening, impermeable filling Unaltered joint walls, surface staining only Slightly altered joint walls, non-softening mineral coatings, sandy particles.

Silty or sandy clay coating, small clay friction Softening or low friction clay minerals coating

(b) Rock wall contact IO cm shear Sandy particles, clay-free disintegrated rock Strongly over-consolidated non-softening clay mineral fillings (continuous, <5 mm thickness)

Medium or low over-consolidation, softening, clay mineral fillings (continuous, <5 mm thickness)

Swelling-clay fillings

(c) No rock-wall contact when sheared (thick mineral filling)

Zones or bands of disintegrated or crushed rock and clay Zones or bands of silty or sandy clay, small clay fraction (non- softening)

Thick, continuous zones or bands of clay 25-35 25-30 20-25 8-16 25-30 16-24 12-16 6-12 6-24 6-24 0.75 8-12 6, 8, 12 10,13-20 Table 5: The Joint Water Reduction Number (Ju,)

Class A B C D E Dry excavations or minor inflow, i.e., < 5 Ilmin locally Medium inflow or pressure, occasional outwash of joint fillings Large inflow or high pressure in competent rock with unfilled joints Large inflow or high pressure Exceptionally high inflow or pressure at blasting, decaying with time Exceptionally high inflow or pressure Water pressure (kg/cm2)

1-2.5 2.5-10 2.5-10 Table 6: The Stress Reduction Factor (SRF)

Class 0.66 0.5 0.33 0.2-0. I o. 1-0.05 SRF A B C D E F G

(a) Weakness zones intersecting excavation, which may cause loosening of rock mass when tunnel is excavated Multiple occurrences of weakness zones containing clay or chemically disintegrated rock, very loose surrounding rock, any depth)

Single weakness zones containing clay or chemically rock (depth < 50 m)

Single weakness zones containing clay or chemically disintegrated rock (depth > 50 m)

Multiple shear zones in competent rock (clay-free), loose surrounding rock (any depth)

Single shear zones in competent rock (clay-free, depth < 50 m)

Single shear zone in competent rock (clay-free, >50 m)

Loose open joints, heavily jointed or sugar cube (any depth)

IO 2.5 7.5 2.5 l. Reduce these values of SRF by 25-30% but only if the relevant shear zones influence does not intersect the excavation Class

(b) Competent rock, rock mass problems SRF ac/01 ce/oc H J K L M N O P R S Low stress, favourable stress condition Medium stress, favourable stress condition High stress, very tight pressure. Usually favourable to stability Moderate slabbing after > lh in massive rock Slabbing and rock burst after a few minutes in massive rocks Heavy rock burst and immediate dynamic deformation in massive rock >200 200-10 10-5 0.65-1 0.65-1

(c) Squeezing rock; plastic flow of incompetent rock under the infl uence of high rock pressure Mild squeezing rock pressure Heavy squeezing rock pressure 0.01-0.3 0.3-0.4 50-200 50-200 200-400 ce/oc 1-5 >5

(d) Squeezing rock; chemical swelling activity depending on presence of water Mild swelling rock pressure Heavy swelling rock pressure 2.5 0.5-2 5-3 3-2 5-10 10-20 5-10 10-15 Table 7: The Rock Mass Classification based on Q-System Q-value 400-1000 100-400 40-100 10-40 4-10 1-4 0.1-1 0.01-0.1 0.001-0.01 Class c Rock mass Exceptionally good Extremely good Very good Good Fair Poor Very poor Extremely poor Exceptional ly poor Table 8: Discontinui orientation factor O factor O factor description Very favourably oriented Set A 2.0 set B 1.5 Quite favourable Unfavourable Very unfavourable Causing failure if unsupported 1.0 0.75 0.5 0.25 Table 9: Environmental and geological condition number wice Stable structure; competent rock Stable structure; incompetent rock Unstable structure; competent rock Wet Desert environment environment 1.0 0.7 0.8 Unstable structure; incompetent rock 0.5 0.7 0.6 0.5 0.3 1.0 0.9 0.8 0.5 Tropical storms 0.5 0.3 0.1 0.05 Ice wedging 0.9 0.5 0.3 0.2 a When drainage measures are installed, apply /wice X 1.5, when slope reinforcement measures are installed, apply X 1.3, and when drainage and reinforcement are installed, apply both factors !wice X 1.5 X 113 Table 10: physical condition Description A Slight loosening due to surface location, disturbance from blasting or excavation B Loose blocks, signs of tension cracks and joint shearing, susceptibility to weathering, severe disturbance from blasting C As B, but strong susceptibility to weathering D Slope is in advanced stage of erosion and loosening due to periodic erosion by water and/or ice-wedging effects E Residual slope with significant transport of material downslope SRFa 2.5 Table 11: SRFb : stress and strength Description Moderate stress-strength range F High stress-strength range G Localized intact rock failure H Crushing or plastic yield J Plastic flow of strain softened material K aoc—unconfined compressive strength (UCS), 01 Table 12: SRFc : major discontinuity SRFc oc/af 50-200 10-50 5-10 2.5-5 1-2.5 Unfavourable Very L M Major discontinuity with little or no clay Major discontinuity with oa due to clay KQDID0 and crushed rock Favourable unfavourable SRFb 2.5-1 5-2.5 10-5 15-10 20-15 Ca using failure of unsupported N Major discontinuity wilh — 0b due to clay RQD300 — and crushed rock — 1 m perpendicular sample of discontinuity, RQD300 = 3 m perpendicular sample of discontinuity