ROCK MASS CLASSIFICATION ROCK MASS CLASSIFICATION Components of rock mass classification system 1 # of joint sets 2 3 Engineering definition of a joint • A joint is an obvious feature that is continuous if its length is greater than the width of the excavation or if it abuts against another joint (after Laubscher, 1990) • Joints create blocks Joint descriptions • Trace length • persistence / continuity • Sets • Roughness • rough • smooth • undulating • planar • Frequency • Orientation • Dip and strike / dip direction • Aperture • Infilling • Weathering Joint sets and scanlines Pole plot of discontinuities in a rock mass Defining joint sets Describing joint roughness Classification of discontinuity roughness Class Description I Rough or irregular, stepped II Smooth stepped III Slickensided, stepped IV Rough or irregular, undulating V Smooth, undulating VI Slickensided, undulating VII Rough or irregular, planar VIII Smooth, planar IX Slickensided, planar Joint and excavation orientations Discontinuity aperture description Joint persistence Joint trace length (m) <1 1-3 3-10 ISRM description of persistence Very low Low Medium 10-20 >20 High Very high Rock mass classification systems Why classifications systems • Classify overall rock mass using parameters relevant for the purpose of the classification • Predict performance of rock mass under load and select appropriate support • Common language for communication • Compares conditions with past experience for strategic planning • Groups sections of rock mass into areas of similar characteristics • Enables tracking of changes in rock mass with time • Preliminary basis for design • Estimation of rock mass mechanical properties, e.g Young’s modulus (E), and m and s in Hoek-Brown criterion Problems with classification systems (1/2) • Databases may be limited and bias towards a particular task • Most based on civil engineering experience • Focused on global conditions • Input parameters are very subjective • Tendency to mislead and obscure necessity for detail design Problems with classification systems (2/2) • Joint persistence inadequately accounted for: assume all joints are continuous • Block size assessment problematic • Geology and mineralogy underplayed • Mix external and intrinsic factors Two systems of classification •Single parameter classification systems •Multi-parameter classification systems Single parameter classification system: RQD RQD is a measure of block size • Significance • Component of most multiparameter systems • Problems • depends on direction but currently implies direction independent • does not account for joint: • weathering • infillings • orientation • high RQD does not always imply competent rock (clay gouge) Other methods for estimating RQD • RQD can be estimated directly in the field from excavation surfaces • Easy to do in scanline surveys • RQD can be estimated from joint frequency measurements in the field • RQD can be obtained from three-dimensional area mapping using the equation: • RQD=115-3.3Jv Where Jv is the number of joints present in cubic meter of rock (i.e. the volumetric joint count) RQD and joint frequency Practical problem! RQD and rock mass rating Example application of RQD as a rock mass quality index Terzaghi’s rock load classification system Overburden weight Movement occurs within this area B1 Terminology Resisting forces transfer most of W1 to abutments Loosened rock Resisting frictional forces Application to tunnel support design Problems with Terzaghi Rock Load Classification System • Too general to • permit objective evaluation of rock quality • Provides no quantitative information on the rock mass properties • Too conservative and only for steel arch support design Lauffer’s Stand-up time classification system Definitions • Stand-up time • length of time which an underground opening will stand unsupported after excavation and scaling. • Active span • largest unsupported span in the tunnel section between the face and the supports. Relationship between active span and stand-up time for different rock mass classes Method is semi-quantitative Rock Structure Rating (RSR) • Wickham et al (1972) described a quantitative method for describing the quality of a rock mass and for selecting appropriate support on the basis of their Rock Structure Rating (RSR) classification. • Most of the case histories, used in the development of this system, were for relatively small tunnels supported by means of steel sets, • Historically this system was the first to make reference to shotcrete support. • It introduced the concept of rating each of the components listed below to arrive at a numerical value of RSR = A + B + C. 1. Parameter A, Geology: General appraisal of geological structure on the basis of: a. Rock type origin (igneous, metamorphic, sedimentary) b. Rock hardness (hard, medium, soft, decomposed) c. Geologic structure (massive, slightly faulted/folded, moderately faulted/folded, intensely faulted/folded). 2. Parameter B, Geometry: Effect of discontinuity pattern with respect to the direction of tunnel drive on the basis of: • Joint spacing. • Joint orientation (strike and dip). • Direction of tunnel drive. 3. Parameter C: Effect of groundwater inflow and joint condition on the basis of: a) Overall rock mass quality on the basis of A and B combined. b) Joint condition (good, fair, poor). c) Amount of water inflow (in gallons per minute per 1000 feet of tunnel). • Note that the RSR classification used Imperial units and these units have been retained in this discussion. • Three tables from Wickham et al (1972 ) are reproduced in Tables 1, 2 and 3. • These tables can be used to evaluate the rating of each of these parameters to arrive at the RSR value (maximum RSR = 100). NOTES: • Dip: flat: 0-20o; dipping: 20-50o; and vertical: 50-90o • Joint condition: good = tight or cemented; fair = slightly weathered or altered; poor = severely weathered, altered or open Rock structure rating system - RSR: Wickham Introduced the system of rating used in most classification systems today Application of RSR to tunnel design and limitations t=1”+Wr/1.25 Rib section • The limitations of the RSR system are: • steel arch supports • data based on small tunnels datum 8WF=8”-deep wide flange I section weighing 31 pounds per foot RMR or Geomechanics Classification system: Sometimes called CSIR system • Parameters • UCS of rock material • RQD • joint spacing • Joint condition • roughness • aperture • persistence • weathering • Groundwater • Relative orientation of joints RMR structure RMR joint persistence rating based on ISRM persistence classification Joint trace length (m) <1 1-3 RMR rating 3-10 10-20 2 1 >20 0 6 4 RMR parameter rating (1/2) (2/2) Evolution of RMR parameter ratings Application of RMR to excavation design RMR support selection Limitations of RMR • UCS rating is insensitive • Double counting by including both joint spacing and RQD • Does not account for • faults • blasting • Whole excavation is assessed en mass • individual excavation surfaces important in mining • Support recommendations by RMR are highly conservative, in particular for mining purposes Geological Strength Index (GSI) •Advantage for estimation of engineering properties of rock masses Geological Strength Index GSI Geological strength Index GSI • GSI can be deduced from RMR from the following equations: For better quality rock masses (GSI>25) GSI RMR1976 OR GSI RMR1989 5 Where groundwater rating and joint orientation adjustments are set to zero for the RMR 1976 OR for RMR 1989, groundwater rating set to 15 and joint orientation adjustment set to zero For poor quality rockmasses (GSI<25 or RMR<25), GSI charts must used as RMR is no longer valid Hoek & Brown parameters and GSI mi values for intact rocks, by rock group Hoek and Brown parameters for the rockmass from GSI (recall: �1 = �3 + ��� �3 + ��� 2 ) GSI 100 m b m i exp 28 For GSI>25: GSI 100 s exp 9 a 0 .5 For GSI<25: s0 and GSI a 0 . 65 200 Mohr-Coulomb parameters from GSI Mohr-Coulomb in effective principal stress space 1 cm k 3 where k is the slope of the line relating 1 and 3 Hence: k 1 sin k 1 and cm 1 sin c 2 cos 1 Hence k=tan 3 Estimation of rock mass properties 1/2 Estimation of rock mass properties 2/2 GSI deficiencies • Inherits deficiencies of RMR • Very qualitative and therefore subjective • block size description • Joint surface qualitative • Joint roughness - inter-block shear strength not included The Mining Rock Mass Rating System • Summary of adjustments • weathering 30-100 • orientation 63-100 • Induced stresses 60-120 • Blasting 80-100 Application of MRMR to stope design Limitation of the system • Major criticism of MRMR is that it is too complicated to use and requires a lot of experience to get it right! Q-System - (TQI or NGI-system) •Q-system (Barton et al. 1974) •Q’-system •Stability graph Q-System • Principal parameters • Rock quality designation (RQD) • Joint set number (Jn) • Joint roughness (Jr) • Joint alteration (Ja) • Joint water reduction factor (Jw) • Stress reduction factor (SRF) (Note: Jr, Ja, Jw are the properties of the critical joint) Combination of factors for rockmass rating • The rock mass is rated according to the following empirical equation: Q=(RQD/Jn)x(Jr/Ja)x(Jw/SRF) • RQD/Jn=average block size • Jr/Ja =joint surface integrity and interblock shear strength • Jw/SRF=active stress state Description of considerations and their ratings in tunnelling quality index (Q system) Classification of the rock mass quality in Q-system Q range Rockmass description 0.001 – 0.01 Exceptionally poor 0.01 – 0.1 Extremely poor 0.1 - 1 Very poor 1-4 Poor 4 – 10 Fair 10 – 40 Good 40 – 100 Very good 100 – 400 Extremely good 400 - 1000 Exceptionally good Limitations of the Q-system • SRF is ambiguous • difficult to determine • Jn is always difficult to determine and often leads to underestimation of rock mass quality • RQD is directional but often used independent of direction • Wrong to include stress and water effects (stress and water to intrinsic rock mass properties - external factors) • Does not account for joint persistence • Assumes all joints are continuous Relationship between Q and RMR Application of Q-system to excavation design Definitions: ESR and De • Excavation Support Ratio (ESR) • accounts for excavation life span • acceptable risk • Equivalent dimension of excavation (De) De=Excavation span, diameter or wall height (m)/ESR Relationship between De and Q Q-system for support design Q-system for support selection Concerns in the use of Q for support design in mining • Too conservative • Insensitive to changes in rock mass quality in hard rock mining where rock mass qualities generally fall within a narrow range • Does not account for new support technologies • spray-on liners e.g. Tekflex • Does not account for relative orientation of excavation with structure • Does not account for blast damage Application of Q-system to stope design •Modified Q (Q’) •Stability graph - Mathews method Modified Q - Q’ • Hard rock mines where open stoping can be used generally dry and Jw=1 • SRF difficult to assess (set to 1) and is replaced with a more realistic factor - Stress factor A • Also wrong to include Jw and SRF in rock mass rating as these are external factors • Hence modified Q defined as: Q’=(RQD/Jn)x(Jr/Ja) Some empirical correlations between classification systems and rock mass mechanical properties Correlation of RMR and Q with E Best fitted Equation For ci<100 GSI 10 40 ci Em 10 100 Stability graph for open stope design 1000 Modified stability number N' • Definitions • stability number N N=Q’xAxBxC • A=stress factor • B=joint orientation factor • C=gravity factor • Hydraulic radius HR=surface area/perimeter • Stability graph is a plot of stability number N against shape factor HR (or S) 100 Stable zone 10 Caving 1 0.1 0 5 10 15 Hydraulic radius HR (m) 20 25 Charts for determining factors A, B and C Problems with stability graph • Did not account for • faults • blast damage • time • tension • complex stope surface geometries • C is not applicable to footwalls • Originally subjective • transition zones • bias towards relaxation
