Instructional Objectives. Why use mass classification? What is rock mass classification? 3 Pillars of empirical design and rock mass classification

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1 GE 6477 DISCONTINUOUS ROCK 5. Rock Mass Classification and Empirical Design Dr. Norbert H. Maerz Missouri University of Science and Technology (573) Instructional Objectives 1. Explain the rational for using rock mass classifications. 2. List the advantages and disadvantages of empirical design based on rock mass classification. 3. Select and justify which classification system you would use for a) tunneling, b) mining, c) block caving mining, mechanical rock excavation. 4. ustify the approach to rock support that is proposed by the new Austrian tunneling method. 5. Barton s Q system is semi-analytical. Explain. What is rock mass classification? CLASSIFICATION: The formal arrangement of attributes in a hierarchy Why use mass classification? Can t measure what we want to, instead measure while we can and classify Empirical Design Empirical design Uses engineering judgement Uses experience 3 Pillars of empirical design and rock mass classification 1) Description of ground quality by a quantitative classification system, based on parameters that are easily and universally measured. 2) Description of ground performance by a formal set of parameters (unsupported stand time, support requirements, bearing capacity etc.). 3) Correlation of the above 2 based on a broad spectrum of case histories, based on local or global experience. 1

2 Empirical design and rock mass classification method of design Approach ADVANTAGES Incorporates everyone s experience Uses parameters that are easy to measure Provides simple, specified, tested solutions DISADVANTAGES No predictive ability Difficult to extrapolate, safe only in the context for which they were formulated Description of the classification system Dependencies/ Emphasis How used in design How relates to discontinuous rock Classification systems 1. Sonic velocity classification 1. Sonic velocity (1 parameter system) 2. Deere s RQD system 3. Franklin s size-strength classification 4. Franklin s Shale rating system 5. Bieniawski s RMR system 6. Laubshers geomechanics rock mass classification 7. Barton s Q system 2. Deere s RQD system Single parameter system Incorporates (somewhat both size and strength) length _ of _ core _ in _ pieces 100mm RQD(%) 100 length _ of _ borehole Picture(s) from Gonzales de Vallejo and Ferrer 2

3 Deere s system Deere: design chart 3. Franklin s size-strength classification New Austrian Tunneling Method Picture(s) from Gonzales de Vallejo and Ferrer 3

4 Principle of stress relaxation New Austrian Tunneling method, design principles 4. Franklin s Shale rating system Strength / Durability Index Not consider discontinuities Durability in the form a slake durability test Strength from point load test for hard shales Strength from plasticity index for soft shales Slake durability apparatus 5. Bieniawski s RMR system 4

5 What must a rock mass classification do: 1) Divide the rock mass into groups of similar behavior 2) Provide a good basis for understanding the characteristics of the rock mass 3) Facilitate the planning and the design of structures in the rock by yielding quantitative data required for the solution of real engineering problems 4) Provide a common basis for effective communication among all persons concerned with a geomechanics problem The classification should be: 1) Simple and meaningful in terms 2) Based on measurable parameters which can be determined quickly and easily in the field. What Bieniawaski though important 1) RQD 2) State of weathering 3) UCS (unconfined compressive strength) 4) Discontinuity spacing 5) Strike and dip 6) oint separation 7) oint persistence 8) Ground water Final Parameters 1) UCS 2) RQD 3) Discontinuity spacing rating 4) oint condition rating 5) Groundwater rating RMR: Rating 5

6 Picture(s) from Gonzales de Vallejo and Ferrer RMR: relationship to discontinuous rock? RQD, spacing, joint condition are all directly related to joints Ground water flow, which in most hard rock is a function of joints Strength is the only intact parameter 6. Laubshers geomechanics rock mass classification 1) RQD 2) IRS (Intact rock strength) 3) oint spacing rating 4) Condition of joint rating 5) Groundwater rating 6

7 Laubscher: Adjustments Weathering - up to 75% Field stresses - 120% for increase in compression, 90% for decrease Stresses in problem area, crown pillars, adjacent to caving, abutments, % Laubscher: Adjustments Laubscher: Adjustments Orientation Adjustment Blasting Adjustment Laubscher: tunnel design chart Laubscher: tunnel design chart explanation a) no support b) grouted bolts on 1 m pattern c) grouted bolts on 0.75 m pattern d) grouted bolts on 1 m pattern, 50 mm shotcrete e) grouted bolts on 1 m pattern, 300 mm concrete f) grouted bolts on 0.75 m pattern, 100 mm shotcrete g) grouted bolts on 1 m pattern, 100 mm mesh reinforced shotcrete h) grouted bolts on 1 m pattern, 450 mm concrete i) grouted bolts on 0.75 m pattern, 100 mm shotcrete, yielding steel arches j) 450 mm concrete k) shotcrete, yielding arches l) AVOID 7

8 Laubscher: cavability chart Caveability Angles of cave Laubscher: pit wall angle design chart Laubsher: relationship to discontinuous rock? 7. Barton s Q system RQD, spacing, joint condition are all directly related to joints Ground water flow, which in most hard rock is a function of joints strength is the only intact parameter Barton s Q-System RQD Q RQD * n r a w * SRF RQD - rock quality designation n - joint set number r - joint roughness number a - joint alteration number w - joint water reduction factor SRF - stress reduction factor 8

9 oint set number Block size component RQD n RQD - rock quality designation, 10 to 100 n - joint set number, 0.5 to 20 RQD/n, 0.5 to 200 oint roughness number Shear strength component r a r - joint roughness number, 0.5 to 4 a - joint alteration number, 0.75 to 20 r/a, 0.05 to 5 Mathew s Modified Barton s Q-System ' RQD Q * n r a RQD - rock quality designation n - joint set number r - joint roughness number a - joint alteration number 9

10 oint water reduction factor Active stress component w SRF w - joint water reduction factor, 0.05 to 1 SRF - stress reduction factor, 0.5 to 20 w/srf, to 2 Q Barton s Q-System RQD n SRF * * * r a * w Q: Design table Excavation support ratio Equivalent dimension of opening Based on Excavation support ratio (ESR) Sort of inverse FACTOR OF SAFETY 10

11 Q: Design table Q: Case histories Q: Design table Picture(s) from Gonzales de Vallejo and Ferrer Design criteria Q: relationship to discontinuous rock? RQD, v, r, a are all directly related to joints w is a measure of water flow, which in most hard rock is a function of joints SRF is under some conditions related to zones of weakness like joints 11

12 Q: relationship to true block size? Hoek Brown-Strength Criterion r-squared= Block Size (m) RQD/n GSI Summary Rock mass classification characterizes rock masses based on easily measurable features Empirical data, or case studies can be used to predict performance base on the classification Rock mass classification can be though of as a model Model is far from perfect, however can result in useful designs, and it is often the best design approach. 12

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