Eng class rock.ppt
- 1. Engineering Classification Systems for Rocks INTACT ROCK ROCK MASS Rock mass refers to in situ rock with all its inherent geomechanical anisotropies
- 2. Engg classification • Geologists: have a genetic bias, provide little information relating to engg behaviour • Tests which are used for engg classification are called Index Test • If right index tests are chosen, then rocks having similar index properties, irrespective of their origin, will probably exhibit similar engg performance • First Major classification by Terzaghi (1946) • After that, there have been many classification = some are oversimplified, misapplied = led to chaos, multiplicity • Last two decades = some effort done, to bring some order from the chaos
- 3. Bases of Engg Classification • Lithology • Strength • Modulus of deformation • Discontinuities, Joint: spacing, inclination, orientation, roughness, groundwater flow • In situ stress
- 4. Terzaghi’s Classification • Terzaghi : first to attempt classification • Bases : discontinuity, weathering Term / Description Intact = neither joints nor hair cracks Stratified = individual strata with little or no resistance against separation Moderately Jointed = blocks between joints are intimately interlocked Blocky and Seamy = rocks consists of chemically unweathered rock fragments which are entirely separated from each other Crushed = chemically unweathered, like crusher run material Squeezing = slowly advances into tunnel Swelling = expansion due to swelling capacity
- 5. Disadvantage of Terzaghi’s Classification • Overlooked the properties of rocks • Granite, mudstone may belong to one group as per Terzaghi • But their mechanical properties differ significantly
- 6. Grade Description Lithology Foundations VI Soil Some organic content, no original structure Unsuitable V Completely weathered Decomposed soil, some remnant structure Assess by soil testing IV Highly weathered Partly changed to soil, soil > rock Variable and unreliable III Moderately weathered Partly changes to soil, rock > soil Good for most small structures II Slightly weathered Increased fractures and mineral staining Good for anything except large dams I Fresh rock Clean rock Sound Engineering classification of weathered rock Based on Weathering
- 7. Classification of Intact Rock • Deere and Miller (1966) Classification of intact rock: • Bases: unconfined (uniaxial) compressive strength ( 1) • Young’s Modulus (E) – Rocks are subdivided into five strength categories on a geometric progression basis based on unconfined (uniaxial) compressive strength ( 1) Description/ Class Very high A High B Medium C Low D very low E
- 8. Based on Young’s Modulus (E) /Modulus Ratio Three ratio intervals are employed for the modulus ratio; high(H) – medium(M) – low(L). Rocks are therefore classed as BH (high strength- high ratio); CM (medium strength – medium ratio), etc.
- 9. C-factor classification (Hansagi, 1974) • Fissuration factor, C = 1/2S(p x H+k x n) • S = length of drill hole • p = number of cylindrical samples which can be obtained from cores corresponding to length S • H = height of the cylindrical samples used for compression testing • k = total length of core fragments with cylindrical heights > the core diameter • n = number of core samples
- 10. Coates Classification 1. Uniaxial com strength : Weak, Strong, Very strong 2. Pre-failure Deformation: Elastic, Viscous 3. Failure Characteristics: Brittle, Plastic 4. Gross Homogeneity: Massive, Layered 5. Continuity: Solid, Blocky, Broken
- 11. USBM Classification (US Bureau of Mines) • Competent Rock – excavation requires no added support • Massive-Elastic – homogeneous and isotropic • Bedded- elastic but laminated, little cohesion b/w beds • Massive-Plastic – Rocks that will have creep flow under low stress
- 12. Rock Quality Designation Index (RQD) (Deere et al. 1967) • Aim : to provide a quantitative estimate of rock mass quality from drill logs • Equal to the percentage of intact core pieces longer than 100mm in the total length of core
- 13. Rock Quality Designation (RQD) and Indices • Quality and deformability = f (discontinuity) • Core Recovery = total core recovered/length of drilling – Expressed in %, good core recovery = good quality rock – Poor core recovery = poor quality of rocks – In core recovery, all lengths of cores are counted • In RQD, only cores with >10cm length are considered • Quantitative estimate of rock mass quality from drill core logs RQD = % intact core pieces >10cm in total length of core Rock Quality Designation index, or RQD, was introduced by Don Deere in 1963. It judges rock quality based solely on measurements of recovered rock core
- 14. Rock Quality Designation (RQD)
- 15. Natural joint surface vs core broken during operation • Mechanical breaking of cores caused by drilling processes • Natural joint surface
- 16. Broken cores • Cores broken during handling or drilling = fresh irregular breaks = can be fitted • Natural Joint surface – broken surface smooth, pieces cannot be fitted • For RQD determination, International Society for Rock Mechanics recommends a core size of at least 54.7 mm (NX) diameter drilled with double tube core barrels
- 18. RQD RQD% A.Very poor 0 – 25 B.Poor 25 – 50 C.Fair 50 – 75 D.Good 75 – 90 E.Excellent 90 - 100
- 19. RQD : when core is unavailable • RQD = 115 – 3.3 Jv, for Jv between 4.5 and 30. • For Jv < 4.5, RQD is 100%, and for Jv > 30, RQD is 0%. Jv = number of joints per m3 volume of rock mass
- 20. Outcrop Measurements of RQD
- 21. RQD • Directionally dependant parameter • Intended to indicate rock mass quality in-situ • Adapted for surface exposures as ‘Jv’ number of discontinuities per unit volume • Used as a component in the RMR and Q systems • Palmstrom (1982) • Priestai Hudsona (1976) l - number of joints per unit length v J RQD 3 . 3 115 l l 1 . 0 1 . 0 1 100 e RQD
- 22. Mass factor = j • j = ratio of deformation modulus of rock mass (in situ, outcrop)/ that of intact rock core comprising the same lithology • j = reflects effects of discontinuities
- 23. Velocity ratio • Vcf/Vcl • Vcf = in situ (field f) compressional (c) wave velocity • Vcl = compressional (c) wave velocity in lab (l), intact rock core • Difference in Vcf and Vcl caused by structural discon
- 24. Quality classification / Rock Quality Description RQD% Mass Factor, j Velocity Ratio, Vcf/Vcl Fracture frequenc y C-factor Very Poor 0-25 - 0-0.2 >15 0-0.15 Poor 25-50 <0.2 0.2-0.4 15-8 0.15- 0.30 Fair 50-75 0.2-0.5 0.4-0.6 8-5 0.30- 0.45 Good 75-90 0.5-0.8 0.6-0.8 5-1 0.45- 0.65 Excellent 90-100 0.8-1.0 0.8-1.0 <1 0.65- 1.0
- 25. Multi parameter Rock Mass Classification Schemes • Rock Mass Structure Rating (RSR) • Rock Mass Rating (RMR) • Rock Tunnelling Quality Index (Q) • Geological Strength Index (GSI)
- 26. Rock Mass Rating (RMR) / Geomechanics Claasification • Rating Concept • Weightage • Total Rating
- 27. Rock Mass Rating (RMR), Geocmechanics Classification (Bieniawski,1976, 1989) • Classifies rock mass according to 6 rated parameters: – UCS (unconfined com strength) – RQD – Spacing of discontinuities – Condition of discontinuities – Groundwater conditions – Discontinuity orientation
- 28. RMR or ‘Geomechanics Classification’ RMR emphasizes STRUCTURAL consideration and Orientation
- 29. Rock Mass Rating System 1976 to 1989 Bieniawski • System refined by greater data • Ratings for parameters changed • Adapted by other workers for different situations • PROJECT SPECIFIC SYSTEMS
- 30. Development of Rock Mass Rating System
- 31. Rock Mass Rating System Rating Class Description 81-100 I Very Good Rock 61-80 II Good Rock 41-60 III Fair Rock 12-40 IV Poor Rock Less than 20 V Very Poor Rock
- 32. Rock Tunnelling Quality Index, Q (or Norwegian Q system), Barton et al., 1974 SRF Jw Ja Jr Jn RQD Q RQD = Rock Quality Designation 100 - 10 Jn = Joint set number 1 – 20 Jr = Joint roughness factor 4 -1 Ja = Joint alteration and clay fillings 1 – 20 Jw = Joint water inflow or pressure 1 – 0.1 SRF = stress reduction factor 1 – 20 Good Bad Q Range 0.001 to 1000 Exceptionally poor squeezing : 0.001 Exceptionally good unjointed Rock : 1000
- 33. Q system • (RQD/Jn) = crude measure of block size • (Jr/Ja) = roughness/friction of surfaces • (Jw/SRF) = ratio of two stress parameters (active stress) SRF Jw Ja Jr Jn RQD Q •Q Does NOT include joint orientation Mistake in Bell New Volume/Indian Ed
- 34. Guideline properties of Rock Mass Classes
- 35. Rock Mass Structure Rating (RSR) (1972) • Introduced the concept of rating components to arrive at a numerical value • Demonstrates the logic in a quasi-quantitative rock mass classification • Has limitations as based on small tunnels supported by steel sets only RSR = A + B + C Maximum value of RSR = 100
- 36. Rock Structure Rating Parameter A: General area geology Considers (a) rock type origin (b) rock ‘hardness’ (c) geotechnical structure
- 37. Considers (a) joint spacing (b) joint orientation (strike and dip) (c) direction of tunnel drive Rock Structure Rating Parameter B: Geometry : Effect of discontinuity pattern
- 38. Considers (a) overall rock mass quality (on the basis of A + B) (b) joint condition (c) water inflow Rock Structure Rating Parameter C: Groundwater, joint condition
- 39. Using Rock Mass Classification Systems • RMR and Q most widely used • Both use similar parameters; difference in weighting • Q Does NOT include joint orientation • RMR emphasizes STRUCTURAL consideration and Orientation