Lecture 1_ Rockmass Characterisation.pptx

Rockmass characterisation and classification

1. Presenter: WB Motlhabane Geotechnical Engineering 3A (GEMIN 3A) Lecture 1 ROCKMASS CHARACTERISATION AND CLASSIFICATION

2. Agenda Definition Safety Hazards (+ visuals) Rockmass Characterisation and Classification Data Collection (and RQD) Q-System by Barton et al (1974) Geomachanics Classification System by Bieniawski (1989). Mining Rock Mass Rating by Laubscher and Taylor (1976) and refined by Laubscher (1994). Design Rockmass Strength (DRMS) Applications Classification Systems for Mine excavation design Worked Examples

3. Rockmass Classification Critical to mine planning: Identification of potential modes of failure and evaluation of stability Evaluation of support requirement Defines geotechnical domains for preparation of Code of Practice to combat rock falls

4. 4 Is the excavation I am planning going to be stable? Will it be stable somewhere else? Will it be stable if I change the shape or orientation? Will it be stable if I change the size? YES NO PROCEED SUPPORT Excavation Design Sequence

5. Potential wedge failure

6. Potential failure along a plane Water in strata

7. Potential failure along a plane Orientation

8. Potential failure along a plane Coal mine

9. Safety Hazards Large scale failure of slopes is a hazard for workers operating within the pit or quarry; Bench failures are a local hazard for workers within the vicinity; Localised failure can lead to subsequent bouncing of boulders down into the pit or quarry; Failure of surface soil horizons or weathered material may disturb other areas and may lead to mud flows and the accumulation of mud in the bottom of the pit or quarry.

10. Data Collection Physical/actual site investigation Core logging Recovery% - (meters drilled/recovered meters)*100 Geotechnical interval, rock type and rock competence

11. Data Collection Rock Quality Designation (RQD)

12. Data Collection… cont Rock Competence M1 - Faults M2 - Shears M3 - Intense fracturing M4 - Intense mineralisation M5 - Deformable material Weathering 1 - Unweathered 2 - Slightly weathered 3 - Moderately weathered 4 - Highly weathered 5 - Completely weathered

13. Data Collection… cont Hardness 1 - Very soft 2 - Soft 3 - Hard 4 - Very hard 5 - Extremely hard Jointing Distribution

14. Data Collection… cont Joint Surface Conditions

15. Data Collection… cont

16. Data Collection… cont Mapping of Exposed Surfaces Joint Mapping Geotechnical classification mapping

17. Data Collection… cont

18. Data Collection… cont Laboratory rock testing Compressive tests (UCS) Tensile tests (UTS) Shear Tests Durability Tests

19. In- Situ Stress Conditions Stress types: virgin, induced, field, residual Stress measurement Stress magnitude impact

20. Rockmass Classification – Q System Q expression:

21. Rockmass Classification – Q System J n : Joint Set Number

22. Rockmass Classification – Q System J r : Joint Roughness Number

23. Rockmass Classification – Q System J a : Joint Alteration Number

24. Rockmass Classification – Q System J w : Joint Water Reduction Factor

25. Rockmass Classification – Q System SRF: Stress Reduction Factor for weak zones

26. Rockmass Classification – Q System SRF: Stress Reduction Factor for competent rock and stress zones

27. Rockmass Classification – Q System Q Calculation:

28. Rockmass Classification – Geomechanics Classification (RMR) RMR Parameters:

29. RMR - Classification

30. RMR - Classification

31. RMR – Visual Estimation Classification RMR – Visual Estimation

32. Rockmass Characterisation and Classification Worked Problems Problem 1 A borehole log was drilled to a depth of 30m, extracted and layed out on surface for inspection. The first 5% of length from surface is soils. Between 19m and 23m there is a peculiar finely crushed rock. Wet mud was also encountered between 29 and 29.5m. Determine the percentage core recovery. Solution: Losses: 1.5m + 4m + 0.5m = 6m % Recovery = 24/30 * 100 = 80%

33. Rockmass Characterisation and Classification Worked Problems Problem 2 A 3m cubic shaped boulder was observed in a mine excavation. Joints spacing along its dimensions are 45cm, 60cm and 70cm, respectively. Estimate the Rock Quality Designation of this geotechnical environment based on this boulder and comment on your answer. Solution: 6/3 + 5/3 + 4/3 = 15/3 = 5 = Jv RQD = 115 – 3.3 Jv = 98.5%

34. Rockmass Characterisation and Classification Worked Problems Problem 3 Rate the rock mass described below according to Q-system where an excavation is to be sited. Also estimate RMR value. Comment on your answers. Depth below surface is 800m and the rock density is 2.5 tons/m 3 There are two observed planar and slicken-sided joint sets RQD = 67 and the intact uniaxial strength of the rock is 60 Mpa. Joint walls are unaltered with some staining Damp to dry environment conditions Major principal stress is the same as the vertical virgin stress

35. Q System Problem Solution J n : Joint Set Number

36. Q System Problem Solution J r : Joint Roughness Number

37. Q System Problem Solution J a : Joint Alteration Number

38. Q System Problem Solution J w : Joint Water Reduction Factor

39. Q System Problem Solution SRF: Stress Reduction Factor for weak zones

40. Q System Problem Solution SRF: Stress Reduction Factor for competent rock and stress zones 60/ 2500X 9.81 X 800 = 3

41. Q System Problem Solution Q - Value Q = 67/4 X 0.5/1 X 1/5 = 1,65 RMR = 48

42. Rockmass Characterisation and Classification Worked Problems Problem 4 An excavation is to be sited in blocky faulted Rockmass conditions in which joint surfaces (of dip 45 degrees) are smooth and moderately weathered. The excavation is to developed against the dip and along the strike of joint. Estimate RMR.

43. RMR – Visual Estimation Classification RMR – Visual Estimation RMR = 40 – unfavourable orientation (10) = 30