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ROCK MECHANICS Team J: Naadirah Custis, Deion Floyd 1.

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Presentation on theme: "ROCK MECHANICS Team J: Naadirah Custis, Deion Floyd 1."— Presentation transcript:

1 ROCK MECHANICS Team J: Naadirah Custis, Deion Floyd 1

2 Introduction  Identifying Rocks  Formation of Rocks  Mine exploration  Explosion/Drilling  Rock sample preparation  Physical Properties  Rock Mechanical Testing  Instrumentation 2

3  What is Rock Mechanics? 3

4 Rock Types The 3 basic rock types are:  Igneous  Metamorphic  Sedimentary 4

5 Identifying Rocks A) B) C) 5

6 Formation of Rocks A) B) C) 6

7 Mine Exploration A) B) C) 7

8 Explosion/Drilling A) B) C) D) 8

9 Rock Sample Preparation  Coring  Cutting  Grinding A) B) D) C) 9

10 Physical Properties  Density  Water content  Porosity A) B) 10

11 Rock Mechanical Testing  UCS Test A) 11

12 Instrumentation  Rock Displacement  LVDT A) B) C) 12

13 Test Procedure 13

14 Test Procedure 14

15 Test Performing 15

16 ROCK QUALITY CLASSIFICATION Having tests that give an idea of the quality of the rock mass is essential for engineering purposes 16

17 What are we calling a rock? GradeDescriptionLithologyExcavationFoundations VI SoilSome organic content, no original structure May need to save and re-use Unsuitable V Completely weathered Decomposed soil, some remnant structure ScrapeAssess by soil testing IV Highly weathered Partly changed to soil, soil > rock Scrape NB corestones Variable and unreliable III Moderately weathered Partly changes to soil, rock > soil RipGood for most small structures II Slightly weathered Increased fractures and mineral staining BlastGood for anything except large dams I Fresh rockClean rockBlastSound Engineering classification of weathered rock 17

18 Rock Mass Strength  Strength depends on the density, nature and extent of the fractures within it 18

19 Rock fractures and their characterization Typically carried out using 3 orthogonal scanlines  orientation  spacing  length  roughness  aperture  filling  block size 19

20 Rock Quality Designation (RQD)  Quantitative estimate of rock mass quality from drill core logs  % intact core pieces >10cm in total length of core  Deere et al.,

21 Rock Quality Designation (RQD) 21

22 RQD A.Very poor0 – 25 B.Poor25 – 50 C.Fair50 – 75 D.Good75 – 90 E.Excellent

23 Terzaghi’s Rock Mass Classification (1946) Rock Mass Descriptions – Intact – Stratified – Moderately jointed – Blocky and Seamy – Crushed – Squeezing – Swelling 23

24  Intact rock contains neither joints nor hair cracks. Hence, if it breaks, it breaks across sound rock.  Stratified rock consists of individual strata with little or no resistance against separation along the boundaries between the strata. The strata may or may not be weakened by transverse joints. In such rock the spalling condition is quite common.  Moderately jointed rock contains joints and hair cracks, but the blocks between joints are locally grown together or so intimately interlocked that vertical walls do not require lateral support. In rocks of this type, both spalling and popping conditions may be encountered. Terzaghi’s Rock Mass Classification (1946) 24

25  Blocky and seamy rock consists of chemically intact or almost intact rock fragments which are entirely separated from each other and imperfectly interlocked. In such rock, vertical walls may require lateral support.  Crushed but chemically intact rock has the character of crusher run. If most or all of the fragments are as small as fine sand grains and no recementation has taken place, crushed rock below the water table exhibits the properties of a water-bearing sand.  Squeezing rock slowly advances into the tunnel without perceptible volume increase. A prerequisite for squeeze is a high percentage of microscopic and sub-microscopic particles of micaceous minerals or clay minerals with a low swelling capacity.  Swelling rock advances into the tunnel chiefly on account of expansion. The capacity to swell seems to be limited to those rocks that contain clay minerals such as montmorillonite, with a high swelling capacity. Terzaghi’s Rock Mass Classification (1946) 25

26 RMR and Q Rock classification systems  Primary use of RQD is as a parameter in more widely used  RMR (Bieniawski, 1976) and  Q System (Barton et al., 1974) classification systems 26

27 Rock Mass Rating (RMR), Bieniawski (1976, 1989)  Classifies rock according to 6 parameters:  UCS  RQD  Spacing of discontinuities  Condition of discontinuities  Groundwater conditions  Discontinuity orientation 27

28 RMR or ‘Geomechanics Classification’ 28

29 Rock Tunnelling Quality Index, Q (or Norwegian Q system), Barton et al., 1974 RQD = Rock Quality Designation Jn = Joint set number1 – 20 Jr = Joint roughness factor4 -1 Ja = Joint alteration and clay fillings1 – 20 Jw = Joint water inflow or pressure1 – 0.1 SRF = stress reduction factor1 – 20 Typically: 0.01 < Q <100 29

30 Q system  (RQD/Jn) = crude measure of block size  (Jr/Ja) = roughness/friction of surfaces  (Jw/SRF) = ratio of two stress parameters (active stress) 30

31 Guideline properties of Rock Mass Classes 31

32 Using Rock Mass Classification Systems  RMR and Q most widely used  Both use similar parameters; difference in weighting 32

33 Using Rock Mass Classification Systems  Good practice to assign a range of values Field example 33


35  The relationship between the peak shear strength and the normal stress can be represented by the Mohr-Coulomb equation: 35

36 36

37  In the case of the residual strength, the cohesion c has dropped to zero and the previous relationship can be represented by: 37

38  The basic friction angle b is a quantity that is fundamental to the understanding of the shear strength of discontinuity surfaces. This is approximately equal to the residual friction angle r but it is generally measured by testing sawn or ground rock surfaces. These tests, which can be carried out on surfaces as small as 50 mm *50 mm, will produce a straight line plot defined by the equation: 38

39 39

40 40

41 Shear strength of rough surfaces  Patton (1966) demonstrated this influence by means of an experiment in which he carried out shear tests on 'saw-tooth' specimens such as the one illustrated in Figure 4. 41

42 42

43 Barton’s estimate of shear strength  While Patton’s approach has the merit of being very simple, it does not reflect the reality that changes in shear strength with increasing normal stress are gradual rather than abrupt. Barton (1973, 1976) studied the behaviour of natural rock joints and proposed that equation (4) could be re- written as: 43

44  Barton developed his first non-linear strength criterion for rock joints (using the basic friction angle): where r is the Schmidt rebound number wet and weathered fracture surfaces and R is the Schmidt rebound number on dry unweathered sawn surfaces. 44

45  Estimate of JRC: 45

46  Estimate of JRC: 46

47  Estimate of JCS: 47

48 Shear strength of filled Discontinuities: 48


50  Consider an element of rock at a depth of 1,000 m below the surface. The weight of the vertical column of rock resting on this element is the product of the depth and the unit weight of the overlying rock mass (typically about 2.7 tonnes/m3 or MN/m3). Hence the vertical stress on the element is 2,700 tonnes/m2 or 27 MPa. This stress is estimated from the simple relationship: 50

51 51

52  The horizontal stresses acting on an element of rock at a depth z below the surface are much more difficult to estimate than the vertical stresses. Normally, the ratio of the average horizontal stress to the vertical stress is denoted by the letter k such that: 52

53  Terzaghi and Richart (1952) suggested that, for a gravitationally loaded rock mass in which no lateral strain was permitted during formation of the overlying strata, the value of k is independent of depth and is given by k = v /(1 − v), where v is the Poisson's ratio of the rock mass. 53

54  Sheorey (1994) developed an elasto-static thermal stress model of the earth.  where z (m) is the depth below surface and Eh (GPa) is the average deformation modulus of the upper part of the earth’s crust measured in a horizontal direction. This direction of measurement is important particularly in layered sedimentary rocks, in which the deformation modulus may be significantly different in different directions. 54

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