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Team J: Naadirah Custis, Deion Floyd

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1 Team J: Naadirah Custis, Deion Floyd
Rock Mechanics Team J: Naadirah Custis, Deion Floyd

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

3 What is Rock Mechanics?

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

5 Identifying Rocks B) A) C)

6 Formation of Rocks C) A) B)

7 Mine Exploration A) B) C)

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

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

10 Physical Properties Density Water content Porosity A) B)

11 Rock Mechanical Testing
UCS Test A)

12 Instrumentation Rock Displacement LVDT B) A) C)

13 Test Procedure

14 Test Procedure

15 Test Performing

16 Rock quality classification
Having tests that give an idea of the quality of the rock mass is essential for engineering purposes

17 What are we calling a rock?
Grade Description Lithology Excavation Foundations VI Soil Some organic content, no original structure May need to save and re-use Unsuitable V Completely weathered Decomposed soil, some remnant structure Scrape Assess 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 Rip Good for most small structures II Slightly weathered Increased fractures and mineral staining Blast Good for anything except large dams I Fresh rock Clean rock Sound Engineering classification of weathered rock

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

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

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., 1967

21 Rock Quality Designation (RQD)

22 RQD RQD Very poor 0 – 25 Poor 25 – 50 Fair 50 – 75 Good 75 – 90

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

24 Terzaghi’s Rock Mass Classification (1946)
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.

25 Terzaghi’s Rock Mass Classification (1946)
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.

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

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

28 RMR or ‘Geomechanics Classification’

29 Rock Tunnelling Quality Index, Q (or Norwegian Q system), Barton et al
RQD = Rock Quality Designation Jn = Joint set number 1 – 20 Jr = Joint roughness factor Ja = Joint alteration and clay fillings 1 – 20 Jw = Joint water inflow or pressure 1 – 0.1 SRF = stress reduction factor 1 – 20 Typically: < Q <100

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)

31 Guideline properties of Rock Mass Classes

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

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

34 Shear strength of discontinuities

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


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

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:



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.


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:

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.

45 Estimate of JRC:

46 Estimate of JRC:

47 Estimate of JCS:

48 Shear strength of filled Discontinuities:

49 In situ stresses

50 Consider an element of rock at a depth of 1,000 m below the surface
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:


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:

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.

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.


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