EXAMINATION PAPER
SUBJECT: CERTIFICATE IN ROCK MECHANICS
PAPER 3.1 : HARD ROCK TABULAR
SUBJECT CODE:
EXAMINER:
MCB STANDER
COMRMC
MODERATOR: KR BRENTLEY
EXAMINATION DATE: 13 OCTOBER 2016
TIME:
14:30 – 17:30
TOTAL MARKS: [100]
PASS MARK:
(60%)
NUMBER OF PAGES:
THIS IS NOT AN OPENBOOK EXAMINATION – ONLY REFERENCES PROVIDED ARE
ALLOWED
SPECIAL REQUIREMENTS:
1. Only answer FIVE (5) of the six questions. Answer the questions legibly in English.
2. Write your ID Number on the outside cover of each book used and on any graph paper or
other loose sheets handed in.
NB:
Your name must not appear on any answer book or loose sheets.
3. Show all calculations and check calculations on which the answers are based.
4. Hand-held electronic calculators may be used for calculations. Reference notes may not be
programmed into calculators.
5. Write legibly in ink on the right hand page only – left hand pages will not be marked.
6. Illustrate your answers by means of sketches or diagrams wherever possible.
7. Final answers must be given to an accuracy which is typical of practical conditions,
however be careful not to use too few decimal places during your calculations, as rounding
errors may result in incorrect answers.
NB: Ensure that the correct unit of measure (SI unit) are recorded as marks will be deducted
from answers if the incorrect unit is used even if the calculated value is correct.
8. In answering the questions, full advantage should be taken of your practical experience as
well as data given.
9. Please note that you are not allowed to contact your examiner or moderator regarding this
examination.
10. Cell phones are NOT allowed in the examination room.
GENERAL
State whether the following statements are True or False?
In your exam book only write the relevant question number and correct answer.
1.1
The Witwatersrand Basin and Bushveld Igneous Complex were formed through (2)
different geological processes. The presence of certain minerals, such as Pyrite and
Chalcopyrite, suggest that the basin was formed during the age of Dinosaurs, some 65
million years ago.
1.2
Examine2D is a 2-dimensional plane strain indirect boundary element program for the (2)
elastic stress analysis of underground excavations. This means that the program
calculates stress in the third dimension but strain remains zero.
1.3
The Mohr-Coulomb and Hoek & Brown failure criteria is used for estimating the (2)
strength behaviour of intact rock and suggests various modes of failure.
1.4
The Brazilian Test is a geotechnical laboratory test for indirect measurement of tensile (2)
strength of rocks. In the Brazilian test, a disc shape specimen of the rock is loaded by
two opposing normal strip loads at the disc periphery.
1.5
Seismic Moment (Mo) is a scalar that measures the coseismic inelastic deformation at (2)
the source. Since seismic moment is proportional to the integral of the far field
displacement pulse, it can easily be derived from recorded waveforms.
Choose the correct answer?
In your exam book only write the relevant question number and correct answer.
1.6
A dark, medium-grained igneous rock, typically with ophitic texture, containing (2)
plagioclase, pyroxene, and olivine and typically occurs in dykes and sills. Is known as?
(a) Norite
1.7
(b) Dolomite
(c) Dolerite
(d) Granite
During field investigations Calcite and Quartz veins contained in a rockmass may (2)
appear similar. On Mohs hardness scale, what are the hardness values for Calcite
and Quartz respectively?
(a) 3 and 7
1.8
(b) 4 and 8
(c) 2 and 7
(d) 4 and 9
Calculate the Rock Quality Designation (RQD) of a section of core recovered from a (2)
cover hole drilled from a development heading. The following values represent the
intact sections of core:
5 cm, 18 cm, 30 cm, 9 cm, 3 cm, 14 cm, 22 cm, 35 cm, 5 cm, 8 cm, 9 cm, 35 cm, 45 cm, 20
cm, 12 cm, 45 cm, 7 cm, 8 cm, 4 cm, 50 cm, 3 cm, 8 cm, 15 cm, 12 cm, 6 cm, and 6cm.
The RQD is:
(a) 82.73%
(b) 75.84%
(c) 85.49%
(d) 81.34%
1.9
A longwall consisting of Four (4) panels, similar in face length, is mining with an overall (2)
underhand face configuration in a western direction. The following lead/lag distances
exist between the panels:
Panel
Panel Length
Lead / Lag
P4
25 m
Leading
P3
28 m
15 m
P2
30 m
12 m
P1
25 m
16 m
If the ideal lead / lag distance between the panels should be 10 m, then the face
shape index for the long wall is:
(a) 6.25
(b) 10.75
(c) 4.33
(d) 8.33
1.10 In a narrow tabular deep underground stope (2800m below surface) calculate the (2)
elastic convergence at a point 10 m from the face. Use the following input parameters.

Mining Span of 200m

Gravitational Constant of 9.81 m/s2

Modulus of Elasticity of 60 GPa

Poisons Ratio of 0.25

Overburden Density of 2750 kg/m3
Formulas:
G
E
21  v 
(a) 0.462 m
sz 
2(1   )q 2 2
l x
G
(b) 0.617 m
(c) 0.144 m
lc 
sm G
2(1   )q
(d) 0.522 m
[20]
ROCK SAMPLE TESTING.
2.1
Suggested techniques for determining the uniaxial compressive strength and (7)
deformability of rock material are given by the International Society for Rock Mechanics
Commission on Standardization of Laboratory and Field Tests (ISRM Commission,
1979). List the essential features of the recommended procedure?
2.2
Young’s modulus of the specimen varies throughout the loading history and so is not a (6)
uniquely determined constant for the material. It may be calculated in a number of ways.
List and discuss three common ways?
2.3
The Mohr-Coulomb and Hoek & Brown failure criteria are used for estimating the strength (7)
behaviour of intact rock. Briefly discuss these criteria and elaborate on their potential
weaknesses.
[20]
ROCK AND ROCKMASS BEHAVIOUR
3.1
As Rock Engineer on a deep level gold mine, you are requested by the Mine Manager (10)
to review a mining layout pertaining to the establishment of a new longwall on the
opposite side of a dyke. It is planned to extend a footwall haulage below the faces of
the stopped longwall. Figure 1 depicts an illustration of the planned layout.
Figure 1: An illustration of the planned mining layout.
You conducted numerical modelling and calculated Rockwall Condition Factor (RCF)
along a 100m section of the planned haulage and obtained the results presented in
figure 2:
Figure 2: A Plot of RCF along the planned Haulage.
It must be noted that the dyke is known to be seismically active.
With reference from the start of the planned haulage you are requested to provide an
opinion regarding the anticipated ground conditions along the excavation, as well
as the primary and secondary support requirements.
3.2
Define Energy Release Rate (ERR) and discuss practical methods to manage ERR?
(5)
3.3
Define Excess Shear Stress (ESS) and discuss practical methods to manage ESS?
(5)
[20]
MINING SUPPORT STRATEGIES.
4.1
You form part of a design team, evaluating a mining project of a shallow chrome mine. (10)
The orebody dips relatively flat and mining is planned to take place at an average
depth of 300m below surface. It is envisaged to mine with 10m wide boards, leaving
square pillars and at an average mining height of 2m. The overburden density is
estimated at 3000 kg/m3. Preliminary laboratory results suggest the following elastic
properties for the Chromitite samples.

Young’s Modulus of 90 GPa.

Poisons ration of 0.22.

Average Uniaxial Compressive Strength of 120 MPA.
Applying a factor of safety of 1.6, determine the required pillar dimensions.
NB: Ensure that any assumptions are stated clearly.
Formulas:
 v  gH
 h  k v
wa
s  k b
h

Fs  s
p
4.2



w2
e  1 2
C
G
E
21  v 
p 
q
1 e
C  w B
In the Chrome Mine, mentioned in 4.1, a weak parting is known to exist 1.5 m above (5)
the Chrome seam with a very competent rock layer above. You are required to design
a systematic support system for the mining boards of 10m. Calculate the following:

The required support resistance when applying a factor of safety of 1.3.

The required spacing of bolts with a breaking capacity of 120 kN.
NB: Ensure that any assumptions are stated clearly.
You have a choice between resin anchored bolts and mechanically end anchored
bolts, which bolt type would you recommend and why?
4.3
List five (5) factors that may affect the design, or influence the performance of pillars?
(5)
[20]
INVESTIGATION TECHNIQUES
5.1
Differential methods rely on modelling a specific volume of the rock mass in its (15)
entirety. In deferential methods, the rock mass is modelled as a volume (3D) or an
area (2D) that is large enough to ensure that the boundary conditions do not
significantly affect the region in which one is interested. If the influence of the
boundaries is significantly small then they may be considered as infinity. As a result
of the volume filling methods required in differential methods, the stresses and strains
in the solid away from any excavation are not continuous but are discrete. There are
three types of differential methods in common use;

The Finite Difference Method (FDM)

The Finite Element Method (FEM)

Distinct Element Method (DEM)
Briefly describe each method?
5.2
Steps are suggested as an outline for a disciplined modelling methodology that can (5)
be applied to a wide range of problem solving situations encountered in Rock
Engineering. List these steps?
[20]
ROCKBREAKING IN HARD ROCK
6.1
Industrial chemical explosives are of two main types, list and define these two main (4)
types?
6.2
In the period during and following the passage of a detonation wave along an (6)
explosive charge, the rock around the blast hole is subjected to three phases of
loading. List the three phases of loading and briefly explain when these phases occur
during blasting?
Note: You may make use of annotated sketches to assist with your explanation.
6.3
Describe Pre-split blasting and Smooth blasting as applied perimeter blasting (10)
techniques?
[20]
Total Mark = [100]