Sab 2712

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FKA.B.PP.03 Ed.2

UNIVERSITI TEKNOLOGI MALAYSIA

FACULTY OF CIVIL ENGINEERING FINAL EXAMINATION

SEMESTER II, SESSION 2008/2009

COURSE CODE : SAB 2712/ SAM 3722

COURSE : SAW

PROGRAMME : GEOLOGY AND ROCK MECHANICS

DURATION : 2 HOURS

DATE : APRIL 2009

INSTRUCTION TO CANDIDATES:

ANSWER ALL QUESTIONS IN SECTION A (4 QUESTIONS) AND

SECTION B (2 QUESTIONS).

USE SEPARATE ANSWER BOOKLET FOR EACH SECTION.

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SECTION A: GEOLOGY

You are required to answer all the questions in SECTION A.

Q1. Sedimentary rocks are formed by the aggregation of almost any solid

particles found on the earth surface. Based on the statement, answer all the

following questions.

a) What is the difference between clastic and non-clastic rock?

(2 marks)

b)

Explain on the lithification process that transforms loose sediment to

sedimentary rock.

(2 marks)

c) Size and shape of the grain particles can be used to evaluate the history of

the sedimentary rock? Discuss.

(4 marks)

d)

Explain and describe the typical feature of sedimentary rock.

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Q2. Discontinuities is one of the major factors that can weaken the strength of

rock mass.

a) Describe discontinuities in rock.

(2 marks)

b) How discontinuity spacing can influence excavation works?

(3 marks)

c) Compare the effect of discontinuities on hard and weak rock.

(4 marks)

d) How does discontinuities orientation can be evaluated using the stereonet

projection?

(6 marks)

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Q3. Weathering effects in tropical climate is significant and should be given

sufficient attention in civil engineering works.

a) Why does thick weathering profile exist in tropical climate?

(2 marks)

b) Evaluate the typical weathering profile in limestone area.

(5 marks)

c) Evaluate the factors and issues to be considered when you are designing a

high rise building in a limestone area.

(6 marks)

Q4. The strength of rock material depends on a number of factors.

a) List and describe five (5) factors that influence the rock material strength.

(5 marks)

b) Evaluate the influence of moisture content to the strength of weathered

rock material?

(5 marks)

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SECTION B: ROCK MECHANICS

This section consists of two (2) questions Q1 and Q2, answer all the questions from

this section.

Q1. Specific rock properties dictate its suitability for use as construction materials

or, as part of structure components. The properties are commonly verified

through laboratory tests. However, careful consideration on the use of rock

properties obtained from laboratory test is essential in designing the related

structure in the rock mass. Factors like small-scale and large-scale

discontinuities in rock, degree of weathering and type of loading impose on the

rock mass all requires careful consideration when lab data is used as input

parameters in design.

Answer the following questions

(25 marks).

(a)

Based on the nature of load/stress that is likely to act on a rock mass, name

one type of laboratory test (index, indirect or strength test) that is important

to verify the relevant rock properties for the construction activity/structure

listed in Table B1? (Write your answer in column 3 of Table B1)

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(c) With respect to discontinuities in rock mass, explain why strength of rock

material obtained from lab test, such as uniaxial compressive strength

(UCS), is always higher than the mass or in situ strength of the rock?

(3 marks)

(d) Types of discontinuities/weakness planes found in rock depend on mode of

formation of the rock; sedimentary, igneous and metamorphic. For the large-

scale and small-scale discontinuities/ listed in Table B2, indicate (√) in

which rock (shale, gabbro and schist) each of this discontinuity is present?

(5 marks)

(e)

When evaluating compressive strength of rock in laboratory it is necessary

to consider the effect of lamination and foliation exhibited by rock samples.

This is particularly important for metamorphic rocks which display

distinctive mineral arrangement. Typical stress-strain data obtained from

compression test on two (2) core samples of schist, Sample X and Sample Y,

is listed Table B3. Loading orientation with respect to the mineral

arrangement during testing of samples is shown in Fig. B2.

Using test data for Sample X and Sample Y in Table B3 plot stress-strain

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Q2. Fig. B3 shows circular-shaped tunnel of 6 m diameter proposed in ROCK

MASS A and ROCK MASS B. As indicated in the figure, ROCK MASS A

displays inclined bedding planes and ROCK MASS B displays intersecting joint

sets. The proposed depth of the tunnel from ground surface at both sites is

similar and it is 140 m. Both sites exhibit similar type of topsoil that consists of

sandy clay of unit weight (γs) 15 kN/m3 and thickness as shown in Fig. B3.

The typical material properties for ROCK MASS A and B, obtained from lab

tests are listed in Table B4 below.

Table B4: Rock material properties based on laboratory tests

Rock type & properties Rock mass A Rock mass B

Compressive strength, UCS 80 MPa 30 MPa

Tensile strength, T 0.8 MPa 0.1 MPa

Slake durability index, Id 99.9 % 20 %

Young’s modulus, E 40 GPa 15 GPa

Primary wave velocity, Vp m/s 4000 – 6000 m/s 1500 – 2500 m/s

Unit weight of rock, γr 28 kN/m3 24 kN/m3

U it i ht f il 28 kN/m3 24 kN/m3

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(b) The rock core samples obtained from ROCK MASS B is shown in Fig. B4

with core recovery of 100 %. Calculate the RQD value for this core?

(4 marks)

(c)

Based purely on the nature of discontinuity/weakness plane in ROCK MASS

A and B, which tunnel will require a more intensive stabilisation system for

stability?

(2 marks)

(d) Name two (2) properties listed in Table B4 which may be used to indicate

that ROCK MASS A is denser than ROCK MASS B?

(2 mark)

(e) Based on the appearance of the in situ rock mass in Fig. B3, give an

explanation why it will be easier to excavate in ROCK MASS B?

(3 marks)

(f) It is predicted that the induced compressive stress (σc) at the walls of the

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(g) For both tunnels A and B, the requirement for rock stabilisation system is

unavoidable, and the expected modes of instability that will occur in the

tunnel walls are listed in Table B5. For each mode of instability listed in the

table, recommend a suitable method for stabilisation? State also whether the

recommended method is ‘rock support’ or ‘rock reinforcement’ system?

(Write your answer in column 3 and 4 in Table B5)

(8 marks)

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FKA.B.PP.06.Ed.1

FACULTY OF CIVIL ENGINEERING


SEMESTER : II SESSION : 2008/2009 PROGRAMME : SAW

COURSE : GEOLOGY & ROCK MECHANICS COURSE CODE : SAB 2712/ SAM 3722

No. Name of structure Type of lab test1. Bedrock as foundation for a large concrete dam (bedrock is 3 m

below ground surface)

2. Rock boulders for use as protection against erosion along a

coastline

3. Slope face excavated in rock with inclined bedding planes and

joints

4. Rock mass surrounding tunnel located at depth of 20 m below

ground surface

5. Excavation in rock mass by fracturing mechanism, e.g. blasting

& ripping.

Table B1: Rock as part of structure components and as construction materials

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WEATHERING DESCRIPTION

RESIDUAL SOIL; ORIGINAL TEXTURE

STRUCTURE AND MINERALOGY

COMPLETELY DESTROYED

COMPLETELY WEATHERED DECOMPOSED

AND FRIABLE, BUT ROCK TEXTURE AND

STRUCTURE PRESERVED

HIGHLY WEATHERED; WEATHERING

EXTENDS THROUGHOUT ROCKMASS AND

ROCK MATERIAL IS PARTLY FRIABLE

MODERATELY WEATHERED; WEATHERING

EXTENDS THROUGHOUT ROCKMASS, BUT

ROCK MATERIAL IS NOT FRIABLE

SLIGHTLY WEATHERED; PENETRATIVE

WEATHERING ON OPEN DISCONTINUITY

SURFACES, BUT ONLY SLIGHT

WEATHERING OF ROCK MATERIAL

FRESH, NO VISIBLE SIGN OF WEATHERING

OR FAINTLY WEATHERED WITH

WEATHERING LIMITED TO SURFACE OF

MAJOR DISCONTINUITIES

VI

V

IV

III

II

I

0.001 0.004 0.01 0.04 0.1 0.4 1.0

STRENGTH REDUCTION FACTOR

Fig. B1: Strength Reduction Factor (SRF) for rock at different weathering grade

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No. Large-scale discontinuities SHALE GABBRO SCHIST

1. Bedding planes

2. Joints

3. Faults

No. Small-scale discontinuities SHALE GABBRO SCHIST1. Lamination (mineral arrangement dueto sedimentation)

2. Flow texture/foliation (mineral

arrangement due to metamorphism)

Table B2: Large-scale and small-scale discontinuities in rock

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Stress (MPa) Strain (%) – Sample X Strain (%) – Sample Y0 0.000 0.000

3 0.038 0.038

6 0.075 0.065

10 0.113 0.103

15 0.155 0.140

22 0.220 0.180

30 0.290 0.225

38 0.360 0.265

44 0.400 -

50 0.450 -

Table B3: Stress strain data obtained from laboratory tests

(1) (2)

Fig. B2: Loading orientation for two

samples of schist; (1) loading is at an

angle to foliation, (2) loading is perpendicular to foliation

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Ground surface Ground surface

ROCK MASS A

ROCK MASS B

SANDY CLAYSANDY CLAY

40 m20 m

140 m

TUNNEL A TUNNEL B

Fig. B3: Proposed circular shaped tunnel in ROCK MASS A and ROCK MASS B

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1500 mm (core length)

130 mm

90 mm

90 mm

75 mm

100 mm

120 mm

135 mm

220 mm

75 mm

95 mm

70

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No. Mode of instability & factor leading to instability Recommended stabilisation

system

Rock reinforcement OR rock

support

1. Extensive groundwater flowing into the tunnel and induces high

pore-water pressure within the fractured rock in the tunnel walls.

2. Low shear strength of joints and sliding of unstable blocks (few

m3 in size) along joints and bedding planes

3. Rock mass with closely spaced joints that induce falling of smallrock blocks (0.3 m3 in size) on the tunnel roof.

4. Intersecting of two faults that lead to an unstable rock blocks of

more than 100 tonne in weight in the tunnel walls

Table B5: Modes of instability in rock mass surrounding excavated tunnel

Sab 2712

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