microcrack pattern propagations and rock quality designation of batu caves

- 591 -

Microcrack Pattern Propagations and

Rock Quality Designation of Batu Caves

Limestone

Nur Irfah Mohd Pauzi

Universiti Putra Malaysia

MTDRC, Level 9, Engineering Block, Faculty of Engineering,

43400 UPM, Serdang, Selangor, Malaysia

e-mail: [email protected]

Husaini Omar

Associate Professor, Universiti Putra Malaysia




Bujang Kim Huat

Professor, Universiti Putra Malaysia




Halina Misran

Senior Lecturer, Universiti Tenaga Nasional

Mechanical Engineering Department, College of Engineering,

KM 7, Jalan Ikram-Uniten, 43009, Kajang, Selangor, Malaysia


Zainuddin Md. Yusoff

Senior Lecturer, Universiti Putra Malaysia




ABSTRACT Microcrack pattern of limestone with various grade of natural weathering were studied to examine the

microcrack initiation and propagations in relation to Rock Quality Designation (RQD) and shear

strength parameters such as shear stress and normal stress. The degree of natural weathering were

determined using Schmidt hammer test and it was found that as the weathering grade increased from

grade I to grade IV, the microcrack length changed from ca. 500 μm to a longer length of ca. 800 μm.

The increased in microcrack length were coupled with the increased in width of the cracks as observed

using Scanning Electron Microscope (SEM). Furthermore, it was suggested that the microcrack length

of limestone is inversely proportional to RQD value with confidence level of 60%. However, the

microcrack length of limestone was observed to be directly proportional with shear stress and normal

stress with confidence level of 95% and 99% respectively.

KEYWORDS: microcrack propagations of limestone, rock quality designation, grade of

weathering, scanning electron microscope

Vol. 16 [2011], Bund. E 592

- 592 -

INTRODUCTION

Cracks were produced when the local stress exceeds the local strength. The local stress may be

augmented by twin lamellae interactions, kink bands and deformation lamellae, stress concentrations at

grain boundary contacts and around inter-crystalline cavities (Krantz, 1983). Microcrack pattern that was

produced around crystalline cavities for limestone rock was the main concern in this research. Limestone

was a sedimentary rock that was encountered in many constructions of engineering projects worldwide.

Limestone was known to have dissolve properties which could easily dissolve in the rain. Limestone rock

consists of calcium and carbonate minerals which define in chemical terms as CaCO3. Most researcher

has classified the rock to have physical properties of slightly resistance than most igneous rocks but more

resistant than most other sedimentary rocks. Limestone was partially soluble especially in acid, and

therefore forms many erosion landforms. Such erosion landscapes were known as karsts. Limestone has a

low water absorption capacity, medium compressive strength, quite low porosity and resistant to

weathering impact.

Limestone can be in crystalline, clastic, granular or massive, depending on the method of formation.

Limestone can be found in many colors especially on weathered surfaces. The colors were different due

to the impurities such as clay, sand, organic remains, iron oxide and other materials that can be found on

the limestone surface. The hardness of limestone based on Moh’s scale was 3-4, the compressive strength

was between 60 to 120 N/mm2, and the density was between 2.5 to 2.7 kg/cm

3.

Limestone rock was made of calcium carbonate in the form of the mineral calcite. Precipitation came

from the rain collects some carbon dioxide and produces a weak acid, known as carbon acid of which

calcite. Thus the calcium carbonate was vulnerable to when this acid reaches the limestone structure that

will cause the limestone to dissolve. Limestone which was categorized as carbonaceous rock weathers

based on the classification given in Fig. 1. The grade of weathering of carbonaceous rock can be divided

into five grade of weathering namely grade I – unweathered rock, grade II and grade III – slight to

moderate weathering and dissolution ,and finally grade IV and grade VI – rock mostly dissolved to

weathered to residual soil. The knowledge of grade of weathering in limestone rock was used as an

indication for collecting samples of different grade of weathering. Based on Fig. 1, the higher the location

of the limestone, the higher the degree of weathering since the top location of the Batu Caves are prone to

rainfall compared to the bottom location.

Vol. 16 [2011], Bund. E 593

- 593 -

Figure 1: Degree of weathering in carbonaceous rocks i.e. Batu Caves Limestone, Kuala

Lumpur

Many studies were done to investigate the effect of weathering on strength of the rock in relation to

change of color of rock and bonding of the rock grains (Deere and Patton, 1971) but not many researches

has been done on the study of crack propagation causes by natural weathering processes. Feng et.al

(2009) has observed the process of initiation, propagation and coalescence on cracks under the influence

of the chemical corrosion. The results indicated that the effect of chemical corrosion was quite

complicated depending on chemical ions and their concentrations and pH values, mineral components of

rock, geometry and the number of flows. Their studies was almost the same as weathering studies of

microcrack pattern but the differences was that they used chemical corrosion as agent of weathering. This

paper attempted to study the microcrack behavior in natural weathering process in relation to RQD and

shear strength parameters such as normal stress and shear stress.

The microcrack propagations in limestone as studied by Pauzi et. al. (2009) at Bukit Chuping

Limestone in Perlis area indicated that the crack which are produced by weathering process are studied

and analyzed in detail using SEM method. Weathering of carbonaceous rock like limestone starts from

the dissolving of the rock into smaller particles. Limestone rock is made of calcium carbonate in the form

of the mineral calcite. Precipitation coming from the rain collects some carbon dioxide and produces a

weak acid, known as carbon acid of which calcite. Thus the calcium carbonate is vulnerable to when this

acid reaches the limestone structure that will cause the limestone to dissolve. The process of dissolve of

the rock can be seen from the micropores, crack length and crack propagations of rocks from collected

samples at different grade of weathering. The increased in crack length and micropores of limestone is

increase as the weathering grade increase from grade I (unweathered rock) to grade VI (dissolved or

residual soil).

Vol. 16 [2011], Bund. E 594

- 594 -

Both studies shows that the crack propagation either by dissolving it in the chemical solutions or by

exposing it to the rain which also causing the dissolving of limestone started to propagates easier with

higher chemical content and high degree of weathering.

EXPERIMENTAL PROCEDURE

The experimental procedure was conducted to correlate the microcrack propagations with RQD

parameters. The samples are collected at Batu Caves, Kuala Lumpur. The detail description, sample

preparation, determination of microcrack propagation, determination of RQD, and determination of shear

strength are explained in details in this section.

DESCRIPTION OF SAMPLES

The samples were taken from Batu Caves area. It was located at Gombak district which is 13

kilometers north of Kuala Lumpur, Malaysia. It was selected as study area for collecting limestone

samples because the limestone there has existed almost 400 million years.

The color of the limestone samples collected from the Batu Caves Karst Formation was mainly white

colored with small beige-colored on the rock surface. Schmidt hammer test was used to determine the

grade of weathering of limestone. The Schmidt hammer was developed for non-destructive testing of

concrete hardness and only later applied to estimate rock strength. It consists of a spring loaded mass that

was released against a plunger when the hammer was pressed onto the stone surface. The plunge impacts

the surface and mass recoils, while the rebound value of the mass was measured by sliding pointer.

Schmidt hammer rebound values were often used to predict the uniaxial compressive strength of rocks

and rate of weathering. The details of the rock descriptions of sample A1 to sample F1 is shown in Table

1. The samples were collected based on different grade of weathering. The higher value of Schmidt

hammer test showed the higher strength of the samples and the less the grade of weathering.

Table 1: Samples description in terms of grain size, grade of weathering and Schmidt hammer Sample No. Grain Size (mm) Grade of Weathering Schmidt Hammer (N)

A1 0.1 – 0.2 I 12-22

B1 0.1 – 0.18 II 12-20

C1 0.15 III - IV 11-18

D1 0.1 – 0.12 IV - V 10-16

E1 0.11 V - VI 6-15

F1 Small grains VI 5-10

SPECIMEN PREPARATION

Specimens were prepared by slicing the samples into thin sliced section with 2 cm thick using cutter

machine. Six specimens were prepared for SEM microcrack analysis. The specimens were then placed on

the microscope stub and left it dried on an oven with 20oC temperature for one day. The specimens were

coated with copper layer with thickness of 0.1 mm using coating machine by Polaron Equipment Ltd. The

coating takes about 30 minutes to 1 hour to complete. The coated specimens were ready to be tested with

the Scanning Electron Microscope (SEM).

Vol. 16 [2011], Bund. E 595

- 595 -

DETERMINATION OF MICROCRACK PROPAGATIONS

SEM images of the specimens were taken using LEO 1455 at variable pressure at acceleration voltage

of 20 kV. The specimens can be observed in their natural state. The SEM is also equipped with 4

quadrant, back-scattered electron and detector type 225, which was manufactured by K.E. Development

LTD, Cambridge, England. The process of capturing microcrack was done by first identifying the

microcrack on the specimen. The crack was measured either horizontally or vertically. Any micropores

were also measured from the image. The different specimens for different grade of weathering were

tested. The author has labeled the specimen as Sample A1 to Sample F1 for easier references in terms of

grade of weathering and location of samples although the sample has been transform into specimen.

DETERMINATION OF RQD

RQD was determined from the rotary percussion drilling. The rotary percussion drilling tools was set

up using tripod, air compressor, drilling bits and split spoon sampler. A hole was drilled to pulverize the

rock using a rapid pneumatic hammer often known as down the hole hammer. Air compressor was needed

to drive the tools. The air also flushes the cuttings and dust from the borehole. Rotation of 10-30 radiuses

per minute to ensure the borehole was straight, and circular in cross section. The readings of the soil strata

are recorded for every 1 meter. Rock samples are also collected. The samples were labeled and put inside

box sampler. RQD was determined using the Equation (1). The drilling procedure was repeated for

another 2 more determinations with the locations not farther away from the original point.

RQD =

x 100% Eq. (1)

DETERMINATION OF SHEAR STRENGTH

Shear strength test was done to determine the friction angle, cohesion, normal stress and shear stress

of limestone rock. The Robertson direct shear box was used for shear strength determinations. The normal

loads used for shear strength testing were 5 kN, 10 kN, 20 kN, 30 kN and 40 kN. The normal load was

applied to the desired load. The loads were locked at constant normal load. The shear forced was applied

and the shear displacement readings were recorded for every increment of 0.5 kN. Shear stress versus

shear displacement was plotted to determine the highest shear stress applied. Furthermore, shear stress

versus normal stress was plotted to determine the friction angle and cohesion values.

RESULTS AND DISCUSSIONS

MICROCRACK INITIATIONS AND PROPAGATION ANALYSIS

The microstructure analysis of SEM image showed that the initiations of microcrack in Batu Caves

limestone occur at the microcrack length of ca. 500 μm or less. This was proved in Sample A1 (Fig. 2a).

It was analyzed that there were three microcracks started to propagate.

As the weathering increased to Grade II, the microcrack length has become smaller with microcrack

length ca. 100 μm. The result in Sample B1 (Fig. 2b) indicated that the microcrack length was measured

to be in the range of 90 μm to 182 μm. The microcrack length has started to have a gap which was called

as micropores with the width of 88 µm. The micropores would be a placed for water to seep through the

Vol. 16 [2011], Bund. E 596

- 596 -

internal bonding. Thus it will reduce the bonding between the grains. But the internal bondings are still

strong at this stage.

In Sample C1 with the grade weathering III to IV, the weathering processed has caused the

microcrack length to be multiple in numbers. The microcrack length has become smaller with the length

ca. 100 µm or less but the micropores has widened with the width of 105 µm. A number of micropores

which was developed as the weathering increased causing the slightly loose bonding between the grains.

The microcrack length was to propagate into a smaller microcrack length but wider micropores in Fig. 2c.

The multiple microcrack length has shortened into ca. less than 50 µm in Sample D1 with grade of

weathering IV to V. There was no sign of micropores in sample D1 (Fig. 2d) but in sample E1, the

micropores has widen in width ca. 100 µm to 400 µm. The widening of micropores has caused the

internal bonding to become weak. The microcrack length was also increased in length to ca. 800 µm in

Sample E1 (Fig 2e).

In Sample F1, the microcrack length was maintained in between 300 µm to 700 µm. However, the

micropores widen with the width of 82 µm to 277 µm. The internal bonding between the grains is weak

and loose because of the larger micropores which reduce the strength of the rock. The large width of

micropores is shown in Fig. 2f.

(a) (b)

Figure 2: (a) Sample A1 (grade I), (b) Sample B1 (grade II), (c) Sample C1(grade III –IV) (d)

Sample D1 (Grade VI –V), (e) Sample E1(Grade V-VI) and (f) Sample F1(Grade VI)

Continued on the next page

Vol. 16 [2011], Bund. E 597

- 597 -

(c) (d)

(e) (f)

Figure 2: (a) Sample A1 (grade I), (b) Sample B1 (grade II), (c) Sample C1(grade III –IV) (d)

Sample D1 (Grade VI –V), (e) Sample E1(Grade V-VI) and (f) Sample F1(Grade VI)

Table 3: SEM microcrack pattern analysis

Sample Label No Grade of Weathering Micropores / Void

(µm)

Crack Length

(µm)

Internal Bonding

A1 I No void 585, 486, 516 Strong

B1 II 88 182, 90, 167 Strong

C1 III-IV 105 197, 145, 110, 94 Slightly loose

D1 IV-V No void 110, 55, 54, 38 Slightly loose

E1 V-VI 402, 137, 150 137, 276, 800 Weak bonding

F1 VI 277, 82, 166, 111 321, 305, 700 Week bonding

Vol. 16 [2011], Bund. E 598

- 598 -

The microcrack pattern can be plotted into a histogram analysis to see the most frequent and

maximum crack length of the limestone rock. The highest microcrack length is 585 micrometers, the

lowest microcrack length is 37.7 micrometers and the average microcrack length is 202.5 micrometers.

The plot of the histogram is shown in Fig. 3. The highest micropore is 402 micrometers, the lowest

micropore is 88.1 micrometers and the average micropore is 176.4 micrometers. The plot of histogram for

micropore is shown in Fig. 4.

Figure 3: Crack length of limestone samples

Figure 4: Micropores of limestone samples

0

100

200

300

400

500

600

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Cra

ck L

en

gth

(m

icro

me

ters

)

Microcrack No.

Crack Length

Crack Length

0

100

200

300

400

500

1 2 3 4 5

Mic

rop

ore

s (m

icro

me

ter)

Micropores No.

Mircopores

Mircopores

Vol. 16 [2011], Bund. E 599

- 599 -

RQD OF LIMESTONE ROCK

Three boreholes logging data are determined at Batu Caves Karsts formation sites. The boreholes

indicated that the rocks are encountered at the depth of 3 meter and below. The depth of the borehole

logging is 6 meters. The soil strata of the borehole logging are mainly limestone rock which has slightly

fractures. RQD of limestone rock at Batu Caves from three different bore hole logging is shown in Table

4. The average value of RQD is 51.67%. The RQD value of 51.67% means that the quality of rock is in

the range of poor to good quality of rocks.

Table 4: RQD for limestone rock at Batu Caves Karsts formation Test Parameters BH 1 BH 2 BH 3

Core Length (cm) 200 200 200

length of core more the

10 cm (cm)

85 + 25 = 110

74 + 33 = 107 30 + 21 + 42 = 93

RQD (%) 55.0 53.5 46.5

SHEAR STRENGTH OF LIMESTONE

The shear strength of limestone rock are tested for 5 different normal loading which are 5 kN, 10 kN,

20 kN, 30 kN and 40 kN. A series of shear strength test are carried out to determine the shear

displacement, shear test, normal stress and shear stress. Results of shear stress versus shear displacement

are plotted in Fig. 5. The highest shear stress is 6.5 MPa; the highest normal stress is 23.7 MPa. The shear

stress versus normal stress is also plotted to get the friction angles and cohesion values of limestone. The

friction angle is 530 and the cohesion is 0.987 MPa. The plot of shear stress versus normal stress with the

equation display is shown in Fig. 6.

Figure 5: Shear stress versus Shear Displacement for Batu Cave limestone

0

1000

2000

3000

4000

5000

6000

7000

0 1 2 3 4 5 6 7

She

ar S

tre

ss (

kN/m

2)

Shear Displacement (mm)

Normal Load 5 kN

Normal Load 10 kN

Normal Load 20 kN

Normal Load 30 kN

Normal Load 40 kN

Vol. 16 [2011], Bund. E 600

- 600 -

Figure 6: Shear stress vs. normal stress for Batu Cave limestone

CORRELATIONS OF MICROCRACK PATTERN AND RQD OF LIMESTONE

Microcrack pattern of limestone is compared with RQD from the borehole logging. The microcrack

length values are plotted against RQD values. In sample D1, there are multiple microcrack can be

observed but only the maximum length is plotted against RQD value. The same case for sample E1 and

Sample F1, only the maximum length are taken for analysis. The details of RQD, microcrack and grade of

weathering are shown in Table 5.

Table 5: RQD, microcrack length, grade of weathering and descriptions of rock BH No. RQD

(%)

Microcrack

Length (μm)

Sample No. Grade of Weathering Descriptions of Rock

BH1 55.0 110 Sample D1 IV – V Rock mostly dissolved

BH2 53.5 276 Sample E1 V-VI Rock mostly dissolved

BH 3 46.5 321 Sample F1 VI Rock mostly dissolved/

residual soil

Theoretically, microcrack length increases with decreasing RQD values. The result in Fig. 7 shows

that RQD is inversely proportional to the microcrack length. The standard error bars together with the R2

= 0.607 are shown in the plotted graph. Microcrack length is at the maximum when the degree of

weathering increased. Since there are other parameters which would also affecting the microcrack length

y = 1.351x + 0.987

0

1

2

3

4

5

6

7

8

9

0 1 2 3 4 5 6

She

ar S

tre

ss (

MP

a)

Normal Stress (MPa)

Shear stress vs Normal Stress

Linear (Shear stress vs Normal Stress)

Vol. 16 [2011], Bund. E 601

- 601 -

such as degree of weathering, the mineral crystallization of limestone rock and micropores. More

experimental series need to be carried out to evaluate the crystalline of minerals in limestone related to its

microcrack length for limestone rock.

Figure 7: RQD versus Microcrack Length

CORRELATIONS OF MICROCRACK PATTERN AND SHEAR STRENGTH OF LIMESTONE

Shear strength parameters are important parameters in determining the stability of limestone rock.

The cohesion and friction angles will determine the safe height or safe angle for Batu Caves formation.

Previously, the cohesion and friction angle have been determined. The limestone rock has high friction

angle but low cohesion values. Further analysis of the data, the shear stress and normal stress is correlated

with its microcrack length to know their correlations. The shear stress is linearly proportional with

microcrack length with confidence level of 95%.Normal stress is also linearly proportional with

microcrack length with confidence level of 99%. Fig. 8 and Fig. 9 show the shear stress and normal stress

plot respectively.

y = -0.0318x + 59.164R² = 0.6073

0

10

20

30

40

50

60

70

0 50 100 150 200 250 300 350

RQ

D (

%)

Microcrack Length (micrometer)

RQD vs Microcrack Length

Vol. 16 [2011], Bund. E 602

- 602 -

Figure 8: Shear Stress versus Microcrack Length

Figure 9: Normal stress versus Microcrack Length

y = 0.0109x + 1.3693R² = 0.9504

0

1

2

3

4

5

6

0 50 100 150 200 250 300 350

She

ar S

tre

ss (

MP

a)


Shear Stress vs. Microcrack Length

y = 0.0402x - 1.2784R² = 0.9998

0

2

4

6

8

10

12

14

0 50 100 150 200 250 300 350

No

rmal

Str

ess

(M

Pa)


Normal Stress vs. Microcrack Length

Vol. 16 [2011], Bund. E 603

- 603 -

CONCLUSIONS

Microcrack pattern that are produced around crystalline cavities for limestone rock are the main

concern in this research. The microcrack pattern of limestone rock is related to its RQD and shear

strength. It can be concluded that RQD value is inversely proportional with microcrack length. The RQD

value is inversely proportional to microcrack length, shear stress is linearly proportional with microcrack

length and normal stress is linearly proportional with microcrack length with confidence level of 61%,

95% and 99% respectively.

ACKNOWLEDGEMENT

The author would like to acknowledge the Ministry of Science and Technology Malaysia (MOSTI)

for supporting the research under FRGS grant with grant number 01101034 FRGS. Thanks to Universiti

Tenaga Nasional and Universiti Putra Malaysia for the laboratory facilities.

REFERENCES 1. Deere D.U. and Patton F.D., 1971, Slope Stability in Residual Soils, Proceedings of the Fourth

Pan-American Conference on Soil Mechanics and Foundation Engineering, San Juan, Puerto

Rico.

2. Feng X.T., Ding W and Zhang D, 2009, Multicrack interaction in limestone subject to

stress and flow of chemical solutions, International Journal of Rock Mechanics and

Mining Sciences 46, 159-171

3. Fredrich J.T., Evans B., Wong T.F., 1990, Effect of grain size on brittle and semi brittle

strength implication for micromechanical modeling of failure in compression, J Geophys

Res 95: 10907-20

4. Griffith, A.A., 1920, The phenomena of rupture and flow in solids: Philosophical Transactions

Royal Society, series A, volume 221, pp 163-198

5. Kranz, R.L., 1983, Microcracks in rocks; a review, In: M. Friedman and M.N. Toksoz

(Editor) Continental Tectonics Structure, Kinematics and Dynamics, Techtonophysics, 100:

449-480

6. Nasseri M.H.B., Mohanty B. and Robin P.Y.F, 2005, Characterization of Microstructure and

Fracture Toughness in Five Granitic Rocks, International Journal of Rock Mechanics & Mining

Sciences 42, 450 – 460

7. Pauzi N.I.M., Omar H., Omar R.C., and Yusoff Z.M., 2009, Microcrack Propagation in

Weathered Limestone in Bukit Chuping, Perlis, Regional Conference On Environmental and

Earth Resources, 190-198

8. Simmons G. and Richter D., 1976, Microcracks in Rocks, In R.G.J. Strens (editors), The Physics

and Chemistry of Minerals and Rocks, Wiley, New York, N.Y. , 105-137

Vol. 16 [2011], Bund. E 604

- 604 -

9. Whittaker B.N., Singh R.N., and Sun G, 1992, Rock fracture mechanics principal, design and

applications, Amsterdam, Elsevier

10. Wong R.H.C, Chau K.T., and Wang P., 1996, Microcracking and grain size effect in Yuen

Long marbles, International Journal of Rock Mechanics & Mining Sciences

Geomechanics Abstr 33 (5): 479 - 85

© 2011 ejge

http://www.ejge.com/copynote.htm

microcrack pattern propagations and rock quality designation of batu caves

Documents