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Introduction
Porosity is one of the important physical properties thatgovern physical attributes of rocks such as strength,deformability and hydraulic conductivity (Franklin andDusseault 1991). Porosity is the representativecharacteristic of rock which shows the particle formationchange due to the weathering process. Such weatheringprocess is more rapid in tropical climate due to the hightemperature and high humidity. The boundary crack thathas been developed during weathering processcontributes to the porosity of weathered rock mass.
The fracture of mineral grains (grains boundary) iscontinuously developed and creates bulk of crack line allover the rock mass. Those cracks can be subdivided intotwo categories as micro-cracks and macro-cracks. Portionof soluble minerals decreases as the weathering degreeincreases. It controls the change of porosity of graniticrock mass.
This study presents the evaluation of micro andmacro level porosities of granitic rock by using analyticalimage processing and saturation and caliper techniquesconsidering the degree of weathering. Furthermore,additional experimental tests were performed tocharacterize the relation between porosity and variousengineering properties such as uniaxial compressivestrength (UCS), resistivity and compressive wave velocity depending on the degree of weathering.
Materials and methods
Four different grade of Tropical granite with no presenceof bedding were taken from typical quarry in SungaiBuloh, Selangor with total of 34 samples. Reboundhammer was used as initial identification of weatheringdegree (WD) of r ock sample (refer Figure 2).
Saturation and caliper method (ISRM 1981) wereconducted to determine macro scale porosity. Theporosity was calculated based on the fluid fill the porespace and microcraack.
AcknowledgmentsThis research was made possible by the support from
Faculty of Civil Engineering, University Technology MARA, Malaysia.
ResultsFrom the study, it can be generalized that the study wassuccesfull characterized the porosity based on the
weathering degree. The porosity increases as the degreeof weathering increases (i.e., rebound number
decreases).
It was found that:-
The macro scale porosity of weathered graniteobtained from the saturation and caliper technique variesbetween 1.5 % and 14.0 %. It can be concluded that porespace expands and pore structure is developed due to the
weathering process. The micro scale porosities of tropical weatheredgranite ranged from 2 % to 8 %.
The porosity of the relatively fresh rock (i.e., WD Iand WD II) was as small as 4 % thus both approachesshowed similar porosity range.
Porosity difference between two different methodsbecame significant in moderate and highly weatheredgranite (i.e., WD III and WD IV): micro scale porosity
was smaller than the macro scale porosity.
From the electro microscopy images, it can be deducedthat micro-cracks occur along the boundaries of mineralgrain and contribute to the development of porestructure before the mineral grains begin to break.
ConclusionsThe evaluation process of micro and macro levelporosities of granitic rock by using analytical imageprocessing and saturation and caliper techniques takinginto consideration the degree of weathering is presentedin this study. The salient findings obtained from thisstudy can be summarized as follows:
• Porosity increases as the weathering degree increases.• Macro scale porosity is relatively higher than micro
scale porosity.• Porosity has significant influence on the engineering
properties: As porosity increases, compressive wave velocity, resistivity, and uniaxial compressive strengthdecrease.
Ahmad Syauqi Md Hasan Zainab Mohamed Ki-Il Song
Faculty of Civil Engineering, University Technology MARA Malaysia, 40450 Shah Alam, Selangor, Malaysia
Literature citedHusaini, O., Pauzi, N.I., Shariah, M.A., Yusof, Z.M., and
Maail,S. 2009. Microcracks pattern and the degree of weathering in granite. Electronic Journal of Geotechnical Engineering 14: 1-21.
Lindqvist, L.E., Akesson, U., and Malaga, K. 2007Microstructure and functional properties of rockmaterials. Materials Characterization 58: 1183-1188.
Takemura ,T., Golshani, A., Oda, M., Suzuki, K. 2003.Preferred orientations of open micrcracks in graniteand their relation with anisotropic elasticity.International Journal of Rock Mechanics and MiningSciences40: 443-454.
Tugrul, A. 2004. The effect of weathering on pore geometry and compressive strength of selected rock types fromTurkey. Engineering Geology 75: 215-227.
Franklin ,J.A., and Dusseault, M.B. 1991. Rock engineeringapplication. McGraw-Hill, New York.
International Society for Rock Mechanics, RockCharacterisation, Testing and Monitoring. In: BrownET(Ed.), ISRM suggested methods. Oxford:Pergamon; 1981. p. 211
Figure 2. Rebound number and dry density of collected granitesamples. Note: WD denotes weathering degree.
Figure 5. Variation of micro and macro scale porosity dependingon the degree of weathering
For further informationPlease contact [email protected].
Fresh rock : porosities in the horizontal and verticaldirection are almost identical, since micro-cracks rarely develop and pore structure is relatively isotropic.
Moderate and highly weathered rock : the micro-cracks arefully developed and pore structure shows anisotropiccharacteristic in horizontal and vertical direction.
Inter-relation between Porosity and Engineering Properties
The changes of porosity due to the weathering process havesignificant influence on the various engineering properties.
• Compressive wave velocity decreases as the porosity increases.
• Resistivity decreases as the porosity increases.
• Uniaxial compressive strength decreases as theporosity increases.
Micro and macro approaches for characterization of porosity
depending on the weathering degree
Figure 1. Presenceof micro-crack inGranite
2.2
2.32.4
2.5
2.6
2.7
0 10 20 30 40 50 60 70
ReboundNumber,R
D r y D e
n s i t y ( g / c m
3 )
.
WDIWDII
WDIII
WDIV
Sampleplacedondessicator
Weightthesample
Dry thesampleintheovenat105°C
Weightthesample Figure 3.Saturation andcaliper methodtechnique
Analytical image processing technique to determine theporosity from the images taken from the petrographicthin section is newly suggested in this study. Porosity calculated in this approach can be considered as microscale porosity
Figure 4. Description of calculation process of analytical imageprocessing from petrographic thin section slide
0
2
4
6
8
10
12
14
16
0 10 20 30 40 50 60 70
ReboundNumber, R
P o r o s i t y
( % )
WDI (Macro)
WDII(Macro)
WDIII(Macro)
WDIV(Macro)
Average (Macro)
WDI (Micro)
WDII(Micro)
WDIII(Micro)
WDIV(Micro)
Average (Micro)
Variation ofmacro
scale porosity
Varition ofmicro
scale porosity
porosity increases as the degree of weathering increases
0
1000
2000
3000
4000
5000
0 3 6 9 12 15
Porosity (%)
C o m p r e s s i v e w a v e v e l o c i t y ( m / s e c ) WDI(Macro)
WDII(Macro)
WDIII(Macro)
WDIV(Macro)
Average (Macro)
WDI(Micro)
WDII(Micro)
WDIII(Micro)
WDIV(Micro)
Average (Micro)
Macro scale porosity vs
compressive wave velocity
Micro scale porosity vs
compressive wave
(a)
0
20
40
60
80
100
0 3 6 9 12 15
Porosity (%)
U n i a x i a l c o m p r e s s i v
e s t r e n g t h ( M P a ) . . Macro scale porosity vs
uniaxialcompressive strength
Micro scale porosity vs
uniaxialcompressive strength
(c)
v
v v
y
/
WDI(Macro)
WDII(Macro)
WDIII(Macro)
WDIV(Macro)
Average (Macro)
WDI(Micro)
WDII(Micro)
WDIII(Micro)
WDIV(Micro)
Average (Micro)
1.0E+00
1.0E+01
1.0E+02
1.0E+03
1.0E+04
1.0E+05
1.0E+06
1.0E+07
0 3 6 9 12 15
Porosity (%)
R e s i s t i v i t y ( O h m · m ) .
Macro scale porosity vs resistivity in dry
Macro scale porosity vs resistivity in saturation
Microscale porosity vs resistivity in dry
Micro scale porosity vs resistivity in saturation
(b)
v
v v
/
WDI(Macro)
WDII(Macro)
WDIII(Macro)
WDIV(Macro)
Average (Macro)
WDI(Micro)
WDII(Micro)
WDIII(Micro)
WDIV(Micro)
Average (Micro)
v
v v v
v
v v
Figure 7. Porosity against a) Compressive wave velocity,b) Resistivity, and c) Uniaxial compressive strength
Figure 6.Direction of micro-craks
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