Interaction Between Rock Minerals, Weathering, Strength Propeties, Etc_rev1a1

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INTERACTION BETWEEN MINERAL CONSTITUENTS, WEATHERING, FRACTURING

AND STRENGTH AND HYDRAULIC PROPERTIES OF BUKIT TIMAH GRANITE

Tint Lwin Swe, M. E. Geo, MMGSS, MGeoSS,

[email protected]

Abstract

Bukit Timah Granite generally consists of quart, feldspar, mica and hornblende and other minor

accessories. Individual mineral grains alter due to weathering of rock mass. The alteration of those

minerals increases with increasing degree of fracturing and brecciation which in turn affects the great

variation of the strength of the rock material.

The intact rock samples are not always be tested for representing typical strength value of the

Singapore Granite. Most of the samples are affected by various degrees of weathering and some

internal cracks appeared as resulted from the tectonics activities in the past. The relationship betweenthe strength values of Singapore Granite and weathering grades with or without the cracks in the

material are described in this study.

The density and network of discontinuities in a rock mass also play major role in determining the

hydraulic properties of the Bukit Timah Granite. Generally, the highly fractured rock mass with a

considerable thickness of in situ rock mass have the larger the transmissivity and the hydraulic

conductivity values. The interaction between those physical processes and properties of Bukit Timah

Granite is disclosed based on the results of drilled core mapping, laboratory tests and field pumping

tests.

Key words: Granite, Bukit Timah, alteration, strength, RQD, transmissivity, hydraulic conductivity

Introduction

The Bukit Timah Granite widely distributed in the central and northern parts of Singapore is named

to a group of granitic rocks consisting of granite to granodiorite and diorite, and some xenoliths or

inclusions of gabbro and dolerite. The granite is generally light grey and medium-grained. The main

minerals are quartz, which often accounts for 30 % of the mineral present, and feldspar, which often

constitutes 60 to 65 % of the rock. Microscopically, the granite consists predominantly of quartz and

varying amounts of alkali feldspar and acid plagioclase with subordinate amounts of biotite andhornblende (DSTA, 2009). As an interlocking nature, the granite is very strong rock but the

constituent minerals alter due to weathering leading to the rock weaker. Normally rock weathering is

classified based on materials or hand specimen collected from bore holes during site investigation

works of various major civil engineering projects in Singapore. On the other hand, the rock strength

measurements are also made on such collected samples in the laboratory. The uniaxial compressive

strength (UCS) test and point load strength (PLS) test are routinely carried out in accordance with

ISRM suggested methods (ISRM, 1979 and 1985).

Water percolation in the rock mass is dominated by the fracture state which is stated in this study in

terms of Rock Quality Designation, RQD (Deere and Deere, 1988). Field pumping tests were carried

out during construction of Downtown Line Stage 2 MRT station construction projects along theUpper Bukit Timah Road, Singapore where the Bukit Timah Granite Formation occupied. Total 5

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pumping tests were done in different locations and pump duration is ranging from 8 hrs to 48 hrs.

The sizes of pumping wells and monitoring wells are 200mm and 100mm respectively. Drawdowns

in monitoring wells were recorded over the time during pumping wells by using water level

indicators. Air lift method is used for pumping wells.

Weathering, fault rocks and classification

The effect of weathering is more obvious in the upper parts of the Formation which can be

understood from the typical weathering profile of the fresh rock through the residual soil. For inner

part of the rock mass, the weathering normally follows the fractures in the rock in connection with

groundwater fluctuation and chemical constituents in the water. Shirlaw et al (2000) also indicated

the role of temperature and consistent precipitation on Singapore Granite weathering. When

weathering took place, the minerals of rock turned to decompose and the strength properties of the

rock alter. The altered granitic mineral assemblages comprises from irregular shape to the more

rounded with decreasing sizes. Alteration of K-feldspar is very obvious transforming into brown

colored clayey products. Biotite altered partially to completely to chlorite and opaque. Minerals

alteration of weathered Bukit Timah Granite is shown in Figure 1.

Figure 1 Photomicrograph of an altered hornblende granite (Bukit Timah Granite). Blue-purple is chloriteafter biotite. Clay and epidote alteration after plagioclase is speckled brown. The grey, central area is quartz.The dominantly black area at the upper left corner is altered hornblende. The breccia vein dominates much ofthe right hand side with its numerous quartz clasts. Diameter of field 2.5mm. (Courtesy of C. M. Gray.)

Fault rocks were observed frequently in the collected core runs. Sometimes; those rocks are

misinterpreted as the highly weathered or completely weathered granite. The observation indicates

that the brecciation if it occurred in the rock mass can also accelerate the rate of decomposition or

mineral alteration. Figure 2 shows brecciated granite of which discoloration of rock: light to dark

yellow color in the original grey, is quite obvious in the brecciated rocks.



Figure 2 Weathering of the brecciated granite (Bukit Timah Granite): Loc. Gambas Avenue,

Singapore.

The weathering grades of granite in the field or on observation of core are classified in accordance

with BS5930. Weathering classification is mainly based on the decomposition of rock and is not

applicable to structural situation. The intensity of weathering increases with increasing fracture

density but the weathering induced fractures are rarely found in this study. There is the only thing to

note the effect of weathering on the highly fractured rock mass that some rock pieces are easily

disintegrated when exposed and lost its natural strength at the interface of discontinuities.

Weathering classification and observation on rock cores of Bukit Timah Granite are summarized in

Table 1.

Table 1. Weathering classification (BS5930) and core observation of Bukit Timah Granite.

Grade Classifier Typical characteristics Graphic Remarks

I Fresh Unchanged from original state Intact

Microcracks

II Slightlyweathered

Slight discoloration, slightweakening

Intact

Fissures

III Moderatelyweathered

Considerably weakened, penetrative discoloration, Large pieces cannot be broken by hand

Microcracks

Joints

IV Highlyweathered

Large pieces cannot be broken byhand. Does not readilydisaggregate (slake) when drysample immersed in water.

Often associatedwith joints andfractures

V Completelyweathered

Considerably weakened, Slake,Original texture apparent

VI Residual soil Soil derived by in situ weathering but retaining none of the originaltexture or fabric



Interaction between weathering and the strength of granite

Drilled core samples of Bukit Timah Granite were collected from various depths and various places

of Singapore for physical observation and the strength tests. These samples were more or less

affected by various degrees of weathering. Sometimes; fissures but intactness is still remaining in the

collected rock specimens which in turn affected on the strength properties. The strength tests werecarried out in an accreted commercial laboratory in Singapore. Among those, 144 USC and PLS

results were selected for analysis. Correlation between UCS and PLS of Bukit Timah Granite is

shown in Figure 3. The relationship can be expressed as

UCS=10.444PLS + 1.1264 1

Standard error = 0.357PLS+2.5729

Where, UCS is the uniaxial compressive strength (MPa) and

PLS is the Point Load Strength (MPa).

Figure 3. The relationship between uniaxial compressive strength and point load strength of Bukit TimahGranite. Colored lines are 95% confident limit. Strength tests were done on drilled core granite samplescollected during geotechnical site investigation works for major civil engineering projects of Singapore.

According to classification and observation, core samples can be categorized as fresh, fresh with

micro-cracks, slightly weathered, slightly weathered with some cracks, moderately weathered,

moderately weathered with cracks and highly weathered. Obviously, the fresh specimen has higher

strength value than the weathered specimen. Table 2 and Figure 4 demonstrate the influences of

weathering and fissures on the strength property of Bukit Timah Granite.



Table 2. Summary of uniaxial compressive strength (MPa) of Bukit Timah Granite of different weatheringgrades and intactness.

Description FreshFresh with

micorcracks

Slight

Slight

with

somecracks

Moderate

Moderate

with

cracks

High

Mean 100.5 41.3 86.5 37.0 34.7 15.9 10.9

Standard Error 2.4 4.1 3.5 2.2 4.4 3.3 1.9

Median 100.0 38.2 79.5 34.3 28.0 11.5 9.9

Standard Deviation 23.7 17.2 19.8 17.6 17.5 11.3 3.8

Minimum 59.1 15.8 60.5 5.9 12.5 3.4 7.8

Maximum 161.1 78.3 127.3 77.7 72.3 48.0 15.9

Count 96 18 32 65 16 12 4

Confidence Level (95.0%) 4.8 8.6 7.1 4.4 9.3 7.2 6.0

Figure 4. Correlation between uniaxial compressive strength, weathering grade and intactness. Details arereferred to Table 1. (I. Fresh, II. Fresh with microcracks, III. Slightly weathered, IV. Slightly weathered with

some cracks, V. Moderately weathered, VI. Moderately weathered with cracks, VII. Highly weathered).

Fractured rock hydraulic properties

Drilled core logging was carried out in accordance with BS5930 and the ISRM suggested methods

for quantitative description of discontinuities in rock masses ISRM (1978). Typical rock core

samples are shown in Figure 5. These fractured rock condition can be expressed in terms of Rock

Quality Designation, RQD. Quantities of fracture and RQD in each core run were recorded properly.

There is a relationship between the RQD and the fracture count per meter of the Bukit Timah

Granite. If the rock is slightly or moderately weathered or fresh state, RQD can be predicted from

fracture count by using the following equation. The relationship between RQD and fracture count isshown in Figure 6.



RQD= 109.95-7.279

Standard error = 1.5163+0.2165

Where, is the fracture count (between 2 and 14).

Figure 5. Core box of fractured granite (Bukit Timah Granite). LTA DTL2 C916 Beauty World Station, Upper

Bukit Timah Road, Singapore.

Figure 6. The relationship between RQD and fracture count of Bukit Timah Granite. Colored lines are 95%confident limit.

The hydraulic properties of the rock mass are strongly influenced by fracture density, network and

apertures, etc. The infilling materials, thickness and cementation nature of those materials more or

less influence the hydraulic condition of the rock mass. But this effect is not considered in this paper.

Zhao et el (1994) and Zhao (1998) also discussed the rock mass permeability of Singapore Granite

based on packer tests that there is the highest permeability in the highly fractured and weatheredzones of G-IV and G-V and lower value in the residual soil.



Field pumping tests were done in Bukit Timah Granite Formation during construction of LTA DTL2

projects in Singapore. The summary results are shown in the form of graphs in figures 7 to 11.

Hydrogeological parameters of fractured Bukit Timah Granite resulted from carefully done pumping

tests such as transmissivity, T, storage coefficient, S and hydraulic conductivity, k are determined by

developing time-drawdown relationship using Theis’s modified non-equilibrium equation (Bear,1979). If the slope of the straight-line segment is expressed as the drawdown difference, s per log

cycle of time.

T = 2.3Q/4s 3

If this line is extended until it intercepts the time-axis where s = 0, the interception point has the

coordinates s=0 and t=t0, Substituting these values to above equation gives

S = 2.25Tt0/r 2 4

The results from monitoring wells by pumping test are summarized in Table 3. Rock mass conditionof well sections were observed and described in terms of RQD. Generally, RQD less than 75% can

be recognized as the fractured rock with regardless of the effect of weathering and infilling.

Table 3 Pumping test results and RQD of rock core for each monitoring well.

Monitoring

Well

Discharge,

Q (l/s)

Thickness,

B (m)

Drawdown,

s (m)

Transmissivity,

T (m/day)

Permeability,

K (m/day)

Storage

coefficient,

S RQD#

MW1 1.3 2.4 1.17 17.57 7.32 0.00025734 *

MW2 1.3 3.3 0.95 21.64 6.56 0.00131088 77

MW3 1.3 3 1.05 19.58 6.53 0.00110114 0

MW4 0.42 1.7 0.32 20.75 12.21 0.00081063 51

MW6 1.62 3.9 1.07 23.94 6.14 0.00014962 47

MW7 1.62 6 1.05 24.39 4.10 0.00051457 31

MW8 1.62 2.2 1.11 23.08 10.49 0.00015324 *

MW8A 1.62 8 1.35 18.97 2.37 0.0000732 *

MW11 0.17 1.5 0.16 16.80 11.20 0.00103044 13

MW1 1.44 4 0.64 35.58 8.89 0.00101755 27

MW1A 1.44 5 0.86 26.47 5.29 0.00102744 20

MW2 1.69 8.25 1.06 25.28 3.10 0.000709 14

# Average value for the monitoring section.

*RQD data are not available.





Figure 9 Time-drawdown results for 4 monitoring wells of pumping test PW2. Loc. Upper Bikit

Timah Road, Beauty World Station and Tunnel Construction Project


Timah Road, Beauty World Station and Tunnel Construction Project




Timah Road, Bukit Panjang Station Construction Project

If RQD is used as an indicator of fracture index or the value representing the fractured rock mass,

then it can be correlated to permeability value of the Bukit Timah Granite and the equation becomes

k = -0.2176RQD + 12.505 5

Where, k is permeability of fracture rock in m/day

RQD is the rock quality designation less than or equals 50%

k = -0.2176RQD + 12.505

R 2 = 0.9111

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

0 10 20 30 40 50 60

RQD (%)

Permeability(m/d

ay

Figure 12 Correlation between RQD and permeability of fractured Bukit Timah Granite.



Conclusion

The rock core samples were carefully observed in order to get information on weathering of granite

and rock fracture in the Bukit Timah Granite. Rock strength test and field pumping test were also

carried out during site investigation works for major civil engineering projects of Singapore where

Bukit Timah Granite occupied. It is obvious that the strength property of Bukit Timah Granite isstrongly influenced by weathering and alteration of constituent minerals as well as the presence of

fissures in the rock. The Rock Quality Designation, RQD is used as an index for the fractured rock.

RQD less than 75% are described as the fractured rock which is correlated to hydraulic properties of

the rock. The lower the RQD value, the poor the rock mass with highly fractured condition and high

water percolation.

References

BS5930:1999. (October 1999). Code of practice for site investigation. BSI.

Bear, J. (1979). Hydraulics of Groundwater, Dover Publications Inc. NY. 569p.Deere, D. U. and Deere, R. W. (1988). The rock quality designation (RQD) index in practice, Rock

Classification Systems for Engineering Purposes, edtr. Louis Kirkaldie, 91-101, ASTM,

1988.

DSTA, 2009. “Geology of Singapore”, 2nd ed., Defence Science and Technology Agency, Singapore.

ISRM. (1978). Suggested method for quantitative description of discontinuities in rock masses, Intl.

J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol. 15, 319-368.

ISRM. (1979). Suggested method for determining uniaxial compressive strength and deformability

of rock materials. Intl. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol. 16, No. 2. 137-140.

ISRM. (1985). Suggested method for determining point load strength, Pp 51-60, 1985.

Shirlaw JN, Hencher S, Zhao J (2000). Invited issue lecture: underground excavations in tropically

weathered soils and rocks. Proc. GeoEng2000 International Conference on Geotechnical,

Geological Engineering , Melbourne, Australia Zhao, J., B.B. Broms, Y. Zhou, and V. Choa. (1994). A study of the weathering of the Bukit Timah

Granite- Part A: Review, field observation and geophysical survey, IAEG, Bulletin of

Engineering Geology and Environment, No. 49, (1) 1994, 97-106, Paris.

Zhao, J. (1998). Rock mass hydraulic conductivity of Bukit Timah Granite, Singapore, Engineering

Geology, 50(1998), 211-216, Elsevier.

Acknowledgements

The author thanks to his colleagues from The West Group Pte Ltd. for their kind supports and

advices during wells installation, pump testing and groundwater monitoring for Singapore DTL2

projects from 2010 to 2012. The grateful thanks are also extended to colleagues from Econ Geotech

Pte Ltd. for their various helps during geotechnical site investigation works for various projects

carried out in Singapore for the period from 2008 to 2010.

Documents

Interaction Between Rock Minerals, Weathering, Strength Propeties, Etc_rev1a1