Comparison of Geological Mapping with Electrical Resistivity ...ASTM D6431-99 as a guide for conducting the survey. The data was processed using RES2DINV software [16, 17]. The length

1346 Chiang Mai J. Sci. 2016; 43(6)

Chiang Mai J. Sci. 2016; 43(6) : 1346-1357http://epg.science.cmu.ac.th/ejournal/Contributed Paper

Comparison of Geological Mapping with ElectricalResistivity and Ground Penetration Radar Methodsfor Rock Fractured System StudyMohd Hariri Arifin [a], Tajul Anuar Jamaluddin* [b], Hamzah Husin [c], Azmi Ismail [d],

Amry Amin Abbas [d], Mohd Nawawi Mohd Nordin [a], Hassan Mohamed Baioumy [e],

John Stephen Kayode* [a], Nadzari Ismail [f], Nor Azura Othman [f], Anita Setu [f] and

Mohd Amir Ashraf Sulaiman [g]

[a] School of Physics, Universiti Sains Malaysia, Pulau Pinang, Malaysia.

[b] Program Geology, Universiti Kebangsaan Malaysia, Selangor,Malaysia.

[c] Fakulti Sains Bumi, Universiti Malaysia Kelantan, Kelantan, Malaysia.

[d] Agensi Nuklear Malaysia, Selangor, Malaysia.

[e] Petroleum Geosciences, Universiti Teknologi Petronas, Perak, Malaysia.

[f] TNB Research Sdn. Bhd., Selangor, Malaysia.

[g] Geo Technology Resources, Taman Mawar, Sepang, Selangor.

* Author for correspondence; e-mail: [email protected]

Received: 26 February 2016

Accepted: 8 July 2016

ABSTRACT

Regional and local geological mapping play an important role in the understandingof fractured rock system. These techniques were applied in this study as well as in a comparisonwith active geophysical methods that respond to the physical properties of objects andmaterials below the earth surface, (i.e., electrical resistivity and ground penetrating radar;GPR, among others). The two geophysical methods were conducted within the tunnel outcrop.In addition, satellite imagery obtained from Google Earth was used to determine the mainlineament pattern present in the area. The geological mapping that consists of the fracturesystem (i.e., discontinuity) mapping was used to compile the catalog of the fractured system.The electrical resistivity method that was applied alongside the tunnel was compared withthe results from the ground penetrating radar method. This study, with the application ofregional geological mapping using satellite imagery, was able to give a valuable comparisonwith in-situ geological mapping. The discontinuity data were plotted in the DIPS softwareto obtain the lineaments’ orientations in rose diagrams. Low resistivity values werecompared with ground penetration radar results and with the lineament pattern. The existenceof water-bearing structures within the fractures system inside the rock mass affects the lowresistivity values, at the same time as the directions of fractures furthermore match that ofthe ground penetration radar results. The high resistivity values, as interpreted from theelectrical resistivity data, are an indication of a lesser amount of water-bearing structuresand therefore a smaller amount of damaged units. The ground penetration radar methodproduces similar results with the lowest amplitude radar reflections from the water free region,and stronger reflections recorded in the area saturated with water. All the parameters used

Chiang Mai J. Sci. 2016; 43(6) 1347

1. INTRODUCTION

Geological mapping for the prupose oftunnel stability is limited when the rocksurface is covered by a shotcrete lining.Geologist face these problems in evaluatingthe stability of a tunnel when no previousrecord of geological mapping data isavailable. . Regional mapping will assist thelineament pattern and could be correlatedwith the fractured system in a rock mass.Geophysics can contribute a lot to thesubsurface study of fractured bedrocks,by helping to identify the fractured andor weak zones in tunnels in which outcropsare unavailable. Electrical resistivity andground penetration radar (GPR) are twomethods used in this study to observeimportant signals related to the geologicalmapping of this area. Many works havebeen reported on faulting and fracturedmechanisms in subsurface rocks, i.e., [2, 3].In these previous studies, some include theapplication of the electrical resistivity tomap the subsurface bedrock units. We haveused some of this previous in this study [4-9].The application is not limited to structuralmapping only; previous researchers haveidentified the usefulness of these geophysicaltools in environmental studies such as seepagemapping and groundwater delineation,[8, 10-14].

In this present study, we applied theelectrical resistivity and ground penetrationradar (GPR) to compare the results fromthese geophysical methods with geological

in this study contribute to a Tunnel Stability Rating System (TSRS) that is in development.The Tunnel Stability Rating System could be a useful method of determining the stabilityof tunnels, particularly tunnels without rock exposure (e.g. that is covered by shotcrete).The overall goal is to make the evaluation process of rock tunnels able to use an alternativeassessment when electrical resistivity and ground penetration radar data are accessible.

Keywords: geological mapping, electrical resistivity, ground penetration radar and tunnel stabilityrating system

mapping to examine the fractured rocksystem and to evaluate the tunnel systemswithin the Peninsula Malaysia. One goalfrom this work is to assess the TunnelStability Rating System as a useful methodof determining the stability of tunnels,particularly tunnels without rock exposure..

2. METHODOLOGY

Regional geological mapping usingGoogle Earth images contribute a betterunderstanding about the in-situ geologicalparameters related to the rock fracture system.Two geophysical methods were used (electricalresistivity and GPR) to capture the subsurfaceinformation (Figure 1). This research alsohighlights the GPR method as a common non-destructive procedure useful in detection ofgeologic hazards including tunnel risks.Observation of support structures (rock bolts,dowels, anchors and shotcrete itself isgathered to establish tunnel stability ratingsystem.

2.1 Satellite ImageryA photo lineaments study was carried

out to visualise the presence of the mainlineaments in the bedrock of the surroundingareas by utilising the satellite imagedownloaded from Google Earth. Thedownloaded image was visualised by usingan image processing software to enhancethe visibility of the topographic featuresand hence the interpretation of lineaments.

1348 Chiang Mai J. Sci. 2016; 43(6)

Image enhancement has been undertakenusing Adobe Photoshop software, and thelineaments were traced manually. DIPS

software was employed to plot the lineaments’orientations into rose diagrams.

Figure 1. Map of the Tunnel showing: a) the geological and geophysical survey lines. b) thesurface inside the tunnel and the sketch diagram of the tunnel used to conduct the geologicaland geophysical survey.

(a)

(b)

Chiang Mai J. Sci. 2016; 43(6) 1349

2.2 Local and Regional GeologicalMapping

In this study, the degree of weatheringfor the rock masses is described using theclassification scheme by IAEG (InternationalAssociation For Engineering Geologists,in [15]. Some of the terminologies used todescribe the discontinuity features and fieldestimation of the rock strength used theInternational Standard Rock Mechanics(ISRM) guideline. Other equipment usedinclude Suunto or Silva Ranger geologicalcompass and clinometers, digital cameraand measurement tapes. The method thatis used for geological and surface mappingis scanline method conducted at each 10 minterval along the entire length of thetunnels by walking along it. All notablegeological features, (Figure 2), (rock types,weathering grade, fault, water or seepages,

and discontinuities) and signs of tunnelinstability, deterioration or degradation arerecorded and noted at each 10 m alongthe tunnels. Beforehand, a photographis captured along the tunnel with a 10 minterval to be inserted into the discontinuitysurvey form to make more precise andorganised recorded geological features.Each geological feature is recorded andnoted according to their symbols. SchmidtHammer is also used at certain locations totest the shotcrete condition or outcrop andpresence of stalactite and stalagmite thatindicates there is a crack at the back of thetunnel walls. It is used to estimate and to testthe strength and integrity of the shotcretelining and exposed outcrop. A point loadtest is used to determine the rock strengthand convert to the international standardclassification of rock strength in MPa unit.

Figure 2. Simplified general Geological Map of the study area after, [19].

1350 Chiang Mai J. Sci. 2016; 43(6)

2.3 Electrical Resistivity SurveyElectrical resistivity used to determine

the electrical properties of material insidethe tunnels. The apparent resistivity iscalculated using the basic formula p

a=2πaR.

An ABEM Terrameter SAS4000 is used toderive the apparent resistivity. The resistivitymeasurement consists of a 41electrode systemthat is laid out along the survey line..This method uses the international standardASTM D6431-99 as a guide for conductingthe survey. The data was processed usingRES2DINV software [16, 17]. The length ofsurvey lines was 200 m each with penetrationdepth target around 30m inside the both(left and right, wall and crown area).

2.4 Ground Penetrating RadarGround Penetrating Radar (GPR)

uses an electromagnetic microwave thatis similar to sound in ultrasonic pulse-echomethods. It is based on the propagationof electromagnetic energy through materialsof different dielectric constants (analogous toacoustic impedance in sound). GPR operatesby transmitting pulses of radio waves downinto the ground through an antenna andmeasuring the reflected energy back to thereceiving antenna. The greater the differencebetween dielectric constants at an interfacebetween two materials, the greater the amountof electromagnetic energy reflected atthe interface. RAMAC/GPR made byMALA GeoScience, Sweden with 250 MHzantenna is used for the survey. The estimatedpenetration depth is 9 metres. GPR method

using [18].

3. RESULTS

Compilation of results used to developan understanding of tunnel stability, andthe Shotcrete Lining covered of the tunnels;this assessment could provide very usefulinformation.

3.1 Major Lineaments PatternThe satellite image interpretation shows

a well-developed lineament system inthe region of the survey area. To objectivelyvisualise the lineament pattern and distribution,the interpretation area was arbitrarily dividedinto 3 sections, namely: the Western/Northwestern, Central and the Easterndomains (Figure 3). The western andnorth-western domain is characterised by3 sets of lineaments, in which the NW-SE setis most prominent, followed by the NE-SWand the N-S set. The same sets of lineamentswere also identified in the Central domain;however some E-W lineaments were alsoencountered, making up a total of 4 sets oflineament in Central domain. The Easterndomain shares much in common withlineaments sets in those of the Central domain;i.e. 4 sets of lineament are present. However,the E-W oriented lineaments are moreprominent in the Eastern domain comparedto the Central domain. Figure 4 shows themajor direction of lineament at the studyarea. The results of the Kinematics stabilityanalysis is as shown in Figures. 5 (i to ii).

Figure 3. Lineament map from the interpretation of Google Earth’s satellite image.

Chiang Mai J. Sci. 2016; 43(6) 1351

Figure 4. The Rose diagram of the lineaments from the study area.

Figure 5. Major planes plots of discontinuity in the data from the outcrops between; i) CH300-350 at left wall in MAT., ii) CH 500-550 at left wall in MAT., and iii) CH 725-800 at leftwall in MAT.

(i) (ii)

(iii)

1352 Chiang Mai J. Sci. 2016; 43(6)

,which indicates that the area is wet andhighly contaminated with fine clayey particles.Areas marked with less fractured (dry)interpreted as relatively much drier comparewith highly fractured [19] areas.

4. DISCUSSION

Groundwater intrusion is a commonissue and problem inside the tunnels.The concrete-lined sections of the tunnelhave been the main concern of watertrapped behind the shotcrete. Once the waterpressure builds up, it induces cracks andmay also trigger failure (roof or wall collapse)in the tunnel. Continuous wetting of thebedrock encourage chemical weathering ormineral decomposition and hence reducesthe rock mass strength in the long run.Concrete spalls, cracking in the shotcretelining and corrosion of the steel supportstructures (bolts, dowels, and anchors) areused as indicators of the weak zone andsuspected highly fractured zone.

4.1 Comparison Between MethodsThe electrical resistivity and the GPR

geophysical methods used in the work

3.2 Low and High Electrical ResistivityResponse

Base on the results from more than30 electrical resistivity survey lines in thisstudy, values less than 100 Ωm are classifiedas a weak zone with water and matchedwith the geological mapping results. Anexample of electrical resistivity result isshown in Figure 6, which has low resistivityvalue for the weak area, containing watery.The high resistivity values (>1000 Ωm) wereclassified as less fractured and dry. Low andhigh electrical resistivity responses are alsointegrated with GPR results.

3.3 Low and High Reflection of RadarWave

The GPR shows the high reflectionsignal associated with the wet area whilelow reflection at dry region and with fewerfractures in the rock mass. In the studyresults (Figure 7), the areas with saturatedfracture have a higher intensity of reflection

Figure 6. Electrical resistivity result with lowand high resistivity value along the survey lineshown inside the Tunnel.

Figure 7. GPR result with low and highreflection signal observed at different rockmass condition.

Chiang Mai J. Sci. 2016; 43(6) 1353

showed that low resistivity and highintensity of reflection signal areas could becorrelated with weak and groundwater-filledin fractured/faulted bedrock zones.Regional and in-situ mapping helped toidentify the directions of fractures/faultsand the major joints present in thebedrocks. The study areas with a total lengthof the tunnels used is about 2 km.

4.2 Tunnel Stability Rating System(TSRS)

Once all the parameter from geologicalmapping and geophysical survey wereobtained, a Tunnel Stability Rating System(TSRS) was proposed. These methods usedfive parameters to establish this ratingsystem with the summary of the results aspresented in Table 1. In this stabilityrating system, geophysical parameterscontribute 30%, and the rating value isgiven based on their classification of

groundwater accumulations and the degreeof bedrock fractures. Meanwhile, structuralor mechanical defect (concrete spall andcorrosion on support structures) contribute20%. Geological mapping includes thegeological defects and discontinuity datarecorded in term of the intensity of theseparameters. Rock material strength frompoint load test and converted value fromSchmidt hammer test (uniaxial compressivetest, USC) used with 10%. Groundwateroccurrence also gave a percentage of 10.Rock Mass Geomechanical Propertiescontributed the balance of 30%. All theparameters observed, i.e., geological mappingand a geophysical study conducted in thesurvey area are in-situ. Normally, the RockMass Rating (RMR) by [20], and theQ-System by [21], was adopted for thisstudy and used to evaluate the rock massinside the tunnels as shown in Figure 8.

Table 1. Tunnel stability rating system proposed to quantify the tunnel integrity, after [20, 21].

Parameter

A) Geophysical Properties (30%)

• Resistivity Value (% of interpreted

water accumulations in single

pseudo section)

Rating Value

• Ground Penetration Radar

(Degree of saturation)

Rating Value

• GPR(degree of fracturing with

water accumulations)

Rating Value

B) Structural/Mechanical Defects (20%)

• Cracks/Concrete Spall

Rating Value

• Corroded/Defective Support

Structures (Bolts/Anchors/etc)

Rating Value

Range value

80-100

15

Unsaturated

@ dry

10

Massive

5

None

5

None

15

60-79

12

Slightly

saturated

8

Low

fractured

3

Minor

4

Minor

12

40-59

10

Moderately

saturated,

6

Moderately

fractured

2

Moderate

3

Moderate

10

20-39

5

Highly

saturated

3

Hifhly

fractured

1

Substantial

2

Significant

7

< 19

0

Completely

saturated

0

Intensely

fractured

0

Extensive

1

Severe

3

1354 Chiang Mai J. Sci. 2016; 43(6)

Table 1. Continued.Parameter

C) Rock Mass Geomechanical Properties (30%)

• Geological Defects & Instabilities

D) Rock Material Strength (10%)

• UCS/Point Load Index (MPa)

E) Ground Water Intrusions (10%)

• Inside tunnel conditions

Range value

Absent

30

> 200

10

Dry

10

100-81

I

Very good

Minor

25

200-100

8

Damp/Minor

7

61-80

II

Good

Moderate

15

99-50

5

Mod. Wet

5

41-60

III

Fair

High

10

49-10

3

Wet

3

21-40

IV

Poor

Intense

5

< 10

0

V. Wet

0

< 20

V

Very Poor

Classification Of Tunnel Stability Rating System (TSRS, 2015)

Rating Value

Rating Value

Rating Value

Total of Rating Value

Class

Figure 8. Rock Mass Rating (RMR) and Q-System, after [20, 21].

5. CONCLUSION

This study has shown the advantagesof the use of integrated geophysical,geological and geomechanical methods asa better approach for the determinationof tunnel stability assessment. Hiddeninformation on the fractured /faulted

bedrock units and the groundwater flowsystem internal of the shotcrete lined wallscould be determined by this approachwithout damage to the walls as demonstratedin this work and presented in Table 1 andFigure 9. The characteristics of the tunnelrating system adopted in this study are;

Chiang Mai J. Sci. 2016; 43(6) 1355

a) Very good. For a tunnel to beadjudged as a very good class, the bedrockconditions must be massive and dry.It must not show any significant signs ofdistress, mechanical or structural defects inthe shotcrete lining. The bedrock must besolid, less jointed structures, unweatheredor slightly weathered (most importantly,Gneissic bedrock materials), and no visiblesigns of instability. The supporting structuresfor the tunnel must also be in excellent orexcellent conditions. Usually, highly strongbedrock materials, especially Graniticmaterials are most recommended. Theremust be no signs of groundwater intrusions.In situations of minor groundwater intrusions,it must be well managed through the adequatedrainage system. Routine maintenance is atthis moment recommended to keep a tunnelin a very good class.

b) Good. A tunnel is termed to be in agood class if the bedrock conditions areless jointed but slightly wet and show someminor defects in the shotcrete lining such as;light cracks, a small degree of fractured/faulted bedrocks and minor or localisedcorrosions in the shotcrete walls. The exposed

bedrock units are widely spaced or jointed,slightly weathered, but no signs of alterationsand evidence of instability in the exposedrocks mass that was adequately taken-care-of. In situations of groundwater seepages,it must be well managed through adequatedrainage system and routine maintenancesystems.

c) Fair. A fair tunnel rating system givesthe following characteristics; when thebedrock units of a tunnel are riddledwith some discontinuities structures thatare significantly wet through seepages ofgroundwater intrusions, some significantamount of mechanical defects, i.e., cracksand spalls were observed in the bedrockunits. If the exposed bedrock units are welljointed, but show some localised wedgesand joints, with seepages of from the wallsand roof but insufficient drainage controlsystems. The bedrock materials must beunyielding to supporting tunnel systems.Adequate routine maintenance must becarried out to arrest the situations.

d) Poor. A tunnel system is judged poorwhen the bedrock units are heavily jointed

Figure 9. Some results from the Electrical Resistivity Survey; a) Evidence of faulted/fracturedbedrock unit, but dried as shown by the red arrow; b) Seepage at shotcrete and groundwaterflow out as observed during the data acquisition as confirmed by the Electrical Resistivityresults as demonstrated by the red arrow.

(a) (b)

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with localised sheared structures. Thesupporting steel materials are heavilyaffected by groundwater intrusions leadingto corrosions. The bedrock is heavilyweathered, altered and shows high degreesof instabilities, and the presence of weakrock materials with a substantial amountof groundwater intrusions leading to wetground and calcite deposition. This class oftunnel needs urgent attention and structuralrepair with additional drainage systems toarrest the situation.

e) Very Poor. In situations where thebedrock units in a tunnel system are heavilyjointed and sheared, widespread defects,direct contacts with the groundwaterintrusions, and heavily weathered with steelsupporting materials corroded, such tunnelsystems are classified as very poor.The presence of feeble rock materialscauses excessive groundwater seepages,highly wet ground, roofs and walls conditionswith calcite deposits due to poor drainagesystems. This situation calls for an immediateshutdown of the system for urgent repairsor replacement of all the supportingmechanical, structural steel materials andthe drainage systems. Reinforcement of theshotcrete lining walls must be carried out.

The tunnel stability rating system,(TSRS), offer a better alternative to theidentification of the stability of tunnels.This study clearly correlated well with theprevious studies as shown.

5. ACKNOWLEDGMENTS

The authors are very grateful to theUniversiti Sains Malaysia, School ofPostgraduate Studies, for the PostgraduateFellowship awards as well as, TenagaNasional Berhad, Malaysia, for supportingthis research program. In conducting this

research, we were in contact with manypeople, researchers, academicians andpractitioners that contributed a lot towardsthe achievements in this study. We alsoappreciate the Editor and the two reviewersthat helped to shape the paper throughtheir suggestions and comments.

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