Application of electrical resistivity

Introduction

Electrical resistivity imaging is a widely used tool in near surface geophysical surveys for investigation of various geological, environmental and engineering problems including landslide. In addition, a 2-D synthetic resistivity modelling study was carried out to understand the response of the resistivity method to a landslide problem before the field surveys (Drahor,MahmutG et al, 2006).

The relevance of electrical surveys is to identify the subsurface resistivity distribution by making measurements on the ground surface. The true resistivity of the subsurface can be quantified from these measurements (Singh et al 2006). The ground resistivity is related to various geological parameters such as the mineral and fluid content, porosity and degree of water saturation in the rock. Electrical resistivity surveys have been used for many decades in hydrogeological, mining and geotechnical investigations. More recently, it has been used for environmental surveys.

The resistivity measurements are normally made by injecting current into the ground through two current electrodes (C1 and C2), and measuring the resulting voltage difference at two potential electrodes (P1 and P2). From the current (I) and voltage (V) values, an apparent resistivity (pa) value is calculated.

pa = k V / I

where k is the geometric factor which depends on the arrangement of the four electrodes.

Resistivity meters normally give a resistance value, R = V/I, so in practice the apparent resistivity value is calculated by

pa = k R

The calculated resistivity value is not the true resistivity of the subsurface, but an “apparent” value which is the resistivity of a homogeneous ground which will give the same resistance value for the same electrode arrangement. The relationship between the “apparent” resistivity and the “true” resistivity is a complex relationship. An according to Singh et al (2006), an inversion of the measured apparent resistivity values using a computer program must be carried out to determine the true subsurface resistivity.

Problem statement

Landslide – Jalan Baru Gap ,Fraser Hill

A recent problem faced in Malaysia is landslides on hill slopes. This harmful situation always occurred in highland area during wet season.

One landslide occurred in km 90,FT055,Kuala Lumpur/Kuala Lipis (Gap Road/Tranum),Fraser Hill starting December 16 2007 until almost recently. This road is the only one that is connecting Kuala Kubu Baru or Fraser Hill to Raub. In recent tragedy on April 5 2008 at 9 pm,sliding of some boulders onto one old bridge in km 38.4,FT055,Kuala Kubu Road-Raub ,Fraser Hill ,Raub district, Pahang had occurred.

The landslides are often triggered by water accumulation within part of the slope which leads to weakening of a section of the slope. Thus, it is important to accurately map the zone of ground water accumulation.

Landslide is one of natural hazards that are often occur all over the world. In tropical climate such as Malaysia, the phenomenon is common especially in hilly areas during monsoon season. According to the report by Jamaludin et al, 2006, serious natural landslides in this country normally occur in monsoon seasons where intense precipitation is the main triggering factor. Early indication of the slope stability prone area such as the landslide hazard maps may help planners and developers to choose favorable locations for locating development schemes. Careful engineering and geologic study could then follow before such specific project could be implemented.

Landslide is a phenomenon that can give the disadvantage for earth. Landslides cause property damage, injury, and death and adversely affect a variety of resources. For instance, water supplies, fisheries, sewage disposal systems, forests, dams, and roadways can be affected for years after a slide event. The bad economic effects of landslides include the cost to repair structures, loss of property value, distruption of lost timber and fish stocks. Landslides can affect water availabiity, quantity, and quality. Geotechnical studies and engineering prospects to assess and stabilize potentially dangerous can be costly.

OBJECTIVE

In this study,there are two main objectives as follow:

1. To determine the subsurface characteristics of the study area based upon electrical resistivity images.
2. To assess the slope stability of Jalan Baru Gap,Fraser’s Hill.

Chapter 2

LITERATURE REVIEW

ELECTRICAL RESISTIVITY

The resistivity method has its origin in the 1920’s due to the work of the Schlumberger brothers. For approximately the next 60 years, for quantitative interpretation, conventional sounding surveys (Koefoed 1979) were normally used. In this method, the centre point of the electrode array remains fixed, but the spacing between the electrodes is increased to obtain more information about the deeper sections of the subsurface.The resistivity method measures the apparent resistivity of the subsurface, including effects of any or all of the following : soil type, bedrock fractures, contaminants, and ground water. A change in electrical resistivity may suggest changes in composition, layer thickness, or pollution levels. The resistivity is useful for detecting lateral and vertical changes in subsurface electrical properties on the same time.

According to Gomiz-Ortez et al (2006), electrical resistivity imaging is a geophysical prospecting technique designed for the investigation of areas of complex geology; this involves measuring a series of resistivity profiles, using a computer to control measurements between selected sets of an electrode array. Since increasing separation between electrodes provides information from increasingly greater depths, the measured apparent resistivity can be processed to provide an image of true resistivity against depth. The principal applications of this technique include (Telford et al., 1990; Reynolds, 1997; Å umanovac, 2006) definition of aquifer boundary units, such as aquitards, bedrock, faults and fractures, the detection of voids in karstic regions, the mapping of saltwater intrusions into coastal aquifers, the identification of contaminated groundwater plumes, the detection of mineralized zones, and the exploration of sand and gravel resources among others.

Majotiry of rock-forming minerals are electrical insulators although some native metals and graphite conduct electricity .The measured resistivities in earth materials are majorly controlled by the movement of charged ions in pore fluids. Eventhough water itself is not a good conductor of electricity, ground water in most cases contains dissolved compounds that greatly improve its skill to conduct electricity. Hence, porosity and fluid saturation tend to dominate electrical resistivity measurements. The cracking within crystalline rock can lead to low resistivities if they are filled with fluids with addition to pores.

The resistivities of various earth materials are shown below.

Several standard electrode arrays are available, with different horizontal and vertical resolution, penetration depth and signal-to-noise ratio. The dipole-dipole array gives good horizontal resolution but may have a poor signal-to-noise ratio because the potential electrodes are outside of the current electrodes. The Wenner array is usually applied for a good vertical resolution, but may also provide a reasonable horizontal resolution (Sasaki, 1992). This method has greater signal-to-noise ratio than the dipole-dipole method because the potential electrodes are placed between the two current electrodes. In order to combine the benefits derived from the standard electrode arrays and/or different electrode spacings in some field profiles, both of them were obtained independently, and then combined in a mixed array that has been jointly inverted and interpreted.

APPLICATION OF ELECTRICAL RESISTIVITY

According to Srinivasamoorthy (2009), the important purpose of electrical surveys is to determine the subsurface resistivity distribution by making measurements on the ground surface. From these measurements, the true resistivity of the subsurface can be estimated. The ground resistivity is related to various geological parameters such as the mineral and fluid content, porosity and degree of water saturation in the rock. Electrical resistivity surveys have been used for many decades in hydro geological, mining and geotechnical investigations. More recently, it has been used for environmental surveys.

A 3-D resistivity survey and interpretation model should be even more accurate in theory. However, at the present time, Dahlin (1996) reported that 2-D surveys are the most effective economic acceptance between obtaining very successful results and keeping the survey expenses down.

Surface geoelectric prospecting techniques detect the effects of induced electric currents flowing through the ground. They are based on the principle that measurements of electrical potential at the surface depend on subsurface distributions of electrical resistivity (in Om).

2-D and even 3-D electrical surveys are now practical commercial method with the relatively nowadays advancement technology of multi-electrode resistivity surveying instruments (Griffiths et al. 1990) and fast computer inversion software (Loke 1994).

The resistivity procedure begins in the 1920’s due to the research of the Schlumberger brothers. For the next 60 years, conventional sounding surveys (Koefoed 1979) were normally used for quantitative understanding. In this method, the centre point of the electrode array remains unchanged, but the spacing between the electrodes is increased to produce more explaination about the deeper sections of the subsurface. Previous studies include monitoring of groundwater ?ow by pro?ling or mapping the electrical response to an injected salinetracer, either lonely from the ground surface (White,1988,1994; Osiensky and Donaldson, 1995; Morrisetal., 1996) or in bore holes (Bevcand Morrison, 1991). In recent years complex ERT inversion techniques have been constructed, making tomographic imaging of subsurface ?uid movement possible.

Accomplished qualitative applications have been studied for porous (Dailyetal.,1992,1995) and fractured (Slateretal., 1997) media. On the other hand, quantitative assessment of transport behaviours in soil sand rocks from ERT data is recorded in poor manner.( Binleyetal. , 1996; Slater et al. ,2000).

THE MEASUREMENT ARRAY TYPE

The free program, RES2DMOD.EXE, is a 2-D forward modeling program which calculates the apparent resistivity pseudosection for a user defined 2-D subsurface model.

With this program, the user can choose the finite-difference (Dey and Morrison 1979a) or finite-element (Silvester and Ferrari 1990) method to calculate the apparent resistivity values. In the program, the subsurface is divided into a large number of small rectangular cells. This program is largely intended for teaching about the use of the 2-D electrical imaging method. The program might also assist the user in choosing the appropriate array for different geological situations or surveys. The arrays supported by this program are the Wenner (Alpha, Beta and Gamma configurations – the Alpha configuration is normally used for field surveys and usually just referred to as the “Wenner” array), Wenner-Schlumberger, pole-pole, inline dipole-dipole, pole-dipole and equatorial dipole-dipole (Edwards 1977). Each type of array has its advantages and disadvantages. This program will hopefully help you in choosing the “best” array for a particular survey area after carefully balancing factors such as the cost, depth of investigation, resolution and practicality.

WENNER ARRAY

This is a robust array that was popularized by the pioneering work carried by The University of Birmingham research group (Griffiths and Turnbull 1985; Griffiths, Turnbull and Olayinka 1990). Many of the early 2-D surveys were carried out with this array. Because of this property, the Wenner array is relatively sensitive to vertical changes in the subsurface resistivity below the centre of the array. However, it is less sensitive to horizontal changes in the subsurface resistivity. In general, the Wenner is good in resolving vertical changes (i.e. horizontal structures), but relatively poor in detecting horizontal changes (i.e. narrow vertical structures.

STRENGTH ANISOTROPY

In the previous work,an attempt is being made to related electrical resistivity measurement to slope stability of a sloping ground via electrical resistivity-shear strength relationships. Hence, it is necessary to briefly describe the soil strength component of the work,as follows:

The shearing behaviour of sand has so far been assumed to be isotropic; that is,the sand at a given density and mean stress level subject to any plane strain shear test has been assumed to have a unique peak angle of friction. Arthur and Menzies (1972),Arthur et al (1977),Symes (1983),Tatsuoka et al (1986) had stated that the soil properties then depend on the direction with respect to deposition,or the bedding planes,in which the soil is subsequently sheared.

The important point for a designer concerned with stability analysis is that the shearing resistance in a sample or direct shear test on a sample deposited vertically in the apparatus develops along the bedding planes in the soil,and this corresponds closely with the minimum plane strain shearing resistance in the soil, Tatsuoka et al (1986). Thus a sample deposited vertically through the top of the apparatus (in the conventional way/ and tested horizontally in the direct shear will yield a plane strain angle of friction close to the minimum value for the soil. This is a lower bound (or safe) value for the plane shearing resistance with respect to strength anisotropy.

The difference between the maximum peak plane strain angle of friction which occurs when the sample is sheared across the bedding planes, and the minimum peak plane strain angle of friction which occurs when the sample is sheared along the bedding planes,is likely to depend on the soil particle shape and grading,and the method of soil deposition,among other factors. According to the paper by Symes (1983) and Tatsuoka (1987),for laboratory prepared samples on clean sand, the tangent of the plane strain angle of friction can vary by as much as 25% due to the orientation of shearing with respect to the bedding planes.

DIRECT-SHEAR BOX

In the previous works,direct shear box had been used to determine the strength properties of the soil. The direct shear box apparatus is simple to use and has a long history in soil mechanics. Shibaya, Mitachi and Tamate (1997) have given an optimum configuration of the shear box apparatus for measuring the strength and dilatancy characteristics in direct shear. Their study shows that proper procedures enable the direct shear box test to provide strength and large-strain dilatancy characteristics that are similar to those measured in a simple shear test. Important aspects of the procedure include measuring the vertical load in the lower half of the box, and maintaining the size of the opening between the two halves of the shear box at a fixed value of approximately 10-20 times the mean particle diameter of the test specimen.

The direct shear test has been criticized for the non-homogeneity of the stress-strain state within the direct shear box; however, numerical modeling using the finite element method has demonstrated close similarity between the direct shear and the simple shear test (Jewell 1989). Potts, Dounias and Vaughan (1987) performed a finite element analysis and concluded that the non-uniformities within the direct shear box sample have little affect on the stress-strain behavior, and with little error the test may be interpreted as if it were a simple shear test. The simplicity of the testing operation and interpretation of test results make the direct shear box a good choice to begin the study of the complicated behavior of unsaturated soil interfaces.

The direct shear type of apparatus is a commonly used device for interface testing. As mentioned, the direct shear test has some inherent problems (principal stress rotation, stress non-uniformity, failure plane definition); however, it is fairly well suited for interface testing. Boulon (1989) and Boulon et al. (1995) describe the use of a direct shear apparatus for developing constitutive models for interfaces. As an elemental test for soil, the direct shear test is subject to problems associated with fabric, stress and strain non-uniformity; however, Boulon and colleagues suggest that for interface testing it is appropriate and practical. In the case of soil testing, the problems with direct shear testing are important from a continuum mechanics perspective; however, if the interface direct shear test is considered as a surface test, then homogeneity is required only along the idealized shear band. As pointed out by Boulon and colleagues, a significant amount of research established that the shear band in a direct shear interface test is geometrically homogenous. However, there are still some uncertainties associated with stress level in the shear band; for this reason, stresses determined using boundary forces from this test are considered average stresses for the interface. While uncertainties exist regarding the applicability of the interface direct shear test as an elemental test, Boulon (1989) has indicated that it is a pragmatic approach to model very complex behavior (i.e., large strains, large rotations, and large relative displacements) associated with interface shearing.

From a practical perspective, the direct shear test has many advantages for examining interface behavior, including the ability to simulate different loading paths (Boulon 1989, Boulon et al. 1995). In particular, the device can be used to model loading and unloading conditions under constant normal stress and constant volume conditions, which represent extreme stress paths. A range of behaviors can be investigated by applying these test conditions to soils with different initial states ranging from very loose to very dense. In addition, “pseudo-oedometric” loading and unloading paths, initiated after shearing can be conducted (involve changes in normal stress with no displacement tangent to the interface). The loading paths possible with direct shear interface testing bracket those expected along a pile interface, i.e., lying somewhere between a constant normal stress and constant volume condition. Therefore, the behavior observed from these tests will provide fundamental insight into the shearing mechanisms along piles and other interfaces in unsaturated soils.

GEOPHYSICAL INVESTIGATION OF A LANDSLIDE

Landslide is defined as the movement of a mass of rock, scattered rubbish or earth down a slope. The word landslide also mentions to the geomorphic characteristics that result from the event. Other terms used to refer to landslide include slope failures, slope instability and terrain instability.Landslide may happen almost anywhere, from anthropogical slopes to natural, original condition of the ground. Most slides often occur in areas that have experienced sliding in the past. All landslides are triggered by similar causes (Bujang B.K. Huat,2005).

Landslides have caused mammoth numbers of casualties and big economic losses in hilly and mountainous areas of the world. In tropical countries where yearly rainfall can reach as high as 4500mm and high temperatures around the year caused extreme weathering to their soil and rock profile where in certain area can stretch out 100 m in depth. Landslide is one of the most destructive natural disasters in tropical region with these set of climate and geological condition, combined with other causative factors. And according to Bujang B.K. Huat (2005), Malaysia is one of the countries located in the tropical region. There were 13 major landslides reported in Malaysia, involving both cut and natural slopes with a total lost of more than 100 lives from 1993 to 2004,.

In Turkey, landslide is one of the causes of natural hazard According to the journal by Gokhan Gokturkler et al. (2007), and one of the most important landslide sites is located in the Altindag district. A geophysical survey including electrical resistivity tomography (ERT) and seismic refraction tomography (SRT) was carried out to study a landslide site. The ERT studies were performed along four profiles over the landslide body in the directions of N-S and E-W. A Wenner-Schlumberger configuration was used during the resistivity measurements. And according to Steeples (2001), there has been an increase in the application of geophysical methods to near-surface problems generally including landslide studies, waste disposal site investigations, groundwater explorations, detecting faults, and determining physical properties of soil.

LANDSLIDE OCCURENCE IN MALAYSIA

Landslide occurrence is common in hilly terrain of the tropical region due to weather conditions, thick soil profile and lack of slope stability analysis. In Malaysia which is rapidly moving towards urbanization, many occurrences of landslides had been reported as a serious hazard which had fatalities and losses and results in severe damages in infrastructure.

Malaysians has recently started to monitor landslides and employed extensive site investigation during the construction of North-South Expressway by Plus Expressways Berhad Company in the eighties. After which, many governmental and private institutions started to be more involved in monitoring. Currently, monitoring tenders are issued by the government institutions which are carried on by the private sector. Monitoring of a critical cut slope during construction or after a landslide is done through Geotechnical instruments; tilt-meter, pizo-meter and tensile-meter are the main instruments that used for monitoring the alignment, groundwater level and soil creep. Within the last few years, IT technology has been applied to monitor critical landslide after the tragedy of landslide in Antarabangsa Hill and Taman Hillview tragedy situated within Kuala Lumpur.

Unfortunately, landslide monitoring schemes could not warn the existence of any impendingfor any landslide or even to detect some obvious pre-sliding signs. There is a lack of clear policy for landslide monitoring from a country scale perspective. In addition, the monitoring instruments were usually installed after the landslide had occurred, in a place where the slope reached equilibrium state.

With rapid urban urbanization by the year 2020; more hilly lands will be occupied by urban landuse and more highways will be constructed between the east and west coast of Peninsula through the mountainous terrain. This will increase the probability of landslides. Therefore, landslides monitoring in nation scale will be highly appropriate. Space techniques; remote sensing and GPS with its wide coverage are the proper candidate to put landslide and other natural disasters under surveillance.

CHAPTER 3

MATERIAL AND METHOD

METHODOLOGY

STUDY AREA

Jalan Baru Gap, Fraser Hill

The study area is located in Fraser Hill (see figure 3.1). Fraser Hill is a popular highland resort in Malaysia. The Fraser Hill geomorphology is characterized by hilly terrain and most of the natural slopes are steep with 50° gradients (IKRAM, 2007). The range of elevation in Fraser Hill is from 400 to 1300 meters above sea level. According to Gassim et al., (2001), the mean annual rainfall for the catchment is about 2624 mm with average of 208 rain- days in a year.

Fraser’s Hill located neatly at 1,524 meters above sea level, substantial to the north of the Genting Highlands and also in Pahang. The attractiveness of Fraser’s Hill is based majorly on its cool mountain air, its abundance of luxuriant vegetation, and its peace and tranquility–all of which have made it a favored destination for birdwatchers.

While there are many other attractions to be found here, they are all pleasantly complementary to Fraser Hill’s tranquil charms. There are jungle trails, waterfalls, and flower nurseries, a very pleasant 9-hole golf course, and a riding stable. Other facilities include a children’s playground and a roller skating rink. (http://www.marimari.com/hotel/malaysia/hill/fraser.html)

For accommodation, Fraser’s Hill has a range of hotels, chalets and colonial bungalows to suit one’s preferences and budget. The resort is about an hour and a half away from Kuala Lumpur. The road to the hill resort winds for about 35 km after the Gap.

MATERIALS AND METHODS

Fieldwork

Resistivity measurement using ABEM Terrameter SAS 4000

Two-dimensional (2D) electrical resistivity imaging surveys were performed along four profiles in the study site. The profile are line 1, line 2, line 3 and line 4.

The resistivity measurements are normally made by injecting current into the ground through two current electrodes (C1 and C2 in figure 3.2), and measuring the resulting voltage difference at two potential electrodes (P1 and P2). From the current (I) and voltage (V) values, an apparent resistivity (pa) value is calculated.

pa = k V / I

where k is the geometric factor which depends on the arrangement of the four electrodes. Resistivity meters normally give a resistance value, R = V/I, so in practice the apparent resistivity value is calculated by

pa = k R

The calculated resistivity value is not the true resistivity of the subsurface, but an “apparent” value which is the resistivity of a homogeneous ground which will give the same resistance value for the same electrode arrangement. The relationship between the “apparent” resistivity and the “true” resistivity is a complex relationship. To determine the true subsurface resistivity, an inversion of the measured apparent resistivity values using a computer program must be carried out.

Desk Study

Computer interpretation and data input and format

After the field survey, the resistance measurements are reduced to apparent resistivity values. Practically all commercial multi-electrode systems come with the computer software to carry out this conversion. In this section, we will look at the steps involved in converting the apparent resistivity values into a resistivity model section will be discussed. To interpret the data from a 2-D imaging survey, a 2-D model for the subsurface which consists of a large number of rectangular blocks is usually used . A computer program is then used to determine the resistivity of the blocks so that the calculated apparent resistivity values agree with the measured values from the field survey. The computer program RES2DINV.EXE will automatically subdivide the subsurface into a number of blocks, and it then uses a least-squares inversion scheme to determine the appropriate resistivity value for each block. The location of the electrodes and apparent resistivity values must be entered into a text file which can be read by the RES2DINV program.

Determining shear strength of soil samples using Direct Shear Box Test

The direct-shear test is one of the oldest methods for determining the shear strength of soil materials. All type of soil can be tested by direct shear. The apparatus described herein is relatively small and is suitable only for fine-grained soils, but larger apparatus can be used satisfactorily to test gravels. Other direct-shear apparatus and test methods are presented elsewhere ( Am. Soc. Testing Mater., 1964 ; Lambe, 1951).

The clearance between the boxes by tightening the wedges were eliminated.A soil specimen 0.5 inch thick and 1-15/16 inches in diameter was trimmed ,weighted and it being placed in the shear device. The remainder of the device was assembled and the initial thickness of the soil specimen were determined from the vertical dial, which has been previously calibrated under the normal load with the blank substituted for specimen.

The desired normal load was then applied. Samples are square in plan.The box is split horizontally in to two halves. Normal force applied from top of shear box by dead weights. Shear force applied to the side of the top half of the box to cause failure. In dense sand, the resisting shear stress increases with shear displacement until it reaches a failure stress, ?f.

?f is called the peak shear strength.

After failure, stress is attained, the resisting shear stress gradually decreases as shear displacement increases until it finally reaches a constant value called the ultimate shear strength. Direct shear are repeated on similar samples at various normal stresses. The normal stresses and the corresponding values of ?f obtained from a number of strength parameters are determined.

From the graph,the highest resistivity value is at point L2C1E7 It is because it has high shear strength values. This value indicates that at this point,the soil is difficult to shear and it makes water to be difficult to enter then contributed to the high resistivity .One of the reason is because its porosity is high. The moisture content and saturation percentage of this soil is low so that its resistivity is high. The lowest resisitivity value is at point L2C2E17 where it has low shear strength values. It contribute to water to be less difficult to enter the soil which is why it has low resistivity values. The porosity of this soil is low and its moisture content and saturation percentage is high.

CHAPTER 5

Conclusion

Electrical resistivity prospecting is a very useful and attractive method for soil characterization. Variations in electrical resistivity may indicate change in composition,layer thickness, or contaminant levels. The advantage of using this resistivity imaging is that it gave sufficient data regarding on how we want to interpret the resultant image. The ground resistivity is related to various geological parameters such as the mineral and fluid content, porosity and degree of water saturation in the rock. The relationship of shear strength of a soil and resistivity can be obtained by knowing the factors that contribute to the shear strength value such as moisture content,porosity,and saturation percentage. According to Lagmanson (2005), some properties which affect the resistivity of rock and soil are porosity,moisture content,dissolved electrolytes, temperature of pore water, and conductivity of minerals The proposed laboratory method

From all of the resistivity profiling images that has been carried out,the subsurface formation of all lines has the electrical resistivity range around 700-ohm to 2421-ohm. Most of the area were consist of granite and higher resistivity indicate it has low saturated water. Line 1 gave almost identical shear strength results with line 2 but line 1 is prone to have landslides because of its slope degree higher than line 2. The cohesion and angle of friction value obtained from shear strength results were important parameter because it give indication on how strong the soil can be shear. The higher the value of cohesion and angle of friction contribute to higher shear strength.

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