Investigating Slope Failures Using Electrical Resistivity: Case Studies

Transcription

1 M.J. Joab and M. Andrews: Investigating Slop Failures Using Electrical Resistivity 66 ISSN The Journal of the Association of Professional Engineers of Trinidad and Tobago Vol.38, No.1, October 2009, pp Investigating Slope Failures Using Electrical Resistivity: Case Studies Malcom J. Joab aψ and Martin Andrews b Geotech Associates Ltd. 4 Niles Street, Tunapuna, Trinidad, West Indies a mjoab@geotechassociates.com b ma@geotechassociates.com Ψ Corresponding Author (Received 30 April 2009; Revised 8 July 2009; Accepted 17 September 2009) Abstract: The purpose of this paper is to present case studies and outline a methodology used to estimate the location of failure surfaces in landslides in clay slopes in Manzanilla and Tarouba, Trinidad. The methodology outlined consists of conducting a borehole investigation in conjunction with a topographic survey of the failed area and a series of 1D electrical resistivity measurements taken along a section line down the slope. When these measurements are inverted, compared with the results of the borehole investigation and plotted on a cross-section of the slope, estimation of the location and shape of the failure surface are improved. Typically, in the back analysis of a failed slope, the only guide in estimating the shape of the failure surface is based on visual observations of the topography and vegetation and the location of the back scarp of the landslide. The depth to the failed surface must be estimated from the results of the borehole investigations at a few locations. The use of electrical resistivity provides a quick and cost-effective means of extending the investigation and improving the confidence in the results of the slope stability back analyses. The routine use of electrical resistivity that supplements the results of a borehole investigation in failed clay slopes is unique to the field of geotechnical engineering in Trinidad and Tobago. Keywords: Electrical resistivity, slope failure, water content, clays 1. Introduction The surficial soil types which predominate in central and south Trinidad consist of stiff to very stiff clays. These soils, however, are prone to slope instability. In fact, slope gradients as gentle as 4:1 (horizontal: vertical) have been known to be unstable. Traditionally, methods of geotechnical investigation of slope failures include: 1) Drilling borehole(s) and retrieval of soil samples 2) Conducting topographic surveys 3) Lab testing of representative samples 4) Recommendation/design of remedial measures Generally, the primary focus of step 1 above is to determine the depth to failure/slip surface. In simple terms, the failure/slip surface refers to the interface between the soil which has moved and the soil which has not (See Figure 1). The shape and location of the failure surface is very important in the recommendation and design of remedial measures. However, one drawback of the conventional borehole method is that it provides an estimated location of the failure surface at one point along an entire surface. Therefore, in order to improve the reliability of the slip surface location, multiple boreholes must be drilled. However, this is time consuming and it is not cost effective. Therefore, the total number of boreholes typically used in investigations of this type is three. Figure 1. How Slip Surface is Defined This paper presents case studies outlining a method of obtaining information on the location and shape of failure surfaces in failed clay slopes in a quick and cost effective manner. The method, it is

2 M.J. Joab and M. Andrews: Investigating Slop Failures Using Electrical Resistivity 67 suggests, should be used to supplement the borehole data. 2. Location of Failure Surface Hutchinson (1981), in his seminal paper outlining methods of locating slip surfaces in landslides, pointed out that the analysis of the water pressure within a soil matrix (known as the porewater pressure) is one means of identifying the location of a slip surface. He indicated that in clays or loose sands, the shear disturbance associated with a slip surface causes a tendency for the soil particles to collapse to a closer packing. In saturated soils this produces a local rise in pore-water pressure, which dissipates with time as the shear zone consolidates. The result of this is a cusp of increased pore-water pressure. In the case of dilatant materials, such as stiff clays, a negative cusp of reduced pore-water pressure would tend to be associated with a slip surface. In the longer term, the positive and negative cusps of pore-water pressure dissipate, leaving behind inversely correlated negative and positive cusps of water content. In other words, in terms of water content, contractant and dilatant materials exhibit decreased and increased water content locally within the shear zone/failure surface, respectively. Another characteristic is the presence of soft zones or layers (Hutchinson, 1981). This is as a result of softening during shearing/failure (which is typical of stiff clays) or re-moulding during shearing (which occurs in soft clays). In these cases there is a concomitant increase in water content. This observation has been made in a number of landslides in clay slopes in Trinidad. In fact, in the clay slopes which predominate in central and south Trinidad, during failure the moving soil is re-worked to the extent that it has a markedly lower consistency (i.e. it is softer) and it exhibits higher moisture contents. With the exception of few minerals, most common rock-forming minerals are insulators. Therefore, rocks and soils conduct electricity via electrolytes within the pore water. Therefore, the resistivity of rocks and soils is largely dependent upon the amount of pore water present, its conductivity, and the manner of its distribution within the material. The electrical resistivity may be quantified as follows (Guyod, 1964): where, ρ ρ w n ρ w ρ = 2 Eq. 1 n = Electrical resistivity of soil/rock = Electrical resistivity of pore water = Porosity of soil/rock Therefore, this suggests that, for a given pore water chemistry, the higher the porosity of the soil/rock, the lower its electrical resistivity. The equation also suggests that, for a given soil porosity, there is a proportional relationship between resistivity and pore water resistivity. The electrolyte or salt content of the pore water reduces its resistivity, and by extension the electrical resistivity of the soil/rock. 3.2 Method for Measuring Electrical Resistivity in the Field The basic method for measuring in-situ electrical resistivity is by using a combination of four electrodes (two electrodes to apply current into the ground and two to measure the potential difference); a current source; current meter and voltmeter (See Figure 2). 3. Electrical Resistivity 3.1 Basic Theory Prior to outlining the methodology used, it would be beneficial to describe basic electrical resistivity theory. Electrical resistivity methods rely on measuring subsurface variations of electrical current flow which is exhibited by an increase or decrease in electrical potential (voltage) between two electrodes. It is commonly used to map lateral and vertical changes in subsurface material. Note: C 1 and C 2, P 1 and P 2 refer to the current and voltage electrodes respectively. Figure 2. Basic Concept of Resistivity Measurement Source: Abstracted from Benson et al. (1988)

3 M.J. Joab and M. Andrews: Investigating Slop Failures Using Electrical Resistivity 68 In this case, the electrical resistivity is calculated according to the following formula which is based on Ohm s Law: ρ = k ΔV I Eq. 2 Where ρ = Electrical resistivity ΔV = Potential difference (voltage) I = Applied current k = Geometric factor There are several standard combinations of electrode geometries which have been developed. The value of the geometric factor, k would depend on the particular electrode geometry used. ASTM D (2005) indicates that the most common electrode geometries used in engineering, environmental and ground-water studies are the Wenner, Schlumberger and dipole-dipole arrays. These arrays are shown in Figure 3. Figure 3. Standard Electrode Geometries Source: Abstracted from ASTM D (2005) When electrical resistivity measurements are conducted in the field, the values obtained are referred to as the apparent resistivity. These apparent resistivity values must be inverted in order to determine the true resistivity. The process of inversion entails comparing plots of apparent resistivity versus depth with master or theoretical curves. This process not only determines the true resistivity, but it also gives an estimate of the respective layer thickness. For the case studies outlined later, the inversion process was conducted using the computer programme W-Geosoft/WinSev version Use of Electrical Resistivity in Landslide Investigation Jongman and Garambois (2007) point out that geophysical methods are applied to subsurface mapping of landslides for two primary reasons. The first is to determine the location of the vertical and lateral boundaries of the slide debris i.e. the failure surface. The second reason is the detection of water within the slide debris. In fact Lebourg et al. (2005), Bruno and Marillier (2000) and Lapenna et al. (2005) indicate that the electrical method is one of two methods most applied to investigate this (the other being electromagnetic). The particular use of electrical resistivity in investigations of clay slopes which is globally homogeneous stems from the fact that the action of slope failure alters the soils characteristics (i.e. moisture content and consistency). Therefore, geophysical contrast then develops between the slide debris and the unaffected mass (Caris and van Asch, 1991; Méric et al., 2005; Lapenna et al., 2005; Schmutz et al., 2000; Lebourg et al., 2005 and; Bruno and Marillier, 2000), from the cumulative or separate action of soil movement, weathering and an increase of water content (Jongman and Garambois, 2007). In terms of the direct correlation between electrical resistivity and soil water content Banton et al. (1997) quoted the findings of Kachanoski et al. (1988) and Vaughan et al. (1995) who established relationships between apparent electrical conductivity (which is the reciprocal of electrical resistivity) and water content. The regression analyses obtained in the Vaughan et al. (1995) and Kachanoski et al. (1988) studies were and , respectively. These suggest moderate to strong correlation. Given, therefore, that there is a correlation between electrical resistivity and water content, there is the potential for the use of electrical resistivity profiling to estimate the location of a failure surface. 4. Case Studies The following is a description of geotechnical investigations conducted for a total of four landslides in clays in which electrical resistivity methods were used to supplement the results of the borehole investigation and to give a further indication of the

4 M.J. Joab and M. Andrews: Investigating Slop Failures Using Electrical Resistivity 69 vertical extent of the slide debris and by extension, the likely location of the failure surfaces. Three of the failures occurred at Manzanilla and the other occurred at Tarouba. 4.1 Slope Failures at Manzanilla 1) Site Description The facility at Manzanilla was constructed between 5 10 years ago in North Manzanilla. It was constructed at the top of a small hill, the top of which was flattened to provide an area for its construction. Shortly thereafter, slope instability was noticed on the southern flank of one building and the car-park area. Two other areas of instability have also been observed nearby. These landslides were located adjacent to one another. At the time of the investigation they were m wide and extended between m down slope each. Visual observations revealed that the vertical displacement between the average ground floor elevation and the slide material varied from 2 4 m. In each case horizontal displacements were not obvious. The ground within the sliding mass was hummocky and large fissures up to 75 mm wide were also observed. Within the slide debris of two of the three failures, 150 mm diameter PVC drainage pipes were observed. These pipes presumably were placed to drain surface runoff from the school. These appeared to issue directly onto the area of instability. The surrounding vegetation consisted of low to high grass with few trees. Based on a visual appreciation of the geometry of the slide, it appeared that the landslide was a rotational slide, which meant that the shape of the failure surface was probably circular. 2) Field Investigation The field investigation consisted of drilling a total of nine (9) boreholes (three per landslide); carrying out a topographic survey of the affected areas and; geophysical survey in the affected areas. The boreholes were advanced with an Acker portable drill rig employing wash boring techniques. Each borehole was drilled to a depth of 8.1 m below the ground surface. Samples were taken at intervals of 0.75 m for the first 3.0 m and at 1.5 m intervals thereafter. Both disturbed split spoon and undisturbed Shelby tube samples were taken. A topographic survey of the affected areas was also conducted. The aim of this exercise was to provide topographic information of the site; to provide input information in the stability analyses and; to provide a basis for the proposed remedial measures. The geophysical profiling consisting of a series of electrical resistivity measurements was conducted using the Schlumberger array. The purpose of these measurements was to aid in the determination of the interface between the soft slide debris and the in-situ material. The measurements were conducted as follows: Landslide 1: Four soundings at 3 m intervals to a depth of 6.5 m below the ground surface each Landslide 2: Five sounding at 3 m intervals to a depth of 6 m below the ground surface each Landslide 3: Four soundings at 3 m intervals ranging from 6-12 m below the ground surface 3) Soil Conditions In each case, the soil profile encountered consisted of fine grained material (e.g., silts and clays). Landslide 1: (Boreholes B1 B3) The soil profile encountered was divided into three (3) major soil units. The first unit extended from the ground surface to depths ranging from m below the ground surface. It consisted of medium stiff silty clays. This unit likely represents slide debris. The samples tested may be classified using the Unified Soil Classification System (USCS) as CH, meaning that they can be described as inorganic clays of high plasticity. These were underlain by stiff to very stiff silty clays, trace sand, which extended to depths ranging from m. These samples were also classified as CH. Further underlying these were hard fissured clays and silty clays. These extended to the end of the boreholes at a depth of 8.1 m. Samples within this unit were also classified as CH. Landslide 2: (Borehole B4 B6) The soil profile encountered was divided into three (3) major soil units. The first unit extended from the ground surface to a depth of 1.5 m below the ground surface. It consisted of medium stiff silty clays. This unit likely represents slide debris. The samples tested may be classified using the Unified Soil Classification System (USCS) as CH, meaning that they can be described as inorganic clays of high plasticity. These were underlain by stiff to very stiff silty clays, trace sand, which extended to depths

5 M.J. Joab and M. Andrews: Investigating Slop Failures Using Electrical Resistivity 70 ranging from m. These samples were also classified as CH. Further underlying these were hard fissured clays and silty clays. These extended to the end of the boreholes at a depth of 8.1 m. Samples within this unit were also classified as CH. Landslide 3: (Boreholes B7 B9) The soil profile encountered was divided into two (2) major soil units. The first unit extended from the ground surface to depths ranging from m below the ground surface. It consisted of stiff to very stiff silty clays. A sub-unit of medium stiff silty clay was also encountered in each of the boreholes at the following depths: Borehole B7: m below the ground surface Borehole B8: m below the ground surface Borehole B9: Ground surface to a depth of 1.5 m This unit likely represents the failure zone i.e. slide debris. The samples tested may be classified using the Unified Soil Classification System (USCS) as CH, meaning that they can be described as inorganic clays of high plasticity. These were underlain by hard-fissured clayey silts and silty clays. These extended to the end of the boreholes at a depth of 8.1 m. Samples within this unit were classified using the USCS as ML and CH. Therefore, they can be described as inorganic silts and clays of low to high plasticity, respectively. 4) Electrical Resistivity Soundings (ERS) The 1D electrical resistivity measurements were taken using the Schlumberger array along three sections (one section per landslide). These are referred to as Section A-A, B-B and C-C for Landslides 1, 2 and 3, respectively. They were conducted wherever possible along a line which corresponded with the location of the boreholes, so that a better correlation of the results could be achieved. In the case of Section C-C, a few soundings either had to be conducted off-centre or had to be omitted all together due to the presence of tall trees and other obstructions along the intended section line. The following is a discussion of the results of the inversion. Landslide 1: The results of the inversion of the field results are summarised in Table 1. Table 1: Summary of Results of Inversion of Field Results Landslide 1 Layer Layer Location Layer No. Thickness Resistivity ID (m) (Ωm) 1A B C D E A review of these results reveals the following: Layer 1 extends from the ground surface to depths ranging from m. This layer has resistivities ranging from Ωm. Layer 2 extends from the base of Layer 1. This has resistivities ranging from Ωm. A comparison with the borehole results clearly suggests that Layer 1 represents the medium stiff clays (slide debris) mentioned above and Layer 2 represents the stiff to very stiff silty clays. Landslide 2: The results of the inversion of the field results are summarised in Table 2. Table 2. Summary of Results of Inversion of Field Results Landslide 2 Layer Layer Location Layer No. Thickness Resistivity ID (m) (Ωm) 1a A 1b B a C 1b c a D 1b A review of these results reveals the following: Layer 1 extends from the ground surface to depths ranging from m. This layer has resistivities ranging from Ωm.

6 M.J. Joab and M. Andrews: Investigating Slop Failures Using Electrical Resistivity 71 Layer 2 extends from the base of Layer 1. This has resistivities ranging from Ωm. A comparison with the borehole results suggests that Layer 1 represents the medium stiff clays (slide debris) mentioned above and Layer 2 represents the stiff to very stiff silty clays. Landslide 3: The results of the inversion of the field results are summarised in Table 3. Table 3. Summary of Results of Inversion of Field Results Landslide 3 Layer Layer Location Layer No. Thickness Resistivity ID (m) (Ωm) A 2a b B 2a b C 2a b a D 2b c A review of these results reveals the following: Layer 1 extends from the ground surface to depths ranging from m. This layer has resistivities ranging from Ωm. Layer 2 extends from the base of Layer 1 to depths ranging from m. This has resistivities ranging from Ωm. Layer 3 extends from the base of Layer 2. This has resistivities ranging from Ωm. A comparison with the borehole results suggests that Layer 1 and 2 represent the medium stiff clays (slide debris) mentioned above. The higher resistivities in Layer 1 are probably due to a higher degree of fissuring. Layer 3 represents the stiff to very stiff silty clays. The stratigraphy at each landslide location was determined based on the results of both the borehole investigation and the electrical resistivity soundings. These are shown in Figures 4, 5 and B9 B6 B3 B8 B Figure 4. Landslide 1 (ERS): Section A-A B5 Figure 5. Landslide 2 (ERS): Section B-B Figure 6. Landslide 3 (ERS): Section C-C B7 B4 B1 Unit 1/ 2 Int erface Resistivity Sounding Existing Grd. Level Unit 1/ 2 Int erface Resistivity Sounding Existing Grd. Level Unit 1/ 2 Int erface Resistivity Sounding Existing Grd. Level 5) Slope Stability Analyses (SSA) Slope stability analyses were performed using the computer programme STABL5M to compute the factors of safety against rotational shear failure using Bishop s Modified Method of analyses (after Bishop, 1955). The analyses were conducted on the following basis: The shear strength parameters were determined from the results of the geotechnical investigation; The pore-water pressure regime varied from dry soil to saturated soil;

7 M.J. Joab and M. Andrews: Investigating Slop Failures Using Electrical Resistivity 72 The soil stratigraphy was as shown in Figures 4, 5 and 6; The pre-failure cross-section was inferred from an appreciation of the topography of the area using the survey information and; The constraint that the location of the failure surfaces analysed coincided with the observed position of the back scarp. The analyses showed a factor of safety of 1, which indicates a valid failure mechanism. Additionally, the most critical failure surface obtained was superimposed on each of the sections above. These combined sections are shown in Figures 7, 8 and B3 B B1 Figure 7. Landslide 1 (SSA): Section A-A B6 B B4 Figure 8. Landslide 2 (SSA): Section B-B B9 B B7 Figure 9. Landslide 3 (SSA): Section C-C Infe rre d OGL Failure P lane Unit 1/2 Interface Resist ivit y S ounding Exist ing Grd. Le ve l Infe rre d OGL Failure P lane Unit 1/2 Interface Resist ivit y S ounding Exist ing Grd. Le ve l Infe rre d OGL Failure P lane Unit 1/2 Interface Resist ivit y S ounding Exist ing Grd. Le ve l A review of the results indicates very good correlation between the location of the failure surface determined from the results of the slope stability analyses and its location estimated from the resistivity measurements and inversion for Landslides 1 and 2. For Landslide 3, the correlation is good. However, it probably could have been improved with additional measurements between Boreholes B7 and B Slope Failure at Tarouba 1) Site Description Visual observations revealed that the failure passed beneath two houses in the development. The maximum vertical displacement was approximately 1.2 m. The landslide caused major damage to the external works to the houses including apron, slipper drains and sewer connections. But there was minimal observed damage to the houses. The landslide was approximately 24 m wide (maximum) and 16 m long. It extended about 6 m beneath the houses to a concrete drain approximately 11 m north of the houses. The ground within the sliding mass was hummocky and very moist. Within the landslide, the slide debris toppled a short retaining wall. This wall consisted of 0.15 m wide, 1.2 m high concrete blocks. Topographically, the site sloped gently downwards from south to north, toward a paved drain at the base of a small valley. The surrounding vegetation consisted of low grass. Based on the site reconnaissance, it appears that the landslide was a shallow rotational landslide. 2) Field Investigation The field investigation consisted of drilling two (2) boreholes and conducting a geophysical survey in the affected area. The boreholes were advanced with an Acker portable drill rig employing wash boring techniques. Each borehole was drilled to a depth of 8.1 m below the ground surface. Samples were taken at intervals of 0.75 m for the first 3.0 m and at 1.5 m intervals thereafter. Both disturbed split spoon and undisturbed Shelby tube samples were taken. The geophysical profiling consisted of a series of 1D electrical resistivity measurements using the Wenner array. The purpose of these measurements was to aid in the determination of the interface between the soft slide debris and the in-situ material. A total of nine (9) soundings were conducted at the following intervals: 1, 1.5, 2, 2.5, 3 and 4 m. A

8 M.J. Joab and M. Andrews: Investigating Slop Failures Using Electrical Resistivity 73 topographic cross-section survey was also conducted along a section line. 3) Soil Conditions The soil profile encountered consisted of fine grained material (silts and clays). Three (3) major soil units were identified. The first unit extended from the ground surface to depths ranging from m below the ground surface. It consisted of soft to medium stiff silty clays. The base of this unit likely represents the zone where the slip surface is located. The samples tested may be classified using the Unified Soil Classification System (USCS) as CH, meaning that they can be described as inorganic clays of high plasticity. These were underlain by stiff to very stiff silty clays, trace sand, which extended to depths ranging from m. These samples were classified as MH and CH, meaning that they can be described as inorganic silts and clays of high plasticity. Further underlying these were hard clays and sandy clays. These extended to the end of the boreholes at a depth of 8.1 m. Samples within this unit were also classified as MH and CH. 4) Electrical Resistivity Soundings The electrical resistivity measurements were taken using the Wenner array along one section referred to as Section D-D. These were conducted along a line which corresponded approximately with the location of the boreholes. The results of the inversion of the field results are summarised in Table 4. Table 4. Summary of Results of Inversion of Field Results Location ID Layer No. Layer Thickness (m) Layer Resistivity (Ωm) No result the results of the iteration did not converge A review of these results reveals the following: Layer 1 extends from the ground surface to depths ranging from m. This layer has resistivities ranging from Ωm. Layer 2 extends from the base of Layer 1. This has resistivities ranging from Ωm. A comparison with the borehole results suggests that Layer 1 represents the medium stiff clays (slide debris) mentioned above and Layer 2 represents the stiff to very stiff silty clays. 5) Determination of Failure Surface Comparison the soil stratigraphy was based on the borehole investigation and the geophysical survey on a plot of a cross-section of the landslide (See Figure 10). It reveals that the interface between Units 1 and 2 obtained from the two methods compare very well. Additionally, closer inspection of the stratigraphy obtained from the electrical resistivity is circular in shape. A circular failure surface is expected based on the visual observations. In fact, drawing a circular arc shows a very good correlation with the data, and confirms that geophysical electrical resistivity can provide a very good estimate of the location of the failure surface in clays. B B2 Location of failed wall Bo reho le Investigatio n Unit 1/2 Interface Resistivity Sounding Existing Grd. Level Likely Failure P lane Figure 10. Section D-D: Likely surface failure location at Tarouba 5. Conclusions Based on the analysis of the study findings, it can be concluded that: 1) Conducting 1D vertical electrical resistivity soundings in clays correlates very well with the location of the failure plane. This method readily shows the likely location and general shape of the failure plane. This finding is supported independently by the results of back analyses of the failures presented using the Bishop Modified Method (Bishop, 1955). 2) The conduct of additional electrical resistivity

9 M.J. Joab and M. Andrews: Investigating Slop Failures Using Electrical Resistivity 74 measurements was very quick and cost effective in comparison to conducting additional boreholes at the site. Additionally, the advancing of boreholes does not provide more than simply general guidance regarding the likely area within which the failure plane may be located. 3) Based on the very good correlation of the electrical resistivity results and the results from the back analyses, it may be concluded that variations in the electrolyte concentration did not have a significant influence, if any, on the results. However, a detailed investigation of its influence is beyond the scope of this study and it could form the basis of future research. 4) It is suggested that the investigation of slope instabilities in clay soils be supplemented, where possible, with electrical resistivity soundings to improve the quality of the back analyses. References: ASTM D (2005), Standard Guide for Using the Direct Current Resistivity Method for Subsurface Investigation. Banton, O., Seguin, M.-K. and Cimon, M.-A. (1997), Mapping field-scale physical properties of soil with electrical resistivity, Soil Science Society of America Journal, No.61, pp Benson, R., Glaccum, R.A. and Noel, M.R. (1988), Geophysical Techniques for Sensing Buried Wastes and Waste Migration, National Water Well Association, Dublin, OH, USA, pp 236. Bishop, A.W. (1955), The use of slip circle in the stability analyses of slopes, Géotechnique, Vol.5, pp Bruno, F. and Marillier, F. (2000), Test of highresolution seismic reflection and other geophysical techniques on the Boup Landslide in the Swiss Alps, Survey Geophysical, Vol. 21, pp Caris, J.P.T. and van Asch, Th.W.J. (1991), Geophysical, geotechnical and hydrological investigations of a small landslide in the French Alps, Engineering Geology, Vol.31, pp Clayton, C.R.I., Mathews, M.C. and Simmons, N.E. (Undated), Site Investigations, Chapter 4, 2 nd Edition, Department of Civil Engineering, University of Surrey, UK; available at Geotech Associates Ltd. (2008a), Soil Investigation of Three (3) Landslides at Manzanilla High School GA , Prepared for National Maintenance Training & Security Company Ltd. Geotech Associates Ltd. (2008b), Soil Investigation of a Landslide at Tarodale Housing Development, Tarouba GA Prepared for Trinidad and Tobago Housing Development Corporation. Guyod, H. (1964), Use of geophysical logs in soil engineering, ASTM Symposium on Soil Exploration, Special Technical Publication No.351, pp Hutchinson, J.N. (1981), Methods of locating slip surfaces in landslides, Proceedings of the Symposium on Investigation and Correction of Landslides, Vol.2, pp Jongman, D. and Garambois, S. (2007), Geophysical investigation of landslides: a review, Bull. Soc. géol. Fr. Vol. 178, No. 2, pp Kachanoski, R.G., Gregorich, E.G. and Van Wesenbeeck, I.J. (1988), Estimating spatial variations of soil water content using non-contacting electromagnetic inductive methods, Canadian Journal of Soil Science, No.68, pp Lebourg, T., Binet, S., Tric, E., Jomard, H. and El Bedoui, S. (2005), Geophysical survey to estimate the 3D sliding surface and the 4D evolution of the water pressure on part of a deep-seated landslide, Terra Nova, Vol.17, pp Lapenna, V., Lorenzo, P., Perrone, A., Piscitelli, S., Rizzo, E. and Sdao, F. (2005), 2D electrical resistivity imaging of some complex landslides in Lucanian Apennine Chain, Southern Italy, Geophysics, No.70, B11 B18. Méric, O., Garambois, S., Jongman, D., Wathelet, M., Chatelain, J.-L. and Vengeon J.-M. (2005), Application of Geophysical methods for the Investigation of the Large Gravitational Mass Movement of Sechilienne, France, Canadian Geotechnical Journal, Vol.42, pp Schmutz, M., Albouy Y., Guérin R., Maquaire, O., Vassal, J., Schott, J.-J. and Descloîtres, M. (2000), Joint electrical and time domain electromagnetism (TDEM) data inversion applied to the Super Sauze Earthflow (France), Surveys in Geophysics, Vol.21, pp Telford, W.M., Geldart, L.P. and Sheriff, R.E. (2004), Applied Geophysics. 2 nd Edition, Cambridge University Press, UK. Vaughn, P.J., Lesch, S.M., Corwin, D.L. and Cone, D.G. (1995), Water content effect on soil salinity prediction: a geostatistical study using Cokriking, Soil Science Society of America Journal, No.59, pp Biographical Notes: Malcom J. Joab has over seventeen years of professional experiences in the field of Civil and Geotechnical Engineering. His experience includes development projects throughout the Caribbean related to commercial buildings, industrial plants, highways, bridges, water supply and sewage, housing, airports, bridge condition surveys and numerous forensic geotechnical engineering studies. He has also appeared as an expert witness and provided expert opinions on landslide litigation matters. At Geotech, he has spearheaded vibration as well as electrical resistivity measurement and analyses. Mr. Joab

10 M.J. Joab and M. Andrews: Investigating Slop Failures Using Electrical Resistivity 75 was a Part-time Lecturer in Geology for Engineers at UWI and served on the Executive Council of APETT as Assistant Secretary. He is also a Director at Geotech Associates Ltd. Martin Andrews has over thirty-two years experience in the field of Civil and Geotechnical Engineering. He has worked on project throughout the Caribbean on development projects related to industrial plants, airports, roads, bridges, water supply and sewage, coastal structure and housing. His experience includes forensic geotechnical studies; as an expert witness for arbitration proceedings and litigation; road pavement condition surveys; slope stability analyses; and earthquake engineering studies. Mr. Andrews is responsible for technical ad administrative management of Geotech s head office in Trinidad. Mr. Andrews was a Part-time Lecturer at UWI in Soil Mechanics and Foundations Engineering, and currently lectures Introduction to Geotechnical Engineering to Year 1 Civil Engineering students.