Investigating Slope Failures Using Electrical Resistivity Case Studies

7/27/2019 Investigating Slope Failures Using Electrical Resistivity Case Studies

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M.J. Joab and M. Andrews: Investigating Slop Failures Using Electrical Resistivity 66

ISSN 1000 7924

The Journal of the Association of Professional Engineers of Trinidad and TobagoVol.38, No.1, October 2009, pp.66-75

Investigating Slope Failures Using Electrical Resistivity: Case Studies

Malcom J. Joab a and Martin Andrews b

Geotech Associates Ltd. 4 Niles Street, Tunapuna, Trinidad, West IndiesaE-mail: [email protected]

bE-mail: [email protected]

Corresponding Author

(Received 30 April 2009; Revised 8 July 2009; Accepted 17September 2009)

Abstract: The purpose of this paper is to present case studies and outline a methodology used to estimate thelocation 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-sectionof 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 soilsamples

2) Conducting topographic surveys

3) Lab testing of representative samples4) 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


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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 waterpressure within a soil matrix (known as the pore-

water 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.

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 changesin subsurface material.

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):

2nw = Eq. 1

where, = Electrical resistivity of soil/rock

w = Electrical resistivity of pore watern = 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. Theequation 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 theground and two to measure the potential difference);

a current source; current meter and voltmeter (See

Figure 2).

Note: C1 and C2, P1 and P2 refer to the current and voltage

electrodes respectively.

Figure 2. Basic Concept of Resistivity MeasurementSource: Abstracted from Benson et al. (1988)


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In this case, the electrical resistivity is calculated

according to the following formula which is based

on Ohms Law:

IVk= Eq. 2

Where = Electrical resistivity

V = Potential difference (voltage)

I = Applied currentk = 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 D6431-99 (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 GeometriesSource: Abstracted from ASTM D6431-99 (2005)

When electrical resistivity measurements are

conducted in the field, the values obtained arereferred 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 6.1.

3.3 Use of Electrical Resistivity in Landslide

InvestigationJongman 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 ofslope 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; Mric 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 etal. (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 0.53 0.60

and 0.88 0.94, 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 locationof 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


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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 DescriptionThe 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

10 18 m wide and extended between 20 40 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 pipespresumably 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 geometryof 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 InvestigationThe 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 thesemeasurements 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 surfaceeach

Landslide 3: Four soundings at 3 m

intervals ranging from 6-12 m below the ground

surface

3) Soil ConditionsIn 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 1.5 3.0 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 4.6 6.1 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 theground 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


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ranging from 3.0 6.1 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 4.6 6.1 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: 1.5-3.0 m below the ground

surface

Borehole B8: 1.5-2.3 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 oftall 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

Location

IDLayer No.

Layer

Thickness

(m)

Layer

Resistivity

(m)1 1.8 12

1A2 - 2.3

1 0.9 301B2 - 3.3

1 1.9 9.51C

2 - 1.6

1 1.0 181D

2 - 2.8

1 2.6 6.41E

2 - 1.7

A review of these results reveals the following:

Layer 1 extends from the ground surface to

depths ranging from 0.9 2.6 m. This layer

has resistivities ranging from 6.4 30 m.

Layer 2 extends from the base of Layer 1. This

has resistivities ranging from 1.6 3.3 m.

A comparison with the borehole results clearlysuggests 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:



Table 2. Summary of Results of Inversion of Field Results Landslide 2

Location

IDLayer No.

Layer

Thickness

(m)

Layer

Resistivity

(m)1a 1.8 12

1b 0.6 6.32A

2 - 1.4

1 0.8 162B

2 - 4.4

1a 0.5 9.2

1b 0.6 9.9

1c 1.5 6.92C

2 - 2.6

1a 0.6 7.5

1b 1.0 7.42D

2 - 2.9




has resistivities ranging from 6.3 16 m.


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125.00

130.00

8 0 8 5 9 0 9 5 10 0 1 05 110 115 12 0 12 5 1 30 13 5 14 0 1 45 15 0 1 55

Distance (m)

Elevation

(m)

B7

B8B9

Unit 1/2 Interfa ce

Resistivity Sounding

Existing Grd. Level

Unit 1/2 Interfa ce


Existing Grd. Level

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8 0 8 5 9 0 9 5 10 0 10 5 110 115 12 0 12 5 13 0 13 5 14 0 14 5 150 15

Distance (m)

Elevation(m)

B4

B5B6

Unit 1/2 Interfa ce


Existing Grd. Level

A comparison with the borehole results

suggests that Layer 1 represents the medium



clays.

Landslide 3:



Table 3. Summary of Results of Inversion of Field Results Landslide 3

Location

IDLayer No.

Layer

Thickness

(m)

Layer

Resistivity

(m)1 0.5 30

2a 1.8 7.4

2b 1.9 7.5

3A

3 - 2.8

1 0.3 21

2a 2.3 8.3

2b 0.75 7.33B

3 - 3.6

1 0.5 21

2a 0.9 10

2b 0.4 8.23C

3 - 2.7

2a 0.1 5.8

2b 1.2 8.3

2c 0.7 7.23D

3 - 3.3




has resistivities ranging from 21 30 m.

Layer 2 extends from the base of Layer 1 to

depths ranging from 1.8 4.2 m. This has

resistivities ranging from 7.2 10 m.



A comparison with the borehole results

suggests that Layer 1 and 2 represent themedium stiff clays (slide debris) mentioned

above. The higher resistivities in Layer 1 are

probably due to a higher degree of fissuring.


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 6.

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8 0 8 5 9 0 9 5 10 0 10 5 110 115 12 0 1 25 13 0 13 5 14 0 14 5 1 50 1

Distance (m)

Ele

vation(m)

B1

B2

B3

5

Figure 4. Landslide 1 (ERS): Section A-A

Figure 5. Landslide 2 (ERS): Section B-B

Figure 6. Landslide 3 (ERS): Section C-C

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 usingBishops 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;


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The soil stratigraphy was as shown in Figures

4, 5 and 6;

The pre-failure cross-section was inferredfrom 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 observedposition of the back scarp.

The analyses showed a factor of safety of1,

which indicates a valid failure mechanism.

Additionally, the most critical failure surface

obtained was superimposed on each of the sectionsabove. These combined sections are shown in

Figures 7, 8 and 9.

Figure 7. Landslide 1 (SSA): Section A-A

Figure 8. Landslide 2 (SSA): Section B-B

Figure 9. Landslide 3 (SSA): Section C-C

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 B8.

4.2 Slope Failure at Tarouba

1) Site DescriptionVisual 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.

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8 0 8 5 9 0 9 5 10 0 10 5 1 10 115 12 0 1 25 1 3 0 1 35 1 4 0 14 5 15 0 1 55

Distance (m)

Elevation

(m) Inferred OGL

Failure Plane

Unit 1/2 Interfac e


Existing Grd. Level

B1

B2

B3

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.

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8 0 8 5 9 0 9 5 10 0 10 5 1 10 115 12 0 1 25 1 3 0 1 35 1 4 0 14 5 15 0 1 55

Distance (m)

Elevation

(m)

Inferred OGL

Failure Plane

Unit 1/2 Interface


Existing Grd. Level

B4

B5

B6

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.

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85.00

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95.00

100.00

105.00

110.00

115.00

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8 0 8 5 9 0 9 5 10 0 10 5 1 10 115 12 0 1 25 1 3 0 1 35 1 4 0 14 5 15 0 1 55

Distance (m)

Elevation(m) Inferred OGL

Failure Plane

Unit 1/2 Interface


Existing Grd. Level

B7

B8B9

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


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topographic cross-section survey was also conducted

along a section line.

3) Soil ConditionsThe 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 2.3

3.0 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 4.6 6.1 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 claysand 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 SoundingsThe electrical resistivity measurements were taken

using the Wenner array along one section referred toas 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.

85.00

90.00

95.00

100.00

105.00

90 95 100 105 110 115 120

Distance (m)

Elevatio

n(m)

Boreho le Investigation

Unit 1/2 Interfac e

Res istivitySo unding

Existing Grd. Level

Likely Failure P lane

B2

B1

Location of failed wall

Table 4. Summary of Results of Inversion of Field Results

Location

IDLayer No.

Layer

Thickness

(m)

Layer

Resistivity

(m)1 2.2 4

12 - 2

1 2.7 4.92

2 - 1.0

1 2.3 5.23

2 - 0.9

1 2 4.84

2 - 1.1

1 2.4 4.75

2 - 0.8

1 1.8 4.36

2 - 1.7

7No result the results of the iteration did not

converge

1 0.9 4.18

2 - 2.0

1 1.7 3.19

2 - 2.8







A comparison with the borehole resultssuggests that Layer 1 represents the medium



clays.

5) Determination of Failure SurfaceComparison 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.

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


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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 6431-99 (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 1010-1017.

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 thestability analyses of slopes, Gotechnique, Vol.5, pp 7-

17.

Bruno, F. and Marillier, F. (2000), Test of high-

resolution seismic reflection and other geophysical

techniques on the Boup Landslide in the Swiss Alps,Survey Geophysical, Vol. 21, pp 333-348.

Caris, J.P.T. and van Asch, Th.W.J. (1991), Geophysical,

geotechnical and hydrological investigations of a smalllandslide in the French Alps, Engineering Geology,

Vol.31, pp 249-276.

Clayton, C.R.I., Mathews, M.C. and Simmons, N.E.

(Undated), Site Investigations, Chapter 4, 2nd Edition,

Department of Civil Engineering, University of Surrey,UK; available at www.geotechnique.info.

Geotech Associates Ltd. (2008a), Soil Investigation of

Three (3) Landslides at Manzanilla High School GA 08

246, Prepared for National Maintenance Training &

Security Company Ltd.

Geotech Associates Ltd. (2008b), Soil Investigation of aLandslide at Tarodale Housing Development, TaroubaGA 08 408. 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.75-85.Hutchinson, J.N. (1981), Methods of locating slip

surfaces in landslides, Proceedings of the Symposium

on Investigation and Correction of Landslides, Vol.2,pp.169-203.

Jongman, D. and Garambois, S. (2007), Geophysicalinvestigation of landslides: a review, Bull. Soc. gol.

Fr. Vol. 178, No. 2, pp. 101-112.

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, pp715-722.

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 399-406.Lapenna, V., Lorenzo, P., Perrone, A., Piscitelli, S.,

Rizzo, E. and Sdao, F. (2005), 2D electrical resistivity

imaging of some complex landslides in LucanianApennine Chain, Southern Italy, Geophysics, No.70,

B11 B18.

Mric, O., Garambois, S., Jongman, D., Wathelet, M.,

Chatelain, J.-L. and Vengeon J.-M. (2005),

Application of Geophysical methods for theInvestigation of the Large Gravitational Mass

Movement of Sechilienne, France, Canadian

Geotechnical Journal, Vol.42, pp 1105-1115.Schmutz, M., Albouy Y., Gurin R., Maquaire, O.,

Vassal, J., Schott, J.-J. and Desclotres, M. (2000),

Joint electrical and time domain electromagnetism

(TDEM) data inversion applied to the Super SauzeEarthflow (France), Surveys in Geophysics, Vol.21, pp371-390.

Telford, W.M., Geldart, L.P. and Sheriff, R.E. (2004),

Applied Geophysics. 2nd 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 ScienceSociety of America Journal, No.59, pp.1146-1156.

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
http://www.geotechnique.info/http://www.geotechnique.info/


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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 Andrewshas over thirty-two years experience inthe field of Civil and Geotechnical Engineering. He has

worked on project throughout the Caribbean ondevelopment 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 Geotechs

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 Engineeringstudents.

Documents

Investigating Slope Failures Using Electrical Resistivity Case Studies