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ELECTRICAL RESISTIVITY TOMOGRAPHY FOR EARTHWORK CONDITION APPRAISAL R. Sellers, N. Dixon & T. Dijkstra, Department fo Civil and Building Engineering, Loughborough University, Loughborough LE11 3TU, United Kingdom D. Gunn, J. Chambers & P. Jackson, , British Geological Survey, Keyworth, Nottingham, NG12 5GG, United Kingdom The condition of infrastructure earthworks can deteriorate over time due to seasonal pore pressure cycles. To gain insights into the potential development of progressive failure mechanisms, gradual changes must be detected and quantified. Soil moisture variations form a good indicator and this can be observed using non-intrusive electrical resistivity tomography (ERT). However, ERT is influenced by, among others, material type, water content, pore fluid chemistry, density and temperature. The challenge is to understand how each factor contributes to resistivity measurements before changes in soil moisture content can be reliably quantified. EARTHWORK CONDITION APPRAISAL IN THE UK The condition appraisal of UK infrastructure earthworks is generally undertaken through a risk assessment involving the geometry, material, history and visual inspection of the site. This is subjective and concentrates on the superficial changes in the structure. The use of a geophysical method to assess the health of an embankment through non-intrusive methods is beneficial as it enables the assessment of the internal structure of the embankment without disturbing it. Large infrastructure embankments of significant lengths were constructed in the UK during the period 1850 to 1900 for the expansion of the rail system. Embankments were also constructed in the 1950’s and 1960’s for motorway construction (Scott et al., 2007). The soils used to construct the embankments were often won locally from cuttings or tunnels (MacDonald, 2005). Even though the materials used are similar, embankments constructed during these two periods presently have different physical properties due to the different construction methods employed, reflecting the geotechnical knowledge available at the time. Railway embankments were generally constructed using an end tipping method. The soil was carried by either horse drawn or steam driven carts to the end of the constructed section of embankment and the carts emptied to extend the structure. The soil was not compacted as part in a controlled way. This method was employed as it was the most cost effective at the time. Storage of soil from nearby cuttings and tunnels was expensive and therefore the material had to be placed in the embankment at the same rate at which it was being won (Skempton, 1996). More recent motorway embankments were constructed using soils that were compacted in layers of known thickness to achieve the maximum density and strength available for the material (as specified by appropriate standards). A summary of the differences in present embankment ‘quality’ is illustrated in Figure 1. The term failure is commonly used but not always clearly defined, and it can mean many things. In the context of this study, it includes both serviceability limit states (shrink, swell; limiting performance) and the ultimate limit states (complete failure). There are approximately 5,000km of embankments in use for railway lines in the UK affected by a variety of failures (Figure 2). Some 3518km of trunk roads have earthworks associated with them which are also affected by a similar range of failures (Department of Transport, 2008a). Progressive failure, a slow accumulation of strains until rupture occurs, is considered especially in this study. Often the external pre-rupture signs in an earthwork slope are hardly visible, making these failures difficult to detect. However, the pre-rupture strains tend to loosen the fabric and this leads to higher moisture content levels along potential slip surfaces. These, in turn, could be potentially be detected by ERT. NON-INTRUSIVE ELECTRICAL RESISTIVITY TOMOGRAPHY (ERT) Electrical resistivity is considered to be a suitable technique to assess the moisture content changes within an embankment as it is a relatively non intrusive technique (only small pegs need to be inserted into the ground) and resistivity is sensitive to changes in moisture (Fukue et al., 1999). Investigations have been undertaken to determine the effect on electrical resistivity of different soil parameters, as illustrated in Table 1. Resistivity sensitivity is different for each parameter and therefore each needs to be considered separately, however, the parameters and sensitivities may be interrelated. Soils that are susceptible to fail by progressive failure are the formations with large amounts of swelling clay minerals (Palmer and Rice, 1973). These materials are often not quantified in laboratory studies due to the added complexity and cost of the tests, and because they change volume as moisture content changes as clay particles absorb water into the interlayer space. Investigation of these soils and their resistivity is crucial to understand the long term moisture content changes, and hence potential for progressive failures, in UK infrastructure earthworks. Figure 1. Comparison of modern highways embankment and Victorian railway embankment (after O'Brien, 2007). temp comp pwc clay m mc psd (Bryson and Bathe, 2009) X X (Abu-Hassanein et al., 1996) X X X X X (Besson et al., 2008) X X (Bryson, 2005). X X X X (Samouelian et al., 2005) X X X X X (Rhoades et al., 1976) X X X (McCarter, 1984) X X (Kalinski and Kelly, 1993) X X (Cassiani et al., 2009) X X X (Brunet et al., 2009) X X X Table 1. A summary of research publications on soil parameters and their effect on resistivity (temp=temperature, comp=degree of compaction, pwc = pore water chemistry, clay m = clay mineralogy, mc = moisture content, psd = particle size distribution. Figure 2. The number and type of earthwork failures in the UK rail network between 2004 and 2008 (Department of Transport, 2008b). LABORATORY TESTING PROCEDURE Resistivity testing equipment has to satisfy a number of criteria to make the data gathered useable and reliable in any subsequent modelling. The equipment has to pass a current of known density through a material and record the voltage potential across a known flow path distance. Also, the system has to be cost effective and reusable. There are a number of soil sample parameters that have to be controlled so test results can be compared. The most important three are moisture content, compaction and temperature. One of the controls placed on a measurement system is that a soil test sample must be compacted to a known and repeatable density. This means any resistivity measurements have to be taken causing minimal disturbance to the sample. To control the amount of compaction used on each sample, and hence the density, a standard compaction method is used. The key soils that develop progressive failure in earthworks in the UK are heavily overconsolidated clays (e.g. London Clay, Gault Clay and Oxford Clay). Standard Proctor compaction was used to form the base samples which were transferred to a specially designed mould constructed from non- conducting PVC to which the resistivity testing equipment can be attached. Once the resistivity measurement caps were put in place, the sample is sealed with wax (a non-conducting medium) to preserve the moisture content during testing (Figure 3). FIELD TESTING: BIONICS TEST FACILITY To achieve an understanding of soil resistivity behaviour a combination of field monitoring and laboratory testing is used. The field monitoring takes place at the full scale test embankment BIONICS (BIOlogical and eNgineering Impacts of Climate change on Slopes). This embankment was constructed during 2005 to investigate the effect of climate change on infrastructure embankments. The site is managed by Newcastle University with a number of other institutions running long-term experiments on the site. The embankment is 90m long, 6m high with a crest width of 5m and the soil used to construct the embankment was a locally won glacial till (Hughes et al., 2008). The embankment is orientated with the long axis aligned east-west. Sections of this embankment are either compacted to existing UK highway specifications for earthwork construction or not actively compacted reflecting Victorian embankment construction. In collaboration with the British Geological Survey (BGS) the embankment has been retrofitted with a number of instruments to aid interpretation of resistivity measurements. Two resistivity arrays have been installed by BGS, one in each of the two types of compaction. These arrays are orientated perpendicular to the long axis of the embankment. The electrodes are spaced at 0.5m across the entire section, giving a total of 32 electrodes on each array. The arrays are set to record once every 24 hours with arrays monitored one after the other, to reduce the possibility of interference. In addition to resistivity arrays, 14 combined temperature and moisture content probes were installed in the less well compacted section close to the resistivity array (Figure 4). PRELIMINARY RESULTS AND CONCLUSIONS The first results from the ER array on the BIONICS embankment look very promising (Figure 5) providing good indications of the internal structure of the embankment. Initial models to understand the fluctuations in temperature in the embankment are also positive, suggesting that curve fitting algorithms such as those of Nofziger (2003) can be applied with some further modifications (Figure 6). Some preliminary results investigating the effects of pore water chemistry on ER suggest a more complex feedback in the clays of the BIONICS embankment that requires further study (Figure 7). Variations in resistivity driven by these variables are significant and can occur both spatially and temporally. This in itself is not such a problem if only one snapshot of resistivity is required. However, when comparing multiple images, either along an embankment or at the same spot over a longer time period, it is essential that these variations are accounted for. We are still some way off the target when all variables are properly quantified and valid geotechnical conclusions can be drawn regarding ER as a proxy for soil moisture variations in time and space. Contact Dr David Gunn British Geological Survey Kingsley Dunham Centre Keyworth, Nottingham NG12 5GG United Kingdom Tel: +44 (0)115 936 3100 Prof Neil Dixon Department of Civil and Building Engineering Loughborough University Loughborough LE11 3TU United Kingdom Tel: +44 (0)1509 228542 Figure 3. Laboratory setup for electrical resistivity measurement on compacted earthworks clay samples. Figure 4. An overview of the instrumentation and electrical resistivity array (‘ladder’ on top of flank) on the south facing flank of the BIONICS embankment Figure 7. Preliminary results from laboratory tests on standard Proctor compacted samples of Mercia Mudstone and BIONICS material, indicating complexities in electrical resistivity response. ACKNOWLEDGEMENTS This paper is published with the permission of the Executive Director of the British Geological Survey (NERC). REFERENCES Abu-Hassanein, Z.S., Benson, C.H. and Blotz, L.R., 1996. Electrical Resistivity of Compacted Clays. Journal of Geotechnical Engineering, 122(5): 397-406. Besson, A., Cousin, I., Dorigny, A., Dabas, M. and King, D., 2008. The temperature correction for the electrical resistivity measurements in undisturbed soil samples: Analysis of the existing conversion models and proposal of a new model. SOIL SCIENCE, 173(10): 707-720. Brunet, P., Clement, R. and Bouvier, 2009. Monitoring soil water content and deficit using Electrical Resistivity. Journal of Hydrology, 380: 146-153. Bryson, L.S., 2005. Evaluation of Geotechnical Parameters Using Electrical Resistivity Measurements, Earthquake Engineering and Soil Dynamics (GSP 133). ASCE, Austin, Texas, USA, pp. 10-10. Bryson, L.S. and Bathe, A., 2009. Determination of selected geotechnical properties of soil using electrical conductivity testing. Geotechnical Testing Journal, 32(3). Cassiani, G. et al., 2009. Monitoring the hydrologic behaviour of a mountain slope via time-lapse electrical resistivity tomography. Near Surface Geophysics: 475-486. Department of Transport, 2008a. Transport Statistics Great Britain. Department of Transport, 2008b. Rail Accident Report - Network Rail's management of Existing Earth-works. 25/2008 R.A.I. Branch. Fukue, M., Minato, T., Horibe, H. and Taya, N., 1999. The micro-structures of clay given by resistivity measurements. Engineering Geology, 54(1-2): 43-53. Hughes, P., Glendinning, S. and Davies, O., 2008. Construction and Monitoring of a test embankment for the evaluation of the impacts of climate change on UK transport infrastructure, Proceedings 1st international conference Advances in Transportation Geotechnics. Taylor and Francis Group, Nottingham. Kalinski, R.J. and Kelly, W.E., 1993. Estimating Water Content of Soils from Electrical Resistivity. Geotechnical Testing Journal, 16(3): 323. MacDonald, M., 2005. Vegetation and its effect on slope stability. Safe management of railways structures(Phase 2). McCarter, W.J., 1984. The electrical resistivity characteristics of compacted clays. Geotechnique, 34: 263-267. Nofziger, DL 2003. Soil temperature changes with time and depth. Soilphysics.okstate.edu/software/ O'Brien, A.S., 2007. rehabilitation of urban railway embankments: investigation, analysis and stabilisation, Proceedings of the 14th International Conference SMGE, Madrid. Palmer, A.C. and Rice, J.R., 1973. The Growth of Slip Surfaces in the Progressive Failure of Over-Consolidated Clay. Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences (1934-1990), 332(1591): 527-548. Perry, J.A., Pedley, M. and Reid, M., 2003. Infrastructure embankments condition appraisal and remedial treatment, C592. CIRIA. Rhoades, J.D., Raats, P.A.C. and Prather, R.J., 1976. Effects of Liquid-phase Electrical Conductivity, Water Content, and Surface Conductivity on Bulk Soil Electrical Conductivity. Soil Sci Soc Am J, 40(5): 651-655. Samouelian, A., Cousin, I., Tabbagh, A., Bruand, A. and Richard, G., 2005. Electrical resistivity survey in soil science: a review. Soil and Tillage Research, 83(2): 173-193. Scott, J.M., Loveridge, F. and O'Brien, A.S., 2007. Influence of Climate and Vegetation on Railway Embankments. In: V. Cuéllar et al. (Editors), Geotechnical Engineering in Urban Environments: Proceedings of the 14th European Conference on Soil Mechanics and Geotechnical Engineering, Rotterdam, pp. 659-664. Skempton, A.W., 1996. Embankments and cuttings on the early railways. Construction History, 11: 33-39. Figure 5. Preliminary electrical resistivity profile of the BIONICS embankment, providing a clear indication of the potential benefits of this method for earthwork condition appraisal. However, subtle variations within limits of 10 to 40 ohm.m are most indicative of moisture condition changes along potential slip surfaces. This is also the range that is affected by temperature variations and other influences such as pore water chemistry. Figure 6. In blue is shown the variation of soil temperature at 1m depth on the north flank of the BIONICS embankment for one year (x-axis shows days; y-axis temperature in degree Celsius). The red line shows the trend for an equivalent point on the south flank of the embankment. Initial results suggest that a an adjustment factor of 0.12% per degree can be applied at this location in NE-England for embankments with a different orientation than N-S.

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ELECTRICAL RESISTIVITY TOMOGRAPHY FOR EARTHWORK CONDITION APPRAISAL

R. Sellers, N. Dixon & T. Dijkstra, Department fo Civil and Building Engineering, Loughborough University, Loughborough LE11 3TU, United KingdomD. Gunn, J. Chambers & P. Jackson, , British Geological Survey, Keyworth, Nottingham, NG12 5GG, United Kingdom

The condition of infrastructure earthworks can deteriorate over time due to seasonal pore pressure cycles. To gaininsights into the potential development of progressive failure mechanisms, gradual changes must be detected andquantified. Soil moisture variations form a good indicator and this can be observed using non-intrusive electricalresistivity tomography (ERT). However, ERT is influenced by, among others, material type, water content, pore fluidchemistry, density and temperature. The challenge is to understand how each factor contributes to resistivitymeasurements before changes in soil moisture content can be reliably quantified.

EARTHWORK CONDITION APPRAISAL IN THE UKThe condition appraisal of UK infrastructure earthworks is generally undertaken through a risk assessment involvingthe geometry, material, history and visual inspection of the site. This is subjective and concentrates on thesuperficial changes in the structure. The use of a geophysical method to assess the health of an embankmentthrough non-intrusive methods is beneficial as it enables the assessment of the internal structure of theembankment without disturbing it.

Large infrastructure embankments of significant lengths were constructed in the UK during the period 1850 to 1900for the expansion of the rail system. Embankments were also constructed in the 1950’s and 1960’s for motorwayconstruction (Scott et al., 2007). The soils used to construct the embankments were often won locally from cuttingsor tunnels (MacDonald, 2005). Even though the materials used are similar, embankments constructed during thesetwo periods presently have different physical properties due to the different construction methods employed,reflecting the geotechnical knowledge available at the time.

Railway embankments were generally constructed using an end tipping method. The soil was carried by either horsedrawn or steam driven carts to the end of the constructed section of embankment and the carts emptied to extendthe structure. The soil was not compacted as part in a controlled way. This method was employed as it was the mostcost effective at the time. Storage of soil from nearby cuttings and tunnels was expensive and therefore the materialhad to be placed in the embankment at the same rate at which it was being won (Skempton, 1996). More recentmotorway embankments were constructed using soils that were compacted in layers of known thickness to achievethe maximum density and strength available for the material (as specified by appropriate standards). A summary ofthe differences in present embankment ‘quality’ is illustrated in Figure 1.

The term failure is commonly used but not always clearly defined, and it can mean many things. In the context ofthis study, it includes both serviceability limit states (shrink, swell; limiting performance) and the ultimate limitstates (complete failure). There are approximately 5,000km of embankments in use for railway lines in the UKaffected by a variety of failures (Figure 2). Some 3518km of trunk roads have earthworks associated with themwhich are also affected by a similar range of failures (Department of Transport, 2008a). Progressive failure, a slowaccumulation of strains until rupture occurs, is considered especially in this study. Often the external pre-rupturesigns in an earthwork slope are hardly visible, making these failures difficult to detect. However, the pre-rupturestrains tend to loosen the fabric and this leads to higher moisture content levels along potential slip surfaces. These,in turn, could be potentially be detected by ERT.

NON-INTRUSIVE ELECTRICAL RESISTIVITY TOMOGRAPHY (ERT)Electrical resistivity is considered to be a suitable technique to assess the moisture content changes within anembankment as it is a relatively non intrusive technique (only small pegs need to be inserted into the ground) andresistivity is sensitive to changes in moisture (Fukue et al., 1999). Investigations have been undertaken to determinethe effect on electrical resistivity of different soil parameters, as illustrated in Table 1.Resistivity sensitivity is different for each parameter and therefore each needs to be considered separately,however, the parameters and sensitivities may be interrelated. Soils that are susceptible to fail by progressive failureare the formations with large amounts of swelling clay minerals (Palmer and Rice, 1973). These materials are oftennot quantified in laboratory studies due to the added complexity and cost of the tests, and because they changevolume as moisture content changes as clay particles absorb water into the interlayer space. Investigation of thesesoils and their resistivity is crucial to understand the long term moisture content changes, and hence potential forprogressive failures, in UK infrastructure earthworks.

Figure 1. Comparison of modern highways embankment and Victorian railway embankment (after O'Brien, 2007).

temp comp pwc clay m mc psd

(Bryson and Bathe, 2009) X X (Abu-Hassanein et al., 1996) X X X X X (Besson et al., 2008) X X (Bryson, 2005). X X X X (Samouelian et al., 2005) X X X X X (Rhoades et al., 1976) X X X (McCarter, 1984) X X (Kalinski and Kelly, 1993) X X (Cassiani et al., 2009) X X X (Brunet et al., 2009) X X X

Table 1. A summary of research publications on soil parameters and their effect on resistivity (temp=temperature, comp=degree of compaction, pwc = pore water chemistry, clay m = clay mineralogy, mc = moisture content, psd = particle size distribution.

Figure 2. The number and type of earthwork failures in the UK rail networkbetween 2004 and 2008 (Department of Transport, 2008b).

LABORATORY TESTING PROCEDUREResistivity testing equipment has to satisfy a number of criteria to make the datagathered useable and reliable in any subsequent modelling. The equipment hasto pass a current of known density through a material and record the voltagepotential across a known flow path distance. Also, the system has to be costeffective and reusable. There are a number of soil sample parameters that haveto be controlled so test results can be compared. The most important three aremoisture content, compaction and temperature. One of the controls placed on ameasurement system is that a soil test sample must be compacted to a knownand repeatable density. This means any resistivity measurements have to betaken causing minimal disturbance to the sample. To control the amount ofcompaction used on each sample, and hence the density, a standard compactionmethod is used. The key soils that develop progressive failure in earthworks inthe UK are heavily overconsolidated clays (e.g. London Clay, Gault Clay andOxford Clay). Standard Proctor compaction was used to form the base sampleswhich were transferred to a specially designed mould constructed from non-conducting PVC to which the resistivity testing equipment can be attached. Oncethe resistivity measurement caps were put in place, the sample is sealed withwax (a non-conducting medium) to preserve the moisture content during testing(Figure 3).

FIELD TESTING: BIONICS TEST FACILITYTo achieve an understanding of soil resistivity behaviour a combination of fieldmonitoring and laboratory testing is used. The field monitoring takes place at thefull scale test embankment BIONICS (BIOlogical and eNgineering Impacts ofClimate change on Slopes). This embankment was constructed during 2005 toinvestigate the effect of climate change on infrastructure embankments. The siteis managed by Newcastle University with a number of other institutions runninglong-term experiments on the site. The embankment is 90m long, 6m high with acrest width of 5m and the soil used to construct the embankment was a locallywon glacial till (Hughes et al., 2008). The embankment is orientated with the longaxis aligned east-west. Sections of this embankment are either compacted toexisting UK highway specifications for earthwork construction or not activelycompacted reflecting Victorian embankment construction. In collaboration withthe British Geological Survey (BGS) the embankment has been retrofitted with anumber of instruments to aid interpretation of resistivity measurements. Tworesistivity arrays have been installed by BGS, one in each of the two types ofcompaction. These arrays are orientated perpendicular to the long axis of theembankment. The electrodes are spaced at 0.5m across the entire section, givinga total of 32 electrodes on each array. The arrays are set to record once every 24hours with arrays monitored one after the other, to reduce the possibility ofinterference. In addition to resistivity arrays, 14 combined temperature andmoisture content probes were installed in the less well compacted section closeto the resistivity array (Figure 4).

PRELIMINARY RESULTS AND CONCLUSIONSThe first results from the ER array on the BIONICS embankment look verypromising (Figure 5) providing good indications of the internal structure of theembankment. Initial models to understand the fluctuations in temperature in theembankment are also positive, suggesting that curve fitting algorithms such asthose of Nofziger (2003) can be applied with some further modifications (Figure6). Some preliminary results investigating the effects of pore water chemistry onER suggest a more complex feedback in the clays of the BIONICS embankmentthat requires further study (Figure 7). Variations in resistivity driven by thesevariables are significant and can occur both spatially and temporally. This in itselfis not such a problem if only one snapshot of resistivity is required. However,when comparing multiple images, either along an embankment or at the samespot over a longer time period, it is essential that these variations are accountedfor. We are still some way off the target when all variables are properly quantifiedand valid geotechnical conclusions can be drawn regarding ER as a proxy for soilmoisture variations in time and space.

Contact

Dr David GunnBritish Geological SurveyKingsley Dunham CentreKeyworth, Nottingham NG12 5GG United KingdomTel: +44 (0)115 936 3100

Prof Neil Dixon Department of Civil and Building EngineeringLoughborough UniversityLoughboroughLE11 3TU United KingdomTel: +44 (0)1509 228542

Figure 3. Laboratory setup for electrical resistivity measurement on compacted earthworks clay samples.

Figure 4. An overview of the instrumentation and electrical resistivity array (‘ladder’ on top of flank) on the south facing flank of the BIONICS embankment

Figure 7. Preliminary results from laboratory tests on standard Proctorcompacted samples of Mercia Mudstone and BIONICS material,indicating complexities in electrical resistivity response.

ACKNOWLEDGEMENTSThis paper is published with the permission of the Executive Directorof the British Geological Survey (NERC).

REFERENCESAbu-Hassanein, Z.S., Benson, C.H. and Blotz, L.R., 1996. Electrical Resistivity of Compacted Clays. Journal ofGeotechnical Engineering, 122(5): 397-406.Besson, A., Cousin, I., Dorigny, A., Dabas, M. and King, D., 2008. The temperature correction for the electricalresistivity measurements in undisturbed soil samples: Analysis of the existing conversion models and proposal of anew model. SOIL SCIENCE, 173(10): 707-720.Brunet, P., Clement, R. and Bouvier, 2009. Monitoring soil water content and deficit using Electrical Resistivity.Journal of Hydrology, 380: 146-153.Bryson, L.S., 2005. Evaluation of Geotechnical Parameters Using Electrical Resistivity Measurements, EarthquakeEngineering and Soil Dynamics (GSP 133). ASCE, Austin, Texas, USA, pp. 10-10.Bryson, L.S. and Bathe, A., 2009. Determination of selected geotechnical properties of soil using electricalconductivity testing. Geotechnical Testing Journal, 32(3).Cassiani, G. et al., 2009. Monitoring the hydrologic behaviour of a mountain slope via time-lapse electrical resistivitytomography. Near Surface Geophysics: 475-486.Department of Transport, 2008a. Transport Statistics Great Britain.Department of Transport, 2008b. Rail Accident Report - Network Rail's management of Existing Earth-works.25/2008 R.A.I. Branch.Fukue, M., Minato, T., Horibe, H. and Taya, N., 1999. The micro-structures of clay given by resistivity measurements.Engineering Geology, 54(1-2): 43-53.Hughes, P., Glendinning, S. and Davies, O., 2008. Construction and Monitoring of a test embankment for theevaluation of the impacts of climate change on UK transport infrastructure, Proceedings 1st international conferenceAdvances in Transportation Geotechnics. Taylor and Francis Group, Nottingham.Kalinski, R.J. and Kelly, W.E., 1993. Estimating Water Content of Soils from Electrical Resistivity. Geotechnical TestingJournal, 16(3): 323.MacDonald, M., 2005. Vegetation and its effect on slope stability. Safe management of railways structures(Phase 2).McCarter, W.J., 1984. The electrical resistivity characteristics of compacted clays. Geotechnique, 34: 263-267.Nofziger, DL 2003. Soil temperature changes with time and depth. Soilphysics.okstate.edu/software/O'Brien, A.S., 2007. rehabilitation of urban railway embankments: investigation, analysis and stabilisation,Proceedings of the 14th International Conference SMGE, Madrid.Palmer, A.C. and Rice, J.R., 1973. The Growth of Slip Surfaces in the Progressive Failure of Over-Consolidated Clay.Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences (1934-1990), 332(1591):527-548.Perry, J.A., Pedley, M. and Reid, M., 2003. Infrastructure embankments condition appraisal and remedial treatment,C592. CIRIA.Rhoades, J.D., Raats, P.A.C. and Prather, R.J., 1976. Effects of Liquid-phase Electrical Conductivity, Water Content, andSurface Conductivity on Bulk Soil Electrical Conductivity. Soil Sci Soc Am J, 40(5): 651-655.Samouelian, A., Cousin, I., Tabbagh, A., Bruand, A. and Richard, G., 2005. Electrical resistivity survey in soil science: areview. Soil and Tillage Research, 83(2): 173-193.Scott, J.M., Loveridge, F. and O'Brien, A.S., 2007. Influence of Climate and Vegetation on Railway Embankments. In: V.Cuéllar et al. (Editors), Geotechnical Engineering in Urban Environments: Proceedings of the 14th EuropeanConference on Soil Mechanics and Geotechnical Engineering, Rotterdam, pp. 659-664.Skempton, A.W., 1996. Embankments and cuttings on the early railways. Construction History, 11: 33-39.

Figure 5. Preliminary electrical resistivity profile of the BIONICS embankment, providing a clear indication of the potential benefits of this method for earthwork condition appraisal. However, subtle variations within limits of 10 to 40 ohm.m are most indicative of moisture condition changes along potential slip surfaces. This is also the range that is affected by temperature variations and other influences such as pore water chemistry.

Figure 6. In blue is shown the variation of soiltemperature at 1m depth on the north flank ofthe BIONICS embankment for one year (x-axisshows days; y-axis temperature in degree Celsius).The red line shows the trend for an equivalentpoint on the south flank of the embankment.Initial results suggest that a an adjustment factorof 0.12% per degree can be applied at thislocation in NE-England for embankments with adifferent orientation than N-S.