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Proceedings of the XVI ECSMGEGeotechnical Engineering for Infrastructure and DevelopmentISBN 978-0-7277-6067-8

© The authors and ICE Publishing: All rights reserved, 2015doi:10.1680/ecsmge.60678

ACKNOWLEDGEMENT

The authors thank Professor Ponomarev A.B. (Perm National Research Polytechnic University) and professor Schwerdt S. (Hochschule Magdeburg-Stendal) for invaluable assistance in the writing of the article.

REFERENCES

Bartolomey, A.A., Kleveko, V.I., Ofrikhter, V.G., Ponomaryov, A.B. & Bogomolov A.N. 1999. The use of synthetic materials in the highway engineering in the Urals. Geotechnical engineering for transportation infrastructure. Proceedings of the 12th European conference on soil mechanics and geotechnical engineering, June 1999. Vol 2 (Ed: F.B.J. Barends, Lindenberg, J. Luger, H.J.,. Quelerij, L. & Verruijt, A.) Netherlands. Amsterdam, 1197-1202. Ponomarev, A.B., Tatiannikov, D.A. & Kleveko, V.I. 2013. De-termination of the linear stiffness of geosynthetics Internet Gazette VolgGASU. Ser.: Polythematic. 2 (27). URL:

http://vestnik.vgasu.ru/attachments/PonomarevTatyannikovKleveko-2013_2(27).pdf. Tatiannikov, D.A., Ponomarev, A.B., Kleveko, V.I., Schlömp, S.H. & Schwerdt, S. 2014 Determination of friction characteris-tics for the two types of geosynthetics through shear tests. Herald Perm national research Polytechnic University. Construction and architecture. № 1. S. 174-186. Tatiannikov, D.A, Kleveko, V.I. 2014. Characterisation of the in-teraction of geosynthetics ground // Modernization and research in the transport sector 1, 526-529. Ponomaryov, A. & Zolotozubov, D. 2014. Several approaches for the design of reinforced bases on karst areas. Geotextiles and Ge-omembranes, 42, 48-51. Tatiannikov, D.A, & Kleveko, V.I. 2014. Analysis of changes in the strength characteristics in operation. 10th International Confer-ence on Geosynthetics. Berlin, Vol.4. Alfaro, M.C., Miura, N. & Bergado, D. T. 1995., Soil Geogrid Reinforcement Interaction by Pullout and Direct Shear Tests. Geotechnical Testing Journal, GTJODJ, 18(2), 157-167 Koerner, R.M. 1999. Designing with Geosynthetics. Upper Saddle River, New Jersey. Melo, D.L.A. & Santos, E.C.G. 2014 Shear strength of RCDW/nonwonen geotextile interface. 10th International Confer-ence on Geosynthetics, Berlin.Vol.7.

Water content characteristics of mechanically compacted clay soil determined using the electrical

resistivity method Teneur en eau caractéristiques des compacté mécaniquement sol

argileux déterminées en utilisant la méthode de résistivité électrique A.A. Hassan* & D.G. Toll

School of Engineering and Computing Sciences/Durham University, Durham, UK * Corresponding Author

ABSTRACT In geotechnical testing, a number of techniques have been developed to investigate soil water characteristics. Among others, the electrical resistivity method provides a non-invasive, quick and low cost estimate of water content. However, electrical resistivity of compacted clay soils is influenced, in addition to water content, by various interlinked parameters that need to be addressed to obtain relia-ble water content measurements. In this work, clay specimens from the source used in the construction of an instrumented embankment were used to investigate the influence of compaction key variables; water content, density, compaction and compaction effort on soil resis-tivity. The specimens were compacted using standard Proctor and Modified compaction methods. To measure soil resistivity, a custom re-sistivity probe based on the square arrangement and a multi-electrode resistivity system were used. The results showed that the resistivity of mechanically compacted clay soil is sensitive to water content, density, compaction and compaction effort. It was found that the resistivity is mainly controlled by the degree of saturation and microstructure changes during compaction. It is suggested, therefore, that the resistivity investigation on remoulded soils must consider a range of specimens with various degrees of saturation for better water content estimates.

RÉSUMÉ Dans les essais géotechniques, un certain nombre de techniques ont été développées pour étudier les caractéristiques de l'eau du sol. Entre autres, le procédé de la résistivité électrique fournit une estimation non invasive, rapide et à faible coût de la teneur en eau. Ce-pendant, la résistivité électrique des sols argileux compactés est influencée, en plus de la teneur en eau, par divers paramètres interdépen-dants qui doivent être adressées à obtenir des mesures de teneur en eau fiables. Dans ces travaux, les échantillons d'argile provenant de la source utilisée pour la construction d’une digue instrumenté ont été utilisés pour étudier l'influence de variables-clés de compactage; teneur en eau, densité, compactage et de l'effort de compactage sur la résistivité du sol. Les spécimens ont été compactés en utilisant Proctor stan-dard et les méthodes de compactage modifiés. Pour mesurer la résistivité du sol, une sonde de résistivité personnalisé basé sur l'arrange-ment carré et un système de résistivité multi-électrodes ont été utilisés. Les résultats montrent que la résistivité de sol argileux compactés mécaniquement est sensible à la teneur en eau, densité, compactage et de l'effort de compression. On a trouvé que la résistivité est principa-lement contrôlée par le degré de saturation et de la microstructure des changements au cours du compactage. Il est donc suggéré que l'en-quête de la résistivité des sols remoulés doit examiner une série de spécimens avec différents degrés de saturation pour de meilleures esti-mations de la teneur en eau.

1 INTRODUCTION

An accurate knowledge of the water content of un-saturated soils is crucial to understand the geotech-nical properties and behaviour of natural and engi-neered earth structures. A wide range of lab and field based techniques have been developed to investigate soil water content characteristics. The advantages and drawbacks of these techniques have been discussed in numerous reviews (e.g. Robinson et al. 2008; Ve-

reecken et al. 2008). However, there is an increasing interest in exploring efficient techniques to measure soil water content with volume integration, prefera-bly in a non invasive manner. The electrical resistivi-ty method has emerged recently as a cost effective technique for quantifying soil water content at vari-ous scales. The technique offers non invasive meas-urements that can be integrated on a large volume by increasing the electrode spacing. It has been adopted to address a wide range of problems related to the

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hydraulic properties of the soil such as soil water content (Muñoz-Castelblanco et al. 2011), saturation (Abu-Hassanein et al. 1996), solute transport (Binley et al. 1996). Recently, the method has been identified as an effective technique for the estimation of the spatiotemporal variability of soil water content (e.g. Calamita et al. 2012; Gunn et al. 2014).

However, in addition to water content, the hydrau-lic properties of compacted clay soils are affected by other compaction key variables such as density, compaction and compaction effort (Mitchell et al. 1965). These variables affect the shape, size and connectivity of pores. Like water, electrical conduc-tion paths are affected by pore properties, therefore, soil resistivity is expected to be influenced by these compaction variables (Bryson, 2005). It is important to address these variables in compacted clay soils commonly used in engineered structures such as road embankments. Thus, this work aims to investigate the water content characteristics of mechanically com-pacted clay soil used in the construction of the BI-ONICS embankment in North East England using the resistivity method.

2 THEORETICAL BACKGROUND

2.1 Electrical resistivity method

The electrical resistivity (the reciprocal of electrical conductivity) is a physical property of a material that describes its ability to resist the flow of electricity. The resistivity method (ASTM G57 2006) is based on the principle that the potential drop Δ𝑉𝑉 (volts) across a pair of electrodes due to a direct (DC) or low frequency current 𝐼𝐼 (Amps) injected via another pair of electrodes is proportional to the electrical resistivi-ty 𝜌𝜌 (Ohm.m), that is:

𝜌𝜌 = 𝐾𝐾 Δ𝑉𝑉𝐼𝐼 (1)

Where K is a geometric factor (m) depends on the

arrangement of the electrodes. As electrical conduc-tion takes place as a result of the movement of the ions in the pore water, electrical properties of soils are mainly controlled by water content (Bryson, 2005). However, the solid phase characteristics affect the relative proportions of water and air and the con-

nectivity of pores (Friedman, 2005). Furthermore, the current conduction is affected by the temperature (Campbell et al., 1948) and pore water salinity (Rinaldi & Cuestas, 2002). Some models (e.g. Wax-man & Smit, 1968) have discussed the influence of the double layers of clay minerals. However, recent papers (e.g. Shah & Singh, 2005) showed that this ef-fect is included in Archie' Law (Archie, 1942).

3 MATERIALS AND EXPERIMENTAL SETUP

3.1 Materials

Sandy clay specimens from the source used in the construction of the BIONICS embankment were used (Hughes et al., 2009). The soil is classified as being intermediate plasticity with Liquid Limit (43.3%), Plastic Limit (23.7%), Plasticity Index of 19.6 and a Liquidity Index of -0.05 (Mendes, 2011).

3.2 Sample preparation The oven dried soil was crushed and sieved through a 2.8 mm sieve to remove the large particles. Distilled water was mixed with the soil to the desired gravi-metric water content and left in a sealed plastic bag for 24 hours for water content homogenisation. The soil was then compacted at different gravimetric wa-ter contents according to BS light (Proctor) and BS heavy (Modified Proctor) compaction standard (BS 1377-4 1990: Tests 3.3 & 3.5), using a drop-hammer compaction machine. In addition, at each gravimetric water content, five specimens were compacted using different non-standard compaction efforts, by in-creasing the number of blows from 15 to 55 to cover the wide range of dry density that can be found in practice.

3.3 Resistivity measurements

A resistivity probe based on the square arrangement and a multi-electrode resistivity system (Toll et al., 2013) were used to measure the soil resistivity. Two perpendicular resistivity readings named 𝜌𝜌𝛼𝛼 and 𝜌𝜌𝛽𝛽, (Habberjam & Watkins, 1967) were collected. The average resistivity of 𝜌𝜌𝛼𝛼 and 𝜌𝜌𝛽𝛽 was used as the resistivity of the specimen (Russell & Barker, 2010).

4 RESULTS AND DISCUSSION

Figure 1 shows the dry density-gravimetric water content of specimens compacted using BS Light and BS heavy compaction methods, and the correspond-ing resistivity data. The resistivity decreases with in-creasing water content. The resistivity is relatively low when the soil is compacted wet of the optimum (OMC=0.156 for BS Light, OMC=0.127 for BS Heavy), while resistivity is high when the soil is compacted dry of the optimum. The resistivity in-creases abruptly just dry of the optimum, while it seems to be independent of water content wet of the optimum. Moreover, increasing the compaction effort decreases soil resistivity. However, resistivity is not sensitive to the compaction, or the compaction effort, used when the soil is compacted wet of the optimum, whereas this effect is more significant when the soil is compacted dry of the optimum. This behaviour can be explained in terms of fabric changes during com-paction. When the soil is compacted dry of the opti-mum, or with low compaction effort, the clods of clay are difficult to remould, with large air-filled pores (Benson & Daniel, 1990) and, therefore, high resistivity. In contrast, when the soil is compacted wet of the optimum, or at higher compaction effort, the clods of clay are easy to remould with small pores that are filled with water and, therefore, low re-sistivity. These observations are consistent with ob-servations by Abu-Hassenein et al. (1996), Seladji et al. (2010) and Kibria & Hossain (2012) for different types of clay.

Figure 1. BS Light and BS Heavy compaction curves and the corresponding resistivity data

The same data presented in Figure 1 is plotted in terms of the degree of saturation in Figure 2. The re-sistivity-degree of saturation relationships show in-dependency of the compaction effort. A similar unique relationship that is independent of compaction is reported by Abu-Hassanein et al. (1996) for ten clays.

Figure 2. The resistivity-degree of saturation of specimens com-pacted using BS Light and BS Heavy compaction methods

To better emphasize the influence of the compac-

tion effort, Figure 3 and Figure 4 present, respective-ly, the dry density-gravimetric water content relation-ships and the corresponding resistivity data of specimens compacted using different compaction ef-forts. Increasing the compaction effort increases the maximum dry density and decreases the optimum water content. Again, increasing the compaction ef-fort helps in reducing the voids that are filled with air and, therefore, lowers soil resistivity. At low water content (e.g. 0.065 or 6.5%), the resistivity decreases from 70 to 41 Ohm.m as the number of the blows in-creases from 15 to 55 blows. However, for specimens compacted at high water content (e.g. 0.25 or 25%), where the pores are almost filled with water with a high degree of saturation, resistivity is low (~12 Ohm.m) and not affected by increasing the compac-tion effort or water content.

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hydraulic properties of the soil such as soil water content (Muñoz-Castelblanco et al. 2011), saturation (Abu-Hassanein et al. 1996), solute transport (Binley et al. 1996). Recently, the method has been identified as an effective technique for the estimation of the spatiotemporal variability of soil water content (e.g. Calamita et al. 2012; Gunn et al. 2014).

However, in addition to water content, the hydrau-lic properties of compacted clay soils are affected by other compaction key variables such as density, compaction and compaction effort (Mitchell et al. 1965). These variables affect the shape, size and connectivity of pores. Like water, electrical conduc-tion paths are affected by pore properties, therefore, soil resistivity is expected to be influenced by these compaction variables (Bryson, 2005). It is important to address these variables in compacted clay soils commonly used in engineered structures such as road embankments. Thus, this work aims to investigate the water content characteristics of mechanically com-pacted clay soil used in the construction of the BI-ONICS embankment in North East England using the resistivity method.

2 THEORETICAL BACKGROUND

2.1 Electrical resistivity method

The electrical resistivity (the reciprocal of electrical conductivity) is a physical property of a material that describes its ability to resist the flow of electricity. The resistivity method (ASTM G57 2006) is based on the principle that the potential drop Δ𝑉𝑉 (volts) across a pair of electrodes due to a direct (DC) or low frequency current 𝐼𝐼 (Amps) injected via another pair of electrodes is proportional to the electrical resistivi-ty 𝜌𝜌 (Ohm.m), that is:

𝜌𝜌 = 𝐾𝐾 Δ𝑉𝑉𝐼𝐼 (1)

Where K is a geometric factor (m) depends on the

arrangement of the electrodes. As electrical conduc-tion takes place as a result of the movement of the ions in the pore water, electrical properties of soils are mainly controlled by water content (Bryson, 2005). However, the solid phase characteristics affect the relative proportions of water and air and the con-

nectivity of pores (Friedman, 2005). Furthermore, the current conduction is affected by the temperature (Campbell et al., 1948) and pore water salinity (Rinaldi & Cuestas, 2002). Some models (e.g. Wax-man & Smit, 1968) have discussed the influence of the double layers of clay minerals. However, recent papers (e.g. Shah & Singh, 2005) showed that this ef-fect is included in Archie' Law (Archie, 1942).

3 MATERIALS AND EXPERIMENTAL SETUP

3.1 Materials

Sandy clay specimens from the source used in the construction of the BIONICS embankment were used (Hughes et al., 2009). The soil is classified as being intermediate plasticity with Liquid Limit (43.3%), Plastic Limit (23.7%), Plasticity Index of 19.6 and a Liquidity Index of -0.05 (Mendes, 2011).

3.2 Sample preparation The oven dried soil was crushed and sieved through a 2.8 mm sieve to remove the large particles. Distilled water was mixed with the soil to the desired gravi-metric water content and left in a sealed plastic bag for 24 hours for water content homogenisation. The soil was then compacted at different gravimetric wa-ter contents according to BS light (Proctor) and BS heavy (Modified Proctor) compaction standard (BS 1377-4 1990: Tests 3.3 & 3.5), using a drop-hammer compaction machine. In addition, at each gravimetric water content, five specimens were compacted using different non-standard compaction efforts, by in-creasing the number of blows from 15 to 55 to cover the wide range of dry density that can be found in practice.

3.3 Resistivity measurements

A resistivity probe based on the square arrangement and a multi-electrode resistivity system (Toll et al., 2013) were used to measure the soil resistivity. Two perpendicular resistivity readings named 𝜌𝜌𝛼𝛼 and 𝜌𝜌𝛽𝛽, (Habberjam & Watkins, 1967) were collected. The average resistivity of 𝜌𝜌𝛼𝛼 and 𝜌𝜌𝛽𝛽 was used as the resistivity of the specimen (Russell & Barker, 2010).

4 RESULTS AND DISCUSSION

Figure 1 shows the dry density-gravimetric water content of specimens compacted using BS Light and BS heavy compaction methods, and the correspond-ing resistivity data. The resistivity decreases with in-creasing water content. The resistivity is relatively low when the soil is compacted wet of the optimum (OMC=0.156 for BS Light, OMC=0.127 for BS Heavy), while resistivity is high when the soil is compacted dry of the optimum. The resistivity in-creases abruptly just dry of the optimum, while it seems to be independent of water content wet of the optimum. Moreover, increasing the compaction effort decreases soil resistivity. However, resistivity is not sensitive to the compaction, or the compaction effort, used when the soil is compacted wet of the optimum, whereas this effect is more significant when the soil is compacted dry of the optimum. This behaviour can be explained in terms of fabric changes during com-paction. When the soil is compacted dry of the opti-mum, or with low compaction effort, the clods of clay are difficult to remould, with large air-filled pores (Benson & Daniel, 1990) and, therefore, high resistivity. In contrast, when the soil is compacted wet of the optimum, or at higher compaction effort, the clods of clay are easy to remould with small pores that are filled with water and, therefore, low re-sistivity. These observations are consistent with ob-servations by Abu-Hassenein et al. (1996), Seladji et al. (2010) and Kibria & Hossain (2012) for different types of clay.

Figure 1. BS Light and BS Heavy compaction curves and the corresponding resistivity data

The same data presented in Figure 1 is plotted in terms of the degree of saturation in Figure 2. The re-sistivity-degree of saturation relationships show in-dependency of the compaction effort. A similar unique relationship that is independent of compaction is reported by Abu-Hassanein et al. (1996) for ten clays.

Figure 2. The resistivity-degree of saturation of specimens com-pacted using BS Light and BS Heavy compaction methods

To better emphasize the influence of the compac-

tion effort, Figure 3 and Figure 4 present, respective-ly, the dry density-gravimetric water content relation-ships and the corresponding resistivity data of specimens compacted using different compaction ef-forts. Increasing the compaction effort increases the maximum dry density and decreases the optimum water content. Again, increasing the compaction ef-fort helps in reducing the voids that are filled with air and, therefore, lowers soil resistivity. At low water content (e.g. 0.065 or 6.5%), the resistivity decreases from 70 to 41 Ohm.m as the number of the blows in-creases from 15 to 55 blows. However, for specimens compacted at high water content (e.g. 0.25 or 25%), where the pores are almost filled with water with a high degree of saturation, resistivity is low (~12 Ohm.m) and not affected by increasing the compac-tion effort or water content.

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Figure 3. Dry density-gravimetric water content of specimens compacted using different compaction efforts

Figure 4. Resistivity-gravimetric water content of specimens compacted using different compaction efforts

To support the above discussion, soil resistivity is plotted against dry density and void ratio in Figure 5 and Figure 6, respectively. For a particular gravimet-ric water content, resistivity decreases linearly with increasing dry density/decreasing void ratio (Beck et al. 2011). However, the slope of the relationship is flattened with increasing water content. Again, this behaviour can be explained by changes in the soil properties during compaction. For soil compacted at low water content, the pores are partially filled with air. At water contents dry of optimum, air is expelled from the soil, making the particles closer to each oth-er in a denser state (dry density increases), while, at high water content (wet of optimum), the compaction

effort cannot expel more air and the water added pre-vents densification. At water content close to satura-tion (e.g. 0.25 or 25%), the dry density varies in a limited range; therefore, resistivity varies in a limited range, too. This suggests that the dry density/void ra-tio has an important role in soil resistivity, due to changes in the degree of saturation, particularly at low water content. This also demonstrates that using gravimetric water content as a criterion to calibrate resistivity against water content in remoulded soils can be erroneous, as soils may be found at identical gravimetric water content, but at different degrees of saturation (McCarter 1984).

Figure 5. Electrical resistivity-dry density of the compacted spec-imens

Figure 6. Electrical resistivity-void ratio of the compacted speci-mens

According to Archie's Law (Archie 1940), a de-crease in the degree of saturation of the soil is ac-companied by an increase in the resistivity, due to the partial replacement of pore water with air. Therefore, the variation of soil resistivity can be interpreted by means of the degree of saturation. Figure 7 shows the resistivity-degree of saturation relationship of the compacted specimens. It can be noticed that increas-ing the degree of saturation decreases soil resistivity and that, at low degree of saturation, the resistivity changes more rapidly (McCarter, 1984; Abu-Hassanein et al., 1996). At low degrees of saturation, the continuity of pore water needed for current con-duction (Fukue et al., 1999) does not reach a satisfac-tory level; therefore, the resistivity is relatively high and changes rapidly. Increasing the degree of satura-tion improves the continuity of pore water and elec-trical conduction, causing a decrease in the resistivi-ty. At a degree of saturation close to 100%, the electrical paths are well achieved for the electrical current. Therefore, the influence of water content on the resistivity becomes insignificant. The high corre-lation coefficient (0.9687) of the resistivity-degree of saturation relationship suggests that the resistivity is strongly dependent on the degree of saturation.

Figure 7. The resistivity-degree of saturation relationship of the compacted specimens

5 CONCLUSIONS

The water content characteristics of a mechanical-ly compacted clay soil have been investigated using the resistivity method. The results showed that the re-sistivity of compacted clay soils is sensitive to the compaction key variables and conditions. The key findings can be summarized as follow: (1) The soil resistivity is strongly affected by compaction. The re-sistivity is relatively low for specimens compacted wet of the optimum, while resistivity is relatively high for specimens compacted dry of the optimum. This behaviour is explained by changes in the soil properties during compaction. (2) Increasing the compaction effort decreases soil resistivity. However, this effect is less significant when soil is compacted at high water content or wet of the optimum, where the pores are almost filled by water with a high de-gree of saturation. (3) For a particular gravimetric water content, resistivity decreases with increasing dry density/decreasing void ratio. Therefore, using gravimetric water content as a criterion to calibrate resistivity against water content can be erroneous, as soils may be found at identical gravimetric water content, but at different degrees of saturation. (4) Soil resistivity is controlled by the degree of saturation. An increase in soil water content, dry density or the compaction effort causes an increase in the degree of saturation. At low degrees of saturation, the disconti-nuity of pore water causes a relatively high resistivi-ty, while, at high degrees of saturation, the continuity of pore water is improved, causing a decrease in the resistivity. At water content levels close to saturation, the influence of water on resistivity is insignificant, as the electrical paths are achieved for electrical con-duction. It is suggested, therefore, that the degree of saturation is a more reliable factor than the gravimet-ric water content to correlate with the resistivity of the soil, and resistivity investigation on remoulded soils must consider a range of specimens with vari-ous degrees of saturation for better water content es-timates.

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Figure 3. Dry density-gravimetric water content of specimens compacted using different compaction efforts

Figure 4. Resistivity-gravimetric water content of specimens compacted using different compaction efforts

To support the above discussion, soil resistivity is plotted against dry density and void ratio in Figure 5 and Figure 6, respectively. For a particular gravimet-ric water content, resistivity decreases linearly with increasing dry density/decreasing void ratio (Beck et al. 2011). However, the slope of the relationship is flattened with increasing water content. Again, this behaviour can be explained by changes in the soil properties during compaction. For soil compacted at low water content, the pores are partially filled with air. At water contents dry of optimum, air is expelled from the soil, making the particles closer to each oth-er in a denser state (dry density increases), while, at high water content (wet of optimum), the compaction

effort cannot expel more air and the water added pre-vents densification. At water content close to satura-tion (e.g. 0.25 or 25%), the dry density varies in a limited range; therefore, resistivity varies in a limited range, too. This suggests that the dry density/void ra-tio has an important role in soil resistivity, due to changes in the degree of saturation, particularly at low water content. This also demonstrates that using gravimetric water content as a criterion to calibrate resistivity against water content in remoulded soils can be erroneous, as soils may be found at identical gravimetric water content, but at different degrees of saturation (McCarter 1984).

Figure 5. Electrical resistivity-dry density of the compacted spec-imens

Figure 6. Electrical resistivity-void ratio of the compacted speci-mens

According to Archie's Law (Archie 1940), a de-crease in the degree of saturation of the soil is ac-companied by an increase in the resistivity, due to the partial replacement of pore water with air. Therefore, the variation of soil resistivity can be interpreted by means of the degree of saturation. Figure 7 shows the resistivity-degree of saturation relationship of the compacted specimens. It can be noticed that increas-ing the degree of saturation decreases soil resistivity and that, at low degree of saturation, the resistivity changes more rapidly (McCarter, 1984; Abu-Hassanein et al., 1996). At low degrees of saturation, the continuity of pore water needed for current con-duction (Fukue et al., 1999) does not reach a satisfac-tory level; therefore, the resistivity is relatively high and changes rapidly. Increasing the degree of satura-tion improves the continuity of pore water and elec-trical conduction, causing a decrease in the resistivi-ty. At a degree of saturation close to 100%, the electrical paths are well achieved for the electrical current. Therefore, the influence of water content on the resistivity becomes insignificant. The high corre-lation coefficient (0.9687) of the resistivity-degree of saturation relationship suggests that the resistivity is strongly dependent on the degree of saturation.

Figure 7. The resistivity-degree of saturation relationship of the compacted specimens

5 CONCLUSIONS

The water content characteristics of a mechanical-ly compacted clay soil have been investigated using the resistivity method. The results showed that the re-sistivity of compacted clay soils is sensitive to the compaction key variables and conditions. The key findings can be summarized as follow: (1) The soil resistivity is strongly affected by compaction. The re-sistivity is relatively low for specimens compacted wet of the optimum, while resistivity is relatively high for specimens compacted dry of the optimum. This behaviour is explained by changes in the soil properties during compaction. (2) Increasing the compaction effort decreases soil resistivity. However, this effect is less significant when soil is compacted at high water content or wet of the optimum, where the pores are almost filled by water with a high de-gree of saturation. (3) For a particular gravimetric water content, resistivity decreases with increasing dry density/decreasing void ratio. Therefore, using gravimetric water content as a criterion to calibrate resistivity against water content can be erroneous, as soils may be found at identical gravimetric water content, but at different degrees of saturation. (4) Soil resistivity is controlled by the degree of saturation. An increase in soil water content, dry density or the compaction effort causes an increase in the degree of saturation. At low degrees of saturation, the disconti-nuity of pore water causes a relatively high resistivi-ty, while, at high degrees of saturation, the continuity of pore water is improved, causing a decrease in the resistivity. At water content levels close to saturation, the influence of water on resistivity is insignificant, as the electrical paths are achieved for electrical con-duction. It is suggested, therefore, that the degree of saturation is a more reliable factor than the gravimet-ric water content to correlate with the resistivity of the soil, and resistivity investigation on remoulded soils must consider a range of specimens with vari-ous degrees of saturation for better water content es-timates.

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The effect of temperature on the physical properties of Mauthausen Granite (Austria)

L'effet de la température sur les propriétés physiques de Mauthausen Granite (Autriche)

Anita Török*1, Ákos Török1, Péter Görög1

1 Dept. of Engineering Geology and Geotechnics, Budapest University of Technology and Economics, Hungary* Corresponding author

ABSTRACT This paper provides test results and interpretation of the changes of mechanical properties of granitic rocks by heat effect.The samples were obtained from the quarry of Mauthausen (Upper Austria), from where the produced granite was used construction of bridges, buildings and cobbled streets in several country. The behavior of Austrian granite shows no similarity to other foreign granite by 300°C heat load. The studied granite’s mechanical properties do not change or even slightly decrease, while other tests have observed an increase in the initial stage temperature (Dwivedi et al. 2008). The Young’s modulus, the compressive strength and the indirect tensile strength strong reduction is observed by further increase of the temperature.

RÉSUMÉ Ce document présente les résultats des tests d'origine et l'interprétation de l'effet des variations de température de propriétés mé-caniques des roches granitiques. Les échantillons ont été obtenus à partir de Mauthausen (Haute-Autriche) carrière d'où le granit excavé plusieurs pays des ponts, des bâtiments de chemins pavés sont utilisés. Le comportement de granit autrichienne 300 ° C de chaleur chargés ne présente aucune similitude à d'autres granit étrangère du. Les caractéristiques de granit de test de résistance restent inchangés ou même légèrement diminuer, tandis que d'autres études ont observé une augmentation de la température de l'étape initiale (Dwivedi et al. 2008). On observe en outre une augmentation de la température de a module, la résistance à la compression et une forte réduction de la résistance à la traction.

1 INTRODUCTION

The construction industry uses rocks for different purposes such as decoration and load bearing, thusthese materials are exposed to various environmental impacts. Besides meteorological conditions (tempera-ture changes, snow, wind) special impacts may also act to our buildings. During the design beyond the impacts of weather the negative consequences of earthquakes, explosions, vehicle collision and fire impacts should be taken into account. The meteoro-logical phenomena exert destructive effect over a longer period in contrast to the extraordinary loadsthat needs shorter time, sometimes moments to cause much more damage. Therefore, it is important tostudy the mechanical properties of rocks influencing by sudden and large changes of increased tempera-ture.

Nowadays there are more and more research about the changes of mechanical properties by heat (Dwivedi et al. 2008, Heuze 1983). This topic has been addressed mainly because of recent dangerous tunnel fires. Nowadays tunnels are built all over the world however, despite the scientific developments fires still occur in large numbers (Beard and Carvel 2005). The consequence was that the problems of fires opened new directions in the area of research:changes in behavior of rocks by heat or fire.

The mechanical behavior of individual rock typesdue to temperature increase differ because of the dif-ferent mineral composition. That is why only trends can be determined and these are valid only for rocks with the same composition.

Heat from any natural disaster or human activity affects the physical properties of rocks. This study