C Campbell paper - Australian Geomechanics Society

AN INVESTIGATION INTO THE EFFECT OF SALT WATER ON THE GEOTECHNICAL PROPERTIES OF A RESIDUAL CLAY Natalie Campbell1, Burt Look1 and Andreas NataAtmadja2 1 Connell Wagner, Brisbane, QLD, Australia 2 School of Urban Development, Queensland University of Technology, QLD, Australia

ABSTRACT During construction, water is utilised both as a dust suppressant and for modification of the water content to aid in the compaction process. With the current high level water restrictions in South East Queensland, measures that can be taken to reduce the quantity of potable water in construction are needed. In this case, the possibility of using saline water in earthworks should warrant proper consideration. It has been known that the presence of salt affects the natural and built environment, including pavements, agriculture, reinforced concrete structures and underground services. The main issues being degradation of bituminous surfacing, impact on vegetation, attack of concrete, corrosion of steel and ground movement. However, some studies in the past indicated that salt water can be used in earthworks. This paper presents the findings of a limited study, which looked into the effect of salt water on the geotechnical properties of a typical clay from Queensland. To investigate the effects of salt on the engineering properties of the clay, a number of laboratory tests were undertaken, these included Atterberg limits, standard compaction, electrical conductivity, dispersion, California Bearing Ratio, shear strength, soil suction and swell. Testing was carried out at three salt concentrations. The results show that the addition of salt water, while increasing the electrical conductivity, did not greatly affect the Atterberg limits, compaction characteristics, dispersion, strength or shrinkage of the soil. The use of salt water has been found to increase the variability of testing results; this was particularly evident in the soil suction tests. There appears to be an increase in swell potential with the increase in salt content. Furthermore, salt content appears to affect the matrix suction.

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INTRODUCTION

The continent of Australia is surrounded by salt water. Numerous seas and oceans border Australia, including the Indian and Pacific Oceans as well as the Timor, Arafura, Coral and Tasman Seas. In its natural state, salt water is deemed unsuitable for many purposes, including drinking and earthworks. Methods of treating salt water to make it fit for human consumption, known as desalination, by processes such as reverse osmosis, are currently used in Australia and around the world. These processes, however, are expensive and unsuitable for application in earthworks. The use of salt water in earthworks could potentially have significant benefits to the community, as it reduces the amount of potable water being used, which is of high importance, considering that South East Queensland is presently experiencing the worst water shortage on record. This idea is often raised by contractors working near to saline water, due to the time and cost savings that could be made. However, since the effect of salt water is often an unknown quantity, it is common practice to err on the side of caution and prohibit salt water from being used in earthworks. This paper provides a summary of the findings from a limited study, undertaken by the first author at Queensland University of Technology. The purpose of this thesis topic was to investigate the effects of salt on the geotechnical properties of a typical clay soil from Queensland.

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CURRENT STATUS

At the beginning of the study, a review of existing literature was conducted. The findings of this review are shown in Table 1. It must be noted that these tests were carried out on a variety of soil types in different geographical locations, using water of different salt concentrations. This may account for some of the discrepancies noticed in the results. For example, the compaction tests by Mahasneh (2004) were conducted using water from the Dead Sea, which has an

Australian Geomechanics Vol 44 No 1 March 2009

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THE EFFECT OF SALT WATER ON THE GEOTECHNICAL PROPERTIES OF A RESIDUAL CLAY

CAMPBELL et al.

approximate salt concentration of 330 g/L. The compaction tests by Ridley et al. (1984) used brine and the testing by Abood et al. (2007) was performed on silty clay from southern Iraq. The findings from this literature review can be used for comparison against the results of this study; however, as the type of material and level of salt may differ from the above investigations, different outcomes would be expected. Table 1: Summary of literature review. Property

Effect of increasing salt content

Atterberg limits

Decreases LL, PL and PI

Lower OMC Compaction characteristics

Lower MDD Higher MDD

Electrical conductivity Dispersion Strength

Increases Increased potential Residual shear strength increases Angle of internal friction increases UCS increases

Suction Swelling

Decreases Decrease in swelling volume

Reference Stipho (1985) Mahasneh (2004) Paassen and Gareau (2004) Abood et al. (2007) Ridley et al. (1984) Mahasneh (2004) Abood et al. (2007) Mahasneh (2004) Ridley et al. (1984) Abood et al. (2007) Rinaldi and Cuestas (2002) Warrence et al. (2003) Di Maio (1996) Tiwari et al. (2005) Mahasneh (2004) Abood et al. (2007) Kenney et al. (1992) Ohtsubo et al. (2006)

MATERIALS

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The material that was considered for this project was clay of intermediate plasticity, obtained from the Macadamia Nut Farm in Beerwah, Queensland. All laboratory testing was carried out on this material, and hence the conclusions drawn are currently valid only for this particular soil. Various types of salt are discussed in literature; however for the purposes of this research, the type of salt used in laboratory testing was Sodium Chloride (NaCl), otherwise known as common salt. As the research was aimed at assessing the acceptability of using locally available water sources in construction, the likely sources would be river water or sea water. Potable tap water was also tested to provide a reference point for comparison. To determine the typical salt content of tap, river and sea water, samples were collected and tested using the Total Dissolved Salts method and using an Electrical Conductivity (EC) meter. The salt concentrations determined from this testing are presented in Table 2. For quality assurance purposes, representative samples of river and sea water were mixed in the laboratory. Tap water was used direct from the laboratory tap. Table 2: Typical Salt Concentrations Water Tap River Sea

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g/L 0.4 30 40

BASIC TESTING

All testing was carried out in accordance with the relevant testing standard (Australian or American) with reference to other research literature where possible. The relevant Australian Standard is AS 1289: Methods of testing soils for engineering purposes.

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4.1 ATTERBERG LIMITS The Atterberg tests were used to classify the soil and to see if the addition of salt water caused any change in these fundamental properties, i.e. liquid limit (LL), plastic limit (PL) and plasticity index (PI). The results of these tests are summarised in Table 3. These results can be compared with testing carried out by Stipho (1985) who found that at a salt content of 40 g/L the liquid limit decreased by 15%. However, in the current study, the differences for this test do not show any clear relationship between salt content and plasticity; therefore the small differences in values were attributed to experimental error. Table 3: Atterberg test summary Water type Tap River Sea

Salt Content (g/L) 0.4 30 40

LL (%) 40 39 39

PL (%) 21 19 24

PI (%) 19 20 15

Classification CI CI CI

4.2 COMPACTION CHARACTERISTICS Standard compaction tests were carried out on the soil, using tap, river and sea water for preparation of the material. The results of these compaction tests are summarised in Table 4. Table 4: Standard compaction data Water Type Tap River Sea

Maximum Dry Density (t/m3) 1.65 1.65 1.66

Optimum Moisture Content (%) 19.1 18.8 17.9

Testing by Mahasneh (2004) showed that by increasing the salt content, the maximum dry density (MDD) decreased. However, testing by Ridley et al. (1984) and Abood et al. (2007) showed that increasing the salt content caused an increase in the MDD. From the results of this test, it appears that the addition of salt had very little influence on the MDD for this particular soil. On the other hand, a trend exists between the salt content and optimum moisture content (OMC) as shown in Figure 1. This observation agrees with trends described by Ridley et al. (1984), Mahasneh (2004) and Abood et al. (2007). Homogenous clay tends to have a coefficient of variation of 15% for the OMC. Therefore the trend could also be due to this testing variation. 19.5

OMC (%)

19

18.5 2

R = 0.69

18

17.5 0

10

20

30

40

50

Salt Content (g/L)

Figure 1: OMC vs. salt content.


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4.3 ELECTRICAL CONDUCTIVITY The electrical conductivity of the soil was measured using a 1:5 soil: water suspension (EC1:5). The results of these tests are shown in Table 5 and Figure 2. Table 5: Converted EC results. Water

Salt (g/L) 0 30 40

Electrical Conductivity (us/cm)

Tap River Sea

Average EC (µS/cm) 50.2 1200 2575

3000

ECse (dS/m) 0.30 7.20 15.45

2

R = 0.99

2500 2000 1500 1000 500 0 0

10

20 30 Salt Content (g/L)

40

50

Figure 2: EC results. The results of the EC testing showed that the electrical conductivity of the soil increases as the salt content increases. This trend agrees with research undertaken by Rinaldi and Cuestas (2002). From the results of this testing, the soils can be given a salinity rating (Natural Resources and Water, 2007), as shown in Table 6. Table 6: Salinity ratings of soil. Water used Tap River Sea

ECse (dS/m) 0.30 7.20 15.45

Salinity Rating Non saline Highly saline Extremely saline

Comment Only salt tolerant plants yield satisfactorily Few salt tolerant plants yield satisfactorily

These ratings may limit the application of salt water to various projects, as it is know that saline soils affect plant growth and increase corrosion rates. 4.4 DISPERSION TESTING The Emerson Class test (AS 1289.3.8.1) grades the soil according to its dispersiveness, from Class 1 being highly dispersive to Class 8 being non-dispersive. Different soil samples were prepared with the tap, river and sea water. The results of the tests showed that for each water type used, the Emerson number was Class 6. A Class 6 rating means that the remoulded soil does not disperse in water and the 1:5 soil/water suspension begins to flocculate within 5 min. As there was no difference between the various water types, it was concluded that the addition of salt did not affect the dispersion characteristics of this soil batch. Note that saline (salt affected) soils, as discussed in this paper, are not the same as sodic soils. Sodic soils are susceptible to gully and tunnel erosion and dispersion testing is one indicator of ‘sodicitiy’.

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STRENGTH TESTING

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5.1 CALIFORNIA BEARING RATIO In the design of base and subbase material for pavements, the CBR (AS 1289.6.1.1) value is widely used. This gives an indication of the soil strength and/or stiffness and bearing capacity, under controlled density and moisture conditions. For each water type (tap, river and sea) 3 soaked and 3 unsoaked samples were prepared and tested. The samples were prepared over a range of compaction levels; varying from 80-99% MDD.

CBR (%)

The results from the unsoaked California bearing ratio (CBR) test are presented in Figure 3. 35 30 25 20 15 10 5 0 80

85

90

95

100

Compaction % MDD 0 g/L R² = 0.98

30 g/L R² = 0.88

40 g/L R² = 0.89

Figure 3: Unsoaked CBR data To determine if the use of salt water affected the CBR value for this soil, the following method was used for analysis. Firstly, a trendline was fitted through all the points, regardless of their salt composition and a coefficient of determination (R2) value was obtained. This was then compared to the R2 value for each water type. If the R2 value for the individual categories of data was found to be higher than the value for all of the data (thus indicating a ‘better’ fit), it can be concluded that the salt has had an effect on the strength of the soil, as measured by the CBR test. The R2 value for all the unsoaked data, using a ‘power’ trendline, was 0.78, this was then compared with the coefficients found for each water type (as shown in Figure 3). As these results do not greatly differ from the value obtained for all the data, it was concluded that the use of salt water did not influence the CBR of the unsoaked samples. The following discussion presents general observations, which are independent of the salt content. To investigate the effect of soaking on the sample, regardless of water type, the same methodology was used. The graphs are displayed in Figure 4 and Figure 5.


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2

R = 0.65 30

CBR (%)

25 20 15 10 5 0 75

80

85

90

95

100

Compaction (% MDD)

Figure 4: All CBR data. 35 R² = 0.78 (Unsoaked) R² = 0.78 (Soaked) 30

CBR (%)

25

20

15

10

5

0 75

80

85

90

95

100

Compaction (% MDD) Unsoaked

Soaked

Figure 5: All CBR data comparison. Soaking of samples decreases the strength, as expected. For these results, the soaked CBR is related to the unsoaked CBR value as shown in Table 7. Table 7: Soaked to Unsoaked CBR Relationships.

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Compaction % MDD

CBR Soaked

CBR Unsoaked

85 90 95 98

1 3 6.5 10

5 10 18 25

Ratio CBR Unsoaked/ CBR Soaked 5 3.5 3 2.5



CAMPBELL et al.

The general trend in the ratios is that for an increase in % compaction, the ratio between unsoaked and soaked CBR values decreases. Another consideration is the estimation of CBR values at different % compactions. Looking at the soaked CBR data, it can be seen that the CBR at 95% is only 70% of the CBR value at 98% and this comparison holds true for each drop in compaction. When no compaction criteria is specified for laboratory testing, the tests will usually be carried out at 100% MDD, yet construction specifications are typically 95% MDD. These results indicate that the lab value would need to be factored down to 70% to estimate the field strength. This highlights the need for laboratory samples to accurately replicate the expected construction conditions. The results of the vane shear test and pocket penetrometer test do not appear to have been affected by the addition of salt water. Therefore, it was concluded that, for this soil, the use of salt water does not affect the shear strength of the soil.

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SUCTION AND SHRINK SWELL TESTING

6.1 SUCTION TESTING The total and matrix suctions were determined by using the filter paper method (ASTM D 5298-03). From these results, the osmotic suction could be calculated. The expected relationship was that an increase in moisture content would cause a decrease in the matrix suction. This relationship is shown in Figure 6.

y = 12.178e -0.0633x R2 = 0.31

Suction (pF)

5.00 4.00 3.00 2.00 1.00 0.00 15.0

17.0

19.0

21.0

23.0

MC (%) Matrix

Osmotic

Figure 6: Suction Results. Figure 7 investigates if there is any change in soil suctions for different salt concentrations. The trendline that was fitted to all the matrix suction data (regardless of water type) had an R2 value of 0.43. This is a quite different compared to the R2 values for each water type separately – shown on the graph. These differences indicate that the relationship is different for each salt level, therefore indicating that salt may affect the matrix suction of the soil.


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5.0 4.5 4.0

Suction (pF)

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 15

16

17 0 R² = 0.85

18

19 MC (%) 30 2 R = 0.23

20

21

22

23

40 R² = 0.76

Figure 7: Matrix Suction Results for Different Salt Levels. Figure 8 presents a graphical representation of the spread of results of the matrix suction at various salt contents for N = number of tests. There is a slight upwards trend in the suction for increased salt content, however the main conclusion from this plot is that testing variability also increases with increasing salt content, therefore for accurate results, laboratory testing should be carried out with tap water, to reduce the testing variability. 5.00 4.50

Matrix Suction (pF)

4.00 3.50 3.00

(N = 15)

2.50 2.00 1.50 1.00 0.50 0.00 0

10

20

30

40

50

Salt Content (g/L)

Figure 8: Salt versus Matrix Suction. 5.2 CBR SWELL TESTING To determine if the use of salt had an influence on the movement of the soil, the swell was determined by measuring the swell of the soaked CBR samples. Figure 9 shows little variation in the swell results between tap water (0 g/L) and river water (20 g/L), but there appears to be an increase in the swell for the sea water (40 g/L). This data suggests that at a certain ‘threshold’ salt level, the swell of this material increases significantly.

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2 1.5

Swell (%)

1 0.5 0 0

10

20

30

40

-0.5 -1 Salt Content (g/L)

Figure 9: Swell results.

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DISCUSSION

A summary of the findings from the laboratory testing is provided in Table 8. For this particular clayey soil, these results show that the addition of salt to the testing water did not have a major influence on the majority of the soil properties tested. The main conclusions that can be drawn from this research are: • There was increased variability in testing results for river and sea water, compared to tap water. This was most evident in the filter paper suction test results. Therefore, for accurate testing results, laboratory tests should be carried out using tap water, to reduce the variability. • There was an increase in swell potential between river water and sea water, indicating that somewhere between these two salt concentrations lies a threshold value that needs to be determined. • Salt content appears to affect matrix suction; however this relationship requires further investigation. • The addition of salt to the testing water increases the electrical conductivity of the soil. This will limit the use of salt water in construction, where corrosion is critical. • The addition of salt did not greatly affect the Atterberg limits, compactions characteristics, dispersion, strength or shrinkage. Table 8: Summary of results. Soil Properties Atterberg Limits Standard Compaction – MDD – OMC Electrical Conductivity Dispersion Strength – California Bearing Ratio Matrix Suction Linear Shrinkage Swell

Effect of Increasing Salt Content Nil Nil Small Decrease Increase Nil Nil Slight increase Nil Increase

The overall conclusion from this investigation is that the use of salt water did not have a significant effect on many of the geotechnical properties of the residual soil under consideration (clay of intermediate plasticity from the Beerwah region). Figure 10 shows the geological history of South- East Queensland. The testing location is approximately 100 kilometres north of Brisbane and would lie within the deep ocean zone, some 370 million years ago. This indicates that the material had already been exposed to, and formed in, salt water, which suggests a possible reason why salt water had no


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CAMPBELL et al.

significant effect on the geotechnical properties in many of the tests. A wider scale testing program is required to see if this effect occurs regionally. Test Site

Figure 10: Geological history of region (approx 370 million years ago.Willmott and Stevens, 1992) From the results of this research, it appears that the use of salt water does not have an adverse effect on the majority of the soil properties tested for this clay material. Therefore, it appears likely that river water could be used in construction projects. Sea water has affected the CBR swell properties and, for ‘swelling’ clays, this effect should be avoided. There are however, some limitations to the use of saline water. These limitations follow the guidelines of the Australian Standards (2007) which state that saline waters should not be used in the upper layers of fill, beneath buried bituminous sealed pavements or areas where vegetation may be established and in fill where steel is buried.

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REFERENCES

Abood. T., Anuar. B., Chik. Z. 2007. Stabilisation of Silty Clay Soil Using Chloride Compounds. Journal of Engineering Science and Technology. Volume 2 (1): pp. 102-110. ASTM International 2003. D5298-03 Standard test method for measurement of soil potential (Suction) using filter paper. ASTM. Di Maio, C. D. 1996. Exposure of bentonite to salt solution: osmotic and mechanical effects. Geotechnique 46 (4): 695707. Mahasneh, B, Z. 2004. Dead sea water as a soil improvement agent. Electronic Journal of Geotechnical Engineering. 113. Natural Resources and Water. 2007. Measuring Salinity. Queensland Government. Ohtsubo, M., Kumar, M. A., Li, L. and Higashi, T. 2006. Effect of salt solution on the permeability of the mixtures of soil and bentonite. International Congress on Environmental Geotechnics 1: 601-607. Paassen, L. A. and Gareau, L. F. 2004. Effect of pore fluid salinity on compressibility and shear strength development of clayey soils. Berlin: Springer-Verlag. Ridley, K. J. D., Bewtra, J, K. and McCorquodale, J, A. 1984. Behaviour of Compacted Fine-Grained Soil in a Brine Environment. Canadian Journal of Civil Engineering. 196-203. Rinaldi, V. A. and Cuestas, G, A. 2002. Ohmic conductivity of a compacted silty clay. Journal of Geotechnical and Geoenvironmental Engineering 128 (10): 824-835. Standards Australia 1997. AS 1289: Method of testing soils for engineering purposes. Stipho. A. S. 1985. On the Engineering Properties of Salina Soil. Quarterly Journal of Engineering Geology London. Volume 18: pp. 129-137. Tiwari, B., Tuladhar, G. R., and Marui, H. 2005. Variation in residual shear strength of soil with the salinity of pore fluid. Journal of Geotechnical and Geoenvironmental Engineering. 1445-1456. Warrence, N. J., Pearson, K. E. and Bauder, J.W. 2003. The basics of salinity and sodicity effects on soil physical properties. http://waterquality.montana.edu/docs/methane/basics_highlight.shtml (accessed April 2, 2008). Willmott. W. and Stevens N. 1992. Rocks and Landscapes of Brisbane and Ipswich. Geological Society of Australia.

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