Liquid limit and the temperature sensitivity of clays

ENG NEER N6 GEOLOGY ELSEVIER

Engineering Geology 49 (1998) 95-109

Liquid limit and the temperature sensitivity of clays Ian Jefferson a,,, Christopher David Foss Rogers b a Department of Civil and Structural Engineering, The Nottingham Trent University, Burton Street, Nottingham, NG1 4BU, UK b Department ofCivilandBuilding Engineering, Loughborough University, Ashby Road, Loughborough, Leicestershire, LEI1 3TU, UK Received 19 February 1997; accepted 15 August 1997

Abstract

The Atterberg Limits have been repeatedly shown to be useful indicators of clay behaviour. This paper proposes that they should also be used to assess the effect of temperature on clays. To improve on previous attempts, this paper describes a new simple and rapid method by which such assessments can be made. The method enables consistent results to be obtained over a larger range of temperatures (i.e. 10-80°C) than previously possible. This method is, therefore, potentially useful when assessing the behaviour of clay soils which are likely to be exposed to elevated temperature, such as landfill liners. Results are presented for kaolinite, smectite and mixtures of these clays of various percentages. These demonstrate that smectites are considerably more sensitive to temperature changes than kaolinites. For smectitic clay the liquid limit increases with temperature, whereas a very slight decrease occurs with kaolinite. The variation in liquid limit appears to be closely related to the specific surface area of the clay, and the resulting nature of inter-particle contacts. © 1998 Elsevier Science B.V. Keywords: Clays; Kaolinite; Liquid limits; Sensitivity; Smectite; Temperature

1. Introduction

The liquid and plastic limits (WL and wp) define the transitions between liquid, plastic and brittle solid soil behaviour (Atterberg, 1911). It was Terzaghi who first realised their engineering potential (Seed et al., 1964a). He noted that these limits depend on exactly the same physical properties as do other soil parameters. Thus potentially these limits can yield significant amounts of information about the behaviour of a soil. It was Casagrande * Corresponding author. Fax: 0044 115 9486450; e-mail: [email protected] 0013-7952/98/$19.00 © 1998 ElsevierScience B.V. All rights reserved. PH S0013-7952 (97) 00077-X

(1948) who standardized the method to determine these limits. This has since been modified in Britain and other countries by the use of the falling cone device (British Standards Institution, BSI, 1990). Many attempts have been made to link these limits, or the difference between them, the plasticity index (Ip), to soil properties empirically [for example, Skempton and Northey (1953); Seed et al. (1964a,b); Youssef et al. (1965)]. Only a limited degree of success has been achieved, however. It was the work of Wroth and Wood (1978) that produced an improved correlation between these limits and the properties of remoulded soil. Moreover, these limits are useful where data on in

96

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parameters can be difficult to obtained. Recently, the Atterberg Limits have been used to assess both consistency and chemical stability of clay soils, particularly soils used in landfill liner construction (Brandl, 1992). Therefore, it seems reasonable to suggest that the temperature susceptibility of a soil could be evaluated using these limits. This was an idea first mooted by Tidfors and Sallfors (1989). Temperature changes induced in soils arise from many different sources, such as hot buried pipes, radioactive waste depositories, buried electricity cables and contaminated landfill sites (Mitchell, 1993; Harbidge et al., 1996: Hueckel and Peano, 1996). The latter source can typically experience temperatures in excess of 60"C (Collins, 1993). A more productive use of heat was suggested by Adolfsson et al. (1985), who proposed that the preheating of clay soils could be a source of heat in the winter. Moreover, Youssef et al. (1961) and Mitchell (1993) noted that changes in temperature can and will occur during sampling. Mitchell further showed that such a change will alter the ion complex that is associated with a soil. Consequently, the parameters measured in the laboratory may be unrepresentative of the true behaviour of a soil in the field. The question then arises as to how these changes will affect the properties of a soil. The authors therefore consequently carried out a materially based study of the effect of temperature on the w~ and w~ in order to indicate the likely effect of any changes in temperature on the properties of soils. However, due to inherent problems with the wp test discussed later, this investigation is limited to the wL and how temperature affects it. situ

2. Literature review

Considering what can be potentially gained from the Atterberg Limits, apparently little work has been carried out on them in conjunction with temperature. This could be due in part both to their (erroneously) perceived diminishing importance in recent years with the advent of more sophisticated tests, and problems encountered when performing Atterberg Limit tests at

L)ig#leering Geology 49 (1998) 95 109

elevated temperatures. However, evaluation of the Atterberg Limits at elevated tempertaures could indicate how temperature affects key design parameters, by ulitization of the various correlations that exist. The Atterberg Limits would thus provide a quick and simple means to indicate preliminary effects of temperature, including relative sensitivity, and help evaluate whether further complex and expensive testing would be required. Of the work conducted the majority of authors observed a reduction in the w~ of a variety of clay soils with temperature (Youssef et al., 1961: Laguros, 1969; Ctori, 1989). Mitchell (1969)considered that these results were consistent with strength reductions observed in clays at elevated temperatures, especially since the wL is also an indirect measure of strength. Less consistency was observed with the wp in conjunction with temperature. Youssef et al. (1961)(testing between 15 and 35"C) and Ctori (1989) (testing between 6 and 35 C) both observed a reduction in w~ at elevated temperatures, whereas Laguros (1969) testing between 2 and 41°C) observed an erratic trend overall. Contradictory evidence has been supplied by Tippet (1976) and Reifer (1977) for temperatures between 8 and 22~'C. Tippet observed that the Atterberg Limits were unaffected by temperature when testing three clays containing varying amounts of well crystallized and disordered kaolinite. Reifer made a similar observation when testing kaolinite, although he noted that elevated temperatures increased the wL of kaolinite-bentonite mixtures. This effect increased as the percentage of bentonite was raised. The wp, however, was unaffected by changes in temperature over the same range. Thus no definite single conclusion can be drawn. Previous authors have suggested that these results could be explained in terms of changes to the double layer, the viscosity of water, coagulation and/or the geometrical re-arrangement of the particles with temperature. Youssef et al. (1961) proposed correction factors for temperature, based on changes in the viscosity of water with temperature. However, these met with only a limited degree of success. It is more likely that the results are produced by a combination of effects.

1. Jefferson, C.D. Foss Rogers / Engineering Geology 49 (1998) 95-109

Mineralogy is clearly a significant factor and one that has often been ignored in the literature. For example highly thixotropic clays such as bentonite may well become coagulated at a higher temperature and thus exhibit an increased WL. This could account for the observations made by Reifer (1977), even though the sample should theoretically be in a completely remoulded state. Thus the thixotropic strength gain can be argued to be not only increased by temperature, but also to occur initially at a very rapid rate. Therefore, the effect may be significant even with relatively quickly conducted wL tests. These conflicting results are further confused by questions concerning the reliability of the Atterberg Limit tests, particularly the wp test. When evaluating the wp an exchange of heat occurs between the hand of the operator and the soil. Laguros (1969), Ctori (1989) and others did try to overcome this problem with the use of gloves. However, this method of testing is also problematic, since some transfer of heat can still occur and the gloves additionally make it much more difficult to ascertain when the wp is reached. Furthermore, the wp test is considerably more subjective and susceptible to human error than either of the WL tests used (Ballard and Weeks, 1963; Liu and Thornburn, 1964). In fact considering the way the test is normally conducted, it seems reasonable to suggest that the Wp has always been evaluated at elevated temperatures, which vary between room and hand temperatures. To overcome these difficulties the cone penetrometer has been proposed as a possible technique to determine the wp as well as the WL, and hence the Ip objectively (Campbell, 1976; Wroth and Wood, 1978). However, no standard method has yet been devised to determine accurately and reliably the wp, particularly at elevated temperatures. This, coupled with the fact that the WLappears to be more significantly altered by increased temperatures, caused the authors to concentrate on the wL only. Measurements of WLwith the cone penetrometer (BSI, 1990) are preferred to those from the Casagrande device because the results have greater repeatability, are easier to determine and are less subjective (Wroth and Wood, 1978; Houlsby,

97

1982). Since the Casagrande device was used in nearly all the observations made in the literature, this could partly explain the greater variation in results than would otherwise have been expected. Recently, Farrell et al. (1997) compared the Swedish and British fall cone methods of WLdetermination and found no significant difference between the WLvalues found by the two techniques. It is therefore, appropriate to use the BSI (1990) fall cone in this study. Thus it can be concluded that no clear trend has been observed from the literature, although this is due in part to the poor material characterization in the previous studies reviewed. In consideration of this and the nature of the Atterberg Limit tests, the authors conducted a series of tests to determine the effect of temperature on the wL of clay soils over a greater temperature range than previously considered. In order, to elucidate behaviour it was necessary to use clay minerals that have been shown to exhibit extremes of response to environmental changes (such as temperature), since these serve to provide the best research indicators for this type of study. Two materials that exhibit such extremes are English China Clay (ECC) and Wyoming Bentonite (WB) (Mitchell, 1993). Both are commercially available in a relatively pure form and thus were chosen as the base materials for this study. The primary aim of this study was therefore to evaluate the effect of temperature on WE, hence giving a better undestanding of how clay soils are affected by temperatures. Due to the limitations with the methods empolyed in previous studies, it was necessary to establish a new methodology to achieve this goal.

3. Geology 3.1. English china clay The sample used in this study was in a white dry powdered form typically produced from the large deposits found in south-west England. These deposits are essentially kaolinite formed by alteration in situ of sodic plagioclase found in the

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variscan granites (Highley, 1984). They are world renowned for their purity and whiteness, which ~s directly attributable to the low iron content. Typically the kaolinite is of a well ordered form, consisting of coarse hexagonal lamella grains (Highley, 1984). Although substitution is rare the surfaces are often coated with amorphous silica gel (Jepson, 1984).

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4. Material characterization The two clay samples were mixed in various percentages according to their dry weights. This produced five different samples in all, as shown Table 1. It should be noted that the manufacturing process will slightly alter the intrinsic nature of these materials compared to their state in situ. However, as the same samples were used throughout the test programme, they can be considered internally consistent. Table 1 shows the results of index and compositional tests, thus giving a comprehensive classification of the materials used. All samples used in this investigation were first oven dried for 24 h and then sieved through a 425 #m sieve. All chemical and index tests were conducted in accordance with BSI (1990) under normal laboratory conditions. The particle size distribution and hence clay fractions were determined by a combination of wet

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1. Jefferson, C.D. Foss Rogers/Engineering Geology 49 (1998) 95-109

sieving and analysis by sedigraph 5000. The specimens were first treated with freshly prepared sodium hexametaphosphate ( BSI, 1990). However, problems were encountered when testing WB samples due to strong particle associations. Inconsistent and non-repeatable results were obtained, even after various combinations of dispersion agents and pHs were tried. As a result the values quoted in Table 1 have been estimated from typical values cited in the literature (Seed et al., 1964a,b). For external surface area, a Micromertitics Accelerated Surface Area and Porosimetry 2000 analyser was used, the measurements being based on the physisorption of nitrogen gas. A five point Brunauer, Emmett and Teller (BET) surface areas analysis was then carried out. Further to this a scanning electron microscope at the Macaulay Land Use Research Institute ( M LUR I ) was used to examine particle morphology. This analysis clearly showed that the materials were consistent with expectation (cf. Mitchell, 1993). Qualitative mineralogical analysis was also conducted at MLURI using infrared spectroscopy and X-ray diffraction methods. The results from these are shown in Fig. 1Fig. 2. Table 1 gives further details of this analysis, which again was consistent with expectation. The final analysis set allowed the determination of the major cations and anions present. Measurements were made by a Dionex series 4500 apparatus. All specimens were first treated with 5 ml of AA grade 0.1M nitric acid. The results from these tests are shown in Table 1. The cation exchange capacity values were determined at MLURI using an ammonium ion saturation technique developed by Bain and Smith (1987).

5. Experimental procedures 5.1. Apparatus used The apparatus used throughout the test programme was the cone penetrometer detailed in BSI (1990). Throughout care was taken to ensure the cleanliness of the cone prior to each test being performed. The point of the cone was also checked regularly and maintained. Houlsby (1982) pointed

99

out that lack of attention to these points could induce very significant errors. The temperature was measured during each test by a portable thermocouple thermometer (accuracy + 0.5°C). This allowed measurements to be taken for each test specimen at close proximity to the penetration zone.

5.2. Main test programme Specimens from each sample were first mixed with distilled water to a paste consistency. It proved necessary to store WB and mixed WB:ECC specimens for 24 h to ensure full water adsorption. However, for ECC it was necessary to mix for only 10-15min (Grim, 1968). Once mixed, the specimens at differing moisture contents were placed into cups normally used for WL determinations. These were sealed in three small plastic bags and then stored in an environmental cabinet in yet another plastic bag at the required temperature. After 24 h, the specimens were individually removed and one penetration per cup recorded as quickly as possible. This ensured that temperature losses were insignificant, losses being further controlled by placing insulation at the base of the cup. Immediately after testing the moisture content was determined. A minimum of 12 specimens was tested for each clay and temperature, giving a range of penetrations between 14 and 27 mm. For any penetrations recorded < 14 ram, the specimens were rejected and the test repeated (Harison, 1988). Using this method a wide range of temperatures could be examined. During this study a range between 10 and 80°C was used. The penetration measured was plotted against moisture content, from which the wL was determined at 20 mm penetration. The maximum variation in temperature during testing was observed to be + 1.5°C and thus was deemed to be insignificant, especially when reference to previous work was made.

5.3. Comparative study A comparative study was carried out to enable the results of work reported in the literature to be related to those from tests carried out in accor-

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dance with BSI (1990) at different temperatures. Samples of ECC and WB only were tested. The tests were conducted in an environmental room set at its m a x i m u m operating temperature, corresponding to a soil temperature of ca 32°C. A lower temperature of 17°C was achieved by testing in a

cool room in the laboratory in winter. All equipment, distilled water and samples were first acclimatized for at least 24 h at the test temperature prior to testing. The WE of the samples was determined in the normal manner (BSI, 1990), and this was repeated to check consistency. Again

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the temperature was monitored using the thermocouple device and was found to be within + 2 ' C of the target temperatures, and hence the variation was deemed to be insignificant. A further study was also performed to examine the possible effects of time of storage. These tests were conducted in exactly the same way as in the main test programme, except the bags were stored under normal laboratory conditions for periods of both 24 and 72 h. ECC was not tested after 72 h since only a very small change was observed over a 24 h period. As before the temperature variation was small at _+2 C throughout the programme.

6. Results and discussions 6.1.

Main results

Upon examination of the index and compositional data shown in Table l, it can be seen that most of the values quoted are typical of the materials tested (Mitchell, 19931. However, for WB the values of wL and CEC were slightly lower than expected. This is attributed to the small amount of silica present in this sample. Overall the organic, sulphate and carbonate contents are sufficiently low to be deemed insignificant. Table 2 presents the Atterberg Limit data for the clays tested, which exhibit some interesting trends. These data clearly show that as the percentage of WB increases so the w~. increased in an approximately proportional manner. This is consistent with observations made by Seed et al. Table 2 Atterberg Limits of samples tested Clay E('( ~

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lz)lgineering

Geoh)L~v 49 (1998) 95 109

(1964a). However, the we of the mixed samples was consistently lower than either of the two pure clays [a finding similarly reported by Seed et al. (1964b). This is attributed to intrinsic nature of the constituent clays, the strongly negative basal surfiaces of WB particles associating with the positive edges of the much larger kaolinite particles of ECC. At the wk, where water adsorption capacity is a maximum (Grim, 1968), WB dominates the wL of the mixture according to its proportion. However, at the we, where water is mostly adsorbed (Seed et al., 1964a), the WB particles impart an added lubrication, thus reducing the frictional resistance at the particle contacts. This in effect reduces the moisture content needed to achieve a certain strength and hence decreases the wp. This also shows that wL of ECC:WB mixtures is more sensitive to changes in clay mineralogy than wp, implying that it is at the wL where most changes due to temperature are likely to occur. This is consistent with the effects of environmental changes ( Mitchell, 1993 ). Typical graphs of penetration versus moisture content produced in the main test programme are shown in Fig. 3. All of these graphs exhibited a strong linear correlation, the coefficient of correlation (R) never falling below 0.9 and little scatter of the data occurring about the regressed line. Thus it can be concluded that a linear relationship exists over the range of penetrations measured. From such graphs the we values were determined and these are presented in Fig. 4. An error band corresponding to the 95% confidence intervals has been included in this figure. Fig. 5 presents the w~, data when normalized by room temperature (between 17 and 2 3 C ) for each clay, respectively. Figs. 4 and 5 show that a clear trend with respect to temperature exists tbr all of the samples tested. With ECC there is a small, though definite, reduction in inter-particle bond strength at elevated temperatures, which thus decreases the moisture content needed to achieve a given strength (i.e. the w~. reduces). These effects are relatively small, as would be expected from a clay dominated by solid contacts (Jefferson, 1994). For WB and the mixed samples the opposite trend is apparent. This is attributed to coagulation

1. Jefferson, C.D. Foss Rogers / Engineering Geology 49 (1998) 95-109

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p r o m o t e d by increased temperature m WB, evidence for which is also presented elsewhere (Marshall, 1964; Jefferson et al., 1997). This effecl dominates any other thermal effects, producing enhanced strengths at a given moisture content and hence an increased ,'~, A further indication of thermally induced structural rearrangements was shown by the slight swelling that occurs when specimens were stored at higher temperatures. If this had been associated with a non-continuous, dispersed fabric, the u'L would have reduced. The influence of temperature was more noticeable as the WB content increased. However, Fig. 5 shows that the percentage increase in %, was erratically related to WB content. Thus it is the presence o f WB, rather than its percentage content, that seems the most significant factor. Table 3 shows how the gradient o f the penetra-

tion versus moisture content plots varied with temperature. Although not consistent throughout, the results for E C C do indicate that as temperature increased so the gradient reduced. This implies that a greater change in moisture content is required to change the strength by a given amount. It equally implies, therefore, that at elevated temperature the If, is increased and the material is more "plastic" in its nature. The above trend was also observed with WB, indicating that the same conclusion applies. This finding has important implications for the one point method developed by Clayton and Jukes ( 1978 ) and, although beyond the scope o f this paper, warrants further study. The above results indicate that the effect o f elevated temperature on smectites is markedly different to that of kaolinites, hence explaining the contradiction in the literature. These changes are

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Table 3 Gradients of penetration versus moisture content plots: main test programme Clay

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associated with a number of concurrent thermal events which control the overall behaviour observed. The response to temperature is a func-

tion of the mineralogy of the clay tested and the nature of its inter-particle contacts. In kaolinites the contacts are more solid in their nature (Jefferson, 1994) and thus thermal effects are slight, and due to a weakening of the bonds in this zone. By comparison smectites, whose behaviour is physico-chemically dominated, have thermal events controlled by coagulation and structural rearrangement. This is consistent with data presented by Sridharan et al. (1988) from tests on kaolinitic soils and by Sridharan et al. (1986) from tests on montmorillonitic soils. However, this could not be due to changes in the mineralogical nature of the clay due to the relatively low elevation of temperature (i.e. < 8 0 ° C ) and the very short space of time changes were measured over (Pusch and Karnland, 1996). Further evidence of

106

1. ,leff~)rson, ('. D. 1.'oss Rogers, f:)Tgmc,cring Ok.oh~Kv49 (1998) 95 109

thermal-agglomeration has recently been supplied by Jefferson et al. (1997), who showed that the zeta potential (or surface charge) was independent of temperature (5 60'C) for a relatively pure kaolinite. However, the smectitic clay tested (bentonite) exhibited more than a 50% reduction in zeta potential over the same temperature range. This clearly shows that not only are smectitic clays considerably more sensitive to temperature changes, but also that coagulation will be promoted in these clay soils by an increase in temperature. This is wholly consistent with the data presented in this paper, further indicating the power of the method of clay evaluation presented here. The effect of temperature is therefore related to the total specific surface areas (SSA). Clays with very low SSA exhibit a reduction in wL at elevated temperatures, while clays with very high SSA show the opposite trend. However, it was noticed that at 8 0 C an additional error was induced due to the tendency for evaporation of water from the sample to occur. This tends to cause the wl, values to be underestimated. Nevertheless the authors consider that it is possible to reduce these errors to a minimal level due to the speed at which this technique is conducted. This method, theretore, enables the relative sensitivity of clay soils to temperature changes to be assessed accurately by allowing a far greater temperature range to be examined than previously possible. Furthermore, it is cheap and does not require any calibration for temperature. Overall, these results imply that other key design parameters may be affected by temperature depending on the mineralogy of the soil. The relatively insignificant effect temperature (in the range of 10-8OC) has on w~ for soils dominated by kaolinite, implies that very little change would occur in strength and compressibility parameters. In fact a slight decrease in strength would be anticipated based on the results presented herein, consistent with observations made in the literature (Ctori, 1989; Kuntiwattanakul et al., 1995). Although the results shown in Table 3 for ECC imply that the compression index may rise slightly, the data presented are limited and the trends

insufficiently consistent to make any firm conclusions. Overall, therefore, it is likely that any changes would be slight and practically insignificant, again consistent with the literature (Virdi, 1984 ). Clearly, the most significant effects for temperatures ranging between 10 and 80<'C were observed with clay of high smectite content. The results imply that the strength of these clays would increase with temperature. The effects observed are consistent with observations made on permeability measurements by Towhata et al. (1993), who showed how elevating the temperature increased the coefficient of permeability above that attributable to alterations in the viscosity of water. This could be a result of changes that occur in the clay's fabric, as discussed above, which are in turn consistent with allowing a greater rate of seepage. The data presented here could have significant consequences for correlations which rely on the w~, lbr example, Del Olmo et al. (1996), who proposed a method to overcome specimen variability effects on the thermo-plastic Cam Clay model. However, such correlations must be used with great care. Proper account must also be taken of" the effect that temperature has on the other factors that influence strength. Temperature affects strength in a complex way (Mitchell, 1993), and knowledge of how temperature affects drainage and consolidation, for example, is required to assess fully any changes that may occur. Lingnau et al. (1996) illustrated this when presenting data from triaxial tests conducted on sand-bentonite buffer soil at temperatures between 26 and 100'C. Furthermore, it is typically the case that either details of the materials tested are not presented or the soils tested are of very complex mineralogy, thus making accurate judgements difficult. From the work reported herein, which was aimed at elucidating which clay soil is most materially susceptible to temperature changes, it has been clearly shown that the sensitivity of smectitic soils to temperature changes is considerable and that kaolinitic soils are relatively insensitive of to such changes. Judgements on whether a particular natural clay, or the correlation with WL for natural clays, need to be checked for temperature variations should thus be made on this basis.

L Jefferson, C D. Foss Rogers/Engineering Geology 49 (1998) 95-109

6.2. Comparative tests

To assess further the results discussed in the previous section, normal BSI (1990) tests were conducted on ECC and WB only (see Table 4). It is clear that the wL of ECC is unaffected by temperature over the range 18-32°C. This confirms the observations made above. For WB a slight increase occurred, although it was much less pronounced than in the main test programme. This similarly is attributed to temperature-promoted coagulation, although, due to the short time span of the test, these changes would have only just started to take place. These results further indicate that the assumption that all clays are at their critical state when tested at the WL may not be valid in all cases. This is especially true for sodium dominated smectitic at elevated temperatures, a conclusion that agrees with similar results discussed in the literature. To investigate the effects of time of storage, specimens of WB and mixed samples were tested after 24 and 72 h at room temperature. The results are shown in Fig. 6, which clearly shows the increase in WL decreasing rapidly after the first 24 h of storage. This further supports the discussion above. ECC was not tested due to the insignificance of the changes that occurred in previous testing (see Section 5.3).

7. Conclusions Overall it is concluded that a sodium dominated smectite is considerably more sensitive to temperature changes than a well crystallized kaolinite. Over the same range of temperatures (10-80°C), the liquid limit of sodium smectite changed from 539 to 615% compared with 62-59% for kaolinite. Table 4 Liquid limit determination using the BSI method Clay

Temperature (°C) 17

ECC WB

18

21

59 424

451

22

32

59

59

33

455

107

These changes are related indirectly to the SSA, since significant changes are associated with physico-chemical activity which masks the much smaller, and opposite, trend of liquid limit reduction caused by mechanical effects. Clays with a larger SSA, and hence inter-particle contacts that are more dominated by adsorbed water, are much more sensitive to temperature changes. This is totally consistent with the results of tests that examined the effect of temperature on the surface charge characteristics of almost identical clay minerals. The work has thus enabled elucidation of the sometimes contradictory literature on the Atterberg Limits and has illustrated the key importance of adequate compositional data being obtained. Moreover, these results have indicated how changes to mechanical properties of clay soils may be affected by temperature. For ECC very little change would be anticipated. However, for WB highly significant changes may occur, especially to its strength and permeability characteristics, thus necessitating the evaluation of these parameters at expected operational temperatures. Data from comparative BSI (1990) tests and tests carried out over both 24 and 72 h indicate still further the advantages of the method used in the main test programme. The British Standard tests confirmed the trends observed, although to a lesser extent due to a much more limited temperature range possible. The extended storage test indicated that the majority of the changes due to temperature occurred with the 24 h period.

Acknowledgment The research reported herein was conducted as part of a project on the temperature effects on soil properties at Loughborough University sponsored by the Science and Engineering Research Council. Their support is gratefully acknowledged, as is the help provided by both the Department of Chemical Engineering at Loughborough University and the Macaulay Land Use Research Institute with the material classification tests.

108

1. dtJl~'r.son. ('. D. I=o.~.~Ro,ecra , Engineering GeoloKi, 49 (1998) 95 109 600

500 •

..... ....i

l ........

". . . .

WB

~400

•=

.... O-'-

75:25

---at--.

50:50

....... 0

25:75

......

300

,,,,I' 200

......................................................

i .....................................................

i........................................................

................................. * .......................................................... i...........................................................

IIO[C: variation in temperature

is +/- 2 °C

100

0 2

0

T i m e (days)

Fig. 6. Liquid limit versus time of" storage.

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