Strength behaviour of lime-treated soils in the presence of sulphate

Transcription

1 1358 Strength behaviour of lime-treated soils in the presence of sulphate P.V. Sivapullaiah, A. Sridharan, and H.N. Ramesh Abstract: Lime has been used extensively to improve the shear strength of fine-grained soils. It has been recently reported that the presence of sulphate causes abnormal volume changes in lime-stabilized soil. The paper presents the strength behaviour of lime-treated montmorillonitic natural black cotton soil in the presence of varying sulphate contents after curing for periods of up to 365 days. Alteration of soil lime reactions in the presence of sulphate affects the strength development by cementation. Consequently, the stress strain behaviour effective stress paths of soil cured with sulphate are similar to those of normally consolidated soil rather than cemented soils. The reduction in shear strength due to a reduction in effective cohesion intercept occurs for lime-treated soil cured with sulphate for long periods. Key words: clays, cohesion, fabric, friction, shear strength. Résumé : La chaux a été abondamment utilisée pour améliorer la résistance des sols à grains fins. On a fait rapport récemment du fait que la présence de sulfate produit des changements anormaux de volume dans le sol stabilisé à la chaux. Cet article présente le comportement en résistance d un sol organique noir naturel montmorillonitique traité à la chaux en présence de diverses teneurs en sulfate après des périodes de mûrissement allant jusqu à 365 jours. L altération des réactions sol chaux en présence de sulfate affecte le développement de la résistance par cimentation. En conséquence, le comportement contrainte déformation, les cheminements de contrainte effective du sol mûri avec le sulfate montrent un comportement de sol normalement consolidé plutôt que de sols cimentés. La réduction de la résistance au cisaillement due à la réduction de l interception cohésion effective se produit pour le sol traité à la chaux mûrissant avec du sulfate pour de longues périodes Mots clés : argiles, cohésion, fabrique, frottement, résistance au cisaillement. [Traduit par la Rédaction] Notes 1367 Introduction Lime has been used extensively to improve the shear strength of fine-grained soils (Bell 1988a, 1988b). The low strength of soils is generally associated with increased moisture content. The addition of lime changes the soil behaviour to that analogous to a granular mass, the particles of which are strongly bound by pozzolanic cementitious compounds formed by reactions with soil silica and lime in the presence of water. The cemented soil particles then resist the internal swelling pressure of the clay. The formation of these compounds and hence the strength increase with an increase in the length of the curing period. Many factors such as soil type, type and amount of lime added, curing period and method, moisture content, method of compaction, and time elapsed between mixing and compaction influence the final strength (Ingles and Metcalf 1972). For any given soil, there is an optimum lime content beyond which the strength gain may be marginal. Also in certain cases, the strength gain may be too slow to meet the Received December 11, Accepted January 10, Published on the NRC Research Press website on December 14, P.V. Sivapullaiah and A. Sridharan. Department of Civil Engineering, Indian Institute of Science, Bangalore , India. H.N. Ramesh. Faculty of Engineering (Civil), UVCE, Bangalore University, Bangalore , India. Can. Geotech. J. 37: (2000) requirement. To enhance the effective use of lime, certain additives such as fly ash, blast furnace slag, and a variety of inorganic chemicals (sodium chloride, sodium hydroxide, sodium metasilicate, sodium sulphate, and sodium hydrogen phosphate, etc.) have been used along with lime (Davidson et al. 1960; Moh 1962; Dan Marks and Halliburton 1970; Kujala 1983; Holm et al. 1983). In certain cases it has been observed that the presence of sulphate is harmful in lime stabilization of soils. Sherwood (1962) observed cracking and swelling in specimens of heavy clay stabilized with 10% lime and cured at constant moisture content for 1 week when immersed in solutions of either sodium sulphate or magnesium sulphate at concentrations below 1.5% as SO 3. Hunter (1988) and Mitchell (1986) reported that lime-treated sulphate-bearing clay swelled and disintegrated after a few years when used for road construction. Recently, Sridharan et al. (1995) have shown that the presence of sulphate increases the compressibility of lime-treated black cotton soil after curing for long periods. In this paper an attempt is made to illustrate the effectiveness of lime treatment for the soil in the presence of sulphate. Experimental program Materials and methods The black cotton soil used in this investigation was obtained from Davangere, Karnataka State, India. The soil was collected by open excavation, from a depth of 1 m below

2 Notes 1359 Table 1. Properties of black cotton soil. Specific gravity 2.7 Liquid limit (%) 81 Plastic limit (%) 33.5 Shrinkage limit (%) 8.9 Plasticity index (%) 47.5 Clay content (%) 35 Exchangeable cations (mequiv./100 g) Sodium 9.3 Calcium 10.2 Potassium 0.5 Magnesium 10.1 Total 30.1 Sulphate (CaSO 4 ) 0.0 (trace) Effective stress shear strength parameters Cohesion intercept c 0 Friction angle φ ( ) 27 ground level. The soil was dried and passed through Indian Standard sieve size of 425 µm. The properties of the soil are given in Table 1. The cation exchange capacity of the soil was determined by ammonium acetate extraction. The individual exchangeable ions were determined by atomic adsorption spectrophotometry. Chemically pure hydrated lime, sodium sulphate, and calcium sulphate were obtained from Glaxo Laboratory, India, and used in the investigation. Black cotton soil was mixed with 6% lime and varying sodium or calcium sulphate contents (0.5, 1.0, or 3.0% by weight) and was brought to its liquid limit consistency. The liquid limit of lime-treated soil varied in the presence of sulphate due to chemical interaction. The liquid limits of soil in Table 2. Effect of sulphate on the liquid limit of lime-treated black cotton soil. Additive Liquid limit (%) No additive % Na 2 SO % Na 2 SO % Na 2 SO % CaSO % CaSO % CaSO the presence of sulphate are shown in Table 2. The wet soil was added to stainless steel tubes of diameter 38.1 mm and length 150 mm. The tubes were covered with polythene sheets and kept in an air-tight container to prevent carbonation. After the remoulded samples gained sufficient strength, they were extruded with a sample extractor. Each sample was trimmed normal to its axis horizontally to a height of 76.2 mm. The samples were then covered with a wet cotton cloth and cured in a plastic container for periods varying from 7 to 365 days. Triaxial testing A conventional triaxial testing system was used to carry out consolidated undrained tests (CU tests with pore-water measurements) on the cured samples. Readings of deviator load, pore-water pressure, and axial deformations were taken. The test was continued until the deviator load reached a maximum level and dropped. The test was repeated on at least three identical samples under the effective cell pressures of kpa. Fig. 1. X-ray diffraction patterns of lime-treated black cotton soil (curve a) and lime-treated black cotton soil with 1% sodium sulphate after 365 days of curing (curve b). CSH, calcium silicate hydrate.

3 1360 Can. Geotech. J. Vol. 37, 2000 Fig. 2. Effect of curing period on the stress strain relationship of lime-treated black cotton soil (a) at a cell pressure of 100 kpa, (b) with 0.5% sodium sulphate at a cell pressure of 100 kpa, and (c) with 3% sodium sulphate at a cell pressure of 100 kpa.

4 Notes 1361 Using the test data, stresses and strains were computed and deviator stress versus axial strain plots were drawn. Peak stresses were taken from these plots and total and effective strength parameters were obtained using a modified Mohr-Coulomb plot. Fig. 3. Effect of sodium sulphate content on the stress strain curves of lime-treated black cotton (BC) soil after 365 days of curing at a cell pressure of 100 kpa. Results and discussion Effect of the presence of sulphate on soil lime reactions Soil lime reactions are altered in the presence of sulphate. In the absence of sulphate the reactions between lime and soil in the presence of water produce calcium silicate hydrate of varying calcium to silica ratios. In the presence of sulphate, the reactions are modified and ettringite and (or) thaumasite are formed (Braga Reis 1981). The sequence of reactions is simplified by Hunter (1988) as follows: [1] 6Ca + 2Al(OH) 4 + (OH) + 3(SO 4 ) H 2 O Ca 6 [Al(OH) 6 ] 2 (SO 4 ) 3 26H 2 O (ettringite) [2] Ca 6 [Al(OH) 6 ] 2 (SO 4 ) 3 26H 2 O+2H 2 SiO (O 3 2 +O 2 ) Ca 6 [Si(OH) 6 ] 2 (SO 4 ) 3 (CO 3 ) H 2 O + 2Al(OH) 4 + (SO 4 ) 2 + 4OH +2H 2 O (thaumasite) Thus normal cementitious calcium silicate hydrate formation is inhibited and ettringite and thaumasite are formed. This is supported by X-ray diffraction studies. X-ray diffraction patterns (Fig. 1) of lime-treated soils cured with sulphate have shown new peaks at 9.6 Å (1 Å = 0.1 nm) and 5.6 Å (corresponding to 2θ angles of 9.2 and 15.8, respectively, where 2θ is the diffraction angle) due to ettringite. There is also a reduction in the intensity of the peak at 3.26 Å (corresponding to a 2θ angle of 27.3 ), which is due to calcium silicate hydrate in samples cured with sulphate compared with lime-treated soil cured for 365 days. The effect of ettringite formation on the stress strain curves, peak stress at a given effective cell pressure, and effective stress path and shear strength parameters of limetreated black cotton soil cured for different periods is discussed in the following sections. Stress strain curves It is known that stress strain curves of all normally consolidated soils are nonlinear except in a very narrow region near the origin (at small strains below 10 4 ) and do not exhibit pronounced peaks (Bowles 1996). As seen in Fig. 2a, the stress strain curves of black cotton soil with 6% lime are linear almost up to 80% of peak stress. This linearity increases with the curing period and cell pressure. In fact, Kondner (1963) observed the hyperbolic nature in these curves. Also, the stress strain curves of lime-treated soils exhibit pronounced peaks due to cementation of soil particles by pozzolanic compounds formed by reactions between soil silica and lime. The effect of cell pressure varies with curing period. For short curing periods peaks become more and more pronounced with an increase in cell pressure, whereas the samples cured for longer periods show sharp peaks even at a lower cell pressure and an increase in cell pressure does not influence the nature of the peaks. The presence of sodium sulphate or calcium sulphate has no influence on the nature of stress strain curves at shorter curing periods, but the stress strain curves of lime-treated soil are significantly altered by the presence of sulphate when the samples are cured for a longer period. As seen from Figs. 2b and 2c, the stress strain curves are almost nonlinear and do not exhibit sharp peaks. Further, the effect of cell pressure is significant. The behaviour of lime-treated soil cured with sulphate for longer durations is more similar to that of normally consolidated soil than to that of cemented soil. Peak stress and the corresponding strain The peak stress at any cell pressure increases steeply with an increase in the curing period up to 90 days and marginally beyond 90 days of curing for lime-treated black cotton soil (Fig. 2a). However, the strain corresponding to the peak stress is not significantly affected by the length of the curing period. Effect of sodium sulphate content Figure 2b shows that the peak stress of lime-treated black cotton soil on curing with 0.5% Na 2 SO 4 increases up to 30

5 1362 Can. Geotech. J. Vol. 37, 2000 Fig. 4. (a) Effect of curing period on the stress strain curves of lime-treated black cotton soil with 0.5% calcium sulphate at a cell pressure of 100 kpa. (b) Effect of sodium sulphate content on the stress strain curves of lime-treated black cotton soil with 3% calcium sulphate at a cell pressure of 100 kpa. Fig. 5. Effect of calcium sulphate content on the stress strain curves of lime-treated black cotton soil after 365 days of curing at a cell pressure of 100 kpa. days but decreases when cured for 365 days. On curing with 1% Na 2 SO 4, the peak stress of lime-treated soil increases steeply from 7 days to 30 days but decrease gradually beyond 30 days. In fact, the peak stress of a sample cured for 365 days is lower than that of a sample cured for 7 days. On curing with 3% Na 2 SO 4 the peak stress increases up to 90 days but decreases when cured for 365 days (Fig. 2c). Although the peak stresses of lime-treated soil with different sulphate contents are comparable after curing for 7 days, they are decreased with any percentage of Na 2 SO 4 for 365 days (Fig. 3). The decrease in peak stress is greatest with 1% Na 2 SO 4. Unlike the case of soil samples treated with lime alone, the samples cured with lime and sodium sulphate show the effect of cell pressure even when cured for a longer period. Effect of calcium sulphate content Curing with any percentage of calcium sulphate reduces the peak stress of lime-treated black cotton soil. The decrease starts after 7 days of curing with 0.5% CaSO 4 (Fig. 4a), whereas with 1 and 3% CaSO 4 the peak stress increases up to 30 days and decreases only beyond 30 days of curing (Fig. 4b). The decrease (Fig. 5) is relatively less with 0.5% CaSO 4 and about the same with 1 and 3% CaSO 4.

6 Notes 1363 Fig. 6. Comparison of the effect of sodium sulphate and calcium sulphate on the stress strain curves of lime-treated black cotton soil cured for 365 days at a cell pressure of 200 kpa. Fig. 7. Effective stress paths of lime-treated black cotton soil (a) after 365 days of curing, (b) with 1% sodium sulphate after 365 days of curing, and (c) with 1% calcium sulphate after 90 days of curing. σ 1, total normal stress; σ 1, effective normal stress; σ 3, total cell pressure; σ 3, effective cell pressure; Ψ,angle of the envelope; d, the intercept of the envelope. As in the case of samples cured with sodium sulphate, the samples cured with calcium sulphate also show the effect of cell pressure on stress strain curves even in samples cured for longer periods. After curing with 1% sulphate for 1 year the decrease in peak stress is significantly greater with sodium sulphate than with calcium sulphate. (Fig. 6). Effective stress paths The nature of the effective stress path can be used to assess the cemented nature of soil. The effective stress paths of cemented soils are similar to those of overconsolidated soils. For normally consolidated samples, the effective stress paths are curved towards the left, but with an increase in the cementation the stress paths tend to become linear towards the right, similar to those of overconsolidated soils. The effective stress paths of lime-treated black cotton soil after curing for periods of more than 7 days are much less rounded. However, the roundness increases with an increase in cell pressure. Thus the development of pore-water pressure is less for lime-treated soil. For samples cured for 365 days the roundness is absent even at higher cell pressures. (Fig. 7a). This indicates that soil particles are strongly bonded. The effective stress paths of lime-treated black cotton soil cured with any form or concentration of sulphate gradually

7 1364 Can. Geotech. J. Vol. 37, 2000 Table 3. Shear strength parameters. Curing period (days) c cu (kpa) φ cu ( ) c (kpa) Black cotton soil + 6% lime Black cotton soil + 6% lime + 0.5% Na 2 SO Black cotton soil + 6% lime + 1.0% Na 2 SO Black cotton soil + 6% lime + 3.0% Na 2 SO Black cotton soil + 6% lime + 0.5% CaSO Black cotton soil + 6% lime + 1.0% CaSO Black cotton soil + 6% lime + 3.0% CaSO Note: c cu, total cohesion intercept; φ cu, total friction angle. φ ( ) become more and more rounded. After 1 year of curing the effective stress paths are similar to those of normally consolidated soil, indicating that the cementation bonds are almost destroyed (Figs. 7b, 7c). This variation in the nature of effective stress paths lines is consistent with variation in the peak stress and nature of stress strain curves. Shear strength parameters The addition of lime to black cotton soil changes the fabric of clay particles from dispersed to flocculated. This is reflected in a considerable increase in the effective friction angle, φ, of from 27 to about 40 even for samples cured for 7 days. With longer curing the flocculated particles are cemented by pozzolanic reaction compounds. With longer curing, the increase in strength is only through an increase in effective cohesion intercept c, which increases from almost 0 to 269 kpa. Thus it is clear that flocculation of particles increases the value φ, whereas cementation of particles increases the value of c. Table 3 shows that curing with any type or amount of sulphate for long periods considerably reduces c. Compared with a c value of 263 kpa for lime-treated soil after 365 days of curing, the same soil with Na 2 SO 4 and 3% CaSO 4 content has c values of 88 and 80 kpa, respectively. At short curing periods the value of c increases with an increase in the Na 2 SO 4 content. This is due to enhanced lime reactions by increased availability of silica because of an increase in ph (Davidson et al. 1960). For longer curing periods the reduction in c occurs with an increase in sodium sulphate content. There is no definite trend with respect to variation in φ, therefore φ values are not altered much. Figures 8a 8c show the effect of 0.5% sodium and calcium sulphate content on modified Mohr s envelopes for different curing periods. These figures show the combined effect of c and φ. CaSO 4 content of greater than 0.5% decreases the value of c even for short curing periods. With any CaSO 4 content, c decreases with an increase in the length of the curing period. Again, the variation in φ is not clear. However, the decrease in strength on curing with CaSO 4 is clear as seen from the modified Mohr s envelopes in Figs. 8a 8c. Bonding of particles is affected by sodium sulphate at relatively short curing periods, whereas calcium sulphate affects the bonding only after curing for longer periods. The reduction in strength is greater with an increase in sodium sulphate content than with an increase in calcium sulphate content. For short curing periods the strength of lime-treated black cotton soil is greater with calcium sulphate than with sodium sulphate. This is because the calcium ions required for cation exchanges of soil are provided by calcium ions of calcium sulphate, whereas with sodium sulphate the cation exchange requirements must be met from added lime. Thus there is a reduction in the amount of lime available for pozzolanic reactions in soil with sodium sulphate compared with that available in soil with calcium sulphate (Mitchell 1986). It is interesting to note that while increasing the concentration of sodium sulphate increases the value of c for short

8 Notes 1365 Fig. 8. Effect of sulphate contents on modified Mohr s envelopes on lime-treated black cotton soil after curing for (a) 7 days, (b) 30 days, and (c) 365 days.

9 1366 Can. Geotech. J. Vol. 37, 2000 Fig. 8. (concluded). curing periods, increasing the concentration of calcium sulphate decreases the value of c (Table 3). The increase in strength with sodium sulphate may also be due to increased availability of soluble silica with increased ph. The increase in ph is due to the formation of sodium hydroxide by the following reaction: [3] Ca(OH) 2 +Na 2 SO 4 2NaOH + CaSO 4 Sodium hydroxide gives a higher ph than calcium hydroxide. The greater availability of silica increases the amount of calcium silicate hydrate formed before conversion to ettringite. Conclusions This study has found that the presence of sulphate in soils considerably reduces the shear strength of lime-treated black cotton soil after curing for long periods. The stress strain curves of lime-treated soils which initially shows cemented soil behaviour exhibit the behaviour of a normally consolidated soil after curing for long periods if sulphate is present in the soil. Peak stress at any cell pressure of lime-treated soil increases with an increase in the length of the curing period, whereas for soils containing sulphate the peak stress increases initially with an increase in the length of the curing time and then decreases with further curing. The effective stress paths of lime-treated soils which are similar to that of an overconsolidated soil become similar to that of a normally consolidated soil for sulphate-containing soils, especially after a long curing period. The reduction in the strength of soils in the presence of sulphate is reflected in the reduction of c rather than φ. This is considered to be due to prevention of cementation of particles by sulphate and formation of ettringite, and has been confirmed by earlier investigators. However, for short curing periods the effect of sulphate is marginal. References Bell, F.G. 1988a. Stabilization and treatment of clay soils with lime. Part 1. Basic principles. Ground Engineering, 21: Bell, F.G. 1988b. Stabilization and treatment of clay soils with lime. Part 2. Some applications. Ground Engineering, 21: Bowles, J.E Foundation analysis and design. 5th ed. McGraw Hill Companies, Inc., New York. Braga Reis, M.O Formation of expansive calcium sulphate aluminate, the action of sulphate ion on weathered granites in a calcium hydroxide saturated medium. Cement and Concrete Research, 11: Dan Marks, B., III, and Halliburton, T.A Effects of sodium chloride and sodium chloride lime admixtures on cohesive Oklahoma soils. Highway Research Record, 315: Davidson, D.T., Mateos, M., and Barnes, H.F Improvement of lime stabilization of montmorillonitic clay soils with chemical additives. Highway Research Board Bulletin, 262: Holm, G., Trank, R., and Ekstrom, A Improving lime column strength with gypsum. In Proceedings of the 8th European

10 Notes 1367 Conference on Soil Mechanics and Foundation Engineering, Helsinki, Finnish Geotechnical Society, pp Hunter, D Lime induced heave in sulphate bearing clay soils. Journal of Geotechnical Engineering, ASCE, 114: Ingles, O.G., and Metcalf, J.B Soil stabilization principles and practice. Butterworths, Melbourne. Kondner, R.L Hyperbolic stress strain response, cohesive soils. Journal of the Soil Mechanics and Foundations Division, ASCE, 89: Kujala, K The use of gypsum in deep stabilization. In Proceedings of the 8th European Conference on Soil Mechanics and Foundation Engineering, Helsinki, Finnish Geotechnical Society, pp Mitchell, J.K Practical problems from surprising soil behaviour. Journal of Geotechnical Engineering, ASCE, 112: Moh, Z.C Soil stabilization with cement and sodium additives. Journal of the Soil Mechanics and Foundations Division, ASCE, 88: Sherwood, P.T Efect of sulphate on cement and lime treated soil. Highway Research Board Bulletin, 353: Sridharan, A., Sivapullaiah, P.V., and Ramesh, H.N Consolidation behaviour of lime treated sulphate soils. In Proceedings of the International Symposium on Compression and Consolidation of Clayey Soils, Hiroshima, Japan, Vol. 1, pp