Effect of Sulphate Content in Soils as a Potential Geohazard
Effect of Sulphate Content in Soils
as a Potential Geohazard
H.N. Ramesh
Professor, Faculty of Engineering-CIVIL, UVCE, Bangalore University, Bangalore–560 056, India.
E-mail:
ABSTRACT: Geohazards includes all earth surface processes with the potential to cause loss or harm to the community or the environment. The resulting risks associated with these hazards depend on many factors. Chemical contamination of soil is caused by the presence of man-made chemicals or other alteration in the natural soil environment. It is widely known that lime can be used to improve the engineering properties of expansive soil. Presence of sulphate increases the free swell volumes and oedometer swelling of lime treated soils. The formation of ettringite and thaumasite in lime treated soils containing sulphate is prevented by the addition of barium chloride. Both deviator stress and strength parameters are restored to its original level by the addition of barium chloride.
979
Effect of Sulphate Content in Soils as a Potential Geohazard
1. INTRODUCTION
1.1 Types of Geohazards
Geohazards are ground-related features that can threaten public safety or can have a large impact on the costs and programme of building and infrastructure developments. Although these are most commonly associated with chemical contamination, they include numerous other hazards. It includes all earth surface processes with the potential to cause loss or harm to the community or the environment. In addition to potentially fatal acute geohazards such as earthquakes, landslides and floods, the importance of more chronic geohazards such as acid sulphate soils, coastal erosion, reactive soils and dry land salinity is also recognized. Some geohazards are naturally occurring, and others man-made. The commonest include: chemical contamination of soils, loose and compressible soils, undermining and natural caverns, landslides, buried obstructions, groundwater, and seismicity.
The resulting risks associated with these hazards depend on many factors, including the end-use of the land. In the development of building and infrastructure projects, geohazards discovered at a late stage are a major cause of cost over-runs and programme delays. They have the potential to overwhelm contingency cost allowances and eliminate development profits.
Potential Mitigation of geohazard falls into a number of categories: Avoid—plan development so that the hazard has no effect; Eliminate—remove the hazard completely; Accommodate—design so that the effect of the hazard is acceptable Minimize—treat the hazard so that the effect is reduced to an acceptable level. The stabilization strategy falls into this category.
1.2 Chemical Contamination of Soil
Chemical contamination of soil is caused by the presence of man-made chemicals or other alteration in the natural soil environment. This type of contamination typically arises from the rupture of underground storage tanks, application of pesticides percolation of contaminated surface water to subsurface strata, oil and fuel dumping, leaching of wastes from landfills or direct discharge of industrial wastes to the soil. The most common chemicals involved are petroleum hydrocarbons, solvents, pesticides, lead and other heavy metals. This occurrence of this phenomenon is correlated with the degree of industrializations and intensities of chemical usage. Soil contaminants can have significant deleterious consequences for ecosystems. There are radical soil chemistry changes which can arise from the presence of many hazardous chemicals even at low concentration of the contaminant species. The concern over soil contamination stems primarily from health risks, from direct contact with the contaminated soil, vapors from the contaminants, and from secondary contamination of water supplies within and underlying the soil. Expansive soil deposits are problematic to engineering structures because of their heaving and shrinkage. These soils occurring above water table swell which will increase the moisture content and shrink. It is widely known in practice that lime can be used to improve the engineering properties of soil.
The use of lime as the soil stabilizer database to the early Roman period. It was used in the construction of roads especially the famous “via Apian”. This road has given outstanding service since its construction and is still maintained as traffic artery. In USA hydrated lime appears to have been used for soil stabilization in laying a road in 1924. Lime stabilization was also used in Europe particularly in Germany. Application of lime stabilization received good attention in Australia.
Though initially lime was used for surface stabilization of clays for road constructions, with the development of equipment and methods for deep stabilization of clays, the use of lime was found in many engineering activities as follows:
· Sub grade and sub base stabilization
· Stabilization for embankments and slopes
· Stabilization for foundation soils
· Lime piles and columns
· Stabilization of sensitive clays.
2. MECHANISM INVOLVED IN LIME TREATED
SOILS
2.1 Effects of Sulphate on Basic and Engineering
Properties of Soil
Improvements in the engineering properties of lime treated soils were attributed to four basic reactions namely:
· Cation exchange
· Flocculation
· Carbonation and
· Pozzolanic reaction
The products of soil-lime reactions fall largely in to two cases: Calcium silicate hydrate and Calcium aluminate hydrate.
Presence of sulphate alters the physical and engineering behavior of lime treated soils. The effects are pronounced depending upon concentration, form of sulphate and curing period. Abnormal increase is seen in the liquid limit of lime treated soils after curing in the presence of sulphate. However, shrinkage limit reduces after curing for longer periods. Presence of sulphate increases the free swell volumes and oedometer swelling of lime treated soils. At short duration of curing, the negative effects of sulphate cannot be noticed in the compressibility behavior of lime treated soils. But after curing for long periods, the compressibility of lime treated soils increases significantly in the presence of sulphate.
Sherwood (1962) observed that cracking and swelling in specimens of heavy clays stabilized with lime and cured at constant moisture content for one week when immersed in solutions of either sodium sulphate at concentrations below 1.5% of SO3. He attributed this due to a reaction between clay fraction and sulphate.
Dal Hunter (1988) indicated that lime treated sulphate bearing clay is risky even at relatively low sulphate concentrations. He reported a case of apparently sound lime stabilized soils swelled and disintegrated after few years of construction of stewast avenue roadway because of lime induced heave. Heaved areas are found to contain abundant thaumasite a complex chemical compound containing calcium-silicate-hydroxide-sulphate-carbonate-hydrated material. It was observed that ettringite which was formed first might have transformed to thaumasite after the heave.
3. LABORATORY INVESTIGATIONS
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 one meter from ground level. The soil was dried and passed through Indian Standard sieve size of 425 micron. The properties of soil are given in Table 1. The cation exchange capacity of the soil was determined by ammonium acetate extraction method. The individual exchangeable ions were determined by atomic adsorption spectrometric method.
3.1 Chemicals Used in the Preparation of Soil Samples
Chemically pure hydrated lime, sodium sulphate and barium chloride are obtained from Glaxo Laboratory, India and used in the investigation. First, black cotton soil was mixed with 1% sodium sulphate by weight of soil and left for equilibrium for a week. Then 6% of lime was mixed along with 2% barium chloride at liquid limit consistency. The wet soil was remolded into stainless steel tubes of 38.1 mm diameter and length of 150 mm. The tubes were covered with polythene sheets and were kept in a airtight container to prevent carbonation. After the remolded sample gained sufficient strength, they were extruded with sample extractor. The samples were 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 plastic container for different periods of 30, 90, 180 and 365 days.
Table 1: Properties of Black Cotton Soil
Property / ValueSpecific Gravity / 2.7
Liquid Limit (%) / 81
Plastic Limit (%) / 33.5
Shrinkage Limit (%) / 9
Plasticity Index (%) / 47.5
Clay Content (%) / 35
CEC meq/100 g,
Total,
as Sodium,
as Calcium,
as Potassium,
as magnesium / 30.1
9.3
10.2
0.5
10.1
Primary Clay mineral / Montmorrillonite
Sulphate / Traces
4. RESULTS AND DISCUSSION
4.1 Soil-lime-Sulphate Reactions
Conversion of part of lime to NaOH as per the equation
Na2SO4 + Ca(OH)2 ® 2NaOH + CaSO4 (1)
Formation of ettringite and thaumasite instead of calcium silicate hydrate and calcium aluminate hydrate in the presence of sulphate as follows:
6Ca2+ + 2 Al(OH)–4 + 4(OH)– + 3(SO4)2– + 26H2O
® Ca6[Al(OH)6]2 (SO4)3 26H2 O
(ettringite) (2)
Ca6[Al(OH)6]2(SO4)3 26H2O + 2H2SiO42– + 2CO32– + O2
® Ca6[Si(OH)6]2 (SO4)2(CO3)224H2O
(thaumasite)
+ 2Al(OH4)– + (SO4)2– + 4OH– + 2H2O (3)
These compounds have high affinity for water and expand.
Figures 1 to 12 shows the variation of stress strain curves and effective stress paths obtained by the addition of barium chloride in lime stabilised sulphate soils. The formation of ettringite and thaumasite in lime treated soils containing sulphate is prevented by the addition of barium chloride in the following way:
The reactions that occur in the presence of barium chloride are:
· Precipitation of soluble sulphate into insoluble barium sulphate by the reaction
Na2SO4 + BaCl2 ® 2NaCl + BaSO4 (4)
· But the NaOH formed as Eq. (1) may be converted to Ba (OH)2 by the equation,
NaOH + BaCl2 ® 2NaCl + Ba(OH)2 (5)
Fig. 1: Stress-Strain Curves of Lime Treated Soil in the Presence of 1% Sodium Sulphate after Curing for Different Curing Periods at a Cell Pressure of 100 kPa
Fig. 2: Effective Stress Path of Lime Treated Soil in the Presence of 1% Sodium Sulphate after 7 Days of Curing
Fig. 3: Effective Stress Path of Lime Treated Soil in the Presence of 1% Sodium Sulphate after 365 Days of Curing
Fig. 4: Stress-Strain Curves of Lime Treated Sulphatic Soil Containing Barium Chloride after Curing for 30 Days at a Cell Pressure of 100 kPa.
Fig. 5: Stress-Strain Curves of Lime Treated Sulphatic Soil Containing Barium Chloride after Curing for 90 Days at a Cell Pressure of 100 kPa.
Fig. 6: Stress-Strain Curves of Lime Treated Sulphatic Soil Containing Barium Chloride after Curing for 365 Days at A Cell Pressure of 100 kPa.
Fig. 7: Effect of Cell Pressure on Stress Strain Curves of Sulphatic Lime Treated Soil Treated with Barium Chloride after 30 Days of Curing
Fig. 8: Effect of Cell Pressure on Stress Strain Curves of Sulphatic Lime Treated Soil Treated with Barium Chloride after 90 Days of Curing
Fig. 9: Effect of Cell Pressure on Stress Strain Curves of Sulphatic Lime Treated Soil Treated with Barium Chloride after 365 Days of Curing
Fig. 10: Effective Stress Paths of Lime Treated
Sulphate Soil Containing Barium Chloride after
Curing for 30 Days
Fig. 11: Effective Stress Paths of Lime Treated
Sulphate Soil Containing Barium Chloride
after Curing for 90 Days
Fig. 12: Effective Stress Paths of Lime Treated
Sulphate Soil Containing Barium Chloride
after Curing for 365 Days
5. CONCLUSIONS
On the basis of the present experimental investigations, the following conclusions were drawn:
(a) Presence of sulphate decreases the strength of lime stabilized soils
(b) The reduction in strength of lime treated sulphatic soil is noticed only in the long period due to the formation of swelling type of compound called ettringite.
(c) Both deviator stress and strength parameters are restored to its original level by the addition of barium chloride. The effective stress path also indicates the strength gain by the addition of barium chloride.
(d) The amount of barium chloride required is less than twice the weight percentage of sulphate content.
references
Bell, F.G. (1988a). “Stabilization and Treatment of Clay Soils with Lime—Part 1. Basic Principles”, Ground Engineering, Vol. 21, No. 1. pp. 10–15.
Bell, F.G. (1988b). “Stabilization and Treatment of Clay Soils with Lime—Part 2. Some Applications”, Ground Engineering, Vol. 21, No. 2, pp. 25–29.
Broms, B. and Boman, P. (1975). “Lime Stabilised Columns”, Proc. of the fifth Asian Regional Conference on Soil Mechanics and Foundation Engineering, Bangalore, India, Vol. I, pp. 227–234.
Burkart, B., Goss, C.G. and Kern, P.J. (1999). “The Role of Gypsum in Production of Sulfate-Induced Deformation of Lime-Stabilized Soils,” Environmental & Engineering Geoscience, Vol. V, No. 2, pp. 173–187.
Dan Marks, III. B and Halliburton, T.A. (1970). “Effects of Sodium Chloride and Sodium Chloride—Lime Admixtures on Cohesive Oklahoma Soils”, Highway Research Record, No. 315, 102–111.
Davidson, D.T., Mateos, M. and Barnes, H.F. (1960). Improvement of Lime Stabilization of Montmorillonitic Clay Soils With Chemical Additives, Highway Research Board Bull. 262: 33–50.
Dermatas, D. (1995). “Ettringite-Induced Swelling in Soils: State-of-the-Art,” Applied Mechanics Review, Vol. 48, No. 10, pp. 659–673.
Hawkins, A.B. (1998). “Engineering Significance of Ground Sulphates,” Proceedings of the First International Conference on Site Characterization—ISC’98, Atlanta, GA, April.
Holm, G., Trank R. and Ekstrom, A. (1983). “Improving Lime Column Strength with Gypsum”, Proc. VIII European Conf. on SMFE, Finnish Geotechnical Soc. pp. 903–907.
Hunter, D. (1988). “Lime Induced Heave in Sulfate-bearing Clays,” ASCE Journal of Geotechnical Engineering, Vol. 114, No. 2, pp. 150–167.
Kota, P.B.V.S., Hazlett, D. and Perrin, L. (1996). “Sulfate-Bearing Soils: Problems with Calcium Based Stabilizers,” TRR 1546, Transportation Research Board, Washington, DC.
Kujala, K. (1983). “The Use of Gypsum in Deep Stabilization,” Proc. VIII European Conf. SM & FE, Organised by Finnish Geotechnical Soc. 929–932.
Kujala, K. and Nieminen (1983). “On the Reactions of Clays Stabilized with Gypsum Lime”, Proc VIII European Conf. on SMFE, Finnish Geotechnical Soc. 929–932.
Mitchell, J.K. and Dermatas, D. (1990). “Clay Soil Heave Caused by Lime Sulfate Reactions,” ASTM Special Publication No. 1135, p. 41–64.
Puppala, A.J., Dunker, C., Hanchanloet, S. and Ghanma, K. (1998). “Swell and Shrinkage Characteristics of Lime Treated Sulfate Soil,” Texas ASCE Spring Meeting Proceedings, Texas, April.
Puppala, A.J., Hanchanloet, S., Jadeja, M. and Burkart, B. (1999). “Evaluation of Sulfate Induced Heave by Mineralogical and Swell Tests,” XI Pan-American Conference on Soil Me- Chanics and Geotechnical Engineering, Foz do Iguacu, Brazil, August.