soil stabilisation with cement and aluminium hydroxide

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Soil stabilisation with cement and aluminium hydroxide

A. Assadi, R. Ayers and J. Ricks

Abstract

This paper presents the results of research carried out to stabilise a medium plastic soil in the Gold Coast area of Queensland, Australia. The main stabiliser used was Portland cement.

For a selected strength criterion of 7-day Unconfined Compressive Strength (UCS) of 1.7 MPa, a cement value of 4% gave a satisfactory California Bearing Ratio (CBR) of 160%. Aluminium hydroxide was added to the cement stabiliser and soil as a flocculating agent. The best admixture ratio was found to be 10% of the weight of the cement. With 4% cement and 10% aluminium hydroxide, the UCS values were 1.0 to 1.2 MPa. At this mixing ratio, a minimum value of shrinkage was achieved.

As aluminium hydroxide is more environmentally friendly than other flocculating agents used, it has the potential to replace other chemical agents for this purpose. Aluminium hydroxide also does not appear to be extremely sensitive to moisture content variations, which is a desirable quality for additives used in soil stabilisation.

Vol 5 No 3 September 1996 Road & Transport Research

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This paper describes the stabilisation of a road pavement material using Portland cement in conjunction with aluminium hydroxide (alum). Other additives, such as lime, hydrochloric acid and caustic soda, were also tried, and the results were compared with those of cement and alum. The pavement material was extracted from a section of a road currently managed and maintained by Queensland Transport, South Coast, Hinterland District.

Soil stabilisation is usually necessary when the available natural soil can not meet the required physical performance criteria. Usually the strength-related parameters, deflection and permeability, control the criteria and determine the method of soil stabilisation. Chemical stabilisation affects the molecular structure of the unsuitable material by improving its bonding characteristics which, in turn, improves strength, durability and volume change potential. Soil stabilisation may be achieved by three methods:

(1) Densification of soil: Densification of soil may be achieved by compaction, vibration, precompression, or drainage. A combination of these techniques may also be used.

(2) Mechanical Stabilisation: Sub-standard soil is improved by the addition of another material that only changes the physical properties of the soil.

(3) Chemical Stabilisation: The soil is mixed or grouted with chemicals, cement or lime. Both the physical and chemical properties of the soil are altered in the process.

CHEMICAL STABILISATION

Chemical stabilisation using cement, lime, or lime and fly ash is widely used to improve soils for roadwork and building construction. Cement, used by itself, increases the strength and reduces the shrinkage characteristics. Lime reduces the soil plasticity, which in turn decreases the volume change potential. Mixtures of cement or lime with chemicals

may also be used and these mixtures can improve the strength characteristics considerably; the chemical component controls the setting time. At the same time, the chemical component may lessen undesirable behaviour such as excessive shrinkage or swelling.

A discussion of the advantages and disadvantages of the various types of chemical stabilising agents is beyond the scope of this paper. A comprehensive bibliography of the chemicals used in stabilisation and grouting is given by ASCE (1966). Chemical materials have been used to improve soil or natural ground since early this century (Bowen 1975; Ingles and Metcalf 1972); and for recent techniques, see also Jewell (1994). Chemical materials were first used to grout natural ground in civil and mining engineering. Many of the chemical grout systems and materials in use earlier this century have disappeared from the market because of environmental problems; this has opened the way to the use of non-toxic materials (Brett and Osborne 1984). In Australia, many chemical materials have been used for soil stabilisation. The National Association of Australian State Road Authorities (NAASRA) has classified successful soil treatment chemicals under three categories (NAASRA 1986):

(1) Synthetic resins: Synthetic resins are organic semi-solid or solid polymeric materials.

(2) Trace chemicals waterproofers: These chemicals do not act as binders, but waterproof the soil so that it can maintain its dry strength. The large cationic (positively charged) organic molecules which are attracted to a clay's negatively charged particles are the most economical and effective agents.

(3) Experimental inorganic stabilisers: These chemicals contain acid, which reacts with calcium or other cations in a clayey soil to produce insoluble salts which behave as cement.

Soil—cement is essentially a low-cost pavement material. It is usually recommended for pavements that are designed to carry a loading of less than 1.5


Soil 2

100

90

80

70

60

50

40

30

20

10

0

Soil 1

0.01 0.1 1

10

100

Particle Size (mm)

a) a) a-

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103 ESAs over the pavement's design life (Osula 1989). In non-stabilised soil, the failure of a pavement system is often associated with cracking in the base course or in foundation material that is above the natural subgrade. One solution to this problem is to stabilise the pavement material with cement. Cement-stabilised soil increases the tensile strength and reduces the shrinkage characteristics of the material (Crockford and Little 1987).

If the Liquid Limit of a soil (50% passing 0.75 mm AS sieve) is above 40% (which is the suitability limit as explained by Osula (1989), the ability of Portland cement to act as a sole stabiliser comes into question because of the large quantity of cement that is necessary, as well as the difficulty of handling the mixture. However, if a second stabiliser is introduced into the mixture as well as Portland cement, both the economic and the strength criteria are satisfied.

For some time, alum has been widely used as an inorganic coagulating and flocculating agent. The main use of this agent is to purify water by 'pulling together' the colloidal particles of any suspended clay. It is therefore obvious that alum will react preferentially with the clay fraction in the soil, rather than with the silt or sand fractions.

LOCATION OF SAMPLES AND PROPERTIES

The soil samples used for this study were collected from a section of an unsealed road under the control

of the Queensland Transport, South Coast, Hinterland District, which has been recently upgraded by using the stabilisation process.

Testing procedures were performed in accordance with Queensland Transport Specifications, which are very similar to Australian Standards. Tests were undertaken at Queensland Transport, South Coast, Hinterland District, Materials Testing Laboratory, Stapylton and at the Soil Mechanics Laboratory, University of Southern Queensland.

The grain size distribution of the soil used in this investigation is shown in Fig. 1. The curves show two extreme bounds for the material under investigation. The Plasticity Index and other physical properties are summarised in Table 1. The X-ray diffraction analysis shows that the expansive components, labelled as Randomly Interstratified Materials (RIM), consist of illites and smectites. There are considerable quantities of expansive clay present in the specimens, which means that considerable amounts of stabiliser may be required. The mineralogical analysis of the soil is shown in Table 2. Using the information in Table 1 and the plasticity chart for the laboratory classification of fine-grained soils, the portion of the soil finer than 0.425 mm may be classified as CL. This is an inorganic clay of low to medium plasticity. Figure 1 shows that 45% of the natural soil is finer than 0.425 mm, which means that the soil would be classified as gravelly clay, sandy clay or lean clay. The strength and CBR tests that were carried out on this material show that it will exhibit poor subgrade characteristics and is thus a good candidate for soil improvement.

Figure 1 Grain size distribution curves, soils 1 and 2


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Table 1

Index and Physical Properties of Non-Stabilised Soil

Liquid Limit (%) 25.0

Plastic Limit (%) 17.4

Plasticity Index (%) 7.6

Linear Shrinkage (%) 4.8

Miniature Abrasion Loss (%) 1.8

Optimum Moisture Content (%) 10.2

Maximum Dry Density (Mg/m3) 2.00

pH 5.8

Table 2

Mineralogical Analysis of Non-Stabilised Soil

Quartz (%) 52

Feldspar (%) 18.5

Olivine (%) 3.5

RIM (%) 16

Chlorite (%) 10

ADDITIVES

Several different additives, including cement, cement and alum, hydrated lime, quick lime, hydrochloric acid and caustic soda, were investigated.

Aluminium hydroxide is variously described as alum, hydrated alumina, and alumina trihydrate. In this paper it will be referred to as alum. Alum is made in different grades, and has been used many industrial processes. However, the use of alum as an additive is primarily as a flocculating agent prior to the incorporation of cement. It is expected that the introduction of alum would improve pavement durability due to a reduction in cracking; it should also improve the waterproofing characteristics of a pavement. There appear to be no lasting deleterious side effects to humans from the continued use of alum if precautions are taken to reduce dust. Alum is not considered to be a dangerous good by the Australian Code for the Transport of Dangerous Goods by Road and Rail; however, its suppliers recommend that it should be stored away from acidic substances. This chemical is not of a combustible nature and there appear to be no significant environmental concerns associated with its use. In this investigation alum is expressed in terms of the percentage of the Portland cement used.

TESTING EVALUATION CRITERIA

Most of the materials stabilised by cement and lime are assessed on the basis of strength and durability. In this study the strength of the stabilised material was accepted as the main requirement. The strength was investigated by Unconfined Compressive Strength (UCS) and California Bearing Ratio (CBR) test results. The durability was evaluated by UCS specimens that were immersed in water for a specified period, the value of which depends on the testing standard. This method is also recommended by British standards in which it is thought that using the conventional wetting and drying tests of Portland Cement Association (PCA) are too harsh (Douglas and Osula 1989). The testing regime for cement was designed to achieve 1.7 MPa for UCS specimens in 7 days. For other additives, a smaller value of 0.45 MPa was accepted as the strength requirement. Note that the strength requirement has to be adopted by the code specifications. For sealed pavements, durability may not be a major problem and may not be assessed. In this study an unsealed pavement was investigated, and durability was therefore thought to be of some importance.

In addition to UCS, compaction tests and Plasticity Index tests were carried out, to investigate the variations in density and soil classification. Figure 2


Cement Additive

1 Trial Cement Quantities

1 Compaction Test, Atterberg Limits,

Unconfined Compressive Test

Select Cement Content

4% Cement and Alum

Trial Alum Quantities

1 Compaction Test, Atterberg Limits, Unconfined Compressive Strength

Tests and CBR

Select Alum Content

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Figure 2 Cement selection process

Figure 3 Alum content selection process

shows a typical testing regime for the stabiliser selection process. A similar regime was used to select the magnitude of the chemical additive and this is shown in Fig. 3.

Two samples from a similar area were used in this investigation (Soil 1 and Soil 2 in Fig. 1). The two samples were prepared in accordance with Queensland Transport Specifications and were then combined using a riffle box. Sample preparation involved drying the soil and then passing it over a 19.5 mm sieve to remove large rocks. The test specifications used are as follows:

(1) Atterberg Limits: Liquid Limit and Plastic Limit of samples with or without additives were determined according to Q104A-1986 and Q105-1987 respectively, to the nearest 0.2%. These tests were carried out on the portion of the samples passing 0.425 mm AS sieve. A cone penetrometer was used for the determination of Liquid Limits.

(2) Linear Shrinkage: The Linear Shrinkage of specimens was carried out on material passing a 0.425 mm AS sieve according to Q106-1989, which is based on the decrease in length of a soil bar when it is dried from the Liquid Limit to the oven-dry state.

(3) Miniature abrasion loss: The Miniature Abrasion Loss of samples was determined according to Q107-1986. The specimens were made from a portion of a soil passing 2.36 mm AS sieve. These tests were carried out to verify Linear Shrinkage results. The numerical values are available but are not reported here.

(4) Moisture - dry density relationships: Moisture—density relationships were determined according to Q110A-1993 and Q110E-1993. The diameter of a typical cylindrical sample was 100 mm.

(5) Unconfined Compressive tests: The Unconfined Compressive Strength after a 7-day curing period was used to evaluate potential increases in strength for both additive-free and stabilised soils. The samples were prepared in compaction moulds with the same Optimum Moisture Content obtained in the compaction tests. Note that the height of a typical UCS sample was not twice the diameter, as recommended by the standard codes. Therefore, the strength results are only representative of UCS.

(6) California Bearing Ratio: California Bearing Ratio tests were carried out according to Q113A-1993 and Q113C-1993. The mould used was a cylinder of 150 mm diameter.

RESULTS AND DISCUSSION

Selected results of the tests are shown in Figs 4 to 9. The Atterberg Limits were carried out on all materials with the exception of the alkaline combination. The results are presented in Fig. 4 in which the Liquid Limit and Plasticity Index of the additive-free and stabilised soils are plotted in the


20 —

Natural Soil

Hydrochloric Acid \ Lime Cement

• \ •

• Cement, Alum

0 5 10 15 20 25 30 35 40

Liquid Limit (%)

A-Line

10%

5%

4 )̀/0 2% 3%

5%

10%

1.85 — Lime 5%

Zero Air Voids (soil)

2.1

Natural Soil

1.8

Cement 4%

■ 1.9 — Cement 4%,

Alum 10%

2.05 — E -(3) 2 2.0

a) 1.95 —

0 N+N

8 9 10 11 12 13 14 15 16

Moisture Content (%)

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Figure 4 Presentation of Atterberg Limits in the Unified Classification Chart

Figure 5 Linear Shrinkage of additive- free and stabilised soils

Figure 6 Dry Density—Moisture Content Relationship


4%

3% 5% 10% 20%

2%

5%

2% 3°/`)

Figure 7 Unconfined compressive strength (7 days)

160

140

120

100

80 co

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Soil Cement Cement Lime Hydrochloric Caustic 4% 4% Acid Soda

Alum 10%

2% 5%

10% 10% 20%

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Figure 8 CBR values of additive- free and stabilised soils


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Figure 9 CBR tests for additive free and stabilised soils

Unified Classification Chart. It is seen that the addition of cement, alum or lime has resulted in a minor movement of the material to the right side of the A-Line. This area includes low-plasticity inorganic and organic silts.

Variation in Linear Shrinkage with the type of the stabiliser is shown in Fig. 5. Linear Shrinkage varied from a low of 2.2 for the cement/alum combination to a high of 4.8 for the natural soil. A wide range of Linear Shrinkage results was obtained for different quantities of cement and lime. The minimum value of Linear Shrinkage was achieved when a combination of 4% cement and 10% alum was used. Note that the weight of alum in terms of the weight of soil is 0.04 x 0.10 = 0.004 or 0.4%.

Figure 6 shows dry density—moisture content relationships for natural soil and three selected stabilisers. The Optimum Moisture Content increases slightly as expected with cement stabiliser, causing a slight reduction in the dry density. Overall, the Optimum Moisture Content and corresponding dry density of natural soil and soil with additives, as shown in Fig. 6, have values ideal for many soil structures.

The strength, as determined by UCS tests, varied from 0.2 MPa for natural soil or 2% lime stabilisation, to 1.7 MPa for 4% cement stabilisation as shown in Fig. 7. The strength for various cement/alum combinations varied between 1.0 MPa and 1.2 Mpa.

Thus, the introduction of alum to 4% cement samples has resulted in a 30% reduction in the 7-day UCS. However, this value is still 6 times the UCS of the natural soil.

The CBR test results are shown in Fig. 8. These values correspond to either CBR or CBR (the smaller value). They also correspon2c1 to the minimum values obtained from soaked or unsoaked samples. Typical load-penetration behaviours for natural soil and selected stabilisers are shown in Fig. 9. As may have been anticipated, the cement and cement/alum combinations gave the larger CBR values.

When all the results were reviewed, and particularly those results for Linear Shrinkage and UCS, a suitable additive incorporation rate was obtained. For cement, the most desirable additive quantity was found to be 4% by mass. The most suitable cement/alum combination was determined to be 10% alum by mass of 4% cement by mass of soil. Finally, 5% lime by mass of soil was found to be the most effective additive content.

CONCLUSIONS

The following conclusions were drawn from the experimental results:

(1) For an unsealed pavement using sandy or silty clay of the Gold Coast area in Australia, with a unified classification of CL, a stabiliser has to be


Soil stabilisation

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introduced to increase the CBR or UCS to the required values of the corresponding specification. Note that the magnitude of required CBR in treated soil may be as high as 180% depending on the specification.

(2) Using the conventional minimum 1.7 MPa UCS, it was found that 4% cement is a suitable additive for effective soil stabilisation. Using this amount of cement produces a CBR value of 160% to 180%.

(3) The results show that the suitable value for alum is 10% of the cement. The authors recommend a minimum of 1.4 MPa for UCS and a minimum of 90% for CBR.

(4) A suitable additive value for lime may be accepted as 5%. However, this will not satisfy the strength criteria quoted in (3) above so, an additional cement value of 2% is recommended.

(5) A chemical stabiliser such as alum may be used in medium and highly plastic soils. In low-plastic soil it may decrease the strength. Improvements in shrinkage and permeability have still to be investigated thoroughly.

REFERENCES

AMERICAN SOCIETY OF CIVIL ENGINEERS. (1966). Revised bibliography on chemical grouting. Journal of Soil Mechanics and Foundation Engineering, 92(SM 6), pp.39-66.

BOWEN, R. (1975). Grouting in Engineering Practice. (Applied Science Publishers: London).

BRETT, D.M. and OSBORNE, T.R. (1984). Chemical grouting of dam foundations in residual laterite soils of the Darling Range, Western Australia. Proceedings Fourth ANZ Conference on Geomechanics, Perth, pp.177-187. (Institution of Engineers, Australia: Canberra).

CROCKFORD, W.W. and LITTLE, D.N. (1987). Tensile fracture and fatigue of cement stabilized soil. Journal of Transportation Engineering, 113(5), pp.520-537.

INGLES, O.G. and METCALF, J.B. (1972). Soil Stabilisation Principles and Practice. (Butterworths: Sydney).

JEWELL, R.J. (1994). Ground Improvement Techniques. (Australian Centre for Geomechanics: Perth).

NATIONAL ASSOCIATION OF AUSTRALIAN STATE ROAD AUTHORITIES. (1986). Guide to Stabilisation in Roadworks. (NAASRA: Sydney).

OSULA, D.O.A. (1989). Evaluation of admixture stabilization for problem laterite. Journal of Transportation Engineering, 115(6), pp.674-687.

RECOMMENDATIONS

Trials involving cement/alum combinations should be undertaken on medium to heavy clays such as montutorillonite and illite and the results of these trials should be compared with trials involving cement/lime combinations. As alum is a flocculating agent like lime, the use of alum may prove to be an economically and environmentally superior additive.•


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Ali Assadi

is the Geotechnical Strand Leader in the Faculty of Engineering and Surveying at the University of Southern Queensland (USQ). He holds the degrees of Master of Science, and Doctor of Philosophy (London University, U.K.) in Geomechanics. Ali has been at USQ since Feb. 1992 teaching in the area of Soil Mechanics and Geotechnical Engineering. Before joining the USQ Ali worked at the University of Tabriz, Eastern Mediterranean University (Cyprus) and The University of Newcastle (N.S.W).

Ron Ayers

is the Dean of the Faculty of Engineering and Surveying at the University of Southern Queensland. He holds the degrees of Bachelor of Science, Bachelor of Engineering (Honours) and Master of Engineering from the University of New South Wales. Ron has been at the University of Southern Queensland for 20 years, lecturing mainly in the area of transport engineering and holding the positions of Head of Civil Engineering (1978-1988) and Associate Dean (1988-1994). Before joining the USQ, Ron worked at Campbelltown City Council, N.S.W. as Construction Engineer and for the Department of Main Roads, N.S.W.

Jason Ricks

graduated from the Faculty of Engineering and Surveying (USQ) in 1995. Currently he is working in the Department of Main Roads in Queensland.

Contact

Dr A. Assadi Faculty of Engineering and Surveying The University of Southern Queensland Toowoomba QLD 4350 Phone (076) 31 1721 Fax (076) 31 2526

Acknowledgements

The authors wish to thank Queensland Transport for their assistance in the physical tests. The assistance of Mr Peter Atkinson is particularly appreciated.


soil stabilisation with cement and aluminium hydroxide

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