In this article we will discuss about how to stabilise soil for road construction. Learn about:- 1. Mechanical Stabilization 2. Stabilisation with Additives.
1. Mechanical Stabilization:
Rearrangement of the soil grains and densification by compaction is the first approach.
The other approach consists of blending two or more available soils to obtain the desired gradation of soil particles for the maximum or a specified density. The appropriate amount of fine soil fraction (or the fraction passing 75- μ I.S. sieve), called the ‘binder’, is an important component in the design of a mechanically stabilised mixture.
[Note – If the primary aim is to reduce the permeability of a soil, a small quantity, say up to 10%, of beutonite clay can be added; even locally available clay may be used for this purpose.]
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For a high value of density and grading to ensure grain-to-grain contact, the grain size distribution should correspond to Fuller’s curve given by the equation-
P = 100 (d/D)n
Where, P = percent passing of sieve
d = aperture size of the particular sieve
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D = the maximum size of the aggregate
n = exponential (ranges from 0.3 to 0.5)
Fuller’s curve is a graph plotted between the percentage of soil passing a certain sieve size, P, and the aperture size, d, of the particular sieve.
For spherical-shaped particles, n is taken as 0.5. For angular-shaped particles, n can be from 0.30 to 0.35 based on the angularity number. (Aggregates with angularity number less than 4 are not considered to be suitable for soil-aggregate mixes for roadwork.)
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Sand-clay mixers, predominantly consisting of sand-silt mixture, as also sand-gravel mixtures with laterite gravel, called ‘moorum’, are considered to be suitable for sub-bases and bases when they are well-designed. They may serve as surfacing for low-cost roads with very small traffic volume.
Graphical methods are also used for determining the proportions in which available soil/ aggregate materials are to be blended to derive a mix of desired gradation for maximum or a high density.
Two of these are given below:
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This is convenient when three materials of different gradations, consisting of fractions—gravel, sand and silt-clay-are available. They are represented on a triangular chart on each side of an equilateral triangle representing the percentages of the three fractions (0 to 100%, Fig. 8.4): Let the three materials available be represented on the triangular chart by points A, B and C, based on their constituent fractions-sand, gravel and silt-clay.
Let D represent the gradation desired of the material to be obtained by blending the materials A, B and C.
Join C and D and extrapolate the line to meet the line AB at E.
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Now the required proportions of A, B and C can be calculated as follows –
These are obtained as percentages by multiplying by 100.
Rothfuch‘s Graphical Method:
This is shown in Fig. 8.5.
Let A, B and C be the grain-size distribution curves of the three materials to be blended. Let the desired gradation line be shown on this. Each of the curves A, B and C are balanced by straight lines as shown.
The opposite ends of the balancing lines of A and B (the zero percent passing point of one with the 100% passing point of the other) and of B and C are joined as shown. The intersection points of these lines with the desired gradation line demarcates the proportions of A, B and C as shown in the figure.
Apart from the graphical methods, trial and error method is also used; further, in some relatively simple cases, observation coupled with simple calculations may also yield the proportions.
Typical MOST specifications for sub-base/base courses are given in Table 8.3.
Two typical IRC specifications for the construction of stabilised soil roads – one with soft aggregates and the other with low grade aggregates and soil-aggregate mixtures are given here. (Only the salient features are given; for complete information, the relevant IRC-code has to be referred.).
Scope:
Rainfall – Not exceeding 150 cm per year.
Depth of subsoil water level – More than 2 m below ground level.
Thickness of the Pavement:
Thickness shall be the same as found satisfactory for a flexible pavement in the area for the traffic required. Thickness of base coat will be about 9 cm, and the remaining thickness will be built with base course specification. It is presumed that while constructing the embankment, the top 30 to 45 cm of local soil is compacted at controlled moisture, depending upon the importance of the road.
For Traffic Intensity of 50t/day (Unsurfaced Road):
Base Course:
Soil with plastic index 4 to 7 and sand content (between 425 μm and 75 μm I.S. sieves) not less than 50% shall be laid at optimum moisture and compacted with 6-8 ton power roller to a minimum of 95% of the maximum value obtained in the laboratory Proctor compaction test. (Sodium sulphate content shall not exceed 0.15% by weight of dry soil).
Wearing Course:
Two parts of soil with plasticity index 9 to 11 and sand content not less than 33% shall be mixed with one part by volume of brick aggregate, kankar, moorum or laterite, except that 10% of the aggregate shall be saved and spread on the layer of soil-aggregate mixture, before rolling.
The size of the aggregate shall be such that all of it passes through 31.5 mm sieve and not more than 20% passes 6.3 mm sieve. The aggregate impact value shall not be more than 50%. The soil-aggregate mixture shall be brought to optimum moisture and rolled with 6-8 ton power roller.
For Traffic Intensity of 200t/day (Stabilised Soil Road with Bituminous Surface Treatment):
Base course – Same as given above for 50t/day.
Base Coat:
Two parts of soil with plasticity index 8 to 10 and sand content not less than 33% shall be mixed with one part by volume of brick aggregate, kankar, moorum or laterite, except that 10% of the aggregate shall be saved and spread on the soil-aggregate mixture, before rolling.
(The comments on the size of the aggregate, the aggregate impact value and rolling given for un-surfaced roads are applicable here also.)
Surface Dressing:
The base coat surface shall be left to controlled traffic for about 10 to 14 days and allowed to dry down to a moisture content of 4 to 6%. It shall then be sprayed with bituminous primer (a mixture of bitumen 80/100 and furnace oil in the ratio 3:7), or Cutback SC-0 as per IS: 217-1961 at the rate of 10 kg per 10 m2. After the primer has soaked in, it shall be finished with a two-coat surface dressing or with premix carpet and seal coat, whichever is preferred.
For Traffic Intensity of 500 t/day [Surface Treated Road Having Stone Grafted Base Coat]:
Base course – Same as given above for 50 t/ day.
Base Coat:
Seven parts of soil with plastic index 8 to 10 and sand content not less than 33% shall be mixed with three parts by volume of brick aggregate, kankar, moorum or laterite. The size of the aggregate shall be such that all of it passes through 31.5 mm IS sieve and not more than 20% passes 6.3 mm IS sieve. The aggregate impact value shall not be more than 50%.
The mixture of soil and aggregate shall be brought to optimum moisture and then covered completely with 25 mm size stone metal or over-burnt brick aggregate at the rate of 0.20 to 0.23 m3 per 10 m2. The aggregate impact value of the stone or over-burnt brick used for grafting shall not be more than 25%. Compaction shall then be carried out with a 6- to 8-ton power roller as per the standard procedure.
Surface Dressing:
This shall consist of two-coat surface dressing or premix carpet, whichever is preferred. If premix carpet is used, care shall be taken to see that the cutback primer covers the entire surface to resist penetration of water. The grit used for surfacing shall have an aggregate impact value of not more than 25% and stripping value 15 to 20 as determined in accordance with IS: 6241.
Rainfall – Exceeding 150 cm per year.
Depth of sub-soil water – Within 2 m from ground level.
Traffic intensity – About 500 t/day average of mixed traffic.
This specification consists of subgrade, sub-base, base course, stone grafted base coat and surface treatment.
The exact thickness of the pavement shall be worked out from the soaked CBR of the subgrade and that of the various layers of the road pavement and from the soaked CBR of soil stabilised with binders such as cement or lime. The thickness of the base coat shall be about 10 cm and the remaining thickness shall consist of base course and sub-base. Loose soil layer more than 22 cm thick shall not be compacted.
Subgrade – The surface shall be rolled with power roller 6-8 ton capacity and brought to a camber of 1 in 48.
Sub-Base:
Local soil, or after mixing requisite quantity of binder, lime or cement, if required, is compacted at optimum moisture content (OMC) using 6-8 ton power roller to a minimum of 90% of laboratory Proctor density. This shall be cured for about a week by sprinkling water over it, 3 to 4 times a day. Base-course, base coat and surface dressing are similar as in Specification-I, expect for the addition of binder, lime or cement. Proper curing shall be done in all cases before allowing traffic.
Low-grade aggregates are those which lose more than 15% of their strength upon wetting, in terms of their aggregate impact value. They can be used as such if their wet aggregate impact value does not exceed 50%. If this value exceeds 50%, they would need to be processed before being used. This could be achieved either through stabilisation in accordance with IRC: 28-1967 (1983), or using the aggregates in a soil-aggregate mixture according to a specified gradation, as given later in this specification.
Since the important characteristic of low-grade aggregates is loss of mechanical Strength upon wetting, such aggregates should invariably be tested in the soaked condition, as per IS: 5640-1970, “Method of test for Determining Aggregate Impact Value of Soft coarse aggregates.”
Common types of Low-Grade Aggregates:
The following low-grade aggregates are normally found in India:
(i) Laterite
(ii) Kankar
(iii) Shale
(iv) Moorum
(v) Soft gravel
(vi) Brick aggregate
(vii) Soft stone.
Testing of Low-grade Aggregates and Soil-aggregate Mixtures:
Some of the appropriate tests are:
1. Aggregate impact value test
2. Sodium sulphate soundness test
3. CBR on samples soaked for 4 days (only for moorum)
For Soil-Aggregate Mixtures:
Gradation test
Liquid limit and plasticity index of soil fraction
CBR on sample soaked for 4 days.
Criteria for Use of Low-grade Aggregates:
Low-grade aggregates can be used for sub-base or base courses of road pavements, or even sometimes as surfacing material. The suitability of aggregates should be based on the wet aggregate impact value, except for materials like moorum.
The recommended limits are given below:
Sub-base – Max. 50%
Base course with bituminous surfacing – Max. 40%
Surface course – Max. 30%
In the case of materials like moorum, suitability should be judged based on the soaked CBR- values. This value should not be less than 20, whether moorum is used as sub-base for high class roads, or a surfacing for lightly trafficked roads. Where this is not satisfied, or where a still higher strength is desired, this could be achieved through cement or lime stabilisation.
Gradation:
Low-grade aggregates should be reasonably well-graded to achieve a dense and well interlocked mass. Any grading for such low-grade aggregates should be taken only as a guidance value, since such aggregates are of a crushable nature.
Criteria for Use of Soil-Aggregates Mixtures:
Soil-aggregates mixtures may be in the form of naturally occurring materials like soil-gravel, or soil blended with suitable aggregate fractions. The primary criteria for acceptability are plasticity index and gradation. Plasticity index should be less than 6 when used as sub-base or base course with bituminous surfacing, and between 6 and 9 when used as a surfacing for lightly trafficked roads.
Criteria for Gradation:
The criteria should be as recommended to achieve maximum possible dry density. A few typical gradings are given in Table 8.4 for general application. The first three gradings are especially suited for base courses whereas the remaining two are suitable for both base course and surfacing.
Apart from Pl-value and gradation, soil aggregate mixtures may also be evaluated on the basis of soaked CBR-value. In this case, CBR should not be less than 20 for use as sub-base. For base courses, the acceptable value for heavy traffic is normally 80, but a somewhat lower value could be permitted for light traffic volume roads.
Low-grade aggregate courses should be laid on well compacted subgrade and should be kept well drained.
2. Stabilisation with Additives:
A soil can be stabilised by using additives, such as modifiers, which modify undesirable properties like swelling, e.g., lime; cementing agents such as Portland cement, pozzolonic fly-ash and lime fly-ash, which act as binders and improve the strength properties; water-proofing agents such as bituminous materials, which improve the stability by preventing the entry of water; water- retaining agents that improve the stability of compacted cohesion-less soils, e.g., calcium chloride; and, certain chemicals which, in small quantities, improve the desirable properties of soils.
Of these, the following methods are considered particularly useful for the construction of low-cost roads:
(i) Lime Stabilisation – Soil-Lime:
Lime, when added to soil, acts both as a modifier for clay soil in reducing its plasticity and swelling characteristics, and as a binder in increasing its strength. Clay soils with the fraction passing 425 μm IS sieve not less than 15% along with a minimum of 10% clay content, and a plasticity index of at least 10 are very much suited to lime stabilisation.
When a clay soil is treated with lime, various chemical reactions take place, resulting in Base Exchange (or ion exchange), flocculation of clay particles, carbonation, and cementing action. There are several factors that affect the properties of soil-lime. These are – soil type, type of lime, lime content, curing and additives.
An important factor is the lime content – there is an optimum lime content for a given soil and moisture content beyond which further strength gain does not accrue.
Curing increases the strength at a significant rate, initially, and at a slow rate for a long time. Additives like fly-ash and surkhi (broken brick) help in increasing the strength of soil-lime. Well-designed lime-soil mixes are considered suitable as sub-bases for superior pavements and as bases for low traffic-volume roads. However, they are unsuitable for surfacing in view of their poor resistance to abrasion and impact.
The relevant IRC specification for soil-lime mixes is “IRC: 51-1973-First Revision (1992) – Guidelines for the Use of Soil-lime mixes in Road Construction.”
Salient Features of IRC: 51 – 1973-First Revision (1992):
Introduction:
Addition of lime to soils is known to improve soil strength and reduce plasticity and volume change. Thus, it improves the quality of the subgrade and produces a high-strength material for sub-bases.
In general, clayey gravels, silty clays and clays – soils belonging to I.S. classification groups, CH, CL, MH, ML, CL-ML, SC, SM, GC, and GM – are most reactive to lime and are suited to lime stabilisation.
Mechanical method of construction yields the best results owing to homogeneous mixing. In the manual method, care shall be taken to ensure proper pulverisation and uniform mixing of lime.
Scope:
Soil-lime mixes are intended for use as improved subgrade or as a sub-base in highway pavements.
Materials:
Properties of lime-soil mixes are dependent on many variables – soil type, lime type, lime percentage, fineness of lime and lime purity in addition to temperature and moisture.
Soil:
At least three soil samples shall be taken for a stretch of one kilometre or when the soil type changes. This is to check the suitability of the soil for lime treatment.
For effective stabilisation, a soil must have a fraction passing 425 μ I.S. sieve not less than 15% and its P.I. should be at least 10%. The fraction retained on 425 μ sieve should be well graded with U not less than 5. Besides, clay minerals should belong to kaolinite, montmorillonite or illite group. Organic matter should not be more than 2% and sulphate content not more than 0.2%. A pH-value of 10 or 11 is desirable.
Lime:
Calcite dry lime is suitable, but not dolomitic lime. Purity, expressed as a percentage of calcium oxide should not be less than 50.
Fineness requirement should be as for class C hydrated lime as specified in IS: 1514 and IS: 712 for effective mixing.
Design Considerations:
Lime Requirement:
With small doses of lime, workability increases due to Base Exchange and/or flocculation, but no appreciable gain of strength results. As the dose of lime is increased, the strength increases. The quantity of lime required to satisfy the affinity of soil for lime without appreciable gain of strength is termed lime fixation point or lime retention point. This is usually 1 to 3%.
As the lime content is increased beyond this point, strength gain occurs, but only up to a certain limit, beyond which the strength may decrease to some extent. This lime content for maximum strength is the optimum value. Some tentative values of this optimum lime content are 4% for kaolinitic soil, 8% for illitic soil, and 10% for montmorillonitic soil.
The optimum value may be got by the pH-method, in which the pH of the lime-soil is tested to ensure that it is 12.4. The other approach is the moisture absorption method, in which the optimum value of lime content at which the soil attains steady moisture absorption is determined.
Strength:
The strength is determined either from CBR or unconfined compression strength, the latter being considered less arbitrary. Either test is done at an age of seven days with three days moist curing followed by four days of immersion.
The following have been specified as guideline criteria for the strength of soil-lime:
CBR – Minimum value for sub-base should be 15% for low-trafficked rural roads, 20% for cumulative traffic up to 2 msa, and 30% for traffic exceeding 2 msa.
UCS – For sub-base it should be at least 700 KN/m2.
Pulverisation – Degree of pulverisation should be high for effective stabilisation. Durability test of wetting and drying and checking strength loss are recommended.
Loss of lime on leaching should be between 5% and 10%. To counter the effect of leaching, a little extra lime content, say up to 4%, is added compared to that required for strength gain.
Pulverisation and Mixing:
Mechanical method is preferred for full benefit of stabilisation. The compacted thickness of soil-lime layer shall be in the range of 75-200 mm, depending on the mixing and laying equipment.
Mechanical methods may be mix-in-place or stationary plant.
In the mix-in-place method, the following operations are involved:
(i) Pulverisation of soil
(ii) Spreading line
(iii) Mixing
(iv) Addition of water
(v) Final grading
(vi) Compaction
(vii) Curing
In the stationary plant method, there are two types: continuous type and batch mix type, the latter being more convenient for small jobs.
The sequence of operations in the plant-mix method is:
(i) Site preparation
(ii) Collection and pulverisation
(iii) Mixing
(iv) Transporting and spreading
(vi) Curing
Addition of Lime:
Lime may be added either in slurry form or in dry state. The moisture content should not exceed the OMC by more than 2%.
Time between Mixing and Compaction:
The time gap between mixing and compaction should not exceed three to four hours so that the strength is not reduced.
Rolling:
Compaction should be done with an 8-10 tonne smooth wheel roller or vibratory roller. Rolling has to proceed from the edges towards the centre. Grade and camber should be frequently checked.
Curing:
Normal curing period varies from 7 to 28 days at normal temperature under wet conditions. A minimum of 7 days curing should be done under moist conditions.
Quality Control:
Depth of treatment and uniformity of mixing may be checked by spraying phenolphthalein alcohol indicator solution. Reddish pink colour indicates the presence of lime; non-uniformity of colour indicates inadequate mixing. Field values of CBR should be at least 60% of laboratory values.
Purity of lime should be checked in accordance with IS: 1514 – 1959 or IS: 712 – 1964.
Degree of pulverisation should also be checked regularly.
Dry density after compaction shall be checked for every 500 sq. m. area.
Layer thickness and longitudinal profile should be checked regularly. Tolerances are ± 25 mm for subgrade and ± 20 mm for sub-base.
Surface regularity shall also be checked regularly. Maximum permissible undulations are 24 mm for subgrade, 15 mm for sub-base, and cross-profile tolerances are 15 mm and 12 mm, respectively.
Limitations:
The physical factors limiting the use of soils for stabilisation are:
(i) It must be possible to break the soil in order to mix in a stabiliser.
(ii) The soil should have an adequately stable grading.
(iii) The necessary clay content and minerals are there in the soil for the reactions to proceed-15% clay with the PI of at least 10%.
(iv) Air temperature in the shade is less than 10°C.
(v) Lime stabilisation should not be carried out during rains.
Cement Stabilisation – Soil-Cement and Cement-Modified Soil:
Soil-Cement:
Ordinary Portland cement, when added to soil and compacted in the presence of moisture, results in a mix, known as soil-cement. This process of cement stabilisation produces a mix suitable for sub-base or base course for all type of pavements. The advantage is that almost all types of soil are amenable to this stabilisation technique.
In the case of granular soils, the products of hydration of cement or cementitious materials develop a bond between the soil grains. In the case of fine-grained soils, the secondary products of hydration formed by the reaction of free lime in the cement paste and the clay mineral particles cause base exchange and flocculation, thereby reducing plasticity. This is in addition to the cementitious bond provided by the primary products of hydration. (The Base Exchange and flocculation processes are just the same as in the case of lime stabilisation).
Soil-cement is a scientifically designed material, cement itself being a material of standard quality. In view of the flexural strength and load distribution capacity, it makes an excellent base for high type of pavements (In view of poor resistance to impact and abrasion, it is not preferred as a surface course; instead, a bituminous surfacing is provided as a wearing course). It is considered as a kind of semi-rigid course.
The strength of soil-cement increases with age and curing. Soil-cement also has good durability. However, the cost is higher than that of other methods such as lime stabilisation, and there is need for good quality control in construction.
Factors Affecting the Properties of Soil-Cement:
The following are the important factors which affect the properties of soil-cement:
(1) Cement Content:
This is the most important factor which affects the strength of soil- cement. It depends on the soil type and the desired strength. In general, the strength increases with increase in cement content. But, beyond a limit, economy is lost and the principle of stabilisation loses its meaning. As a guideline, a cement content of 4 to 14% by weight of the dry soil is adequate.
The criterion is the unconfined compression strength of cylindrical specimens (height to diameter ratio 2:1) after moist-curing for 7 days. A value of 1.7 MN/ m2 (17 kg/cm2) was considered adequate to serve as a base course for light-to- medium traffic. However, the recent specification is that it should be 2.8 MN/m2; this can serve as a base course even for relatively high traffic.
The design of the soil-cement mix is a trial-and-error process. Trial mixes with different cement contents are prepared and moulded into cylindrical specimens. After a 7-day curing period, they are tested for compressive strength, in a machine. A graph is plotted between cement content and compressive strength, from which the appropriate cement content for the desired strength can be chosen.
In the case of PCA design approach, durability test with cycles of wetting-drying and freezing-thawing is conducted.
Loss in weight due to brushing the surface is specified so as not to exceed a pre-decided value.
Even CBR-value may be used for specifying soil-cement mixes. For bases, 80-100 is desirable, though 20-30 is adequate for sub-bases.
(2) Soil:
The type of soil has great influence on the extent of cement stabilisation. Organic matter should not be more than 2%. Sulphate content has deleterious effects on cement; so it should not exceed 0.25% for clayey soils and 1% for granular soils. Well-graded soils need less cement content to achieve a certain specified strength.
Appropriate gradings have been recommended by relevant authorities based on experience for achieving success in cement stabilisation.
(3) Water Content:
The importance of water in hydration of cement is well known. Similar to soil compaction, there is an optimum moisture content at which the soil cement mix attains the maximum dry density. Generally, maximum compressive strength is obtained when compacted at lower moisture content than required for maximum density.
(4) Pulverisation and Mixing:
Lumps inhibit effective stabilisation; so pulverisation of soil, especially clay, must be done before mixing. It is usually specified that, after pulverisation, 100% must pass 25 mm sieve and 80% must pass 4.75 mm sieve.
Uniform mixing of cement and soil is necessary for best results.
(5) Compaction:
Compaction shall be done as early as possible since hydration set in soon after mixing is completed. It is stipulated that compaction should be done within two hours of mixing, to avoid loss of strength.
(6) Additives:
Small quantities of additives like lime, and chemicals such as sodium carbonate and calcium chloride will make the cement stabilisation much more effective.
(7) Curing:
A seven-day moist curing is the minimum required for soil cement, although strength gain continues for a long period.
Cement-Modified Soil:
When the cement content added is in small quantities only to improve the strength properties or to modify the undesirable properties of the soil, the mix is called cement-modified soil. Such a mix can be used as sub-base. The design, in this case, is based on CBR-value of the mix. A value of 20-30 in the field is considered adequate for use as a sub-base.
The relevant IRC specification is “IRC: 50-1973: Recommended design criteria for the use of cement-modified soil in road construction.”
The salient features of this specification are given below:
Salient Features of “IRC: 50-1973”:
Scope:
The extent to which soil properties get modified by cement action depends on the concentration of cement. The addition of small quantities of cement results in limited improvement of a soil with considerable advantage. A soil processed thus is known as ‘cement-modified soil.’
It has been found that the addition of even 2 to 3% of cement to a soil could improve the strength sufficiently for use as a road sub-base.
An example (Table 8.5) shows the results of CBR-values of an untreated soil and soil treated with cement quantities ranging from 1 to 4%:
Design Considerations:
Soil Type:
Organic matter should be preferably absent.
The criteria for checking the suitability of soil are:
(i) Plasticity modulus – The product of PI of soil and percent fraction passing 425-μ sieve should not exceed 250.
(ii) Uniformity coefficient of soil should be greater than 5, and should be preferably greater than 10.
Soils not suitable for cement stabilisation are:
(i) Heavy clays including black cotton soil with PI > 30.
(ii) Soils with organic content greater than 2%.
(iii) Highly micaceous soils.
(iv) Soils with soluble sulphate or carbonate concentration greater than 0.2%.
Concentration of Cement:
This will depend upon the type of soil, design requirements, etc. Because of difficulty of uniform mixing even 2% may be sufficient for hand mixing (percent by weight of dry soil).
Degree of Pulverisation:
The soil must be well pulverised before mixing cement; this is necessary for effective stabilisation. The degree of pulverisation should be such that 80% shall pass through 4.75-μ sieve and 100% through 25 mm sieve.
Strength Criteria:
Soaked CBR-value is the strength criterion. Field CBR should be taken only as 60% of laboratory value. CBR must be checked after 3-day curing and 4-day soaking. At least 3 specimens shall be tested for cement content.
The design mix shall be chosen on the basis of the criteria laid down and the test results obtained as above.
The addition of a bituminous material to a soil improves its properties significantly – if the soil is granular, bitumen coats the grains and binds them together, supplying cohesion. If the soil is cohesive, bitumen being a water proofing agent, prevents the entry of water and avoids the adverse effects of water on cohesive soil. In addition, spraying of a bituminous binder on the dry surface of a low-cost road such as earth/gravel road prevents dust in summer and the entry of water in the rainy season.
For achieving these goals, it is important to select the correct type and the appropriate quantity of the bituminous binder and to grade the particles of the soil to be stabilised suitably. The variations in the bitumen stabilisation are – sand-bitumen, soil-bitumen, soil-aggregate- bitumen, and spraying bitumen on earth/gravel road.
Desert regions abound in sand, with very meagre supplies of gravel and stone aggregates within easy reach and economical leads. Examples of such areas are the arid zones of Rajasthan and coastal plains where beach sand is abundant. In such regions, a convenient specification is sand-bitumen. Based on the experience of engineers in its successful use, IRC has come up with a recommended practice for sand-bitumen base courses [IRC: 55-1974].
A typical gradation of wind-blown dune sand from the desert region of Rajasthan, which has been successfully stabilised with bitumen, is given below:
Typical gradation of Rajasthan sand for bitumen-stabilisation-
Binder:
The types of binder used are:
Penetration grade bitumen – 30/40 or 80/100 can be used if the sand is preheated (135°-165°C).
Road Tar – RT-3 is also used.
Cold-application cut-back – MC-1, MC-2, RC-1, RC-2, SC-1, SC-2 have been used. RC-3 has been used in India.
Emulsions – Ideal for dry conditions in desert regions. However, their use under wet conditions is difficult.
Quantity of binder – This is selected after carrying out stability tests with different binder contents. The optimum binder content at which the stability is the maximum is determined. 4 to 10% by weight of total mix is considered satisfactory.
Course aggregate – Addition of about 30% by weight of hard stone aggregate, if available, will improve the quality of the sand bitumen mixture, especially if the sand is not well-graded.
Salient Features of IRC: 55-1974-Recommended Practice for Sand-bitumen Base Courses:
Scope:
The practice is mainly intended for desert areas such as in Rajasthan. A 10 cm-thick base course with a wearing course of 20 mm-thick premix carpet has been found to be satisfactory for normal traffic up to about 150 vehicles per day (exceeding 3t weight) and occasional heavy traffic.
Design Criteria:
The sand-bitumen mix should be properly designed based on the Hubbard Field Stability Method with the following requirements:
Sand grading – Typical Rajasthan desert sand with not more than 10% passing 75- μ I.S. Sieve.
Coarse aggregates – May be incorporated up to 25% by weight. The whole passing through 4.75 mm I.S. Sieve.
Binder – Suitable penetration grade or cut-back bitumen.
Proportion of bitumen – Shall be designed so as to satisfy the requirements given above for stability values.
Edge key – Grooves 8 cm wide and 5 cm deep shall be dug at the edges on either side to give a thickened edge, for giving additional strength at the edge.
Heating of the materials – Normal temperature foe paving grade bitumen is 150°-165° C and for cutbacks, it is 80°-120° C. Sand (and aggregate, if any) shall be dry and heated to 135°-163° C if penetration grade bitumen is used.
Mixing – Mixing is done in a twin-shaft paddle-type mixer for 1 to 2 minutes and laid immediately thereafter.
Laying – The mix shall be laid directly over the subgrade to a loose thickness of 15 cm so as to be compacted to a 10 cm-thickness. The material shall be laid uniformly and rolled.
Curing – For cut-back bitumen, the curing period is 1 to 7 days, or until such time when initial rolling could be started.
Rolling – When penetration grade bitumen is used, rolling should start immediately after laying; but when cut-backs are used, rolling shall be done after at least 24 hrs of laying.