B. R. Malhotra (Dr.), Scientist and K. A. John, Scientist
Central Road Research Institute, New Delhi

ABSTRACT

The paper describes the use of mechanical equipment in the construction of four stretches of lime stabilised roads extending over a length of twenty kilometers. The roads, catering to rural needs, were constructed in black cotton soil areas of the Akola-Amraoti districts in Maharashtra and their service behaviour was observed over a period of seven years. During the construction, disc harrows of two types for pulverizing the B. C. soil, rotavator for mixing lime and soil, tanker for sprinkling water for subsequent mixing, were employed in lieu of more manual effort. A comprehensive assessment of operations like breaking of clods and pulverizing of soil, mixing of lime and soil, addition of water, etc, shows that use of appropriate mechanical contrivances is not only more conducive to achieving better qualitative results but also helps in economizing on the cost of construction to some extent. Besides, the satisfactory performance and service behaviour of these roads lend credence to the success and reliability of the technique of lime-black cotton soil stabilisation. Other salient features of the study like the economy, optimum requirement of lime and construction procedure are also discussed.

1. Introduction

Stabilisation of black cotton soil with lime is acknowledged as an effective technique to improve its engineering properties. By this treatment, the B. C. soil becomes more amenable for use as a suitable material in road construction 3,5,7. Generally the lime soil mix layer is used as a sub-base or base course in light trafficked roads. Addition of lime to the soil is believed to result in aggregation of soil particles, exchange of captions, carbonation and formation of cementitious compounds, and these phenomena are responsible for the improvement in the engineering properties of the black cotton soil1,2. Despite the adoption of this technique in performance of lime-soil mix layer depends on a number of factors like adequate pulverization of black cotton soil, uniform intimate mixing of lime and soil, equitable distribution of required quantity of water etc. At present, most of these operations are accomplished with manual effort.

Since black cotton soil is a hard material in the dry state, the breaking of dry clods is an arduous job. As per IRC guideliness6, 100 per cent of the black cotton soil should be passing 25 mm. and at least 50 per cent should be passing 4.75 mm. for successful stabilization. This specification of pulverization is extremely stringent to be adhered to in the field for constraints of time and money, so clods bigger than 25 mm. are also admitted for lime treatment and which tend to lessen the ameliorating effect of lime. Similarly mixing of lime and soil is done manually and this manual operation rarely achieves a truly uniform mixing. Lack of even distribution of lime leaves some pockets with a relatively lesser quantity of lime which fails to develop the design strength. This results in the formation of depressions on the surface of the road and ends up in cracking of the crust. In the same way, distribution of moisture in the lime-soil mix when added manually is seldom uniform. This means faulty compaction which in turn, leads to the failure of the road because of these weak spots. To obviate these deficiencies which invariably are present when the above mentioned jobs are done with manual effort, it was thought expedient to employ appropriate mechanical equipment for their execution.

Scope

The study is intended to appraise

the gains achieved by using mechanical aids like disc harrows for pulverizing the black cotton soil, rotavator for mixing lime and soil, and a water tanker for adding water to the mix vis-à-vis the manual efforts in executing these jobs in terms of economy and efficiency. The expedience of using these mechanical contrivances would also be gauged by observing the serviceable life and performance of lime stabilised roads in comparison to conventional roads. The study would also consider other aspects like adequate percentage of lime for stabilisation and other associate construction problems.

Field Study

Four roads in Amraoti circle (three in Amraoti Distt. And one in Akola Distt.) were selected for this investigation. Experimental stretches with limestabilised layers, were constructed on each of the four roads with respective lengths as give below.

• Amraoti Asra Road, 9.6 kms
• Asra Mana Road, 7.4 kms
• Daryapur Amla Road, 1.6 kms
• Achaopur Kakda Road, 3.2 kms.

All the roads pass through the agricultural fields having crops of cotton and jowar. The Amraoti Asra road, longest of the four, starts from Amraoti and passes through the Bhatkul village, crosses the river ‘Pedi’ and approaches village Asra. Since the area was lying in an agricultural belt, traffic as seasonal and was mainly comprising iron-tyred bullock carts.

Properties of the soil. The whole area is having black cotton soil at the top; it is highly argillaceous and some what calcareous. The clay mineral is mainly montmorillonitic in nature8. From the physical properties given in Table 1, it can be discerned that soil in this area can be classified as belonging to CH group.

Table 1 Showing the physical characteristics of the black cotton soil
Property	Amraoti Asra Road	Daryapur Amla Road	Asra Mana Road	Achalapur Kakda Road
Liquid Limit (%)	65.0	63.9	50.6	51.3
Publicity Index (%)	27.0	29.9	28.4	27.4
Sand content (%)	5.2	8.2	8.0	10.0
Optimum moisture content (%)	23.0	23.0	23.5	22.5
Max. density (gm/cc)	1.47	1.47	1.46	1.45
CBR(untreated)	2.0	2.0	2.0	2.0
CBR (3% lime)	42.0	54.0	50.0	51.0
CRB (5% lime)	112.0	120.0	110.0	108.0

4 Design of the Roads

4.1 The height of the embankment was 60 cms and the alignment of the road was straight. Widths of the different layers are as shown below:

width of the embankment 8.1 m

width of the sub-base 4.2 m

width of the base-course 3.9 m

width of the base coat 3.6 m

In the absence of any other method which is rational and completely satisfactory for the design of

TABLE 2
Specifications of the four experimental stretches

Layer	Amraoti Asra Road	Asra Mana Road	Daryapur Amla Road	Achalapur Kakda Road
Base Coat	15 cm loose WBM, two layers compacted	20.0 cm loose WBM compacted to 15 cm in two layers	15 cm loose WBM in two layers	15 cm loose WBM compacted in two layers
Base Course	10 cm loose oversize metal compacted	13.5 loose boulder compacted to 10 cm	13.5 cm loose boulder compacted	13.5 cm loose boulder compacted to 10 cm

Sub-base 12.7 cm compacted lime treated B. C. soil (2% on the basis of hundred per cent purity) 19 cm loose.

Though the specifications for all the four roads were basically the same, minor differences in their construction designs were made at the time of actual execution. Specifications for all the experimental stretches are as given in Table 2.

Conventional method of construction as adopted in the present study is shown in Table 3.

Table 3 Design of conventional road
Layer	Specifications
	(i)	(ii)
Base coat	15 cm compacted WBM in 2 layers	15 cm compacted WBM in 2 layers
Base course	10 cms compacted boulder soling	17.7 cms compacted boulder soling
Sub-base	7.6 cm sand blanket

Rolling of the subgrade was done to make it firm but intensive compaction was not considered advisable for black cotton soil subgrade.

Collection and pulverization of the soil were done manually as the irregular shape of borrow pits did not permit the use of mould board plough, etc. The soil was excavated and collected at the subgrade itself. Breaking of clods was done manually in the beginning. Bigger clods were broken to a size of 75-100 mm. In that process, an appreciable portion of the soil got pulverized. The pulverized soil was carried to the subgrade and was spread. Then the clods with size greater than 25 mm were removed to the shoulders by hand picking. Though the degree of pulverization achieved by this process was satisfactory, the cost of pulverization was exorbitant to the tune of Rs. 1.80 per cu. M. and the process was time consuming. Considering these facts, it was thought expedient to make use of off-set disc harrows.

Off-set Disc Harrow-Type 1

Bigger clods were broken at the borrow pit itself to maximum size of 75-100 mm. And then carried to the subgrade.

Off-set disc harrow type 1 attached to a tractor was used for pulverization, photo 1. Ordinarily, discs were not able to penetrate more than 75-100 mm into the soil layer and so satisfactory pulverization was not achieved with this tool without increasing the number of passes. The penetration depth of the disc harrow was increased by making four laborers sit on it. With the increased weight, the disc harrow could push more into the soil to the required depth of 19 cm and thus satisfactory pulverization could be achieved. Results of the degree of pulverization achieved with and without men sitting on the off-set harrow are shown in Table 4.

Table 4 Design of pulverization with disc harrow type 1
Use of disc harrow	Cumulative per cent passing differenet IS sieves
	8 passes		12 passes
	25 mm	4.75 mm	25 mm	4.75 mm
Without men sitting	80.0	39.0	90.0	57.0
With men sitting	93.0	60.0	96.0	> 70.0

ff set Disc Harrow-Type 2

4.7.1 Since the arrangement of seating four persons on the off-set harrow is quite cumbersome and escalates the cost of pulverization, another type of offset harrow was improvised. Except for Amraoti Asra road, pulverization of soil on the roads was accomplished with this offset harrow.

This type of off-set disc harrow had a different design with the provision of having a lever arrangement to adjust the penetration to the desired depth of 19 cm. Besides, this had an additional advantage of having a contrivance to be towed to a tractor. Being wieldy in size and having the arrangement to be raised above the ground completely, transportation and maneuverability of this type of off-set disc harrow were much more convenient. Though the pulverization was much easy at a near optimum moisture content, the same degree of pulverization could be achieved at lower moisture contents with slightly more number of passes. Fig. 1 shows the degree of pulverization with different number of the degree of pulverization with different number of passes of this disc harrow. The degree of pulverization required to be achieved was that atleast ninety per cent should pass through 25 mm IS sieve and fifty per cent should pass through 4.75 mm IS sieve. The cost of pulverization came to be Rs. 0.35 per cubic metre, out of which cost of breaking the clods manually to 75-100mm size was about Rs. 0.25 per cu metre and thus the actual cost of pulverization with this type of off-set disc harrow was only Rs. 0.10 per cu metre.

Slaking and Testing of lime

In the construction of these experimental stretches, usually slaked lime was used, Bigger pieces were broken to a size of 100-125 mm. The an estimated quantity of water was poured and some raking was done for uniform distribution of water. The slaked lime was then screened through 6 mm sieve, photo 3. The retained fraction was collected and again mixed with water till the remaining lime also was slaked. Screening was done again as clods of bigger size if got slaked after compaction cause crumbling of the compacted crust. The percentage of lime to be added to the soil was

determined on the basis of Ca(OH)² content present in lime. The percentage of calcium hydroxide in lime was determined by iodine method (7). As shown by tests, the purity of lime used for these four roads varied from 50 to 60% with the exception of Asra Mana road where the lime used was poor in quality. However lime content was kept at 2 per cent on the basis of 100% purity of lime.

Spreading and Dry Mixing of Lime

From an estimation of lime required, a known quantity of lime was spread manually and dry mixing was done with a rotavator. Slight sprinkling of water was made to reduce the loss of lime by flying.

Use of Sprinkler for Addition of Water

Optimum moisure content of the soil-lime mix was determined by Abbot’s apparatus because of its easy availability in the field. Calculated quantity of water was added after taking into consideration the initial moisture content and the loss due to evaporation. In the case of road No. 1, over night soaking of the soil-lime mix was done while for other roads, lime was added to the moist soil having optimum moisture content and the compaction was done within three hours. This was in consideration of the fact that delayed compaction reduces the development of strength appreciably. Addition of water was done directly from truck tanker through hose pipe determined by iodine method7. As shown by tests, time consuming, photo 4. The span of length for one tanker was calculated and sprinkling of water was controlled by visual inspection. The quantity of water sprinkled was controlled in such a way that some ten per cent water was left and the same was collected in empty tar drums kept on either side of the stretch. This water was utilized for sprinkling on the spots which looked relatively less wet during the process of mixing. Ultimately one full tanker of water was actually used for the stretch earmarked for the same. Water added was always on the higher side of OMC and was brought down by repeated mixing with the help of the rotavator. This process ensured uniform water distribution. However some shoveling was done to attain still better mixing.

About sixteen to eighteen passes of the rotavator were generally required for proper mixing,photos 5 & 6. Uniformity of mixing was checked by visual inspection but degree of pulverization and moisture content were checked by performing appropriate tests. A variation of 3.0% was permitted on either side of the optimum moisure content and any deviation beyond this limit was rectified. Rolling was done by a 8-10 ton smooth wheel roller and on average, about thirty two passes were sufficient to compact 4.2 m wide road. Field density was measured at the rate of four tests over 150 m stretch. Average test results are shown in Table 5.

Table 5 
Laboratory density (standard proctor) and field density attained with
corresponding moisture content

Name of the road	Stretch No	Laboratory Density (gm/cc)	O.M.C. %	Moisture (%) before rolling	Insitu Density (gm/cc)
Amroati Asra Road	1	1.621	21.9	22.1	1.618
	2	1.583	22.2	23.1	1.572
	3	1.592	22.4	22.2	1.582
	4	1.625	22.0	22.4	1.625
	5	1.583	22.9	21.4	1.558
	6	1.522	24.4	23.0	1.570
	7	1.518	23.5	24.0	1.511
Asra Mana Road	1	1.456	21.8	20.2	1.480
	2	1.451	21.4	19.6	1.440
	3	1.466	21.7	19.8	1.465
	4	1.482	21.0	20.0	1.457
	5	1.492	20.8	19.8	1.481
Daryapur Amla Road	1	1.452	22.0	21.0	1.461
Achalpur Kokda Road	1	1.442	25.1	24.8	1.43
	2	1.454	25.4	24.6	1.420

(Note: One stretch is 1.6 km)

Curing

Curing of the lime treated surface was done by sprinkling of water for a period of seven days. Wherever possible, the lime treated surface was immediately covered with material of next layer which comprised boulder or oversize metal. Rolling was only done after a minimum period of fourteen days when the stabilised layer was placed to avoid any possibility of shuffling of the material

Measurement of Strength Development

The strength developed was measured with the help of Proctor’s needle test after 28 days of lime stabilisation. Penetration resistance test results are shown in Table 6. All these tests were performed after 24 hrs of soaking.

Cost Analysis

The design thickness of all the stretches was kept the same throughout. In the case of lime stabilised stretches, requirement of stone metal was reduced equivalent to the thickness of lime stabilised layer. So the cost of lime stabilised layer is to be compared with that of stone soling. It was estimated that cost of the lime stabilised layer was Rs. 13/- per cu. Metre and that of stone soling was Rs. 16/- per cubic metre when the lead of the stone metal was 5 km. The cost of WBM was Rs. 27 per cu metre with lead of stone equal to 5 km. it is quite obvious that use of lime stabilised layer effects economy in the cost of construction of roads. The extent of saving depends on the distance over which the stone metal is transported. If the hauling distance is five kilometers, the saving is quite significant but for greater leads, the economy is further augmented.

Performance of Roads

Performance level of roads was judged by inspection of the surface at regular intervals of time. Based on these inspections and reports received from local authorities, it was observed that behaviour of time stabilised stretches was better than the conventional roads in the beginning. On Amraoti - Asra road (13/72,14/1), a few undulations and damages were reported on the conventional stretch while the stabilised stretches showed no damages. For a period of four years, stabilised sections behaved very satisfactorily but thereafter, the lime treated stretches started deteriorating. After a period of seven years, the magnitude of deterioration was more in lime stabilised stretches that in conventional stretches.

Discussion

As mentioned earlier and shown in table 4, disc harrow type 1 was quite effective in giving the required degree of pulverization. With 12 passes of this disc harrow and having four persons seated on it, it gave 96% soil passing 25 mm sieve and 70% passing 4.75 mm sieve. Though the requirements of pulverization as laid down in IRC Code (4) are fulfilled with disc harrow of this type, the necessity of four persons seated on it not only makes it cumbersome but also the cost of pulverization gets escalated. This encumbrance of requiring four persons has been dispensed with in disc harrow type 2. This type has been contrived so as to be able to penetrate to a depth of 19 cm and thus the results achieved on pulverization are more desirable. As can be seen from Fig. 1, about 22 passes of this type of disc harrow give more than 90% fraction passing 25 mm sieve and about 70% fraction passing 4.75 mm. The cost of braking the clods and pulverizing the soil to the desired degree of pulverization comes to about Rs. 0.35 per cu. metre. This is much less than the cost involved in manual pulverization which is Rs. 1.80 per cu. metre. Thus the use of disc harrow is advantageous in giving the desired degree of pulverization more expeditiously and at a much economical rate. Obviously pulverization done mechanically with disc harrows is to be preferred to manual effort both for consideration of time and economy.

Similarly use of rotavator is quite gainful in obtaining uniform mixing. As observed, about 16-20 passes are enough to get proper mixing of the lime and soil at the optimum moisture content. The employment of tanker for sprinkling of water and bringing the lime-soil mix to the optimum-moisture content is quite expedient.

From the foregoing information,it appears that lime stabilised roads give a satisfactory performance for a period of 3-4 years and then start showing signs of deterioration. This deterioration was attributed to inadequate quantity of lime added to the soil. When lime is added to the soil, first it modifies the properties of soil and the process to effect this change is called lime fixation. It is believed that first 1-2 per cent of lime is utilized in modifying the properties of the black cotton soil and then the excess percentage reacts with clay minerals. In the present case, the lime added was only 2.0 per cent which is utilized in modifying the properties of black cotton soil and very little lime is left to react with clay minerals to form cementitious compounds. It is contended quite logically that lime stabilised stretches would have exhibited more durability with longer serviceable life if the percentage of lime added had been increased to higher values.

(Note: One stretch is 1.6 km)

TABLE 6
Penetration resistance (proctor needle method) for different depths with needles
of different cross sectional areas on stabilised soil (kg/cm²⁾ (Amroati-Asra road)

Area of Cross section of the needle	.387 cm2			.710 cm2			1.355 cm2			2.194 cm2
Depth of Penetration(cms) Stretch No.	1.27	2.54	5.08	1.27	2.54	5.08	1.27	2.54	5.08	1.27	2.54	5.08	Remarks
1.	18.9	32.0	45.0	24.3	37.5	43.3	30.0	30.3	34.1	16.7	17.2	26.9
2.	14.7	23.3	37.8	17.2	16.0	22.0	26.8	34.5	41.9	15.0	17.9	23.8	Test was performed after 24 hours of soaking. Untreated specimens did not offer any resistance.
3.	47.5	40.0	33.3	47.0	41.2	30.0	46.7	28.9	23.8	26.6	19.0	21.8
4.	31.6	46.8	55.2	17.2	30.0	36.0	16.0	27.8	30.0	15.5	20.3	22.2
5.	76.0	11.2	14.7	6.2	12.5	15.5	6.3	7.8	9.0	5.5	9.0	11.2
6.	6.0	8.0	10.5	4.8	8.8	12.6	5.3	6.3	6.9	4.1	6.8	8.3

Data on penetration resistance listed in Table 6 show that lime treated sections, under saturated conditions, developed considerable strength while untreated portion showed very little strength. This demonstrates the fact that lime reacts with soil and produces aggregation of soil particles, base exchange, carbonation and forms cementitious compounds of calcium aluminiate hydrate and calcium silicate hydrate. In the present case, it is surmised that lime percentage was not adequate to complete all the reactions to achieve full stabilisation. Despite this, lime stabilised stretches performed reasonably well for a few years. It is felt that mechanical contrivances do help in executing different jobs more efficiently and also help in achieving economy to some extent.

Conclusions

Lime stabilisation is a scientific method to improve the physical and engineering properties of some soils provided the quantity of lime added is adequate and stabilisation is done with appropriate quality control.

Use of mechanical equipment like disc harrows of different type for pulverization of soil to the right degree, rotavator for uniform mixing of lime and soil, is quite desirable both in respect of quality of results and economy. Use of tanker for adding required quantity of water is more efficient in ensuring uniform distribution of moisture and saving time.

Lime treatment with 2.0 per cent of lime is not very durable. Concentration of lime should be fairly above the fixation limit so as to allow the formation of cementitious compounds.

Construction of lime stabilised roads is some what economical as compared to the cost of construction of conventional roads involving the use of stone metal.

Acknowledgements

Authors owe thanks to Shri T.K. Natarajan, Deputy Director & Head, Geotechnical Engineering for his help and guidance in the completion of this study. The paper is submitted for publication with the permission of Director, Central Road Research Institute, New Delhi.

References

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