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    Practical Problems of Highway Construction in Black Cotton Soil Areas and In-Place Remedial Measures: A Case Study


    Dr. S.S.Seehra, Chief Pavement Specialist–cum–Materials and Geotechnical Engineer, Span Consultants Pvt. Ltd. (Member of the SNC–LAVALIN Group Inc.) and (Former Director–Grade Scientist and Head, Rigid Pavements Division, CRRI), New Delhi.

    Black cotton soils are inorganic clays of medium to high compressibility and form a major soil group in India. They are characterized by high shrinkage and swelling properties. This Black cotton soils occurs mostly in the central and western parts and covers approximately 20% of the total area of India. Because of its high swelling and shrinkage characteristics, the Black cotton soils (BC soils) has been a challenge to the highway engineers. The Black cotton soils is very hard when dry, but loses its strength completely when in wet condition.

    It is observed that on drying, the black cotton soil develops cracks of varying depth. Figure 1 shows the typical cracks in Black cotton soils (BC soils) in a dried state. As a result of wetting and drying process, vertical movement takes place in the soil mass. All these movements lead to failure of pavement, in the form of settlement, heavy depression, cracking and unevenness. This article covers highway construction in Black cotton soils (BC soils) and also describes a case history of highway construction in highway construction in Black cotton soils.

    Black Cotton Soil Peculiar Characteristics

    Black cotton soil (BC soil) is a highly clayey soil. It is so hard that the clods cannot be easily pulverized for treatment for its use in road construction. This poses serious problems as regards to subsequent performance of the road. Moreover, the softened sub grade has a tendency to up heave into the upper layers of the pavement, especially when the sub-base consists of stone soling with lot of voids. Gradual intrusion of wet Black cotton soil (BC soil) invariably leads to failure of the road.

    The roads laid on Black cotton soil (BC soil) bases develop undulations at the road surface due to loss of strength of the sub grade through softening during monsoon. The black color in Black cotton soil (BC soil) is due to the presence of titanium oxide in small concentration. The Black cotton soil (BC soil) has a high percentage of clay, which is predominantly montmorillonite in structure and black or blackish grey in color. The physical properties of Black cotton soil (BC soil) vary from place to place. Its engineering properties are given in Table 1.

    40 to 60% of the Black cotton soil (BC soil) has a size less than 0.001 mm. At the liquid limit, the volume change is of the order of 200 to 300% and results in swelling pressure as high as 8 kg/cm2/ to 10 kg/cm2. As such Black cotton soil (BC soil) has very low bearing capacity and high swelling and shrinkage characteristics. Due to its peculiar characteristics, it forms a very poor foundation material for road construction. Soaked laboratory CBR values of Black Cotton soils are generally found in the range of 2 to 4%. Due to very low CBR values of Black cotton soil (BC soil), excessive pavement thickness is required for designing for flexible pavement. Research & Development (R&D) efforts have been made to improve the strength characteristics of Black cotton soil (BC soil) with new technologies.

    Problems of Highway Construction in Black Cotton Soil Areas

    Problems Arising out of Water Saturation

    It is a well-known fact that water is the worst enemy of road pavement, particularly in expansive soil areas. Water penetrates into the road pavement from three sides viz. top surface, side berms and from sub grade due to capillary action. Therefore, road specifications in expansive soil areas must take these factors into consideration. The road surfacing must be impervious, side berms paved and sub grade well treated to check capillary rise of water.

    It has been found during handling of various road investigation project assignments for assessing causes of road failures that water has got easy access into the pavement. It saturates the sub grade soil and thus lowers its bearing capacity, ultimately resulting in heavy depressions and settlement. In the base course layers comprising of Water Bound Macadam (WBM), water lubricates the binding material and makes the mechanical interlock unstable. In the top bituminous surfacing, raveling, stripping and cracking develop due to water stagnation and its seepage into these layers.

    Generally, road construction agencies do not pay sufficient attention to the aspects of construction and maintenance of side berms. It is emphasized that road formation consisting of carriageway and berms must be considered as one single unit. In expansive soil areas, unpaved berms pose the maximum problem as they become slushy during rains, as they are most neglected lot. Fig 2 and Fig 3 show development of alligator cracks and extensive depression as well as upheavals respectively in bituminous surfacing in Black cotton soil (BC soil) areas.

    Design Problems in Black cotton soils

    In India, CBR method developed in USA is generally used for the design of crust thickness. This method stipulates that while determining the CBR values in the laboratory and in the field, a surcharge weight of 15 kg and 5 kg per 62 mm and 25 mm thickness respectively should be used to counteract the swelling pressure of Black cotton soils (BC soils). BC soils produce swelling pressure in the range of 20-80 tons/m2 and swelling in the range of 10-20%.Therefore, CBR values obtained are not rational and scientific modification is required for determining CBR values of expansive soil.

    Having heavy-duty traffic of 4500 commercial vehicles per day and msa 150 as generally found on our National Highways and taking CBR value of 2%, total crust thickness of flexible pavement works out to 830 mm which is practically an impossible preposition. It is felt that CBR design curves require modification for expansive soils.

    Assuming heavy traffic intensity of 4500 commercial vehicles per day and msa 150, crust thickness of rigid pavement works out approximately 300-320 mm, which is about one third of thickness needed for flexible pavement. Therefore, it sounds reasonable to adopt cement concrete pavement in Black cotton soil areas. This type of pavement may save the engineers from day to day maintenance problems also.

    Another approach to the problem can be in having semi rigid sub-bases. It is suggested that the CBR value of the BC soil be improved by giving a suitable treatment with the appropriate technology and then work out the crust thickness. This will substantially reduce the required crust thickness.

    Uncompacted berms without any treatment cannot withstand the traffic stresses. It is a common sight and experience that heavy vehicles get stuck up while overtaking and sometimes results in serious accidents. Development of separate specifications for berms need to be evolved.

    Technologies for Improving CBR of Black cotton soils

    Materials for Soil Stabilization The materials for Black cotton soil (BC soil) stabilization shall comprise lime or Ordinary Portland Cement (OPC)43 grade, moorum of approved quality, sand and Cohesive Non swelling Soil (CNS) having properties given below:
    1. The Black cotton soil (BC soil) having characteristics as given in Table 1.
    2. OPC 43 grade as per IS:8112- 1989.
    3. Well graded granular moorum having minimum 4 day soaked CBR of 10% and maximum laboratory dry unit weight when tested as per IS:2720 (Part-8) shall not be less than 17.50 kN/m3.
    4. The sand shall be as per IS:383-1970.
    5. The material for CNS soil should be good quality soil having laboratory dry unit weight when tested as per IS:2720 (Part-8) not less than 16kN/m3.

    Cement/ Lime-Soil Stabilization

    The engineering properties of Black cotton soil (BC soil) can significantly be improved with lime or cement treatment. This technology has been very common at global level and is in vogue for the last several years. Cement or hydrated lime in the range of 3 to 5 per cent brings remarkable improvement in the engineering characteristics of Black cotton soil (BC soil). The test results of typical Black cotton soil (BC soil) samples are given in Table 2 to indicate the improvement in its characteristics.

    Cement/ Lime–soil stabilization technology has been found useful cost-effective and suited to manual methods of construction. This technology has been found 20-30% cheaper than conventional WBM construction. The cement or lime treatment is being utilized for the following purposes:
    1. To provide a pavement foundation of marginally weaker in strength than that of concrete pavement, but much improved strength than natural Black cotton soil (BC soil).
    2. To consolidate subgrades and base courses for concrete pavement in order to make them resistant to volume changes and displacement or erosion in the presence of moisture even under the rocking action of curled slabs, if any.
    3. To overcome the susceptibility of foundations to volume change and to increase their shearing resistance and bearing capacity.

    Pulverization and Mixing Methodology

    The method of pulverization of Black cotton soil (BC soil) and mixing with cement and moorum or lime and moorum shall be as follows:

    The Black cotton soil (BC soil) is dug from the ground where the embankment is to be constructed and clods broken with pick-axes so as to reduce them to a maximum size of 50 mm. Alternatively disc harrows with tractor could be employed. Soil clods are spread over the prepared and compacted surface of excavation and a smooth wheeled 8 tonne power roller passed over them a number of times, accompanied by frequent raking of the crushed material. About 8 passes of the roller combined with raking should normally be able to achieve the degree of pulverization. The degree of pulverization should be such that at least 80 per cent of soil passes through 475 micron sieve and there are no lumps larger than 25 mm size.

    The pulverized Black cotton soil (BC soil) is mixed uniformly in given proportion by weight with cement and moorum in-situ field conditions by using rotavator machine or motor grader. Portland cement or lime and Black cotton soil (BC soil) mixed at the proper moisture content has been used to build stabilized bases under concrete pavements for highways/ expressways and air fields .Cement/ lime-modified Black cotton soil (BC soil) is a mix that generally contains less than 5 per cent cement by volume.

    This forms a semi-rigid system, improves the engineering properties of the soil and reduces the potential of the soil to expand by absorbing water. OPC or lime, when mixed with pulverized Black cotton soil (BC soil) reduces the liquid limit (LL), the plasticity index (PI) and the potential for volume change. It increases the shrinkage limit and shear strength. Due to the strength increase incorporated by the use of cement, the Black cotton soil (BC soil) cement mixture can increase the subgrade bearing capacity and strength significantly. This technique is relatively cheap and quite effective. At the ingress of moisture in cement stabilized black cotton soil mix, the water increases the strength of cement matrix and imparts strength to the mix.

    Road Embankment Construction

    The methodology for improving the engineering characteristics of Black cotton soil (BC soil) for road embankment construction is as follows:
    1. After identifying the stretches where a treatment to strengthen the foundation embankment is required, the unsuitable Black cotton soil (BC soil) should be removed to the depth of 1600 mm below natural ground level (NGL).
    2. After excavation, the ground should be checked for density. In case the density is found to be less than 95 per cent of Max. Dry Density (MDD), a further depth of 225 mm should be loosened and recompacted at Optimum Moisture Content (OMC) till it attains the density of 95 percent of MDD and CBR of 5 per cent.
    3. Construction of embankment in layers of 200 mm compacted thickness or part thereof, should be continued up to 600 mm thickness with pulverized Black cotton soil (BC soil) (60% by wt.), cement (OPC) 43 grade or lime (3% by wt.) and moorum (37% by wt.) uniformaly mixed by using rotavator machine or by any other farm equipment and rolled to achieve density of 95 per cent of MDD and CBR of 5 percent.
    4. A layer of 225 mm thick sand filter should be provided over the stabilized Black cotton soil (BC soil) as given at (c) above and also to give uniform support. Over this a layer of 1000 mm thick–compacted moorum in layers of200 mm each should be provided.
    5. Compacted CNS soil of size 3000 mm in width and 2000 mm in depth should be provided at the edges of the compacted sublayers to prevent the entry of ground water to the sub-layers.
    6. Proper pucca drains on both sides of the road at the NGL along the embankment should be provided as per the standards.
    7. Construction details of the remaining upper layers should be as per pavement design based on the CBR results/ modulus of subgrade reaction.
    8. Use of good flyash in embankment construction should be permitted wherever available near the construction site. Flyash should be conforming to IS:3812 (Grade-1) and it should be ensured that availability of flyash is of consistent quality and in required quantity.
    Use of geo-textile fabric has found extensive application in highway engineering, particularly in expansive soil area. It was used in USA in early 70’s and in India it is initial stage. Geo-textiles are manufactured from petroleum derivatives such as polypropylenes, polyesters, polyamides etc. These have high tensile strength, burst and puncture strength, permeability and abrasion resistance etc. The use of geotextile fabric is made to provide a separation barrier between subgrade and sub-base courses. This technique has tremendous potential for its application in Black cotton soil (BC soil) area, and it has further been found that this geo-textile layer acts as a reinforcing layer, acts and can also be used to combat reflective cracking in the pavement structure. Also with the use of geo-textile, a reduction in the thickness of the pavement has been predicted ranging from 10 to 25 per cent. The cost of the geotextile has been the stumbling factor for its wider application in India. However, in special areas, such as Black cotton soil (BC soil) area, geo-textile offers a solution to highway engineering problems.

    Use of moorum layer as a barrier between subgrade and subbase layer has been in practice in view of its cost–effectiveness. Moorum of low plasticity index has been found effective and does not allow intrusion up of soft subgrade soil into the interstices of stone aggregates and ingress of water through it. It is a common old practice to provide about 225 mm thick sand blanket layer on soft soils as a barrier to stop intrusion of subgrade soil into interstices of granular base/ sub-base layer and serves as drainage layer, and also to give uniform support.

    Choice of Top Wearing Courses on Flexible Pavement

    In Black cotton soil (BC soil) areas, choice of bituminous surfacing on road crust is an important parameter in enhancing the life of the pavement. In order to eliminate the ingress of water into the pavement crust, it is preferred to adopt impervious surfacing of low void content, such as dense asphaltic concrete, premix carpet with liquid seal coat, bituminous macadam with seal coat etc. It has been found that open graded bituminous surfacings develop unevenness, waviness and depression during rainy season due to softening of subgrade caused by seeping of water through such open graded bituminous surfacings. In case of construction of new surfacings, it should be ensured that it is laid much before the monsoon season. It is felt that time slot of at least one month should be given between the end of construction and beginning of monsoon so that surface course gets additional compaction due to traffic. It ensures consolidationand compaction.

    Shoulders of roads need special treatment, as these are the vulnerable points for vehicles getting stuck up during overtaking. It is felt that special treatment is needed to make the berms paved and separate specifications should be formulated.

    Case History of Road Construction in Black Cotton Soil Areas

    4 laning of National Highway No. 4 passing through Black cotton soil (BC soil) Area

    National Highway No. 4 (NH-4) connects Chennai and Mumbai via Bangalore. NH-4 is one of the four arms of the National Highway Network popularly known as the Golden Quadrilateral having length of about 6000 km as a component of National Highways Development Project (NHDP).
    The Ministry of Road Transport & Highways (MoRT&H) has authorized National Highways Authority of India (NHAI) to undertake the widening of the existing 2 lane stretch mostly passing through Black cotton soil (BC soil) area from Satara (km 725.00) to Maharashtra State Border Section of NH-4 (km 592.240), to 4-lane divided carriageway and strengthening of the existing 2-lane section through private sector participation on BOT basis. The Black cotton soil (BC soil) deposits existing on the road alignment were fully replaced with Cohesive Non- Swelling (CNS) soil having Plasticity Index (PI) value ranging between 5 to 15 and Free-Swelling Index (FSI) ranging between 7 to 35 in the entire length of new carriageway (Fig 4). Since the good quality CNS soil (moorum) was readily available in abundance from the nearby moorum quarries along the existing carriageway, therefore, no Black cotton soil (BC soil) stabilization was required to be carried out either with lime or cement. Fig 5 shows compaction of CNS soil (moorum) in the construction of embankment for the new 2 lane carriageway. Along this road, sampling of soil had been done at regular intervals of distance through test pitting and had been tested for FSI and Atterberg Limits to verify how much it was expansive. After that it was checked for other properties such as MDD/OMC, CBR and grain size analysis etc. Every borrow soil/ moorum other than Black cotton soil (BC soil) was used after testing their required properties as per codal specifications. Quality acceptance tests were carried out for common borrow embankment and subgrade materials. The methodology of embankment and subgrade construction where there was filling and cutting throughout the existing alignment is described under Case-I and Case-II respectively hereinafter.

    Methodology of Embankment Construction in Filling (Case-I)

    Where the stretches of highway embankment are in filling on Black cotton soil (BC soil) (Fig 6) and are above NGL, the methodology for improving the engineering characteristics of Black cotton soil (BC soil) was as follows:
    1. After location of the stretches where embankment was to be raised in filling, the formation level was prepared after clearing and grubbing to a depth of 150 mm.
    2. Construction of embankment in layers of 200 mm compacted thickness or part thereof, was continued upto thickness–h (Fig 6) with good soil other than Black cotton soil (BC soil) having FSI less than 50 per cent, PI less than 30, Field Dry Density (FDD) equal to 95 per cent of MDD and minimum CBR of 5 per cent.
    3. The remaining height of embankment would be layer of 500 mm thickness–compacted CNS soil in two layers of 200 mm each and the remaining layer of 100 mm was provided over the compacted layer as given at (b) above, having zero FSI, FDD of 95 per cent of MDD and minimum CBR of 5 per cent.
    4. A further layer of 500 mm subgrade thickness of compacted CNS soil in two layers of 200 mm each and 100 mm was provided over the compacted layer as given at (c) above, having zero FSI, FDD of 97 per cent of MDD and minimum CBR of 8 per cent.
    5. The construction details of the remaining upper layers shall be as per pavement design based on the CBR results/ modulus of subgrade reaction.

    Methodology of Embankment Construction in Cutting (Case-II)

    Where the stretches of highway embankment are in cutting on Black cotton soil (BC soil) (Fig 7) and are below NGL, the methodology for improving the engineering characteristics of Black cotton soil (BC soil) for road embankment construction was as follows:
    1. After location of the stretches where embankment was to be raised in cutting, the formation level was prepared after excavation of the soil to a depth of 1600 mm below NGL. The formation level was checked for density. In case the density was found to be less than 95 percent of MDD, the soil would be loosened and recompacted at OMC till it attained the density of 95 per cent of MDD. Over this, a layer of 225 mm thick sand filter or blanket course consisting of non-plastic granular material or local sand was provided.
    2. Construction of embankment in layers of 200 mm compacted thickness or part thereof, was continued upto thickness–h (Fig 7) with good soil other than Black cotton soil (BC soil) having FSI less than 50 percent, PI less than 30, FDD equal to 95 per cent of MDD and minimum CBR of 5 per cent.
    3. The remaining height of embankment was layer of 500 mm thickness of CNS soil as described in para (c) of Case 1.
    4. A further layer of 500 mm subgrade thickness of compacted CNS soil in two layers of 200 mm thickness each and the remaining layer of 100 mm or two layers of 250 mm thickness each (as directed by the Engineer-in-Charge at site to reduce time of construction) shall be provided over the compacted layer as given at (c) above, having zero FSI, FDD of 97 per cent of MDD and CBR of 8 per cent.
    The construction details of the remaining upper layers shall be as per pavement design based on the CBR results/ modulus of subgrade reaction.

    Conclusion

    1. There is a need for providing the engineering characteristics of B.C. soil for road construction.
    2. Flexible pavement design method involving use of CBR method needs modification due to high swelling characteristics of Black cotton soil (BC soil). Rigid pavement construction can be employed to bring out overall economy in Black cotton soil (BC soil) areas.
    3. Use of lime-soil stabilization technology has a great potential in Black cotton soil (BC soil) areas. Use of moorum layer as a barrier between subgrade and sub-base layer has been in practice in view of its cost-effectiveness. Moorum of low PI has been found effective and does not allow intrusion up of soft subgrade soil into the interstices of stone aggregates and ingress of water through it.
    4. Use of 225 mm thick sand filter layer on soft soils as a barrier to stop intrusion of subgrade soil into interstices of granular base/ sub-base layer and serve as drainage layer, and to give uniform support.
    5. Incorporation of geo-textile fabric between subgrade and sub-base layer is an emerging technology. It not only stops seepage of water but also brings reduction in thickness.
    6. Dense bituminous surfacings are essential to stop the ingress of water from top surfacing.
    7. Roadside berms require pucca treatment including use of primer and stone grafting; a steep camber of 1:36 is recommended.
    8. A layer of 1000 mm thick compacted moorum should be provided over the sand filter layer as given at (d.) above.
    The compacted CNS soil of size 3000 mm in width and 2000 mm in depth should be provided at the edges of the compacted sub-layers to prevent the entry of ground water to the sub-layers.

    References

    1. Chen, F. H (1998): “Foundationson Expansive Soils,” American Elsevier Science Publication, New York.
    2. CRRI Report (1980): Investigation of certain stretcheson NH-3 near Biora in Madhya Pradesh.
    3. Dunn I. S., Anderson, L. R. & Kiefer, F.W. (1980): “Fundamentals of Geotechnical Analysis,” John Wiley & Sons, Inc. New York.
    4. John Nelson, D & Debora Millar. J (1991): “Expansive Soils,” John Wiley & Sons, Inc. New York.
    5. National Get–together on Road Research and its Utilization, CRRI, October 1987, Basic Document.
    6. Road Research Paper No. 149: Lime Requirement in Soil- Stabilization.
    7. Technical Specifications (Package-IV): Four-laning of Satara–Kolhapur upto Maharashtra State Border Section of NH-4 (km 592/240 to km 725/000), Vol. – 3.

    NBMCW March 2008

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