Practical Problems of Highway Construction in Black Cotton Soil Areas and In-Place Remedial Measures: A Case Study
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
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 AreasProblems Arising out of Water Saturation
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
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 soilsMaterials 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:
- The Black cotton soil (BC soil) having characteristics as given in Table 1.
- OPC 43 grade as per IS:8112- 1989.
- 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.
- The sand shall be as per IS:383-1970.
- 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 StabilizationThe 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:
- 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).
- 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.
- To overcome the susceptibility of foundations to volume change and to increase their shearing resistance and bearing capacity.
Pulverization and Mixing MethodologyThe 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.
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 ConstructionThe methodology for improving the engineering characteristics of Black cotton soil (BC soil) for road embankment construction is as follows:
- 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).
- 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.
- 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.
- 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.
- 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.
- Proper pucca drains on both sides of the road at the NGL along the embankment should be provided as per the standards.
- Construction details of the remaining upper layers should be as per pavement design based on the CBR results/ modulus of subgrade reaction.
- 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 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 PavementIn 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 Areas4 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).
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:
- 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.
- 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.
- 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.
- 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.
- 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:
- 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.
- 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.
- The remaining height of embankment was layer of 500 mm thickness of CNS soil as described in para (c) of Case 1.
- 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.
- There is a need for providing the engineering characteristics of B.C. soil for road construction.
- 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.
- 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.
- 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.
- 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.
- Dense bituminous surfacings are essential to stop the ingress of water from top surfacing.
- Roadside berms require pucca treatment including use of primer and stone grafting; a steep camber of 1:36 is recommended.
- A layer of 1000 mm thick compacted moorum should be provided over the sand filter layer as given at (d.) above.
- Chen, F. H (1998): “Foundationson Expansive Soils,” American Elsevier Science Publication, New York.
- CRRI Report (1980): Investigation of certain stretcheson NH-3 near Biora in Madhya Pradesh.
- Dunn I. S., Anderson, L. R. & Kiefer, F.W. (1980): “Fundamentals of Geotechnical Analysis,” John Wiley & Sons, Inc. New York.
- John Nelson, D & Debora Millar. J (1991): “Expansive Soils,” John Wiley & Sons, Inc. New York.
- National Get–together on Road Research and its Utilization, CRRI, October 1987, Basic Document.
- Road Research Paper No. 149: Lime Requirement in Soil- Stabilization.
- 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.