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Practical Problems of Highway Construction in Black Cotton Soil Area.pdf
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  LOGIN CREATE AN ACCOUNTADVERTISE WITH USCONTACT USABOUT USHOME Practical Problems of Highway Construction in Black CottonSoil 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 (FormerDirector–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 soilgroup in India. They are characterized by high shrinkage and swelling properties. This Black cottonsoils occurs mostly in the central and western parts and covers approximately 20% of the totalarea of India. Because of its high swelling and shrinkage characteristics, the Black cotton soils (BCsoils) 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 1shows the typical cracks in Black cotton soils (BC soils) in a dried state. As a result of wetting anddrying process, vertical movement takes place in the soil mass. All these movements lead to failureof pavement, in the form of settlement, heavy depression, cracking and unevenness. This articlecovers 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. Itis so hard that the clods cannot be easilypulverized for treatment for its use in roadconstruction. This poses serious problems asregards to subsequent performance of the road.Moreover, the softened sub grade has a tendencyto up heave into the upper layers of thepavement, especially when the sub-base consistsof stone soling with lot of voids. Gradual intrusionof wet Black cotton soil (BC soil) invariably leadsto failure of the road.The roads laid on Black cotton soil (BC soil) basesdevelop undulations at the road surface due toloss of strength of the sub grade throughsoftening during monsoon. The black color inBlack cotton soil (BC soil) is due to the presenceof titanium oxide in small concentration. The Blackcotton soil (BC soil) has a high percentage of clay,which is predominantly montmorillonite instructure and black or blackish grey in color. Thephysical properties of Black cotton soil (BC soil)vary from place to place. Its engineeringproperties 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, thevolume change is of the order of 200 to 300% and results in swelling pressure as high as 8kg/cm 2  / to 10 kg/cm 2 . As such Black cotton soil (BC soil) has very low bearing capacity and highswelling and shrinkage characteristics. Due to its peculiar characteristics, it forms a very poorfoundation material for road construction. Soaked laboratory CBR values of Black Cotton soils aregenerally found in the range of 2 to 4%. Due to very low CBR values of Black cotton soil (BC soil), Search..... LATEST ISSUES MAGAZINESNEWSARTICLESPRODUCTSEQUIPMENTSREPORTSINTERVIEWSSUBSCRIPTIONPEOPLE WATCHEVENTS Practical Problems of Highway Construction in Black Cotton Soil Areahttp://www.nbmcw.com/articles/roads/307-practical-problems-of-highwa...1 of 66/24/2013 12:07 AM  View OnlineDownload Issue (PDF)Previous IssuesView OnlineDownload Issue (PDF)Previous IssuesView OnlineDownload Issue (PDF)Previous Issuesexcessive pavement thickness is required for designing for flexible pavement. Research & Development (R&D) efforts have been made to improve the strength characteristics of Black cottonsoil (BC soil) with new technologies. Problems of Highway Construction in Black Cotton Soil AreasProblems Arising out of Water Saturation It is a well-known fact that water is the worstenemy of road pavement, particularly in expansivesoil areas. Water penetrates into the roadpavement from three sides viz. top surface, sideberms and from sub grade due to capillary action.Therefore, road specifications in expansive soilareas must take these factors into consideration.The road surfacing must be impervious, sideberms paved and sub grade well treated to checkcapillary rise of water.It has been found during handling of various roadinvestigation project assignments for assessingcauses of road failures that water has got easy access into the pavement. It saturates the subgrade soil and thus lowers its bearing capacity, ultimately resulting in heavy depressions andsettlement. In the base course layers comprising of Water Bound Macadam (WBM), waterlubricates the binding material and makes the mechanical interlock unstable. In the top bituminoussurfacing, raveling, stripping and cracking develop due to water stagnation and its seepage intothese layers.Generally, road construction agencies do not pay sufficient attention to the aspects of constructionand maintenance of side berms. It is emphasized that road formation consisting of carriageway andberms must be considered as one single unit. In expansive soil areas, unpaved berms pose themaximum problem as they become slushy during rains, as they are most neglected lot. Fig 2 andFig 3 show development of alligator cracks and extensive depression as well as upheavalsrespectively in bituminous surfacing in Black cotton soil (BC soil) areas. Design Problems in Black cotton soils In India, CBR method developed in USA isgenerally used for the design of crust thickness.This method stipulates that while determining theCBR values in the laboratory and in the field, asurcharge weight of 15 kg and 5 kg per 62 mmand 25 mm thickness respectively should be usedto counteract the swelling pressure of Black cottonsoils (BC soils). BC soils produce swelling pressurein the range of 20-80 tons/m 2 and swelling in therange of 10-20%.Therefore, CBR values obtainedare not rational and scientific modification isrequired for determining CBR values of expansive soil.Having heavy-duty traffic of 4500 commercial vehicles per day and msa 150 as generally found onour National Highways and taking CBR value of 2%, total crust thickness of flexible pavementworks out to 830 mm which is practically an impossible preposition. It is felt that CBR designcurves require modification for expansive soils.Assuming heavy traffic intensity of 4500 commercial vehicles per day and msa 150, crust thicknessof rigid pavement works out approximately 300-320 mm, which is about one third of thicknessneeded for flexible pavement. Therefore, it sounds reasonable to adopt cement concrete pavementin Black cotton soil areas. This type of pavement may save the engineers from day to daymaintenance problems also.Another approach to the problem can be in having semi rigid sub-bases. It is suggested that theCBR value of the BC soil be improved by giving a suitable treatment with the appropriatetechnology and then work out the crust thickness. This will substantially reduce the required crustthickness.Uncompacted berms without any treatment cannot withstand the traffic stresses. It is a commonsight and experience that heavy vehicles get stuck up while overtaking and sometimes results inserious 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 shallcomprise lime or Ordinary Portland Cement (OPC)43 grade, moorum of approved quality, sand andCohesive Non swelling Soil (CNS) having properties given below:The Black cotton soil (BC soil) having characteristics as given in Table 1.a.OPC 43 grade as per IS:8112- 1989.b.Well graded granular moorum having minimum 4 day soaked CBR of 10% and maximumlaboratory dry unit weight when tested as per IS:2720 (Part-8) shall not be less than 17.50kN/m3.c.The sand shall be as per IS:383-1970.d.We have 323 guests online Currently OnlineAlexa Rank Practical Problems of Highway Construction in Black Cotton Soil Areahttp://www.nbmcw.com/articles/roads/307-practical-problems-of-highwa...2 of 66/24/2013 12:07 AM  The material for CNS soil should be goodquality soil having laboratory dry unitweight when tested as per IS:2720 (Part-8)not less than 16kN/m3.e. Cement/ Lime-Soil Stabilization The engineering properties of Black cotton soil(BC soil) can significantly be improved with limeor cement treatment. This technology has beenvery common at global level and is in vogue forthe last several years. Cement or hydrated lime inthe range of 3 to 5 per cent brings remarkableimprovement in the engineering characteristics of Black cotton soil (BC soil). The test results of typical Black cotton soil (BC soil) samples aregiven in Table 2 to indicate the improvement in itscharacteristics.Cement/ Lime–soil stabilization technology hasbeen found useful cost-effective and suited tomanual methods of construction. This technologyhas been found 20-30% cheaper thanconventional WBM construction. The cement or lime treatment is being utilized for the followingpurposes:To provide a pavement foundation of marginally weaker in strength than that of concretepavement, but much improved strength than natural Black cotton soil (BC soil).a.To consolidate subgrades and base courses for concrete pavement in order to make themresistant to volume changes and displacement or erosion in the presence of moisture evenunder the rocking action of curled slabs, if any.b.To overcome the susceptibility of foundations to volume change and to increase theirshearing resistance and bearing capacity.c. Pulverization and Mixing Methodology The method of pulverization of Black cotton soil (BC soil) and mixing with cement and moorum orlime and moorum shall be as follows:The Black cotton soil (BC soil) is dug from the ground where the embankment is to be constructedand clods broken with pick-axes so as to reduce them to a maximum size of 50 mm. Alternativelydisc harrows with tractor could be employed. Soil clods are spread over the prepared andcompacted surface of excavation and a smooth wheeled 8 tonne power roller passed over them anumber of times, accompanied by frequent raking of the crushed material. About 8 passes of theroller combined with raking should normally be able to achieve the degree of pulverization. Thedegree of pulverization should be such that at least 80 per cent of soil passes through 475 micronsieve and there are no lumps larger than 25 mm size.The pulverized Black cotton soil (BC soil) is mixeduniformly in given proportion by weight withcement and moorum in-situ field conditions byusing rotavator machine or motor grader. Portlandcement or lime and Black cotton soil (BC soil)mixed at the proper moisture content has beenused to build stabilized bases under concretepavements for highways/ expressways and airfields .Cement/ lime-modified Black cotton soil(BC soil) is a mix that generally contains less than5 per cent cement by volume.This forms a semi-rigid system, improves the engineering properties of the soil and reduces thepotential of the soil to expand by absorbing water. OPC or lime, when mixed with pulverized Blackcotton soil (BC soil) reduces the liquid limit (LL), the plasticity index (PI) and the potential forvolume change. It increases the shrinkage limit and shear strength. Due to the strength increaseincorporated by the use of cement, the Black cotton soil (BC soil) cement mixture can increase thesubgrade bearing capacity and strength significantly. This technique is relatively cheap and quiteeffective. At the ingress of moisture in cement stabilized black cotton soil mix, the water increasesthe 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) forroad embankment construction is as follows:After identifying the stretches where a treatment to strengthen the foundation embankmentis required, the unsuitable Black cotton soil (BC soil) should be removed to the depth of 1600 mm below natural ground level (NGL).a.After excavation, the ground should be checked for density. In case the density is found tobe less than 95 per cent of Max. Dry Density (MDD), a further depth of 225 mm should beloosened and recompacted at Optimum Moisture Content (OMC) till it attains the density of 95 percent of MDD and CBR of 5 per cent.b.Construction of embankment in layers of 200 mm compacted thickness or part thereof,c. Practical Problems of Highway Construction in Black Cotton Soil Areahttp://www.nbmcw.com/articles/roads/307-practical-problems-of-highwa...3 of 66/24/2013 12:07 AM  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.) uniformalymixed by using rotavator machine or by any other farm equipment and rolled to achievedensity 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 1000mm thick–compacted moorum in layers of200 mm each should be provided.d.Compacted CNS soil of size 3000 mm in width and 2000 mm in depth should be provided atthe edges of the compacted sublayers to prevent the entry of ground water to thesub-layers.e.Proper pucca drains on both sides of the road at the NGL along the embankment should beprovided as per the standards.f.Construction details of the remaining upper layers should be as per pavement design basedon the CBR results/ modulus of subgrade reaction.g.Use of good flyash in embankment construction should be permitted wherever available nearthe construction site. Flyash should be conforming to IS:3812 (Grade-1) and it should beensured that availability of flyash is of consistent quality and in required quantity.h.Use of geo-textile fabric has found extensive application in highway engineering, particularly inexpansive soil area. It was used in USA in early 70’s and in India it is initial stage. Geo-textiles aremanufactured from petroleum derivatives such as polypropylenes, polyesters, polyamides etc.These have high tensile strength, burst and puncture strength, permeability and abrasionresistance etc. The use of geotextile fabric is made to provide a separation barrier betweensubgrade and sub-base courses. This technique has tremendous potential for its application inBlack cotton soil (BC soil) area, and it has further been found that this geo-textile layer acts as areinforcing layer, acts and can also be used to combat reflective cracking in the pavementstructure. Also with the use of geo-textile, a reduction in the thickness of the pavement has beenpredicted ranging from 10 to 25 per cent. The cost of the geotextile has been the stumbling factorfor 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 viewof its cost–effectiveness. Moorum of low plasticity index has been found effective and does notallow intrusion up of soft subgrade soil into the interstices of stone aggregates and ingress of waterthrough it. It is a common old practice to provide about 225 mm thick sand blanket layer on softsoils as a barrier to stop intrusion of subgrade soil into interstices of granular base/ sub-base layerand 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 importantparameter in enhancing the life of the pavement. In order to eliminate the ingress of water into thepavement crust, it is preferred to adopt impervious surfacing of low void content, such as denseasphaltic concrete, premix carpet with liquid seal coat, bituminous macadam with seal coat etc. Ithas been found that open graded bituminous surfacings develop unevenness, waviness anddepression during rainy season due to softening of subgrade caused by seeping of water throughsuch open graded bituminous surfacings. In case of construction of new surfacings, it should beensured that it is laid much before the monsoon season. It is felt that time slot of at least onemonth should be given between the end of construction and beginning of monsoon so that surfacecourse gets additional compaction due to traffic. It ensures consolidationand compaction.Shoulders of roads need special treatment, as these are the vulnerable points for vehicles gettingstuck up during overtaking. It is felt that special treatment is needed to make the berms paved andseparate 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 thefour arms of the National Highway Network popularly known as the Golden Quadrilateral havinglength of about 6000 km as a component of National Highways Development Project (NHDP). TheMinistry of Road Transport & Highways (MoRT&H)has authorized National Highways Authority of India (NHAI) to undertake the widening of theexisting 2 lane stretch mostly passing throughBlack cotton soil (BC soil) area from Satara (km725.00) to Maharashtra State Border Section of NH-4 (km 592.240), to 4-lane divided carriagewayand strengthening of the existing 2-lane sectionthrough private sector participation on BOT basis.The Black cotton soil (BC soil) deposits existing onthe road alignment were fully replaced with Cohesive Non- Swelling (CNS) soil having PlasticityIndex (PI) value ranging between 5 to 15 and Free-Swelling Index (FSI) ranging between 7 to 35in the entire length of new carriageway (Fig 4). Since the good quality CNS soil (moorum) wasreadily 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 limeor cement. Fig 5 shows compaction of CNS soil (moorum) in the construction of embankment forthe 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 muchit was expansive. After that it was checked for other properties such as MDD/OMC, CBR and grainsize analysis etc. Every borrow soil/ moorum other than Black cotton soil (BC soil) was used after Practical Problems of Highway Construction in Black Cotton Soil Areahttp://www.nbmcw.com/articles/roads/307-practical-problems-of-highwa...4 of 66/24/2013 12:07 AM
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