STUDY ON SUBBASE MATERIALS FOR RURAL ROADS

Abstract

In recent years there has been growing emphasis and interest worldover towards promoting the use of marginal materials in road construction in order to effect cost savings, reduce pressure on good quality aggregates and also to protect environment. These include natural non-traditional materials, soils, rocks, local materials (river bed materials and stone dust), wastes and industrial by products which fall outside the normal specifications and are not in common use as road construction materials. The specifications for rural roads published by the Indian Roads Congress (MORD, 2004) provide for use of local materials in road construction where quality materials are not available within economic lead. These specifications also recommend use of industrial wastes such as fly ash, a waste material from thermal power plants and slags, waste materials from iron and steel industry, in road construction. With a view to save the costly aggregates and preserve the environment, it is desirable that local materials including industrial waste are used to the maximum extent in the rural roads constructed under State or centrally sponsored schemes. The use of local materials would result in economy in unit rates, thus within the same budget, additional work can be executed. The generation of waste materials like steel slag and fly ash is associated with production of iron and steel. The generation of slag and fly ash is so high during production of iron and steel that most of the steel plants are now facing shortage of dumping space for these waste materials and also it is causing serious environmental pollution. A serious attempt has been made in the last few years by Indian steel industries and thermal power plants to utilize fly ash in brick-making and steel slag as rail ballast. However, the utilization rate has not improved significantly yet. The utilization of wastes from steel industry (LD slag, granulated blast furnace slag and blast furnace slag) and thermal power plants (fly ash) in lower layer (subbase) of a flexible pavement has not received much attention of road designers and construction agencies in India. It is mainly due to lack of information on behaviour of these materials under static and dynamic load conditions. Although fly ash has been successfully used as filler material in road embankments, its application 11 in subbase layers has not yet been attempted. The present research is aimed at developing experimental data base for utilization of these wastes in subbase layer of a pavement by examining their static and dynamic strength characteristics through laboratory and FE analysis. Seven materials are evaluated in this study. These are conventional subbase material (CSM), blast furnace slag (BFS), coarse sand, Linz-Donawitz (LD) slag, stone dust, granular blast furnace slag (GBFS) and reinforced fly ash. Conventional subbase material consisted of mixture of river bed material and crushed stones. BFS, GBFS and LD slag were obtained from Tata Steel Plant, Jamshedpur (India). Fly ash (Class F) used in this study was brought from National Thermal Power Corporation Ltd. Dadri (Ghazibad) in Uttar Pradesh, India. CSM, stone dust and coarse sand were obtained from local stone crusher and the river. Since the behaviour of granular materials is extremely complex and cannot be evaluated by a single test, the CBR tests, static triaxial and cyclic triaxial tests were conducted to study their behaviour in terms of shear strength parameters, resilient strain, permanent strain and the resilient modulus. Three types of subgrade soils are used. These are referred as soil A, B & C in this report. Their CBR values are 2.0 %, 4.15 % and 6.20 % respectively. Static triaxial tests were conducted on all the three types of subgrade soils to determine the stress-strain behaviour, failure stress and failure strain and modulus of elasticity as per ASTM D 2850. CBR, static triaxial and cyclic triaxial tests were conducted on all the seven subbase materials to study their stress-strain behaviour, resilient modulus, resilient strain and permanent strain behaviour. As FE analysis is used in the present study to compare the performance of different subbase materials, a flexible pavement was designed as per IRC:37, 2001 with layers of subbase, base (WBM type), bituminous base course (Dense bituminous macadam) and wearing course (bituminous concrete). Triaxial tests were conducted on these materials also (WBM, DBM and BC) for input in the FE analysis. The results of soaked CBR tests conducted on various subbase materials showed that BFS has the highest and reinforced fly ash, GBFS and LD slag have the minimum CBR value. The results of static triaxial tests conducted on subbase, base (WBM), DBM and BC at 40 kPa confining pressure revealed that the modulus of elasticity increases with confining pressure. Straight line relations are developed for all the seven materials between modulus of elasticity and confining pressure. These in results are used as input parameters in ANSYS for calculating the vertical axial strain at top of subgrade. The initial tangent modulus (Ej) for soil A,B and C are obtained from triaxial tests and these are 11.18, 18.0 and 24.0 MPa respectively. Cyclic triaxial tests were conducted on subbase materials to simulate the repetitive nature of actual loads imposed by the moving vehicular traffic. It is the most reliable and suitable method for determining the resilient modulus of soils and other unbounded materials and also for studying the deformation characteristics of such materials under repeated cyclic stress. The cyclic triaxial tests were conducted at three confining pressures of 40, 70 and 120 kPa and two vertical cyclic stresses (o\) of 108 kPa and 195 kPa to produce three deviator stresses (ov 03) of 68, 95 and 125 kPa. The test was conducted up to 10000 cycles of load application and resilient and permanent strains were observed after 1, 10, 100, 1000, 5000 and 10000 cycles of load applications. A five layer flexible pavement system was considered and analyzed using finite element software ANSYS. Roller supports were provided along the axis of symmetry, right boundary and bottom boundary to achieve the boundary conditions. The right boundary was placed at 110 cm away from the outer edge of loaded area and a uniform pressure of 575 kPa was applied on a circular contact area of radius 150 mm. The pavement sections are designed for 100 million standard axles, and the thickness of layers are decided based on the Indian code of practice, IRC: 37-2001. Three types of subgrade soils are considered to investigate the effect of subgrade quality on pavement response. The vertical compressive strain at the top of subgrade with varying thickness of subbase, base and DBM were calculated with help of ANSYS. In all analyses, the thickness of Bituminous Concrete (BC) layer was kept constant. The laboratory results indicate that out of the seven materials used in the present study, the CSM is the strongest material and reinforced fly ash is the weakest. Pavement section with CSM in its subbase was considered as standard section and the life of other pavement sections with remaining six subbase materials are estimated. The results suggested that the life of a flexible pavement reduces when industrial waste materials are used in its subbase layer. The equivalent thickness of industrial waste materials in terms CSM for same service life of a pavement sections are calculated. Other options are also suggested by keeping the subbase thickness as per design and compensating remaining thickness by either base layer or in some cases by iv the DBM layer. The cost analysis ofthe pavements with different subbase materials is also carried out. The initial construction cost of each item was worked out in detail and it is found that the cost of construction reduces by 6.3 % to 17.6 % when waste materials are used in subbase layer of a pavement.