A rational approach of the design for highway formations

Highways are important components for bolstering the economic development and industrial growth of every country, and hence, should be well designed. The formations should be stable enough to facilitate increased efficiency while ensuring longer life coupled with low maintenance costs, thus, safely allowing the running of heavier and faster vehicles. Two approaches in improving highway formations have been investigated in this thesis. In the first approach, studies on improving the subgrade strength by adding natural agricultural wastes as additives was carried out. In the second approach, a more realistic methodology for the design of highway formations was investigated. Accordingly, the improvement of soil subgrade by adding different percentages of Oil Palm Shell aggregate (OPS agg), Oil Palm Shell granule (OPS granule), Oil Palm Shell ash (OPS ash), and lime was studied. The California Bearing Ratio (CBR) test was used to investigate this improvement and to determine soil suitability for highway formations. It is suggested that optimal improvement in the CBR values can be attained by adding 10% OPS agg, S% OPS granule, 30% OPS ash and 3% lime for subgrade soil in unsoaked condition, and 30% OPS aggregate, 10% OPS ash, 10% lime in soaked condition. The empirical CBR test is traditionally used for the design of highway formations. However, pavements and runways are also designed using the elastic foundation approach, I.e., the modulus of subgrade reaction, ks, and Modulus of Elasticity, E. Both ks and E are determined by plate load tests and widely used by Geotechnical engineers for the design of substructures. In this thesis, to take advantage of several available advanced methods using the elasticity approach, the CBR test is correlated with E and ks using the Finite Element Method. Thus, the choices available for the design of highway formations can be widened and methods used in highway engineering and geotechnical engineering could be Integrated. In this context, with vehicles actually transmitting cyclic stresses onto the highway formations, it is crucial to determine the threshold stress of the soil. This approach is developed by conducting cyclic triaxial tests on soils. The design of highways using threshold stress approach proposed to take into account a more realistic assessment of the soil behaviour that is subjected to repeated loadings due to running vehicles compared to the CBR test-based design. Cyclic triaxial test is used to determine the subgrade properties needed, including the threshold stress. Accordingly, Unconfined Cyclic Triaxial tests, Unconsolidated Undrained Cyclic Triaxial tests and Consolidated Undrained Cyclic Triaxial tests are conducted on the soil. Stresses caused by wheel loads of vehicles are computed using Boussinesq's solutions for Simplicity. The formations thickness is then obtained in such a manner that the induced stresses on top of the subgrade are less than the threshold stress as determined in the above-mentioned laboratory tests. Design charts are developed for designing highway formations using threshold stress of the soil. Typical designs are compared to the CBR values-based design.