CHARACTERIZATION OF SUBGRADE SOIL FOR FLEXIBLE PAVEMENT DESIGN

Abstract

A pavement when subjected to a wheel load under-goes both recoverable (elastic) and irrecoverable (plastic) deformations. The most significant factor influencing the design of a flexible pavement for given traffic and environ-mental conddi.tions is subgrade soil support. Subgrade soil support refers to strength of soil and deformation proper-ties which ,significantly influence (a) the response of sub-grade to the heavy repeated loading of construction traffic and operations,(b) the future success of placement and compaction of overlying layers, and (c) the long-term per-formance of the pavement subgrade. Ideally the subgrade should be strong enough to prevent excessive rutting and shoving and sufficiently stiff to minimize resilient def- lection. To ensure adequate stability and to ensure satis•- factory in-service performance of highway pavements, adequate characterization_ of subgrade soil is essential. Without correct characterization, solutions to boundary value problems cenno t be used with confidence in predicting pave-ment response. Therefore, a systematic study has been made in the present investigation for evaluation of subgrade response to traffic loading and environmental conditions. Four types of soils having IS classifications of SM, SC, T'lL and CH have been selected for the present study. Since CBR method of design is most widely used in India, an in-depth study of CBR values for its possible correlation with index properties of soils has been conduct--ed. Influence of soaking period on CBR values has also been studied. The stresses in subgrade soil under a wheel load can be best simulated by a triaxial test. As such triaxial tests have been conducted on each soil at confinding pre-ssures of 0.4 to 1.0 kg/cm2, depending upon the stress conditions in the subgrade soil. Because pavement materials are subjected to repetitive applications of traffic loads, it was felt that the dynamic modulus could best characterize subgrade behaviour under traffic loads compared to other methods of characterization. Oscillatory shear tests were conducted on each soil under the simulated highway loading conditions. Influences of applied shear stresses, verti-cal stresses9 number of loading cycles etc on dynamic modulus of elasticity have been studied for each type of soil. Corre-lations of dynamic E and conventional CBR values have been studied. Froi:, the tests conducted in the oscillatory shear-apparatus, it has been found that the dynamic modulus values of subgrade soils are in close conformity with the values reported by other.researchers.