Finding sub-base material as per required specifications is a major issue in many parts of Sri Lanka. Therefore, in some cases, crushed stone is used as an alternative to sub-base material. Since good quality material is scarce, it is worthwhile to study the use of marginal materials to ensure sustainable development of the highway sector. The specification used in Sri Lanka for road constructions is the Standard Specification for Construction and Maintenance of Roads and Bridges (SCA/5) (SSCM) (ICTAD, 2009). Sub-base material is classified as lower sub-base material and upper sub-base material according to their Liquid Limit (LL), Plastic Limit (PL), Maximum Dry Density (MDD), California Bearing Ratio (CBR), and Sieve Analysis. A questionnaire survey was conducted among those engaged in the construction industry, to collect information on sub-base construction and on issues connected with the current specification relating to the passing percentage of the fine fraction. As a result, an experimental study was conducted by altering the fine fraction of soil from 0% to 40%. It was also verified whether the grading band of No. 200 sieve passing can be relaxed up to a certain percentage if the soil sample satisfies the specified CBR, Plasticity Index (PI) value and swell percentage requirement. It was found that the proposed changes have been adopted in AASHTO and Road Note 31 specifications. Furthermore, linear regression models were fitted to assess the CBR of material in relation to fine fractions (percentage passing of 425 mu m, 300 mu m, 75 mu m sieves). These passing percentages were identified as the most important percentages in predicting the CBR of the soil tested.
