Effect of fine percentage on properties of subbase material

Abstract

With the huge infrastructure development in Sri Lanka, road construction plays a vital role. Massive quantities of construction materials are required for these highway and expressway constructions. Finding Subbase material as per specification is a major issue in most part of the country. Therefore, in some road construction projects, crushed stone is used as an alternative material to replace Subbase material. Due to the scarcity of good quality material, there is a need of research to use marginal materials for sustainable development in the highway industry. Standard Specification for Construction and Maintenance of Roads and Bridges(SCA/5) (SSCM) (ICTAD,2009) is used as a road construction specification in Sri Lanka. Liquid limit(LL), plastic limit(PL), maximum dry density(MDD), California Bearing Ratio (CBR), and sieve analysis are specified in selection of gravel Subbase material. According to sieve analysis requirements in SSCM, percentage of passing 75μm sieve should be 5-25% by weight. This grading limit for Subbase material was adapted to the specification in second edition of SSCM in 2009.Questionnaire survey conducted among senior engineers has expressed that one of the least important parameters in material selections was grading (84% of the participants) and 16% of the engineers have expressed grading as the most difficult parameter to meet. This study was conducted to evaluate the possibility of relaxing the passing percentage of fine fraction. Experimental study was conducted by altering the fine fraction of soils, varying from 0-40%. Properties of those samples were tested and it revealed a linear relationship with high correlation factor between fine fraction of the material and its properties (CBR, MDD, OMC). Only three samples out of ten samples were within the grading band requirement and nine samples out of ten samples satisfied CBR requirements. By scrutinizing the findings and available literature, it can be recommended that grading band of No.200 sieve passing can be relaxed up to 35% if soil sample satisfy the specified CBR requirement (30), PI value is less than or equal to 10, and swell percentage is less than 2%. Further, linear regression models were fitted to assess the CBR of material with reference to fine fraction(Percentage passing of 425μm, 300μm, 75μm sieves). Statistical analysis explained that material passing 425μm and retained on 300μm, and 75μm passing percentage are the significant parameters when predicting CBR of the selected soil in this study.