An investigation into the correlation between California bearing ratio and thermal conductivity of soil

Abstract

Californian Bearing Ratio (CBR) value of sub-grade is used often for design of flexible pavements. In practice, only limited number of such tests could be performed because of high unit cost and time required for such testing. As a result, in many cases, it is difficult to reveal detailed variations in the CBR values over the length of roads. In such cases if the estimation of the CBR could be done on the basis of some tests which are easy to perform, less time consuming and cheap, then it would be easy to get the information about the strength of sub-grade over the length of roads. By considering these aspects, our study focused on an investigation into a correlation between CBR and thermal conductivity of soils with specific aims of classifying the soil samples, establishing the relationship between soaked CBR and dry density of the soil samples and establishing the relationship between soaked CBR and thermal conductivity of the soil samples. The soil properties were determined in the laboratory with reference to international standards of American Society for Testing and Materials (ASTM) and British Standards Institute (BSI) such as BS 1377 for particle size distribution, moisture content, liquid limit, plastic limit, CBR-three point and compaction test and ASTM D5334 for thermal conductivity test. The sample soils before compaction were generally classified as GC: clayey gravels (gravel-sand-clay mixtures), GM: silty gravels (gravel-sand-silt mixtures), MH: inorganic silts and CH: inorganic clays of high plasticity whereas on compaction the soils were generally classified as SW: well-graded sands and SC: Clayey sands (sand-clay mixtures) using the Unified Soil Classification System (USCS). A linear positive correlation between CBR and thermal conductivity for a given soil and a linear negative correlation for different soils on both side of the compaction curve is observed. The later observation is explained by classifying the soils after compaction and studying the change in particle size distribution properties such as grading modulus, grading coefficient, coefficient of uniformity and coefficient of curvature and their effect on thermal conductivity and CBR. Given the correlations established between CBR and thermal conductivity of soils, much more data should be collected for a wide range of varying soil classes or types to increase the reliability of the correlations for predicting soil CBR using soil thermal conductivity and swell indices should be integrated into the models.