Wetting Effect on CBR Properties of Compacted Fine-Grained Residual Soils

Abstract

Fine-grained residual soils are widely used as compacted fill materials, although they are well-known to be vulnerable to weakening when wetted, leading to fill slope failures. A practical approach to evaluate this wetting effect based on typically available properties is being developed. Extensive evaluations of laboratory compacted soils and variability of field compacted soils, followed by the development of a simple deformation-based stability model, are reported. The wetting effect is represented by the CBR soaked to unsoaked ratio, and the soil activity can be used as a predictor of this ratio. Compaction effort has a significant effect on this ratio. Effects of variability of compacted soils and uncertainty of prediction equation are incorporated into the stability model to evaluate the wetting effect on slope probability of failure. INTRODUCTION Fine-grained residual soils are widely used in Java, Indonesia as compacted fill materials due to their availability and associated significantly lower costs. However, these materials are wellknown to be vulnerable to wetting, and their mechanical properties tend to weaken with an increase in water content. In some cases, these weakened materials are suspected to lead to fill slope failures. To reduce future risk of failures, quantifying this weakening process and its uncertainty is important. Research on this problem has progressed (e.g., Melinda et al. 2004), but most of the approaches are technically prohibitive for typical compacted soil projects. Therefore, developing a practical approach to this problem is of interest. This study is an attempt to develop a practical approach to evaluate the wetting effect on the mechanical properties of these materials. As the emphasis is on the practical aspect, only typically available properties in compacted soil projects –– such as index properties, compaction properties, and California Bearing Ratio (CBR) ratio –– are evaluated to find relationships among these groups of properties. To limit the effect of geological background, only fine-grained residual soils from quarries in western Java are considered. In this paper, properties of laboratory compacted fine-grained residual soils are evaluated, followed by a brief discussion on field variability. A simple deformation-based stability model for evaluating the wetting effect of compacted finegrained residual soil slopes is subsequently described, followed by a discussion on the wetting effect on the probability of failure of those slopes. LABORATORY COMPACTED FINE-GRAINED RESIDUAL SOILS The wetting effect on the properties of compacted fine-grained residual soils was evaluated through laboratory tests. The number of data sets tested by means of the standard Proctor compaction method (ASTM 698, 2000) is 40 data sets, while that tested by means of the modified Proctor compaction method (ASTM 1557, 2002) is 25 data sets. The data are from different project files maintained by the Geotechnical Laboratory of Universitas Indonesia. All soils were