Unconfined compression strength as consistency index for compacted clay in construction – a statistical approach

ABSTRACT: In this paper, unconfined compression strength (UCS) as obtained from an unconsolidated-undrained test is used as a consistency index for classification of natural clay. For saturated clay, the UCS or undrained shear strength is independent of the confining pressure and is an indicator of consistency in stiffness. However, the structure of compacted clay varies as a function of compaction effort and water content and as such is more heterogeneous than that of natural clay. This study investigates whether UCS can be used as a consistency index for compacted clay. Two different types of clay, Calgary till and Regina clay, were used with clay contents of about 15 and 45%, respectively. Compacted clay specimens were prepared at varying water contents using the standard Proctor method. Computer X-ray scanning technique and filter paper method were used to quantify the variation of bulk density and matric suction along the specimen’s height, respectively followed by UCS determination. Empirical correlations among bulk density, matric suction and UCS were developed. In addition, these data were analyzed using a statistical approach. The results provide confidence levels for the use of UCS as a consistency index for compacted clay used in construction.