CORRELATION OF THE UNDRAINED SHEAR STRENGTH AND PLASTICITY INDEX OF TROPICAL CLAYS

Abstract

This paper attempts to establish a relationship between the undrained shear strength and plasticity index of tropical clays. Its theoretical basis lies with the previous work of Skempton and North (1953) and Atkinson and Bransby (1978). The data obtained from independently carried out laboratory tests on some clay samples in several actual project locations in Eastern Nigeria, were subjected to statistical least squares regression analysis, after the samples had been grouped into CL, CI and CH using the Unified System of Soil Classification. The following regression equations between undrained shear strength (q₂) and plasticity index (PI) were obtained:

CL Soils:     log (q_u\O₃) = 0.0488 – 1.01 (PI/100)
                    with a correlation coefficient of -0.88

CI Soils:      log (q_u\O₃) = 0.0328 – 1.47 (PI/100)
                    with a correlation coefficient of -0.93

CH Soils:    log (q_u\O₃)= -02292 – 0.96 (PI/100)
                     with a correlation coefficient – 0.88

These regression equations are seen to possess high correlation coefficients and can be used effectively by geotechnical engineering to predict the shear strength of tropical days for foundation designs.