Determination of liquid limit and plastic limit of clays using cone penetrometer

Abstract

This study employed an alternative method to determine soils' two significant geotechnical index properties, the liquid limit (LL) and the plastic limit (PL). As the traditional methods are time-consuming, need much human labor, and have faced criticism for being subjective, the study attempted to use cone penetration methods as an alternative to determine the LL and PL of soils. The study aims to reduce the operator variability and improve consistency in determining these two geotechnical properties of soils. The study also examined the relationship between soil minerals and soil type to determine further insights into soil behavior. Soil samples were collected from 20 locations across Bangladesh and then tested using ASTM standard procedures. The LL was determined using the Casagrande method, and the PL was determined through the hand-rolling method (ASTM D4318). The LL obtained from the cone penetrometer method was then compared with the Casagrande method. To determine the PL by cone penetrometer, first, the standard penetration depth for plasticity was determined using the 30-degree standard cone utilizing the hand-rolling method. Then, the standard penetration depth was used to determine the PL of the soils, which was subsequently compared with the standard method. Modifications were made to conventional penetrometer cones, with two additional cone angles of 20 and 16 degrees fabricated alongside the standard 30-degree cones to determine the liquid and plastic limits. The study findings revealed that the cone penetrometer method measurements of LL and PL, well enough and matches with the standard method. For LL determination, the 30-degree cone penetrometer measurement deviates from -10% to +7% for low to high plasticity soil by comparing with the standard Casagrande method. For soil with very high plasticity, the deviation was higher (>25%). For PL determination, the results showed much better, with the measurement deviation varying between -3.28% and +6.63% by comparing with the standard hand-rolling method. The standard 30-degree cone proved to be the most reliable for the simultaneous determination of liquid and plastic limits. Among the soil minerals identified from the X-ray diffraction (XRD) analysis, the Illite clay mineral was the most abundant (11.21% to 25.50%) for most of the soil samples, correlating with plasticity and grain size results. The study's findings are expected to be an alternative to the standard methods in determining the liquid and plastic limits of the soil, which will reduce human labor and the complicacy of the standard procedure.