A new method of measuring plastic limit of fine materials

Abstract

Index properties such as the liquid limit and plastic limit are widely used to evaluate certain geotechnical parameters of fine-grained soils. Measurement of the liquid limit is a mechanical process, and the possibility of errors occurring during measurement is not significant. However, this is not the case for plastic limit testing, despite the fact that the current method of measurement is embraced by many standards around the world. The method in question relies on a fairly crude procedure known widely as the ‘thread rolling' test, though it has been the subject of much criticism in recent years. It is essential that a new, more reliable method of measuring the plastic limit is developed using a mechanical process that is both consistent and easily reproducible. The work reported in this paper concerns the development of a new device to measure the plastic limit, based on the existing falling cone apparatus. The force required for the test is equivalent to the application of a 54 N fast-static load acting on the existing cone used in liquid limit measurements. The test is complete when the relevant water content of the soil specimen allows the cone to achieve a penetration of 20 mm. The new technique was used to measure the plastic limit of 16 different clays from around the world. The plastic limit measured using the new method identified reasonably well the water content at which the soil phase changes from the plastic to the semi-solid state. Further evaluation was undertaken by conducting plastic limit tests using the new method on selected samples and comparing the results with values reported by local site investigation laboratories. Again, reasonable agreement was found.

Index and strength properties of clay–gravel mixtures

“…This value reflects a reasonable 'average' value to those reported in the literature (e.g. Brown and Huxley, 1996;Karlsson, 1961;Sharma and Bora, 2003;Sivakumar et al, 2009), and when used with Equation 10 corresponds to an undrained strength at the liquid limit of 1 . 7 kPa.…”

Use of fall cones to determine Atterberg limits: a review

“…Sherwood, 1970;Sherwood & Ryley, 1970;Whyte, 1982;Belviso et al, 1985;Sivakumar et al, 2009). To investigate this point, reported PLs determined independently by four laboratories for 11 inorganic fine-grained soils of intermediate to very high plasticity (see Table 1) were considered.…”

“…Simply based on analysis of historical data, as the ratio of strengths at the PL and LL varies substantially between soils, these strength-based approaches can only coincidentally give correct PL values, actually measuring what might be termed the plastic strength limit (PL 100 ); that is the water content corresponding to s uFC ¼ 100 Â s uFC(LL) . Fall cone Belviso et al, 1985;Wasti, 1987;Harison, 1988;Feng, 2000Feng, , 2001Feng, , 2004Koumoto & Houlsby, 2001;Sharma & Bora, 2003;Lee & Freeman, 2009;Shimobe, 2010;Sivakumar et al, 2015), steady monotonic penetration (Stone & Phan, 1995;Stone & Kyambadde, 2007), fast-static loading (Sivakumar et al, 2009) and extrusion (Timár, 1974;Whyte, 1982;Medhat & Whyte, 1986;Kayabali & Tufenkci, 2010a, 2010bKayabali, 2011aKayabali, , 2011bKayabali, , 2012Kayabali et al, 2016) approaches for PL determination have all been suggested as alternatives to the standard thread-rolling approach. As mechanical tests, these strength-based approaches are seen by some researchers as means of achieving higher degrees of repeatability and reproducibility of results, although, to date, most fall-cone research has been conducted on well-behaved clay-rich soils that lie above the A-line on the standard plasticity chart.…”

Discussion: Measuring the plastic limit of fine soils: an experimental study

“…Sample preparation Sivakumar et al (2015) state that sample preparation was done in accordance with the procedures outlined by Sivakumar et al (2009), utilising oven drying during the process. However, Mesri and Peck (2011: pp.…”

“…However, Mesri and Peck (2011: pp. 89-90), in their discussion of the paper by Sivakumar et al (2009), cast doubt on the use of oven drying for sample preparation. Does the sample preparation process cause the material to enter at least a partially brittle (or cracked) state?…”