Citation: O’Kelly, B.C. (2022) Appraisal of novel power -based extrusion methodology for consistency limits determinations of fine- grained soils. In: Proceedings of the Civil Engineering Research in Ireland 2022 Conference, 25–26 August 2022, Dublin, Ireland (Holmes, N., de Paor, C., and West, R.P. (Eds.)). Civil Engineering Research Association of Ireland. Vol. 1, pp. 317 –322. 317 ABSTRACT: The consistency limits (liquid limit LL and plastic limit PL) are among the most commonly performed tests in geotechnical engineering practice, being used for classification of fine-grained soils and in deducing other parameters (e.g., shear strength, permeability, compressibility), necessary for preliminary design/assessments, via numerous correlations built up over the decades. Depending on geographic region and/or referenced standard, the LL testing is performed using either the fall-cone or Casagrande (percussion-cup) approaches, while Atterberg’s PL is universally determined using the thread-rolling method. Because of its dependence on operator judgement regarding the crumbling condition during the rolling-out procedure, the accuracy of the PL test has been called into question by some researchers. This has prompted various alternative proposals, including undrained strength-based fall cone and extrusion approaches, but these are not appropriate for the determination of Atterberg’s PL which defines the water content at the plastic–brittle transition point (i.e., not strength-based). This paper presents the culmination of a two-year research project performed at Trinity College Dublin on the investigation and development of a novel power-based extrusion methodology for determination of the consistency limits of fine-grained soils. KEY WORDS: Atterberg limits; cohesive soil; extrusion; strength; workability. 1 INTRODUCTION Plasticity of fine-grained soils is considered a function of the liquid limit (LL) and plastic limit (PL), these consistency limit parameters having wide importance for civil/geotechnical engineering and agronomic applications, and in the ceramic industry and brick manufacturing process. The LL, notionally understood as the water content below which fine-grained soil ceases to flow as a liquid, is invariably determined using the fall-cone or Casagrande (percussion-cup) approach. The PL (i.e., the water content at the plastic–brittle transition point)  originally proposed by Atterberg (1911)  identifies a genuine observable transition in soil behaviour, as conventionally determined by rolling-out on a glass plate of soil threads for the standard PL test. Various alternative experimental approaches have been proposed, particularly for PL determination, typically involving variants of the fall-cone approach, but also investigating the extrusion method (see the review papers by O’Kelly (2019, 2021b) and O’Kelly et al. (2018)). Proposed fall-cone approaches define a ‘plastic strength limit’ PSL (term coined by Haigh et al. (2013)) water content, typically associated with a remoulded undrained shear strength (sur) value of 100-fold greater than that mobilised at the fall-cone LL (i.e., LLFC) water content. It must be emphasised that the PSL and Atterberg’s PL are fundamentally different parameters (Haigh et al., 2013; O’Kelly, 2013; Sivakumar et al., 2016). Extrusion involves the reduction in cross-sectional area of a billet (fine-grained soil test-specimen for the purposes of this investigation) by forcing it to flow through a die orifice under the action of an extrusion pressure. There are two approaches, direct extrusion and reverse extrusion (see Figure 1); the latter generally being preferred for soil mechanics’ applications, since the friction component (mobilised between the test- specimen and the chamber side-wall) associated with direct extrusion does not arise for the reverse extrusion approach. (a) (b) Figure 1. Schematic diagram of typical experimental set-up for (a) direct extrusion, and (b) reverse extrusion (after O’Kelly, 2019). The first attempts at using the direct and reverse extrusion methods for consistency limits determinations of fine-grained soils were reported by Timár (1974) and Whyte (1982), respectively. Several testing parameters affect the extrusion force Fe (extrusion pressure pe), such as the cylindrical chamber’s geometry/dimensions and extrusion ratio R, the ram displacement rate (velocity), ram–chamber side-wall friction, and (for direct extrusion) the soil specimen–side-wall friction. Note: pe =  e  o ⁄ and = o  f ⁄ , where Ao is the bore cross- sectional area of the cylindrical chamber and Af is the die orifice area. The principal material parameter affecting the extrusion Appraisal of novel power-based extrusion methodology for consistency limits determinations of fine-grained soils Brendan C. O’Kelly 1 1 Department of Civil, Structural and Environmental Engineering, Trinity College Dublin, Dublin D02 PN40, Ireland email: bokelly@tcd.ie