###### Abstract
The ASTMC181-11 standard [44] presents a test to determine a \"workability index\" for fireclays and high alumina refractory plastics by measuring the plastic deformation of a molded test specimen when subjected to impact. This approach is comparatively similar in function and outcome to the moisture condition apparatus and test [45] that are used to assess the suitability of mainly cohesive soils for incorporation into earthworks. With tougher soil lumps being more resistant to remolding, thereby requiring more compaction energy and giving higher moisture condition values (MCVs), [PERSON] [8] suggested that the moisture condition test could potentially be used to assess soil toughness. [PERSON] and [PERSON] [46] performed extrusion tests on brick clays, measuring their extrusion rates as a function of the applied extrusion pressure (\(p_{\mathrm{e}}\)) for different clay contents. Subsequently, CERAM Research [47; 48] developed a ceramic rheology tester, termed the Martin flow instrument (MFI), which is a modified version of the melt flow indexer used for testing the flow properties of plastics and polymers. For the MFI, a standard length of cored soil sample is inserted into a cylindrical barrel that contains a die at the base. A piston placed inside the barrel applies pressure via a deadweight load to cause the extrusion of the soil to occur through the die orifice. The time period required for a known soil volume to flow through the die orifice is measured, with the flow time (extrusion rate) dependent on the soil toughness, with tougher materials taking longer [47]. The soil sample, from soft earthquake clays to some advanced ceramic materials, is tested within a preferred shear rate range (relevant to the subsequent processing of the material) by employing an appropriate deadweight in the range of 2-30 kg applied to the piston [47]. In soil mechanics, the extrusion approach has also been investigated for the testing of fine-grained soils to possibly assess/measure their toughness [12; 13], including proposed power-based extrusion approaches for consistency limits determination [13; 49]. These aspects will be discussed in detail later in the paper. Closely replicating [PERSON]'s hand rolling of threads method, [PERSON] [8; 17; 18] developed a mechanical thread rolling apparatus and test that allow nominal applied stress and diametrical strain measurements for a uniform soil cylinder (thread) during the rolling out procedure. The toughness-related soil properties and the value of \(w_{\mathrm{P}}\) can be determined from the obtained experimental \(T\)-\(w\) plots (associated methods are elaborated later in Section 2.1). Based on this research, [PERSON] [17] proposed a tentative toughness classification based on the remolding toughness mobilized at the \(w_{\mathrm{P}}\) water content (i.e., the maximum remolding toughness, \(T_{\mathrm{max}}\)). Finally, the work of [PERSON] and [PERSON] [11] is briefly introduced at this point. They obtained a quantitative relationship between the \(I_{\mathrm{P}}/w_{\mathrm{L}}\) ratio and \(T_{\mathrm{max}}\), thereby allowing toughness estimation for fine-grained soil based solely on consistency limits measurements, allowing them to develop a soil plasticity classification system in which the determining factor is toughness. The aim of the present paper is to present a state-of-the-art review of soil remolding toughness, with emphasis on its experimental determination and the quantification of soil toughness using various related index properties and coefficients. This review focuses particularly on various extrusion proposals [12; 13; 47; 48; 49] and [PERSON] mechanical thread rolling apparatus and methodology [8; 17; 18], which are presently the only means available for obtaining quantitative soil remolding toughness measurements. Challenges arise in the quantification of, and in assigning units of measurement to, the soil remolding toughness. These aspects are discussed in the context of [PERSON] thread rolling approach.