## 1 Introduction  
In the analysis of slopes that have experienced large shear displacements, the relevant factor is the residual shear strength of the soil. This is most commonly determined based on laboratory tests using a direct shear apparatus by performing a reverse test, or using an apparatus that allows unlimited horizontal deformation (ring shear apparatus). In geotechnical practice in Serbia, as well as globally, the use of the ring shear apparatus is not common, given that it falls under research equipment rather than standard laboratory equipment. The direct shear test is the oldest and simplest type of laboratory testing for shear strength. This conceptually simple test has been used to assess soil since 1776 by [PERSON] [1], and it was notably highlighted by French engineer [PERSON] in 1846 [2]. In Britain, [PERSON] (1915) recorded the first measurements and constructed a device intended to be a prototype for the later development of the direct shear apparatus. [PERSON] was the first to conduct and publish practical results of shear strength testing for different soil types [3,4]. The modern shear box was designed by [PERSON] at Harvard University (USA) in 1932 [5]. An apparatus with a constant deformation rate applying the \"controlled deformation\" principle using a fixed-speed motor was developed in 1946 [6], [PERSON], 1946 presented detailed design improvements using this principle. [PERSON], 1984 developed another shear apparatus capable of performing both drained and undrained tests [7]. Today, there are many devices from different manufacturers used for conducting direct shear tests, which fundamentally do not differ much in their testing mechanisms. It was in 1936 and 1939 that [PERSON] proposed the first general concept for constructing a ring shear apparatus [8,9]. Following this, several different designs of the apparatus were developed based on this concept; examples include: La Gata (1970), [PERSON] et al. (1971), [PERSON] (1979), [PERSON] (1984), [PERSON] and Morgenstern (1984), [PERSON] (1989), and [PERSON] (2002) [10, 11, 12, 13, 14, 15, 16]. However, the ring shear apparatus constructed by [PERSON] in 1979 has become increasingly popular due to its affordability and ease of use [12]. [PERSON] and [PERSON] (1983) demonstrated that this apparatus produces results consistent with those obtained using more sophisticated devices developed by the Norwegian Geotechnical Institute and Imperial College [17]. Since 1984, Prof. [PERSON] et al. from the Disaster Prevention Research Institute (DPRI) at Kyoto University have developed seven designs of ring shear apparatuses that allow for the laboratory simulation of seismic impacts and the measurement of pore pressures during undrained tests through a control system [18]. The concept of residual shear strength first appeared in the literature in 1937 [19]. The residual shear strength of soil is an important parameter in geotechnical engineering. It is the strength reached after large displacements. Further shearing in the same direction does not lead to a decrease in shear strength, as it remains constant; thus, there are no further changes in volume or changes in excess pore pressures. Understanding residual shear strength at large displacements is crucial in explaining the mechanism of sliding [2,16,20] Conventional laboratory methods for defining shear strength, such as the direct shear test, allow for maximum displacements up to 10 mm for samples measuring 6 \(\times\) 6 cm [21, 22, 23]. For these reasons, assessing soil behavior at large displacements is challenging since residual shear strength can only be defined through multiple shear cycles. The ring shear test, which allows unlimited displacement of the sample, is a much more reliable method for determining residual shear strength [22,24], as evidenced by several scientific studies [25, 26, 27, 28]. For example, [PERSON] et al. (2007) applied a new concept using the ring shear test to define shear zones that occur at large displacements.