# Application of Direct Shear Test to Analysis of the Rate of Soil Improvement with Polyester Fibres
## 1 Introduction  
In practice, in many cases, an improvement of soil properties proves necessary. Since soil shear strength is one of the most important properties, it is often required to be improved. To evaluate the improvement rate of soil shear strength, despite some shortcomings (predefined shear plan, uneven distribution of shear stress, small shear box, etc.) of the direct shear test (DST), many authors apply the DST in their research due to its simplicity. The DST has been used, e.g., to evaluate soil improvement with fibres [1; 2; 3; 4; 5], lime and fly ash [6], and palm oil fuel ash (POFA)-based geopolymer [7]. In addition to the use of DST in evaluation of the improvement rate of soil shear strength, DST has been used, e.g., for evaluation of the reinforcement effects of conventional georelites, geogrids, and novel georelites [8], evaluation of the influence of the scale effect on the shear strength of scaled coarse-grained soil (CGS) [9], and analysis of coarse-grained soil behavior during shearing [10]. Moreover, DST has been used, e.g., for the determination of soil shear strength parameters, which are further used in machine learning (ML) models built for their predicting for other soils without the need to carry out DST [11]. [PERSON] et al. [1] investigated the effects of discrete short polypropylene fibre (PP-fibre) on the strength and mechanical behaviour of uncomented and cemented clayey soil (CL). In the presented investigation, 12 groups of soil samples were prepared at three different percentages of PP-fibre content (i.e., 0.05%, 0.15%, and 0.25% by weight of soil) and two different percentages of cement content (i.e., 5% and 8% by weight of soil), and unconfined compression and direct shear tests were carried out after 7-, 14-, and 28-day curing periods. The specimens for DST were shaped in a cylindrical mould of 20 mm in height with an inner diameter of 61.8 mm. The tests were performed at four vertical normal stresses, 50, 100, 200, and 300 kPa, in order to define the shear strength parameters. The authors stated that the test results indicated that the inclusion of fibre reinforcement within uncomented and cemented soil caused an increase in shear strength. The introduced increase in the values of the angle of internal friction was very small. So, e.g., after curing for 28 days, the values of the angle of internal friction of soil with 5% cement content and 0.05%, 0.15%, and 0.25% fibre content were 35.1\({}^{\circ}\), 36.3\({}^{\circ}\), and 36.7\({}^{\circ}\), respectively (differences of 1.2\({}^{\circ}\) and 0.4\({}^{\circ}\)); the values of the angle of internal friction of soil with 8% cement content and 0.05%, 0.15%, and 0.25% fibre content were 37.0\({}^{\circ}\), 37.5\({}^{\circ}\), and 39.3\({}^{\circ}\), respectively (differences of 0.5\({}^{\circ}\) and 1.8\({}^{\circ}\)). Since the differences were very small, it was also possible that such small changes could be smaller than the result uncertainty; therefore, uncertainty analysis could be useful. [PERSON] et al. [3] investigated the effects of recycled tire polymer fibres (RTPFs) and glass fibres (GFs) on enhancing the strength/deformation properties of clays. A series of compaction, unconfined compression, and DST were performed on precisely prepared composite soils comprising clay with different amounts (0.5, 1.0 and 1.5%) of RTPFs and GFs having varying lengths (5 and 10 mm). For the clay reinforced with GFs and RTPFs, the shear strength was measured by a direct shear apparatus implementing the method of ASTM D3080 [12]. All specimens were sheared under constant vertical stresses of 55, 105, and 205 kPa. The differences between the peak friction angle of soil and soil composite were very small (from 0.1\({}^{\circ}\) to 3.3\({}^{\circ}\); smaller than 1\({}^{\circ}\) in four cases). Similar small differences between the peak friction angle of composite with various amounts of RTPFs and GFs could be seen (from 0.4\({}^{\circ}\) to 2.7\({}^{\circ}\); smaller than 1\({}^{\circ}\) in six cases).