# Application of Direct Shear Test to Analysis of the Rate of Soil Improvement with Polyester Fibres
## 4 Discussion  
The obtained values of the shear strength parameters of soils CI and CS1 improved with fibres TEXZEM PES 200, showing significant improvement. Similarly as in previous studies [18; 19; 20], the optimal amounts of fibres were \(0.5\%\) or \(1\%\), depending on the type of soil. In the case of soil CS2, even if there was no simultaneous increase in both shear strength parameters, it is proposed that, in many cases, regarding subsoil bearing capacity, slope stability, and soil pressure, an increase in the angle of internal friction of soil CS2 by \(9.755^{\circ}\) (\(47.00\%\)) and decrease in cohesion by \(4.293\) kPa (\(6.88\%\)) can be considered as an improvement. However, a calculation check is required. It is also necessary to note that the mentioned improvement only applies to the cases of static loads. In the cases of dynamic loads induced from, e.g., urban vibration, industrial machinery, earthquakes, wind loads, and sea waves, among others [27], further tests should be carried out. In any case, based on the findings presented in [27], the issue regarding the specific strain level required for mobilisation of the tensile strength of the fibers should be investigated. The SEM images taken in this study showed visible scratches on the fibre surface, which were present not only locally but also along the fibre's longitudinal direction on the surface, similarly as presented in [24]. It can also be proposed that this may result from the plowing of angular hard particles into the fibre body during the shear process, due to the suitable scratch resistance of the tested fibres. The SEM images also confirmed the conclusion presented in [25] that soil properties are very important since the surfaces of the fibres were very different after shearing in soil CI and CS1. The contact area between the soil and fibres is also important since, as can be seen in [26], higher consolidation stress makes the tested material denser with less pores of smaller size. So, it is proposed that higher normal stress provides larger contact area between the soil and fibres compared to lower normal stress; therefore, in the case of higher normal stress, there are more scratches of larger depth. However, the issues presented above are very complex and require further study. It is planned to test the same soil with fibres TEXZEM PES 200 and other fibres. In this study, the minimal uncertainty of the angle of internal friction \(\varphi\) was \(0.422^{\circ}\) (see expanded uncertainty with covariance in Table 6; 0% fibres; four specimens) and the minimal uncertainty of cohesion c was \(1.250\) kPa (see expanded uncertainty with covariance in Table 6; 0% fibres; four specimens). When comparing improvement, the difference between two cases should be calculated and the difference (improvement) should be larger than the sum of the uncertainties of two cases, e.g., larger than \(2.0.422^{\circ}\) = \(0.844^{\circ}\) and \(2\). \(1.250\) kPa = \(2.500\) kPa. In this study, the differences should be larger than the mentioned values \(0.844^{\circ}\) and \(2.500\) kPa since the values of \(0.422^{\circ}\) and \(1.250\) kPa were minimal (specific values to be compared are calculated and presented in the Section 3.1). It is therefore useful to check if the difference (improvement) in shear strength parameters of \(0.5^{\circ}\)[1]; \(0.1^{\circ}\) and \(0.4^{\circ}\), \(0.2\) kPa [3]; \(-0.17^{\circ}\) and \(-0.45^{\circ}\)[4]; \(0.3^{\circ}\) and \(0.5^{\circ}\)[6] can be confirmed based on result uncertainty analysis.