# Application of Direct Shear Test to Analysis of the Rate of Soil Improvement with Polyester Fibres
## 3 Results
### Improvement of Soil Shear Strength Parameters
The increase in the angle of internal friction could be confirmed since 3.81\({}^{\circ}\) was larger than the uncertainty of 0.520\({}^{\circ}\) + 0.540\({}^{\circ}\) = 1.060\({}^{\circ}\) (see expanded uncertainty with covariance in Table 4 for 0% and 1% fibres; four specimens). The increase in cohesion could be also confirmed since 7.186 kPa was larger than the uncertainty of 1.604 kPa + 1.820 kPa = 3.424 kPa (see expanded uncertainty with covariance in Table 4 for 0% and 1% fibres; four specimens). The increases in both the angle of internal friction and cohesion in the case of fibres in the amount of 1.0% was caused by the fact that all shear stresses were higher in comparison with the case of 0% fibres (compare blue numbers with red numbers in Table 3 and see a yellow line in Figure 4) and there was the highest shear stress from all shear stress values (274.2 kPa, corresponding to a normal stress of 300 kPa). It is possible that such a high value of shear stress was reached due to friction between the soil particles and fibres (see a great number of depth scratches on the fibre surface in Figure 8d). Regarding soil CS1 (see blue coloured numbers in Tables 3 and 7 and a green line in Figure 5), fibres in the amount of 0.5% increased the angle of internal friction of soil CS1 by 3.141\({}^{\circ}\) (9.70%) and also increased cohesion by 49.063 kPa (86.63%). The increase in the angle of internal friction could be confirmed since 3.141\({}^{\circ}\) was larger than the uncertainty of 0.522\({}^{\circ}\) + 0.622\({}^{\circ}\) = 1.144\({}^{\circ}\) (see expanded uncertainty with covariance in Table 5 for 0% and 0.5% fibres; four specimens). The increase in cohesion could be also confirmed since 49.063 kPa was much larger than the uncertainty of 1.636 kPa + 2.250 kPa = 3.886 kPa (see expanded uncertainty with covariance in Table 5 for 0% and 0.5% fibres; four specimens). These increases were caused by high values of all maximal shear stresses corresponding to all normal stresses. It should be noted that the mentioned maximal shear stresses were the highest in comparison with the maximal shear stresses obtained from other amounts of fibres (1.0% and 1.5%). It is possible that such high values of shear stresses were reached due to friction between the soil particles and fibres (see scratches on the fibre surface in Figure 9a,b). It is worth noting that the maximal shear stresses in the case of fibres in the amount of 1.5% were very low in comparison with the other amounts of fibres (see a red line in Figure 5). This could be caused by a specific behaviour of this soil CS1, which had a higher liquid limit, plastic limit, and optimal water content, but a lower maximum dry  
Figure 7: The SEM images of fibres before testing: (a) 50\(\times\) magnification; (b) 2000\(\times\) magnification. density than soil CI (see Table 1). It can be proposed that the specific behaviour of this soil CS1 also caused more soil particles to attach to the fibre surface and less scratches on the fibre surface, as shown in Figure 9c-f.  
Figure 8: The SEM images of fibres applied in soil CI after shearing: (**a**) 0.5% fibres, normal stress of 50 kPa; (**b**) 0.5% fibres, normal stress of 300 kPa; (**c**) 1.0% fibres, normal stress of 50 kPa; (**d**) 1.0% fibres, normal stress of 300 kPa; (**e**) 1.5% fibres, normal stress of 50 kPa; (**f**) 1.5% fibres, normal stress of 300 kPa. Figure 9: The SEM images of fibres applied in soil CS1 after shearing: (**a**) 0.5% fibres, normal stress of 50 kPa; (**b**) 0.5% fibres, normal stress of 300 kPa; (**c**) 1.0% fibres, normal stress of 50 kPa; (**d**) 1.0% fibres, normal stress of 300 kPa; (**e**) 1.5% fibres, normal stress of 50 kPa; (**f**) 1.5% fibres, normal stress of 300 kPa. The behaviour of soil CS2 was different in comparison with the aforementioned soils (see Tables 3, 6 and 7 and Figure 6). Fibres in the amount of 0.5% increased the angle of internal friction of soil CS2 by 9.755\({}^{\circ}\) (47.00%) but decreased cohesion by 4.293 kPa (6.88%).