# Application of Direct Shear Test to Analysis of the Rate of Soil Improvement with Polyester Fibres
## 1 Introduction
Also, the simulations proved that the greatest MU values were obtained when the DST measurands were totally negatively correlated. Therefore, the authors stated that further research was needed for accurate estimation of the correlation coefficient values. Concerning the use of scanning electron microscopy (SEM) in an analysis of specimens after shearing, [PERSON] et al. [23], on the basis of the micrographs of the shear plane of a polypropylene fibre-line specimen, stated that it was clearly seen that, after shearing, some fibres were left in the soil with part of the length exposed to the air and, on the other hand, some threadlike grooves appeared in the shear plane. This was likely due to the strong resistance of the fibres against tension. Part of the fibre was pulled out from the soil when shearing occurred and the fibre itself was not sheared off. From the abrasion trace in the fibre surface, it was indicated that the fibres strengthened the soil due to friction between the fibres and soil particles. To evaluate the factors affecting the interfacial strength properties of polypropylene fibre-reinforced soil, [PERSON] et al. [24] carried out a single fibre pull-out test using a modified special apparatus. The SEM image taken from a single fibre-reinforced specimen after the pull-out test showed a number of visible scratches along the fibre's longitudinal direction on the surface. The authors stated that this may have resulted from the plowing of angular hard particles into the fibre body during the shear process. It confirmed that an interlock force developed between the soil particles and the fibre surface. [PERSON] et al. [25] introduced an analysis of the influence of the fibre type on the polymer matrix/fibre bond using a natural organic polymer stabiliser. The study compared the effect of polypropylene and wool fibres on the mechanical properties of natural polymer-based stabilised soils. The authors stated that SEM proved to be a useful tool for the direct study of polymer-soil matrix interfaces. According to the authors, the main factors that affected the adhesion between the fibres and the soil were: (a) the cohesive properties of the polymer-soil matrix; (b) the compression friction forces appearing on the surface of the reinforcing fibre due to the shrinkage of the soil; and (c) the shear resistance of the polymer-soil matrix due to the surface form and roughness of the fibre. The authors also came to the conclusion that the loss of strength observed was not only caused by the variation in the fibre type but was more importantly the result of the effect of the differing properties of different soil types. [PERSON] et al. [26] applied X-ray diffraction and SEM analyses to reveal the micromechanics of the deformation and strength of grout. In the paper cited, the SEM results of concrete grout in sand for 28 days under 0 kPa, 100 kPa, and 200 kPa are presented. In the case of a pressure of 0 kPa, one can see that there was a large amount of pores and many pores were connected. In the case of a pressure of 100 kPa, there were fewer pores that were less connected. The maximal pore size was about 25 \(\upmu\)m. The SEM images of the grout under a pressure of 200 kPa showed many fewer pores that had smaller pore sizes (about 7-15 \(\upmu\)m). The authors stated that when the pressure was increased, the grout material particles became denser and the amount of hydration products rose, resulting in an increase in the consolidation deformation and UCS of the grout. Another significant study showing the importance of the roughness of the fibre surface for improving the properties of the soil-cement-fibre composite is presented in [27]. To investigate the influence of the roughness of the fibre surface on improvement, cylindrical specimens of stabilised sandy soil, unreinforced and reinforced with polypropylene and fossil fibres, were tested.