# Application of Direct Shear Test to Analysis of the Rate of Soil Improvement with Polyester Fibres
## 1 Introduction
Small changes in both the peak friction angle and cohesion could be seen between 0.5% and 1% composite, e.g., 1\({}^{\circ}\) and 0.2 kPa (RTPF--L 10 mm), and 0.4\({}^{\circ}\) and 2.6 kPa (GF--L 5 mm). Therefore, it was also possible in this case that such small changes could be smaller than the result uncertainty, so uncertainty analysis could be useful. [PERSON] et al. [4] studied the mechanical characteristics of clay improved with shredded polyethylene terephthalate (PET). PET was provided by a local plastic waste repository. It comes from recycled water, beer, and soda bottles and was cleaned using specific methods for cleaning and recycling plastic waste. PET was shredded into irregular shapes with sizes ranging from 3 mm to 12 mm and was randomly distributed in percentages of 2%, 4%, and 6%. DST was performed according to the Romanian standard STAS 8942/2-82 [13] applying three normal stresses (100 kPa, 200 kPa, and 300 kPa). The values of the peak angle of internal friction of soil with 2%, 4%, and 6% PET were 20.93\({}^{\circ}\), 20.20\({}^{\circ}\), and 24.40\({}^{\circ}\), respectively, so the improvement could be 2.10\({}^{\circ}\), 1.37\({}^{\circ}\) and 5.57\({}^{\circ}\), respectively (the peak angle of internal friction of soil without PET was 18.83\({}^{\circ}\)). Values of the residual angle of internal friction of soil with 2%, 4% and 6% PET were 12.21\({}^{\circ}\), 11.33\({}^{\circ}\), and 11.92\({}^{\circ}\), respectively, so the improvement could be \(-0.17^{\circ}\), \(-1.04^{\circ}\), and \(-0.45^{\circ}\), respectively (the residual angle of internal friction of soil without PET was 12.37\({}^{\circ}\)). It was also possible in this case that such small changes could be smaller than the result uncertainty; therefore, uncertainty analysis could be useful. [PERSON] et al. [6] studied the mechanical properties of Xinyang red clay improved by lime and fly ash. The dry red clay soil that passed through the 2 mm sieve was divided into four groups. The first group was the single lime group, adding lime by 2%, 5%, 8%, and 11%. The second group was the single fly ash group, adding fly ash by 5%, 10%, 15%, and 20%. The third group was the mixed lime-fly ash group, and the mass ratios of lime-fly ash to dry soil were 1:2:7, 1:3:6, and 1:4:5. The fourth group was the control group, without lime or fly ash. DST was used to determine the soil shear strength parameters of the improved soil, applying three normal stresses (100 kPa, 200 kPa, and 300 kPa). The values of the angle of internal friction of soil with 2%, 5%, 8%, and 11% lime content were 14\({}^{\circ}\), 14\({}^{\circ}\), 15\({}^{\circ}\), and 17\({}^{\circ}\), respectively, so the improvement could be 1.5\({}^{\circ}\), 1.5\({}^{\circ}\), 2.5\({}^{\circ}\), and 4.5\({}^{\circ}\), respectively (angle of internal friction of soil was 12.5\({}^{\circ}\)). The values of the angle of internal friction of soil with 5%, 10%, 15%, and 20% fly ash content were 11\({}^{\circ}\), 13\({}^{\circ}\), 12\({}^{\circ}\), and 10\({}^{\circ}\), respectively, so the improvement could be \(-\)1.5\({}^{\circ}\), 0.5\({}^{\circ}\), \(-\)0.5\({}^{\circ}\), and \(-\)2.5\({}^{\circ}\), respectively. The values of the angle of internal friction of soil with lime-fly ash-dry soil ratios of 1:2:7, 1:3:6, and 1:4:5 were 12.8\({}^{\circ}\), 10.2\({}^{\circ}\), and 13\({}^{\circ}\), respectively, so the improvement could be 0.3\({}^{\circ}\), \(-\)2.3\({}^{\circ}\), and 0.5\({}^{\circ}\), respectively. Similarly, in this case, it was possible that such small changes could be smaller than the result uncertainty, so uncertainty analysis could be useful. [PERSON] et al. [11] used the shear strength parameters obtained from DST (with four applied normal stresses of 100, 200, 300, and 400 kPa) as guide values for predicting soil shear strength parameters using combined data and three different machine learning models. In total, 83 values of the angle of internal friction and 83 values of cohesion were obtained.