# Application of Direct Shear Test to Analysis of the Rate of Soil Improvement with Polyester Fibres
## 5 Conclusions  
From the obtained results, the following conclusions can be made:  
1. The tested fibres improved the soil shear strength parameters differently; both the angle of internal friction and cohesion were increased for soil CI using 1.0% fibres and for soil CS1 using 0.5% fibres. For soil CS2, the angle of internal friction increased but cohesion decreased.
2. SEM is a useful tool for exploring the fibre surface after shearing. Suitable fibre scratch resistance can significantly increase the shear strength parameters, but there are additional fibre properties contributing to the improvement rate, such as fibre surface roughness, adhesion of soil particles to the fibre surface, etc., which requires further study.
3. It is not recommended to carry out DST with only three specimens since different combinations of three specimens provide different results.
4. Improvement rates of shear strength parameters of soil using various materials require determination of their uncertainty so that the improvement can be confirmed. Determination of the uncertainty of shear strength parameters is also important in the case when they are used as guide values for the prediction of soil shear strength parameters using machine learning models.
5. In this study, correlation coefficients between normal stresses and shear stresses existed and were not negative. Covariance between normal stresses and shear stresses has a negligible influence on shear strength parameter uncertainty.
6. The worst-case strategy, considering linearly added uncertainty contributions \(c(\sigma_{i}).u(\sigma_{i})\) and \(c(\tau_{i}).u(\tau_{i})\) and not considering correlation coefficients, provides too high uncertainty and is not recommended.
7. This study also has various limitations. The tests were carried out only for clayey soils with insufficient shear strength. It will be useful to carry out further tests for silty soils and sandy soils.
8. A further limitation of this study is the focus of the research only on shear strength parameters, without dealing with deformation properties. Since soil mixed with fibres can be compacted to reach a maximum dry soil density obtained from a standard Proctor test, it is proposed that there will not be a problem with excessive deformation. It is planned to carry out an oedometer test of soil mixed with fibres using an oedometer with a diameter of 30 cm so that the deformation in practical applications can be quantified.
* This study presents the results of laboratory tests where the preparation of specimens can be different from the preparation of soil in practical applications. Using a modern soil stabiliser with a powerful milling and mixing rotor can minimise the difference. In the case of soil where the preparation of soil in practical applications does not provide similar conditions as in the laboratory, verification using in situ testing is required.  
Conceptualization, G.N., S.M., F.G. and J.G.-P.; methodology, G.N., S.M., F.G. and J.G.-P.; formal analysis, [PERSON], S.M., F.G. and J.G.-P.; investigation, S.M., F.G. and J.G.-P.; writing--original draft preparation, G.N.; writing--review and editing, G.N., S.M., F.G. and J.G.-P.; project administration, F.G.; funding acquisition, F.G. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Operational Programme Integrated Infrastructure: Application of innovative technologies focused on the interaction of engineering constructions of transport infrastructure and the geological environment, ITMS2014+ code 313011 BWS1, and the project was cofounded by the European Regional Development Fund. The APC was funded by 313011 BWS1.  
Not applicable.  
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
The authors declare no conflicts of interest.