## 4 Conclusions  
The main goal of this paper was to study the resilient response of a fine soil reinforced with natural fibres from the forest value chain (pine needles) and to evaluate their potential use on unpaved forest roads. Repeated load CBR tests (RL-CBR) were carried out on soil and soil-pine needles composite material (with 1 and 2% of needles incorporated). The force-penetration response, CBR value, recoverable and permanent displacements, and equivalent modulus were estimated. The effect of different load stages on the elastic response of both unreinforced and reinforced soil was also evaluated. The results of the initial phase (phase 0) of the tests, which is equivalent to the standard CBR tests, allowed us to conclude the following:  
* The force-penetration response and CBR value of the soil improved with the inclusion of pine needles. * The best response was obtained for specimens with 1% of needles since the use of a higher percentage of incorporation did not result in further improvement. Several causes for this are outlined in the document. Figure 8: Relation between E\({}_{\text{equ}}\) and maximum force at each repeated load phase (1 to 3). The three phases of repeated loading carried out in RL-CBR tests showed the following:  
* For each repeated load phase (1 to 3), as the number of cycles increased, the permanent displacement decreased, approaching a resilient response. * The total displacement of the unreinforced soil was the largest and decreased with the inclusion of the natural reinforcement. The total displacements caused by phases 2 and 3 (i.e., with the increment of load applied to the specimen) were smaller because most of the plastic displacement had already occurred during the initial phase of the test (phase 0). * The equivalent modulus quantified for the unreinforced soil was within the range of the [PERSON]'s modulus values reported in literature for the same type of soil (2 to 20 MPa for silts). * The equivalent modulus of the soil reinforced with 1% or 2% of pine needles increased relative to that of the unreinforced soil, confirming the beneficial effect of the natural fibre reinforcement. This parameter also indicates that the reinforced specimens exhibited a stiffer response. * The criterion used to define the number of cycles per repeated load phase may need to be adjusted to achieve a constant elastic displacement, i.e., a resilient response. The experiments conducted in this work showed that for unpaved forest roads, where the investment in soil characterization is often very limited (and often unused), RL-CBR tests can be a promising (simple, quick, and affordable) approach to obtain current design parameters. However, there are some open questions that need to be further investigated. The effect of reinforcement geometry, soaking process, specimens dimensions, and of applying other types of repeated loads should be included in future studies. In addition, the test procedure should be revised regarding the number of cycles per repeated load phase to ensure that the resilient response can be properly assessed. It should also be noted that aspects related to the durability (effect of mechanical, physical, chemical, environmental agents, etc.) of the natural fibres considered in this study were not evaluated (as they were beyond the scope of the work). Conceptualization, D.M.C., J.M. and M.P.-L.; methodology, D.M.C., J.M. and M.P.-L.; validation, D.M.C., D.R., C.M., J.M. and M.P.-L.; formal analysis, D.M.C., J.M. and M.P.-L.; investigation, D.M.C. ; resources, J.M. and M.P.-L.; data curation, D.M.C., J.M. and M.P.-L.; writing--original draft preparation, D.M.C., D.R., C.M. and M.P.-L.; writing--review and editing, D.M.C., D.R., C.M., J.M. and M.P.-L.; visualization, D.M.C., D.R., C.M. and M.P.-L.; supervision, J.M. and M.P.-L.; project administration, J.M. and M.P.-L.; funding acquisition, J.M. and M.P.-L. All authors have read and agreed to the published version of the manuscript.