## 1 Introduction  
Currently, the demand for sustainable and resilient infrastructure are paramount. This paper analyses the case of unpaved forest roads, particularly focusing on considering the incorporation of pine needles as an alternative and sustainable solution, to promote a reduction of the need for maintenance works. Traditional solutions for unpaved roads make use of natural resources, namely good-quality aggregates. Typically, unpaved roads are formed of a base layer overlaying the natural subgrade. The base layer is unbound and often is formed of good-quality aggregates. The quantity of natural aggregates needed in such traditional solutions impose large impacts on the environment. As an alternative to these materials, residues and byproducts should be considered [1]. The incorporation of these residues in new solutions can be beneficial, as it promotes circularity and sustainability. The advantages of such solutions include lower costs and reduced environmental impacts, as residues are reassessed to be utilised as building materials. For the case of unpaved forest roads, it is important to evaluate how alternative solutions respond to loads associated with logging operations [2]. To ensure forest roads exhibit an adequate and resilient response, when considering new solutions, it is essential to assess the road resistance to prevent (or at least minimise) deterioration problems [3]. Alternative solutions have been studied. For example, [PERSON] [1] and [PERSON] et al. [3] considered the use of industrial byproducts, assessing the soil behaviour, studyingthe performance of the solution, and conducting triaxial and California Bearing Ratio (CBR) tests, respectively. Byproducts from the paper industry can also be incorporated into road solutions. [PERSON] et al. [4] created a mixture between clay soil, waste, and agglomerant in different conditions and assessed the compressive strength in relation to the proportions of this mixture. [PERSON] et al. [5] considered the use of fly ash as an alternative hydraulic binder replacing the cement in road projects. Most design methods available in the literature refer to paved roads. These are usually based on mechanical-empirical approaches. However, the procedure of this type of design method does not focus on granular layers, but on bound materials. To optimize the performance of granular layers, how they behave when subjected to cyclic traffic loads must be considered [6]. Given that, in most unpaved roads, the materials are unbound, this is a crucial aspect in design [7]. Developing a straightforward laboratory test that can provide relevant mechanical characteristics of granular materials is important. However, some of the test equipment is expensive, so it is valuable to find a more affordable test equipment and method that provides these mechanical characteristics of the materials [7]. A possible solution for a simple laboratory test that can provide relevant information is the repeated load CBR test (RL-CBR). One of the most often utilised parameters in road design, for both paved [8; 9] and unpaved roads [10], is the CBR value, measured through CBR tests. The conventional CBR test is a penetration test, where a plunger moves at a constant axial displacement rate into a cylindrical soil specimen, formed of layers of compacted soil placed in a steel mould. There are different test standards for the CBR test; the most common is ASTM D 1883-07 [11]. While the conventional CBR test is only carried out with one load cycle (monotonic), the RL-CBR test is based on a standard CBR test with an increased number of load cycles [12]. There is no standardised procedure for the RL-CBR test; however, a number of studies including this type of test are available in the literature [6].