## 3 Results and Discussion
### CBR Test--Phases 1 to 3, Repeated Loading  
The results of phases 1 to 3 of the RL-CBR tests allowed the assessment of the evolution of the permanent and recoverable displacements suffered by the specimens until a resilient response is reached, i.e., when the application of a load only causes elastic deformations in the material. Figure 6 illustrates the total displacement, as well as the recoverable (elastic) and permanent (plastic) components of the penetration displacement observed during each loading cycle (average for three specimens per condition). The largest component of the displacement is the recoverable portion, except for the first cycle (phase 0) in which most of the permanent displacement takes place. Both the  
Figure 6: Total, recoverable, and permanent displacement per cycle for different PNF (average of three specimens): (**a**) total accumulated displacement; (**b**) percentage of permanent and recoverable displacement; (**c**) recoverable displacement; (**d**) permanent displacement. recoverable and permanent displacement decreased as the number of cycles increased. In theory, the permanent displacement will tend to decrease towards zero while the recoverable displacement will also decrease until it reaches a constant minimum value as the number of loading cycles increases. Although the permanent displacement at the last cycle of phase 1 was very small (\(\sim\)0.02 mm), the corresponding recoverable displacement had not yet stabilised (after 35 loading cycles). This shows that the criterion used to define the number of cycles may need to be adjusted to achieve a constant elastic displacement, i.e., a resilient response. The total displacement of the unreinforced soil (PNF = 0%) was the largest and decreased with the inclusion of the natural reinforcement (pine needles). The lowest total displacement was observed for PNF = 2%. The presence of pine needles (PNF = 1% or PNF = 2%) did not change the proportion of recoverable and permanent displacement per cycle observed for the unreinforced solution (PNF = 0%). A similar behaviour was observed for the following repeated load stages considered (phases 2 and 3). However, the total displacement due to phase 1 is substantially greater than that caused by phases 2 and 3 (Figure 6). This is because the plastic displacement at the beginning of the test (phase 0, monotonic loading) is extremely high and around 68 to 70% of the total displacement (depending on the PNF considered). A densification of the specimen below the plunger is observed, which influences the behaviour of the material and leads to a stiffer response. In the following cycles, the reduction in total displacement is substantial because much of the plastic displacement has already occurred. The total displacements caused by phases 2 and 3 are smaller because, as mentioned before, most of the plastic displacement has already occurred (the plastic displacement caused by the first cycle of phase 2 and of phase 3 was 12 to 14% of the total displacement (varied with PNF)). To assess the soil response to repeated loading, an equivalent elastic modulus (Eequ) was estimated (Equation (2)). This parameter was calculated for all loading cycles of phases 0 to 3, although the most representative value refers to the last cycle of each phase (Eequ for the resilient response of the material). Figure 7 shows the evolution of Eequ with the number of loading cycles for the different conditions studied (PNF of 0%, 1%, and 2%). The results obtained for phase 1 show that the equivalent modulus tends to increase with the number of load cycles. This parameter is influenced by the recoverable displacement and should reach a maximum constant value when this portion of the displacement is constant, i.e., when the material reaches its resilient response.