## 3 Results and Discussion  
### CBR Test--Phase 0, Monotonic Loading  
Figure 5 summarises the main results of the initial phase of the RL-CBR tests performed (phase 0, with monotonic loading). As mentioned before, this phase corresponds to the CBR standard test. For this reason, the force-penetration response (Figure 5a) and the CBR values (Figure 5b) for three specimens per condition are defined. Herein, the CBR values are obtained as the ratio of the force measured during the test for a value of penetration of 2.5 mm to the corresponding force for a standard material, typically a well-graded crushed stone (1355 kgf or 13.29 kN, defined in LNEC E198 [23]); the mean values of CBR are also included in Figure 5b. Figure 4: Force–penetration response of unreinforced soil (specimen 1) subjected to a standard CBR loading (phase 0), followed by 3 stages of repeated loading (phases 1 to 3). The force-penetration response and the CBR value are improved by the incorporation of the natural fibres in the soil. The inclusion of the pine needles led to a stiffer response for all reinforced specimens. The CBR value increased by 45% for PNF = 1% and 18% for PNF = 2%, compared to unreinforced soil. The specimens with 1.0% of pine needles (PNF = 1%) showed the best force-penetration response and the highest CBR values, indicating that this is the ideal PNF to improve the behaviour of this soil. The redistribution of stresses and strains within the soil, induced by the inclusion of fibres, is the main cause of the improved soil response. This process occurs when the fibres transfer tensile forces to the soil through interaction mechanisms. However, this process is limited because the inclusion of fibres changes other relevant parameters, such as density and water content of the mix, which greatly influence its behaviour. The bearing capacity of fine soils is greatly influenced by their water content and density. Figure 5c illustrates the density of the specimens, dry, wet before soaking (as prepared), and wet after soaking. The density of the reinforced specimens decreases, particularly for PNF = 2%. The reduced density can be attributed to several factors: (1) the fibres tend to restrict the packing of soil grains; (2) the excessive length of the fibres makes it difficult to mix them into the soil, resulting in heterogeneous orientation and spatial distribution of the fibres in the specimen; (3) part of the specimen is occupied by fibres that are less dense than the soil; (4) the compaction process is made more difficult due to the elastic behaviour of the fibres and the voids created next to the fibres, which absorb the impact forces generated by the rammer. In contrast, the water content of the reinforced specimens is higher than in the unreinforced soil. This change is more pronounced for PNF = 2% (see Figure 5d). Possible reasons  
Figure 5: Influence of the percentage of natural fibres (PNF) on monotonic loading phase (phase 0) of the CBR tests carried out: (**a**) force–penetration response; (**b**) CBR value (95% confidence interval in brackets); (**c**) density, before the test (average of three specimens; 95% confidence interval in brackets); (**d**) water content, before the test (average of three specimens; 95% confidence interval in brackets). for the increase in water content include the following: (1) during the soaking process, water is absorbed by both the soil and fibres; (2) the needles, being hydrophilic with high absorbent capacity, expand more than the soil, creating preferential paths for water to flow through; (3) the increase in voids near the fibres also contribute to an increase in water flow into the specimens. Higher PNF may have also reduced the mobilisation and transfer of stresses between the fibres and the soil, thereby decreasing the soil response. For instance, the agglomeration of fibres (overlapping in certain areas of a specimen) and the bending and breaking of some fibres contribute to decreasing the effectiveness of the mobilisation of the fibres.