## 2 Materials and Methods
### Repeated Load CBR Tests (RL-CBR)  
In the preparation of the specimens for testing, the soil, in its dry state, was mixed with the amount of water necessary to reach the optimum water content (\(w_{opt}\) = 13.9%), simulating the compaction characteristics of the soil obtained from modified Proctor tests [20]. The soil was then kept in controlled conditions (temperature: 20 \({}^{\circ}\)C, relative humidity: 65%) for 24 h, allowing uniformization of the soil moisture. The amount of soil necessary for one specimen was then divided into 5 portions. For the reinforced specimens, each portion  
Figure 1: Materials studied: (a) soil—sandy silt; (b) reinforcement—pine needles. Figure 2: Grain size distribution of the sandy silt collected from a salt pan wall of the Aveiro Lagoon, Portugal. of soil was mixed with the correspondent percentage of pine needles to reach the desired PNF. Each layer was then placed inside the test mould and compacted with 25 blows of a rammer (4.54 kg; drop height: 457 mm). After the 5 layers had been compacted into the cylindrical mould, illustrated in Figure 3b (5) (diameter: 152 mm; height: 125 mm), the specimen underwent a 96 h soaking process. Throughout this process, it is possible to monitor the swelling of the specimen. For levels of PNF \(\leq\) 1%, after that period, the swelling had stabilized; for PNF = 2%, the soaking process was terminated after 96 h, despite the swelling still varying. Between the end of the soaking process and the beginning of the RL-CBR test, a resting period of 15 min was given. After the specimen preparation, the mould was placed into the loading frame (Figure 3a, (2)) with 100 kN capacity. Throughout the test, an axial displacement was applied at a constant rate. The load cell (Figure 3a, (1)), 5 kN capacity, measures the force applied to the specimen, and the displacement linear variable differential transformer, LVDT with 25 mm stroke, (Figure 3b (4)), reads the displacement of the plunger. The data logger (Figure 3a, (3)) registers both variables in real time. The RL-CBR tests carried out followed a sequence similar to that found in the literature (Table 1): an initial phase similar to a standard CBR test, followed by phases of repeated loading. The standard procedure adopted herein is described in LNEC E198 [23]. Relative to the procedure described in ASTM D 1883-07 [11], the test carried out differs in the following aspects: the plunger moves at an axial displacement rate of 1.00 mm.min\({}^{-1}\)[23], instead of 1.27 mm.min\({}^{-1}\)[11]; the steel cylinder mould has a diameter of 152 mm [23], rather than 152.4 mm [11]. The initial phase (phase 0) of the RL-CBR test involves the loading of the specimen at a constant axial displacement rate of 1.00 mm.min\({}^{-1}\) up to a penetration of 2.5 mm (which is the reference value for calculating the CBR according LNEC E198 [23]) (Figure 4). Then, the maximum force reached is recorded and used as a reference for the remaining test. The load was applied at the upper surface of the specimen, using a 50 mm diameter plunger and under constant surcharge by three annular plates (24.52 kN each). This phase of the test is similar to the standard CBR test (with monotonic loading). Afterwards, the specimen is unloaded (at the same axial displacement rate of 1.00 mm.min\({}^{-1}\)) to the force of \(\sim\)20 N, a value that was obtained from a trial CBR test on an unreinforced soil specimen. This load ensured that the plunger is still in contact with the specimen and was considered to be smaller than 5% of the maximum force applied during the trial test (28 N) and the pre-load defined in LNEC E198 [23] procedure (49 N). Figure 3: RL-CBR test apparatus: (**a**) overview of the test system; (**b**) detail of the test setup (1—load cell; 2—mechanical load frame; 3—data logger; 4—LVDT; 5—mould with specimen). Once this displacement is reached, phase 1 begins and several cycles of loading and unloading are applied until the criteria for defining the required number of cycles is met.