**2. Methods and Tools for GPR Survey**

#### *2.1. Experimental Setup*

Birhane et al. [11] pointed out that fouled ballast shows more signal scattering, especially at the boundary of the ballast and reinforced subgrade. In this study, to assess the fouling level of the ballast meticulously, further GPR surveys were conducted on lab-built and actual ballast tracks.

In the lab, the ballast tracks were simulated using four ballast boxes with distnict fouling levels. The four ballast boxes represent the stages of fouling on a ballast track: (1) clean (Box 1); (2) partially fouled (Box 2); (3) fully fouled (Box 3); and (4) Box 4, which has increasing fouling material content according to the depth, as illustrated in Table 1. The fouling material is added to the ballast aggregate in all cases at predetermined ratios and mixed before the boxes are filled.


**Table 1.** Test Track Ballast Boxes with Various Fouling Levels.

\* Bottom to upper layer (1st layer = bottom, 5th layer = surface layer), () = ratio of fouling material to clean ballast.

The ballast tracks consisted of 2 m × 2 m × 0.6 m wooden frames. The front side wall was composed of a transparent material to facilitate visual inspections. Figure 1 illustrates the construction procedure of the ballast boxes. Using an excavator, the gravel and fouling materials were mixed. This mixed material was then poured into the boxes, as shown in Figure 1.

**Figure 1.** Construction of ballast boxes.

Figure 2 shows the gradation of the ballast aggregates and the fouled materials. Only 0.2% of the ballast aggregates can pass through 4.75 mm and 0.075 mm sieves. In contrast, in the fouling silt material, 100% and 1.9% of the material can pass through the 4.75 mm and 0.075 mm sieves, respectively. The fouling index can be calculated by Equation (1), suggested by Selig and Waters [22]:

$$FI = P\_{\%}^4 + P\_{\%}^{200} \tag{1}$$

where *P*<sup>4</sup> % is the percentage by mass of the sampled ballast material finer than the 4.75 mm (No. 4) sieve, and *P*<sup>200</sup> % is the percentage by mass finer than the 0.075 mm (No. 200) sieve.

The corresponding fouling indices of each of the ballast boxes were 0.4, 34.23, 51.15, and 30.08 for Boxes 1 to 4, respectively, as shown in Table 1.

**Figure 2.** Grain size distribution of the ballast gravel and fouling silt.
