## 3 Methods  
### Development of Log Data Collection System Related to Truck Transport Time  
Bluetooth beacons and tablet computers were used to collect log data related to truck transport times, and the collected log data were uploaded to the cloud server and managed (Figure 2). A tablet computer mounted on the truck received signals from Bluetooth beacons installed on major routes inside and outside the mine where the haulage was performed. The tablet computer stored the information (beacon ID, location, and time) received from the Bluetooth beacon in its built-in memory. Log data stored during haulage operations were uploaded to a cloud server in a wireless communication area located outside the mine, which then stored and managed the data transmitted to it from tablet computers installed on multiple trucks. The Bluetooth beacons and tablet computers used to collect log data related to truck transport times should operate reliably even in harsh environments with high humidity and dusty underground mines. Bluetooth beacons and tablet computers are easy to maintain and manage, have good durability and after-service (A/S), and can be purchased easily in the market. Moreover, it is important that Bluetooth beacons have a minimum specified signal strength because tunnel conditions are not constant at all points in the mine. In this study, Beacon i3 (Hyunseung; Seoul, Korea) was selected, being convenient for controlling the signal strength and transmission period of beacons, having a long battery life, and including dustproof and waterproof design features. Beacon i3 proved its reliability in a study conducted by [PERSON] and [PERSON] [20]. For detailed specifications, please refer to the study mentioned above. Tablet computers that can receive signals from Bluetooth beacons and store log data are available in various price ranges; they should have sufficient storage capacity, the latest Wi-Fi and Bluetooth specifications, be based on the Android operating system version, and be easy to use. In this study, Galaxy Tab A 8.0 (Samsung; Seoul, Korea) was selected, and the detailed specifications are shown in Table 1. Moreover, the EAP110-Outdoor, which can cover a wide range and provide sufficient communication speed, was selected for the Wi-Fi access point (AP) to establish a wireless communication area in the study area. The product is equipped with waterproofing and dustproofing features; thus, it can be installed outdoors, providing a stable wireless communication range of approximately 200 m (Table 2). Figure 2: Conceptual diagram showing the operating principle of the log data collection system related to truck transport times using Bluetooth beacons and tablet computers. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{2}{c}{**Specifications**} \\ \hline \multirow{2}{*}{General} & Model & Galaxy Tab A 8.0 (2019) Wi-Fi \\ \cline{2-3}  & Battery capacity (mAh) & 5100 \\ \hline \multirow{4}{*}{Hardware} & Processor & 2 GHz octa-core \\ \cline{2-3}  & RAM & 2 GB \\ \cline{2-3}  & Internal storage & 32 GB \\ \cline{2-3}  & Expandable storage & Yes (microSD, up to 512 GB) \\ \hline Software & Operating system & Android 9 Pie \\ \hline \multirow{2}{*}{Connectivity} & Wi-Fi standards supported & 802.11 a/b/g/n \\ \cline{2-3}  & Bluetooth version & 4.20 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Specifications of the Galaxy Tab A 8.0 (Samsung; Seoul, Korea) used to receive Bluetooth beacon signals and store log data. The installation points of the Bluetooth beacons were selected based on the results of an analysis of the loading-haulage system, aerial photography, 3D drawings, and production logs. As a result, it was found that installing beacons in 8 loading areas, 4 dumping areas, and 10 major transport routes would be a suitable choice (Figure 3a). Figure 3b shows a schematic of the truck transport route. The locations and altitudes of Bluetooth beacons installed in the study area are also illustrated.