## 2 Project Overview  
The city of Shijiazhuang, being the capital of the Hebei Province, is 300 km away from Beijing. Similar to other large cities in China, Shijiazhuang also suffers significant mobility issues. To tackle these issues, a subway transportation network is under construction. The majority of the subway system is built underground. A segment of the Metro Line 1 was constructed beneath the Hutuo River (Figure 1a) during April-June 2018, which is a major river in north China that discharges into the sea through the Hai River. The Hutuo River is blocked by an overflow dam in the Shijiazhuang area and becomes a man-made lake, which is a part of the Urban Forest Park of Shijiazhuang. The water level is quite steady with an average water depth of 3 m. Water movements in the shallow lake are usually minor. The length of the twin tunnels crossing the river is approximately 450 m (Figure 1b). Each tunnel was excavated by an earth pressure balance (EPB) shield machine. The shield is 6.24 m in diameter and 8.18 m in length. Shield driven tunneling, especially EPB tunneling, has been widely adopted in the construction of metro tunnels in China [16; 17; 18; 19]. It took approximately 50 min for the shield machine to bore (~20 min) and install (~30 min) one concrete lining if the process was smooth. The width of the tunnel lining with segmental rings is 1.2 m. However, the operation of boring and lining installation was often disturbed by various reasons. The shield machine bored 20 m on its most productive day, and 100 m during its best week. The time that the 8.18-m-long shield machine passed GPS sites varied from 6.5 to 34 h (Table 1). Figure 1: (**a**) A map showing the locations of the twin-tunnel project and the reference Global Positioning System (GPS) station in Shijiazhuang; (**b**) a sketch showing the locations of six rover GPS stations within the Hutuo River. The distance between the centerlines of the twin tunnels is 15.2 m (Figure 2a). The average depth of the tunnel centerline is 17 m below the riverbed, slightly less than three times of the diameter of the tunnels. The outside and inside diameters of the tunnels are 6 m and 5.4 m, respectively (Figure 2a,b). The excavations were conducted mainly in heterogeneous alluvial deposits (see Figure 2c) composed of silty clay and sand deposits. The groundwater level in the Shijiazhuang area is about 40 m below the land surface [20]. Thus, the excavation sections of the tunnels are approximately 20 m above the regional groundwater level. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Rover Stations** & **Distance to Base (m)** & **Shield-Head Arriving** & **Shield-Tail Leaving** & **Total Time Passing** \\  & & & **(Beijing Time \({}^{\star}\))** & **(Beijing Time \({}^{\star}\))** & **the Site** \\ \hline \multirow{3}{*}{**First Tunnel**} & GPS2 & 1679.584 & 16:16, April 28 & 12:28, April 29 & 20 h \\  & GPS4 & 1794.432 & 05:13, May 04 & 13:44, May 04 & 8.5 h \\  & GPS6 & 1917.414 & 12:25, May 14 & 16:17, May 15 & 28 h \\ \hline \multirow{3}{*}{**Second Tunnel**} & GPS1 & 1672.586 & 10:14, May 26 & 17:08, May 26 & 7 h \\  & GPS3 & 1795.117 & 02:56, Jun 03 & 12:40, Jun 04 & 34 h \\ \cline{1-1}  & GPS5 & 1912.585 & 21:10, Jun 12 & 03:44, Jun 13 & 6.5 h \\ \hline \hline \end{tabular}  
* Beijing time is 8 h ahead of the Universal Time Coordinated (UTC).  
\end{table} 
Table 1: Date and time that the shield machine passed each rover GPS. Figure 2: (a) A sketch illustrating the major parameters of the twin tunnels; (b) a site photo showing the twin tunnels; (c) a sketch illustrating the GPS-antenna monuments above the centerline of the second tunnel. Owing to the fact that this project is the first shield driven tunnel beneath the Hutuo River in Shijiazhuang, a significant concern regarding the riverbed deformation and tunneling safety occurred during the excavations.