The world-famous Youth Olympic Games were held in Nanjing in summer 2014, bringing 40,000 visitors. This number, combined with the 750,000 people who rode the city’s rails on a daily basis, put the city in need of an extended metro system.
Two new metro lines were built to accommodate the anticipated increase in passengers. Robbins supplied four 6.5 m (21.4 ft) diameter EPBs with mixed ground cutterheads for Line 3 Lot 11 and Line 10 Lot 5, 40.2 km (25.0 mi) and 41.4 km (25.7 mi) long, respectively. The EPBs were part of about 40 TBMs used for the Nanjing Metro expansion.
Both lines were bored in soft ground consisting of silty sand, sandstone, small pebbles and man-made materials. The Qinhuai River, old historical structures, a railway and a viaduct were in close vicinity of the lines. Due to the ground conditions and nearby structural constraints, customized excavation methods were employed. The thrust force was kept very low at 6,000 to 9,000 kN (1.4 to 2.0 million lb), continuous monitoring was used to maintain proper alignment and earth pressure, and foam additive was applied to make the ground less sticky and reduce torque.
Two of the four 6.5 m (21.4 ft) diameter TBMs were launched for the 2,500 m (8,202 ft) Line 10 Lot 5 in December 2011 and February 2012. The second pair of machines for 2,030 m (6,660 ft) Line 3 Lot 11 began boring soon after in January and March 2012. All machines were assembled and launched from 10 m (33 ft) shafts.
By August 2012, the Line 10 EPBs had excavated 720 m and 600 m (2,362 ft and 1,968 ft), with advance rates of 40 mm (1.6 in) per minute. Over at Line 3, the excavation had progressed as far as 600 m (1,968 ft) with rates topping 60 mm (2.4 in) per minute. Settlement for both lines stayed below the strict required limits of less than 20 mm (0.8 in). By 2013, all four machines had completed their respective tunnels with very favorable results.
The new Lines 3 and 10, now complete, combine with Nanjing’s two existing metro lines to handle the increase in rail riders. The lines will also serve as part of the municipal government’s extensive plan to build 17 new rail routes totaling 600 km (370 mi) across the Yangtze River Basin by 2030.