EIC Associates were contracted in 2006 to construct over 13 km (8 mi) of a 115 kV power transmission line through Stamford, Darien, and Norwalk in Connecticut, USA. The final system includes three circuits consisting of 92 vaults and 62 handholds. EIC used a Robbins 1.5 m (60 in) Double Shield Rockhead (SBU-RHDS) to excavate two crossings under the Metro North Railroad in Darien, which turned out to be particularly difficult rock.
Due to the very poor ground conditions, G. Donaldson, a division of Hayward-Baker, Inc., was contracted to pre-grout utilizing two types of pre-grouting. The two types of grouting used were: horizontal grouting to stabilize the tunnel alignment, and vertical grouting around the road and rail structures to help reduce the risk of surface settlement.
Exploratory testing of the bore area found highly fractured meta-quartz monzonite ranging from 5,000 to 20,000 psi (35 to 140 MPa) UCS with Rock Quality Designations averaging 45%. Due to the unstable ground conditions, the location of the tunnel was lowered an additional 3 m (9 ft). However, further testing found extensive fractures here as well.
Rockheads are best suited for longer tunnels, where line-and-grade are critical such as in gravity sewers. EIC chose to use the best available option, the Robbins Double Shield Rockhead, due to the instability of the rock as well as the length of the bore.
The 1.5 m (60 in) diameter Double Shield machine, designed specifically for the highly fractured rock, featured a breasted plate cutterhead, which consisted of a steel plate in front of the standard cutterhead with slots for the single disc cutters. Grill bars were also added across the muck chutes to limit the size of rock allowed onto the conveyor.
Excavation started in the spring of 2008, operating six days a week in two ten-hour shifts. The machine bored the two crossings averaging 254 to 635 mm (10 to 25 in) per hour, due to rock hardness. Using articulation cylinders and stabilizer pads, the machine was able to maintain line and grade throughout the bore. EIC elected to use reinforced concrete pipe for the lining, which was laid behind the machine using a pipe jacking system.
Throughout the bore, field service technicians adjusted the penetration rate of the machine, as pre-grouted sections made the rock face softer and easier to excavate. This adjustment required either a decrease in thrust cylinder pressure or cutterhead speed to avoid plugging the muck removal system with too much debris. The machine’s variable speed electric drive allowed for fine tuning throughout the drive.