Despite the conversation taking place over the phone, one could see just how excited he was about the breakthrough. Chris Hodgkins, vice president of the MAT Concessionaire, talked about the celebration by the workers on site, the number of jobs this project has created in the area — 6,000 — and the work ahead.

It was 31 July when the 12.8m-diameter TBM, Harriet, finished the eastern bore of 3,693ft (1,126m) to breakthrough into bright Miami sunshine. It was much like a baseball game in the dog days of summer. Your team has a rough start in the early innings, grabbed a few runs in the fifth and takes the lead just in time for the seventh inning stretch. There is a brief rest, but with the anticipation of waiting to see if the momentum lasts through the next two innings.

The Miami Access Tunnel project nearly saw cancellation during the financial crisis when an equity partner dropped out, and delayed its first launch thanks to a rogue council member from a nearby town who questioned the dumping location for spoil. But the project moved forward, nonetheless, to complete the first bore this summer and relaunch Harriet on October 29 for the second of two dual tunnels.

"We’ve certainly had some very challenging geotechnical issues that we were able to address in a timely way. There were a lot of firsts on this project," Hodgkins said in August.

It’s the first tunnel project in North America for Bouygues, whose Bouygues Civil Works Florida (BCWF) is the contractor for the MAT Concessionaire of Bouygues Travaux Publics and Meridiam Infrastructure. MAT is part of a P3 with the City of Miami, Miami-Dade County and the Florida Department of Transportation. And it’s the first big tunnel for Florida.

According to Roger Storry, geotechnical manager for Bouygues, the local geology, proximity to the coast and high groundwater levels have previously deterred major underground construction in south Florida. "Prior to this project, tunnelling in Florida was limited to small diameter (less than 8ft [2.4m]) utility microtunnels and a few short and shallow cut-cover type road tunnels," he says.

At 37ft (11.3m) internal diameter, these large diameter tunnels constructed through Florida’s carbonate sedimentary strata, required one of the state’s most comprehensive ground investigations, and one of the nation’s largest TBMs. The Herrenknecht machine arrived in Miami in summer 2011 and began its journey below the sea the following November.

The Turnaround

To stay within FDOT’s right of way, the job site is located within the median of the MacArthur Causeway.

Contractor BCWF has launch and reception pits of roughly 100ft (30.5m) wide and 400ft (121.9m) long, each located on Watson and Dodge Island (the Port of Miami), respectively.

"Because of limited project site spacing and to improve future traffic flow, the tunnels were brought together at both ends into one single shaft, which resulted in a separation between the tunnels of only 14ft [4m] at break in/ out," explains Roger Storry, geotechnical manager for Bouygues.

To ensure the tunnels remained stable during construction, subcontractor Malcolm Drilling installed cutter soil mixing (CSM) barrettes between the two tunnels: 820ft [250m] on Watson Island where the TBM launched and 740ft [225m] on Dodge Island.

In the reception pit of approximately 100ft (30.5m) wide, BCWF designed a turntable to rotate the TBM shield, before reattaching the 419ft (127.7m) of trailing gear for a total TBM length of 457ft (139.3m).

A turntable would reduce impact to traffic and operations at the Port of Miami. Bouygues had used a similar concept on the Boulevard Peripherique Nord de Lyon project in France — a dual tunnel of 3.2km that opened in 1997. Design work started in November 2011 for the turntable, made up of Tefi on on stainless steel. This solution is regularly used by Bouygues for shift bridge spans into position.

Fabrication took place in China from April to June, and the final product, weighing 120t and measuring 32ft (9.7m) in diameter and 5ft (1.5m) high, shipped out, just in time for the breakthrough later in the summer.

It took BCWF a total of 29 days to detach the 2,000t shield from the rest of the TBM, place it on the turntable and complete the u-turn. Actual sliding and turning of the shield lasted for nine days.

Each of the six gantries, weighing 300t, was pulled one at a time by a Goldhofer SPMT vehicle, with the aid of a crane, to be placed on rails for the westbound tunnel. The TBM began operation again for the 3,980ft (1,213m) bore back to Watson Island on October 29.

Layer Up

On both bores, the TBM passes through eight different geological layers, one of which is the Key Largo Limestone. "From the ground investigations, this geology was found to exist on one third of the length of the tunnel alignment conceding with the deepest point under the Biscayne Bay," Storry says.

This highly dissolved, porous and permeable corraline limestone would not permit the traditional routine cutterhead inspections and maintenance on the TBM, explains Storry.

Instead, Malcolm Drilling constructed eight refuge chambers on the alignment using CSM panels, allowing crews to safely access the cutterhead. Chambers are located, one on each tube, at cross passages 1, 2, 4, and 5. Depending on location, they varied from 65ft to 100ft (19.8m to 30.5m) deep and were approximately 40ft to 50ft (12.2m to 15.2m) wide by approximately 11ft to 50ft (3.4m to 15.2m) long, says BCWF’s commercial manager Joe Folco.

But that’s not all. The Key Largo Limestone is so extremely porous, BCWF has adapted the EPBM to excavate without confinement overpressure, using a water pressure balance/ control mode on the TBM — water control process (WCP).

"For the WCP mode, a hydraulic circuit, much like that of slurry TBMs, was connected to the end of the TBM’s screw conveyor with an inline crusher. Spoils were evacuated through the piping to a separation treatment plant in which the water and solids were separated. The water was re-injected into the circuit and the solids disposed of," Folco explains.

That is in addition to extensive pre-excavation grouting on both drives. "Nicholson Construction performed this formation grouting program using a unique thyxotropic grout we developed for the specific conditions," Storry says.

The program was executed on-shore and over-water, and consisted of more than 1,000 holes 125ft (38.1m) deep, and 65,000cu. yards (49,700cu.m of grout. Grout traveled from the onshore batching plant to the offshore equipment through a pipeline bridge. This concept met environmental regulations for the port’s shoreline, and allowed the process to be unaffected by changing tides.

Final breakthrough at Watson Island is planned for April 2013.

"Although uniquely challenging, the mining of the first bore was quite successful," Folco says. "We expect the mining of the second bore to be equally as successful. Notwithstanding that, we are looking at optimising our spoil handling operation while the TBM mines in WCP mode as well as managing the overall cutterhead clogging while the TBM mines through finer material."

The Port of Miami Tunnel is a design-build- finance-operate-and-maintain project. Under the concession agreement, FDOT will make milestone payments during the construction period, upon the achievement of contractual milestones. Once construction is completed, the concessionaire will receive payments contingent upon service quality. The tunnel will be returned to FDOT at the end of the contract in October 2044